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Root parasitic weeds of the Orobanchaceae, such as witchweeds (Striga spp.) and broomrapes (Orobanche and Phelipanche spp.), cause serious losses in agriculture worldwide, and efforts have been made to control these parasitic weeds. Understanding the characteristic physiological processes in the life cycle of root parasitic weeds is particularly important to identify specific targets for growth modulators. In our previous study, planteose metabolism was revealed to be activated soon after the perception of strigolactones in germinating seeds of O. minor. Nojirimycin inhibited planteose metabolism and impeded seed germination of O. minor, indicating a possible target for root parasitic weed control. In the present study, we investigated the distribution of planteose in dry seeds of O. minor by matrix-assisted laser desorption/ionization–mass spectrometry imaging. Planteose was detected in tissues surrounding—but not within—the embryo, supporting its suggested role as a storage carbohydrate. Biochemical assays and molecular characterization of an α-galactosidase family member, OmAGAL2, indicated that the enzyme is involved in planteose hydrolysis in the apoplast around the embryo after the perception of strigolactones, to provide the embryo with essential hexoses for germination. These results indicate that OmAGAL2 is a potential molecular target for root parasitic weed control.

Damage caused by Orobanchaceae root parasitic weeds is a substantial agricultural problem for global food security. Many studies have been conducted to establish practical methods of control, but efforts are still required for successful management. Seed germination of root parasitic weeds requires host-derived germination stimulants including strigolactones (SLs). Studies on SLs have revealed that a butenolide ring is the essential moiety for SL activity as a germination stimulant. Interestingly, recent studies have revealed that butenolide hormones regulate the biosynthesis of secondary metabolites and mediate communication in actinomycete bacteria. Because of the structural similarity between SLs and the bacterial butenolides, we evaluated the germination stimulatory activity of butenolides isolated from Streptomyces albus J1074 on root parasitic weeds. These butenolides were found to specifically induce seed germination of Orobanche minor. Our findings contribute to understanding the molecular mechanisms of germination stimulant perception and to the development of a method for their biological control.

Orobanchaceae root parasitic weeds cause serious agricultural damage worldwide. Although numerous studies have been conducted to establish an effective control strategy for the growth and spread of root parasitic weeds, no practical method has been developed so far. Previously, metabolomic analyses were conducted on germinating seeds of a broomrape, Orobanche minor, to find novel targets for its selective control. Interestingly, planteose metabolism was identified as a possible target, and nojirimycin (NJ) selectively inhibited the germination of O. minor by intercepting planteose metabolism, although its precise mode of action was unclear. Here, transcriptome analysis by RNA-Seq was conducted to obtain molecular insight into the effects of NJ on germinating O. minor seeds. Differential gene expression analysis results suggest that NJ alters sugar metabolism and/or signaling, which is required to promote seed germination. This finding will contribute to understanding the effect of NJ and establishing a novel strategy for parasitic weed control.

Root parasitic weeds of the Orobanchaceae, such as witchweeds (Striga spp.) and broomrapes (Orobanche and Phelipanche spp.), cause serious losses in agriculture worldwide. No practical method to control these parasitic weeds has been developed to date. Understanding the characteristic physiological processes in the life cycles of root parasitic weeds is particularly important to identify specific targets for growth modulators. In our previous study, planteose metabolism was revealed to be activated soon after the perception of strigolactones in germinating seeds of O. minor. Nojirimycin inhibited planteose metabolism and impeded seed germination of O. minor, indicating that planteose metabolism is a possible target for root parasitic weed control. In the present study, we investigated the distribution of planteose in dry seeds of O. minor by matrix-assisted laser desorption/ionization–mass spectrometry imaging. Planteose was detected in tissues surrounding—but not within—the embryo, supporting its suggested role as a storage carbohydrate. Biochemical assays and molecular characterization of an α-galactosidase family member, OmAGAL2, indicated the enzyme is involved in planteose hydrolysis in the apoplast around the embryo after the perception of strigolactones to provide the embryo with essential hexoses for germination. These results indicated that OmAGAL2 is a potential molecular target for root parasitic weed control.HighlightPlanteose accumulated in tissues surrounding the embryo in Orobanche minor dry seeds and was indicated to be hydrolyzed in the apoplast around the embryo by α-galactosidase during germination.

BACKGROUND Strigolactones (SLs) have a vast number of ecological implications because of the broad spectrum of their biological activities. Unfortunately, the limited availability of SLs restricts their applicability for the benefit of humanity and renders synthesis the only option for their production. However, the structural complexity of SLs impedes their economical synthesis, which is unfeasible on a large scale. Synthesis of SL analogues and mimics with a simpler structure, but with retention of bioactivity, is the solution to this problem. RESULTS Here, we present eight new hybrid‐type SL analogues derived from auxin, synthesized via coupling of auxin ester [ethyl 2‐(1H‐indol‐3‐yl)acetate] and of ethyl 2‐phenylacetate with four D‐rings (mono‐, two di‐ and trimethylated). The new hybrid‐type SL analogues were bioassayed to assess the germination activity of seeds of the parasitic weeds Striga hermonthica, Orobanche minor and Phelipanche ramosa using the classical method of counting germinated seeds and a colorimetric method. The bioassays revealed that analogues with a natural monomethylated D‐ring had appreciable to good activity towards the three species and were the most active derivatives. By contrast, derivatives with the trimethylated D‐ring showed no activity. The dimethylated derivatives (2,4‐dimethyl and 3,4‐dimethyl) were slightly active, especially towards P. ramosa. CONCLUSIONS New hybrid‐type analogues derived from auxins have been prepared. These analogues may be attractive as potential suicidal germination agents for parasitic weed control because of their ease of preparation and relevant bioactivity. © 2019 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry., Hybrid‐type strigolactone (SL) analogues were prepared from auxin ethyl ester in a two‐step operation. These SL analogues were bioassayed as germination agents for Striga hermonthica, Pelipanche ramosa and Orobanche minor.

Root exudates from Lotus japonicus were found to contain at least three different hyphal branching-inducing compounds for the arbuscular mycorrhizal (AM) fungus Gigaspora margarita, one of which had been previously identified as (+)-5-deoxystrigol (5DS), a canonical strigolactone (SL). One of the two remaining unknown hyphal branching inducers was purified and named lotuslactone. Its structure was determined as methyl (E)-2-(3-acetoxy-2-hydroxy-7-methyl-1-oxo-1,2,3,4,5,6-hexahydroazulen-2-yl)-3-(((R)-4-methyl-5-oxo-2,5-dihydrofuran-2-yl)oxy)acrylate, by 1D and 2D NMR spectroscopy, and HR-ESI- and EI-MS. Although lotuslactone, a non-canonical SL, contains the AB-ring and the enol ether-bridged D-ring, it lacks the C-ring and has a seven-membered cycloheptadiene in the A-ring part as in medicaol, a major SL of Medicago truncatula. Lotuslactone was much less active than 5DS, but showed comparable activity to methyl carlactonoate (MeCLA) in inducing hyphal branching of G. margarita. Other natural non-canonical SLs including avenaol, heliolactone, and zealactone (methyl zealactonoate) were also found to be moderate to weak inducers of hyphal branching in the AM fungus. Lotuslactone strongly elicited seed germination in Phelipanche ramosa and Orobanche minor, but Striga hermonthica seeds were 100-fold less sensitive to this stimulant.

Root parasitic plant germination is induced by the host-derived chemical, strigolactone (SL). We found that a major microbial culture broth component, tryptone, inhibits the SL-inducible germination of a root parasitic plant, Orobanche minor. l-tryptophan (1a, l-Trp) was isolated as the active compound from tryptone. We further found that l-Trp related compounds (1b-11), such as a major plant hormone auxin (8, indole-3-acetic acid; IAA), also inhibit the germination and post-radicle growth of O. minor. We designed a hybrid chemical (13), in which IAA is attached to a part of SL, and found that this synthetic analog induced the germination of O. minor, and also inhibited post-radicle growth. Moreover, we found that N-acetyl Trp (9) showed germination stimulating activity, and introduction of a substitution at C-5 position incresed its activity (12a-12f). Our data, in particular, the discovery of a structurally hybrid compound that has two activities that induce spontaneous germination and inhibit subsequent radical growth, would provide new types of germination regulators for root parasitic plants.

This article belongs to the Special Issue Parasitic Plants Management., The holoparasitic broomrape weeds (Orobanche and Phelipanche species) cause severe yield losses throughout North Africa, the Middle East, and Southern and Eastern Europe. These parasitic weeds form an haustorium at the tip of their radicles to infect the crop upon detection of the host-derived haustorium-inducing factors. Until now, the haustorial induction in the broomrapes remains less studied than in other parasitic plant species. Known haustorium-inducing factors active in hemiparasites, such as Striga and Triphysaria species, were reported to be inefficient for the induction of haustoria in broomrape radicles. In this work, the haustorium-inducing activity of p-benzoquinone and 2,6-dimethoxy-p-benzoquinone (BQ and DMBQ) on radicles of three different broomrapes, namely Orobanche cumana, Orobanche minor and Phelipanche ramosa, is reported. Additional allelopathic effects of benzoquinones on radicle growth and radicle necrosis were studied. The results of this work suggest that benzoquinones play a role in the induction of haustorium in broomrapes. Although dependent on the broomrape species assayed and the concentration of quinones used in the test, the activity of BQ appeared to be stronger than that of DMBQ. The redox property represented by p-benzoquinone, which operates in several physiological processes of plants, insects and animals, is invoked to explain this different activity. This work confirms the usefulness of benzoquinones as haustorium-inducing factors for holoparasitic plant research. The findings of this work could facilitate future studies in the infection process, such as host-plant recognition and haustorial formation., This research was funded by the Spanish Ministry of Science and Innovation (RYC-2015-18961 and AGL2017-87693-R).

Common broomrape (Orobanche minor Sm.) is the most widespread and variable species in the British Isles and is the subject of much taxonomic confusion. Poor preservation in herbaria, coupled with the presence of cryptic host-specific races, have contributed to this. Here we review the taxonomic status of infraspecific taxa of O. minor in the British Isles, and provide a revised identification key, informed by morphology, ecology and molecular data. We describe two new varieties of O. minor s.l. that are ecologically distinct and discuss within the broader context of cryptic taxa in the subsection Minores (Beck-Mannagetta) Teryokhin in the British Isles and continental Europe. We suggest that delineating taxa objectively and reliably will be important for informing conservation priorities. Host identity and ecology, besides morphology, are essential considerations when identifying infraspecific taxa in this taxonomically challenging species.

One of the germination stimulants for root parasitic weeds produced by maize (Zea mays) was isolated and named methyl zealactonoate (1). Its structure was determined to be methyl (2E,3E)-4-((RS)-3,3-dimethyl-2-(3-methylbut-2-en-2-yl)-5-oxotetrahydrofuran-2-yl)-2-((((R)-4-methyl-5-oxo-2,5-dihydrofran-2-yl)oxy)methylene)but-3-enoate using by 1D and 2D NMR spectroscopy and ESI and EI–MS spectrometry. Feeding experiments with 13C-carlactone (CL), a biosynthetic intermediate for strigolactones, confirmed that 1 is produced from CL in maize. Methyl zealactonoate strongly elicits Striga hermonthica and Phelipanche ramosa seed germination, while Orobanche minor seeds are 100-fold less sensitive to this stimulant.

This article belongs to the Special Issue Natural Compound to Biocontrol Agrarian Pests., A new sesquiterpenoid belonging to the subgroup seco-eudesmanolides and named inuloxin E was isolated from Dittrichia viscosa, together with the already known sesquiterpenoids inuloxins A–D and α-costic acid. Inuloxin E was characterized by spectroscopic data (essentially NMR and ESI MS) as 3-methylene-6-(1-methyl-4-oxo-pentyl)-3a,4,7,7a-tetrahydro-3H-benzofuran-2-one. Its relative configuration was determined by comparison with the closely related inuloxin D and chemical conversion of inuloxin E into inuloxin D and by the observed significant correlation in the NOESY spectrum. Both inuloxins D and E induced germination of the parasitic weed Orobanche cumana, but were inactive on the seeds of Orobanche minor and Phelipanche ramosa. The germination activity of some hemisynthetic esters of inuloxin D was also investigated., This research was funded by Programme STAR 2017, financially supported by UniNA and Compagnia di San Paolo grant number E62F16001250003. M. Fernández-Aparicio was supported by projects AGL2017-87693-R and RYC-2015-18961, from the Spanish Ministry of Science, Innovation and Universities.

Strigolactones (SLs) are carotenoid-derived plant secondary metabolites that play important roles in various aspects of plant growth and development as plant hormones, and in rhizosphere communications with symbiotic microbes and also root parasitic weeds. Therefore, sophisticated regulation of the biosynthesis, perception and functions of SLs is expected to promote symbiosis of beneficial microbes including arbuscular mycorrhizal (AM) fungi and also to retard parasitism by devastating root parasitic weeds. We have developed SL mimics with different skeletons, SL biosynthesis inhibitors acting at different biosynthetic steps, SL perception inhibitors that covalently bind to the SL receptor D14, and SL function inhibitors that bind to the serine residue at the catalytic site. In greenhouse pot tests, TIS108, an azole-type SL biosynthesis inhibitor effectively reduced numbers of attached root parasites Orobanche minor and Striga hermonthica without affecting their host plants; tomato and rice, respectively. AM colonization resulted in weak but distinctly enhanced plant resistance to pathogens. SL mimics can be used to promote AM symbiosis and to reduce the application rate of systemic-acquired resistance inducers which are generally phytotoxic to horticultural crops. © 2019 Society of Chemical Industry.

Background Broomrapes attack important crops, cause severe yield losses and are difficult to eliminate because their seed bank is virtually indestructible. In the absence of a host, the induction of seed germination leads to inevitable death due to nutrient starvation. Synthetic analogues of germination-inducing factors may constitute a cheap and feasible strategy to control the seed bank. These compounds should be easy and cheap to synthesise, as this will allow their mass production. The aim of this work is to obtain new synthethic germinating agents. Results Nineteen N-substituted phthalimides containing a butenolide ring and different substituents in the aromatic ring were synthesised. The synthesis started with commercially available phthalimides. The complete collection was assayed against the parasitic weeds Orobanche minor, O. cumana, Phelipanche ramosa and P. aegyptiaca, with the synthetic strigolactone analogue GR24 used as a positive control. These compounds offered low EC50 values: O. cumana 38.3 μM, O. minor 3.77 μM, P. aegyptiaca 1.35 μM and P. ramosa 1.49 μM. Conclusions The synthesis was carried out in a few steps and provided the target compounds in good yields. The compounds tested showed great selectivity, and low EC50 values were obtained for structures that were simpler than GR24. © 2016 Society of Chemical Industry.

Strigolactones (SLs), a group of plant hormones, induce germination of root-parasitic plants and inhibit shoot branching in many plants. Shoot branching is an important trait that affects the number and quality of flowers and fruits. Root-parasitic plants, such as Phelipanche spp., infect tomato roots and cause economic damage in Europe and North Africa&mdash, hence why resistant tomato cultivars are needed. In this study, we found carotenoid cleavage dioxygenase 8-defective mutants of Micro-Tom tomato (slccd8) by the &ldquo, targeting induced local lesions in genomes&rdquo, (TILLING) method. The mutants showed excess branching, which was suppressed by exogenously applied SL. Grafting shoot scions of the slccd8 mutants onto wild-type (WT) rootstocks restored normal branching in the scions. The levels of endogenous orobanchol and solanacol in WT were enough detectable, whereas that in the slccd8 mutants were below the detection limit of quantification analysis. Accordingly, root exudates of the slccd8 mutants hardly stimulated seed germination of root parasitic plants. In addition, SL deficiency did not critically affect the fruit traits of Micro-Tom. Using a rhizotron system, we also found that Phelipanche aegyptiaca infection was lower in the slccd8 mutants than in wild-type Micro-Tom because of the low germination. We propose that the slccd8 mutants might be useful as new tomato lines resistant to P. aegyptiaca.

Highlight Planteose metabolism was uncovered as a key metabolic pathway for germination of Orobanche minor. Inhibition of planteose metabolism by nojirimycin resulted in selective inhibition of O. minor germination., Root parasitic weeds in Orobanchaceae cause serious damage to worldwide agriculture. Germination of the parasites requires host-derived germination stimulants, such as strigolactones, as indicators of host roots within reach of the parasite’s radicles. This unique germination process was focused on to identify metabolic pathways required for germination, and to design a selective control strategy. A metabolomic analysis of germinating seeds of clover broomrape, Orobanche minor, was conducted to identify its distinctive metabolites. Consequently, a galactosyl-sucrose trisaccharide, planteose (α-d-galactopyranosyl-(1→6)-β-d-fructofuranosyl-(2→1)-α-d-glucopyranoside), was identified as a metabolite that decreased promptly after reception of the germination stimulant. To investigate the importance of planteose metabolism, the effects of several glycosidase inhibitors were examined, and nojirimycin bisulfite (NJ) was found to alter the sugar metabolism and to selectively inhibit the germination of O. minor. Planteose consumption was similar in NJ-treated seeds and non-treated germinating seeds; however, NJ-treated seeds showed lower consumption of sucrose, a possible intermediate of planteose metabolism, resulting in significantly less glucose and fructose. This inhibitory effect was recovered by adding glucose. These results suggest that planteose is a storage carbohydrate required for early stage of germination of O. minor, and NJ inhibits germination by blocking the supply of essential glucose from planteose and sucrose. Additionally, NJ selectively inhibited radicle elongation of germinated seeds of Orobanchaceae plants (Striga hermonthica and Phtheirospermum japonicum). Thus, NJ will be a promising tool to develop specific herbicides to the parasites, especially broomrapes, and to improve our understanding of the molecular mechanisms of this unique germination.

Obligate root-parasitic plants belonging to the Orobanchaceae family are deadly pests for major crops all over the world. Because these heterotrophic plants severely damage their hosts even before emerging from the soil, there is an unequivocal need to design early and efficient methods for their control. The germination process of these species has probably undergone numerous selective pressure events in the course of evolution, in that the perception of host-derived molecules is a necessary condition for seeds to germinate. Although most of these molecules belong to the strigolactones, structurally different molecules have been identified. Since strigolactones are also classified as novel plant hormones that regulate several physiological processes other than germination, the use of autotrophic model plant species has allowed the identification of many actors involved in the strigolactone biosynthesis, perception, and signal transduction pathways. Nevertheless, many questions remain to be answered regarding the germination process of parasitic plants. For instance, how did parasitic plants evolve to germinate in response to a wide variety of molecules, while autotrophic plants do not? What particular features are associated with their lack of spontaneous germination? In this review, we attempt to illustrate to what extent conclusions from research into strigolactones could be applied to better understand the biology of parasitic plants.

Certain amino acids induce inhibitory effects in plant growth due to feedback inhibition of metabolic pathways. The inhibition patterns depend on plant species and the plant developmental stage. Those amino acids with inhibitory action on specific weeds could be utilized as herbicides, however, their use for weed control has not been put into practice. Orobanche minor is a weed that parasitizes red clover. O. minor germination is stimulated by clover root exudates. The subsequent seedling is an obligated parasite that must attach quickly to the clover root to withdraw its nutrients. Early development of O. minor is vulnerable to amino acid inhibition and therefore, a series of in vitro, rhizotron, and field experiments were conducted to investigate the potential of amino acids to inhibit O. minor parasitism. In in vitro experiments it was found that among a collection of 20 protein amino acids, lysine, methionine and tryptophan strongly interfere with O. minor early development. Field research confirmed their inhibitory effect but revealed that methionine was more effective than lysine and tryptophan, and that two successive methionine applications at 308 and 543 growing degree days inhibited O. minor emergence in red clover up to 67%. We investigated additional effects with potential to influence the practical use of amino acids against broomrape weeds, whether the herbicidal effect may be reversible by other amino acids exuded by host plants or may be amplified by inducing host resistance barriers against O. minor penetration. This paper suggests that amino acids may have the potential to be integrated into biorational programs of broomrape management., This work was funded by INRA ‘Division Santé des Plantes et Environnement. Appel à projets scientifiques 2015. Catégorie Biocontrôle’. MF-A was supported by the Marie-Curie FP7 COFUND People Programme, through the award of an AgreenSkills fellowship (under grant agreement n° PCOFUND-GA-2010-267196).

Root exudates from the allelopathic plant, black oat (Avena strigosa Schreb.), were found to contain at least six different germination stimulants for root parasitic plants, but no known strigolactones (SLs). One of these germination stimulants was purified and named avenaol. Its HR-ESI-TOFMS analysis indicated that the molecular formula of avenaol is C20H24O7, and thus it contains an additional carbon compared with known C19-SLs. Its structure was determined as 5-((E)-(5-(3-hydroxy-1,5,5-trimethyl-2-oxobicyclo[4.1.0]heptan-7-yl)-2-oxodihydrofuran-3(2H)-ylidene)methoxy)-3-methylfuran-2(5H)-one, by 1D and 2D NMR spectroscopy, and ESI- and EI-MS spectrometry. Although avenaol contains the C-D moiety, the common structural feature for all known SLs, it lacks the B ring and has an additional carbon atom between the A and C rings. Avenaol is a potent germination stimulant of Phelipanche ramosa seeds, but only a weak stimulant for seeds of Striga hermonthica and Orobanche minor.

Allelopathy has been considered not only as an environmentally friendly approach for weed control but also a potential reason causing autotoxicity in crop production. In this study, the responses of seeds of lettuce, wheat, rice, clover broomrape (CB), and sunflower broomrape (SB) to the root exudates of rice cultivars were studied. Lettuce germination was promoted by root exudates of Yliangyou 3218 and I-Kung-Pao. Wheat seedling growth was inhibited by all nine rice species. I-Kung-Pao and Ganxin 203 exerted greater autotoxicity than other cultivars. Yongyou 15 and I-Kung-Pao induced the highest germination rate of CB, while Yongyou 13, Zhongzao 22, and I-Kung-Pao induced the highest germination rate of SB. A significantly correlation was noted between germination-inducing ability on broomrape seeds and allelopathic effects on target plants. It is suggested that using broomrape seeds germination is a better receptor for the identification of rice allelopathic potential.

Cucumber (Cucumis sativus) plants were found to exude at least 12 germination stimulants for root parasitic weeds including 5 known strigolactones, 7-oxoorobanchol, 7-oxoorobanchyl acetate, orobanchol, orobanchyl acetate, and 4-deoxyorobanchol. Two novel germination stimulants were purified from cucumber root exudates and their structures were determined to be 7αand 7β-hydroxyorobanchyl acetate by 1D and 2D NMR spectroscopy, and ESIand EI-MS spectrometry. The stereochemistry was determined by NOE measurement and by comparing the CD spectra with those of the synthetic standards of four stereoisomers of orobanchol. 7αand 7β-Hydroxyorobanchol were detected by LC-MS/MS, and GC-MS analysis of purified sample confirmed their structures. The germination stimulation activities of 7αand 7β-hydroxyorobanchyl acetate on Orobanche minor were comparable to those of orobanchyl acetate and 7-oxoorobanchyl acetate. By contrast, 7β-hydroxyorobanchyl acetate was a highly potent germination stimulant for Phelipanche ramosa inducing more than 50% germination at 10 pM. © Pesticide Science Society of Japan

Infection by holoparasitic plants typically causes decreases in host mass, thought to be primarily as a result of resource abstraction. Inverse relationships have been noted between the number of Orobanche spp. parasites infecting a host and their mass, suggesting that the parasites compete for a shared resource pool, assumed to be recently fixed carbon (C). In clover, nitrogen (N) fixation requires a high proportion of daily photosynthate and represents a potential competitor for recently fixed C. We grew Trifolium pratense, either singly or parasitised by Orobanche minor, under high or low light levels, and with or without exogenous N supply. Low light and N deficiency led to decreased host biomass, while the damage caused by parasitism was proportionate to host mass. Parasitism caused reductions in host leaf mass, area, photosynthetic rates and shoot N concentration, but did not affect starch accumulation. Parasite mass as a proportion of system biomass was significantly higher when attached to plants grown at high light, which was attributed to higher photoassimilate supply, while the N supply had no effect. While both N limitation and parasitism caused reductions in host growth, little evidence of competition for C between N fixation and the parasites was noted.

(+)-Strigone was described earlier in a paper on isolation of strigol and then recently examined for hyphal branching activity in arbuscular mycorrhizal fungi as a strigolactone. Herein, it was isolated from root exudates of Houttuynia cordata, and its structure was confirmed by direct comparison with synthetic standards in LC-MS/MS, GC-MS, and (1)H and (13)C NMR analyses. The stereochemistry of strigone was determined by comparing the CD spectra and RR(t) in chiral LC-MS/MS with those of synthetic (+)-strigone and (-)-strigone. Four stereoisomers of strigone exhibited clearly different levels of stimulation activity on the seeds of three root parasitic plants, Orobanche minor, Phelipanche ramosa, and Striga hermonthica. (+)-Strigone was a highly potent germination stimulant on S. hermonthica and also on P. ramosa, but less active than ent-2'-epi-strigone on O. minor. In addition to strigone, H. cordata was found to produce strigol, sorgomol, and 5-deoxystrigol, indicating that this plant produces mainly strigol-type strigolactones derived from 5-deoxystrigol.

The holoparasitic angiosperm Orobanche minor parasitizes a diverse range of flowering plants from at least 16 orders in both the monocots and eudicots. However, populations of O. minor show host specificity at a local level. Our previous work identified the potential for host specificity to act as a catalyst for genetic divergence among populations of O. minor. Here we have extended this investigation by sampling populations from multiple hosts, across a broad geographic range. Sequence characterised amplified region (SCAR) data identified an exotic host-generalist lineage and a native host-specialist lineage of O. minor, suggesting genetic structure in this species is defined by both host specificity and geography. In addition, host-range overlap, discordant tree topologies, and cryptic morphology indicate the presence of gene flow between alien races and endemic populations. Therefore, repeated introductions of alien races of O. minor from disparate sources leading to introgression with native populations, and cryptic race formation, seem to have contributed to the taxonomic confusion associated with this species. We speculate that radiations associated with broad host range and divergent host ecologies may have promoted the unusually wide geographic distribution and diversification of this species. Finally, evidence of multiple shifts to exotic hosts, coupled with the predicted northward shift in climatic suitability, identify the potential for range expansion in alien races of O. minor, which may threaten nationally scarce native taxa with genetic assimilation. Our phylogenetic analysis provides a framework for identifying host races in Orobanche with a view to setting conservation priorities. © Springer-Verlag 2009.

Small broomrape is a federally listed noxious weed that was first identified in Oregon in 1923. Between 1923 and 1997, there were only six reports of the species in Oregon. Small broomrape is a holoparasitic weed that attaches to the root of its host and can lead to crop failure depending on the level of infestation. Small broomrape was identified in a red clover field in Oregon in 1998. In 2000 and 2001, the Oregon Department of Agriculture surveyed red and white clover fields in Oregon for the presence of small broomrape; 15 infested fields were identified in 2000 and 22 in 2001. The source of the small broomrape seed was not identified although there was speculation that small broomrape seed may have been contaminated clover seed stock. In 2000, a quarantine was established for small broomrape in red clover seed. Research identified effective, easy to implement management strategies for small broomrape including the use of false host crops and imazamox. In 2003, the ODA amended the quarantine to eliminate the mandatory seed sampling and testing and the requirement of reporting infested fields and seed lots. In 2011, there are still populations of small broomrape present in red clover fields in Oregon.

Nine-cis-epoxycarotenoid dioxygenase (NCED), belonging to the family of carotenoid-cleaving dioxygenases (CCDs), is the rate-limiting step in the abscisic acid (ABA) biosynthetic pathway in plants. Abamine is the first reported abscisic acid biosynthesis inhibitor to target NCED. Its utility has also been considered in the inhibition of other enzymes. As CCDs other than NCED are also involved in the strigolactone biosynthetic pathway, abamine may affect strigolactone biosynthesis. In this study, we found that abamine decreases the level of strigolactones in several plant species and reduces the germination rate of Orobanche minor seeds co-grown with tobacco. Taken together, these data suggest that abamine may be used as a scaffold for the development of specific regulators of strigolactone production.

Eleven naturally occurring strigolactones (SLs) were examined for their germination-stimulating activity on the seeds of a root parasitic plant Orobanche minor Sm. Based on their activity, SLs are classified into 3 groups (A–C). Group A, the most active germination stimulant, consists of 3 monohydroxy-SLs, orobanchol, 2′-epiorobanchol, and sorgomol, inducing >80% germination of O. minor seeds at 10 pM. Group B includes 5 SLs, which were ca. 10-fold less active than those in group A. The 3 least active SLs in group C were either more lipophilic or probably less stable than SLs in groups A and B. These results indicate that the germination-stimulating activity of SLs depends on the lipophilicity of the SL molecules and their stability also influences activity. Other structural features for germination-stimulating activity are also discussed.

A number of broomrapes (Orobanche spp.) are major threats for grain legumes in the Mediterranean area. Previous studies have shown very high levels of resistance to Orobanche crenata in Medicago truncatula with little variation among accessions hampering, therefore, its use for genetic analysis. In order to identify alternative systems for ulterior genetic and genomic analysis, we studied early stages of the interaction between M. truncatula accessions and a range of Orobanche species. We found significant differences in the induction of germination on Orobanche aegyptiaca, Orobanche foetida var. broteri, Orobanche minor, Orobanche nana and Orobanche ramosa seeds. Accessions also varied in the number of O. aegyptiaca, O. crenata, O. foetida var. broteri, O. nana and 0. ramosa attachments supported. Applications of the synthetic germination stimulant GR24 increased the germination level of Orobanche cumana and 0. minor. No attachments were observed for any of these species because of physical barriers typical of host resistance. On the contrary, increase of 0. nana germination by GR24 led to an increase in number of attachments albeit the normal development of the nodules was stopped in a later stage. The genetic variation observed for induction of germination and subsequent attachment will be useful for isolating and characterising genes involved in early stages of Orobanche-host plant interaction and for the study of the biosynthetic pathways of production of germination stimulants.

The main germination active compound in smoke, 3-methyl-2H-furo[2,3-c]pyran-2-one (butenolide), has structural similarities with strigolactones that function as germination stimulants for root parasitic plants such as Orobanche spp. and Striga spp. (Scrophulariaceae). Consequently, we tested whether butenolide also functions as a germination stimulant for parasitic weeds. Butenolide stimulated germination of both Orobanche minor and Striga hermonthica to similar levels as the synthetic strigol analogue GR24 and was effective at similar concentrations (10− 5 to 10− 11 M). Both butenolide and GR24 were more effective than the synthetic strigol analogue Nijmegan-1. Across eight further Orobanche spp., and for species from the root parasitic genera Cistanche, Conopholis and Lathraea, butenolide also had a similar level of activity to GR24. These results suggest that the germination stimulatory activity of butenolide may result from analogy with strigolactones.

Parasitic plants cause devastating losses to crop yields in several parts of the world. The root parasites, Striga and Orobanche species, use chemical signalling molecules that are exuded by the roots of plants in extremely low concentrations, and that can induce germination of the seeds of these parasites, to detect the vicinity of a suitable host. The majority of the so far identified germination stimulants belong to the strigolactones. It was recently discovered that this class of compounds can also induce hyphal branching in the symbiotic arbuscular mycorrhizal fungi, a process involved in root colonisation. The elucidation of the structure of new strigolactones is hindered by their low abundance and instability. In the present paper, we have used existing knowledge on the structure of strigolactones and combined it with recently obtained insight in the biosynthetic origin of these signalling compounds. This enabled us to postulate structures for strigolactones that have been isolated but for which so far the structure has not been elucidated, but also to propose structures of strigolactones that may be discovered in the future. Considering the strongly increased importance of the strigolactones, we expect that more groups will look for these compounds and also in systems so far not exploited. This could lead to the discovery of new strigolactones for which we expect the present biogenetic considerations will facilitate identification and structure elucidation.

Alectrol, a germination stimulant for root parasitic plants, was purified from root exudates of red clover (Trifolium pratense L.) and identified as a strigolactone, (+)-orobanchyl acetate [(3aS,4S,8bS,E)-8,8-dimethyl-3-(((R)-4-methyl-5-oxo-2,5-dihydrofuran-2-yloxy)methylene)-2-oxo-3,3a,4,5,6,7,8,8b-octahydro-2H-indeno[1,2-b]furan-4-yl acetate], by 1D and 2D NMR spectroscopy and ESI- and EI-MS spectrometry. Orobanchyl acetate afforded an [M-42](+) ion in EI-MS and thus had been recognized as an isomer of strigol. Orobanchyl acetate was detected in root exudates of soybean (Glycine max L.) and cowpea (Vigina unguiculata L.) along with orobanchol.

Small broomrape is a parasite of several broadleaf plant species. Consequences of small broomrape infestation in host cropping systems include seed contamination, reduction in crop seed yield, and host plant death. The effect of small broomrape parasitism on the biomass partitioning of its primary host, red clover, has not been documented. Greenhouse experiments were conducted to determine the relationship between small broomrape and red clover biomass accumulation. Total biomass of parasitized red clover plants was 15 to 51% less than nonparasitized red clover plants. Small broomrape parasitism reduced the amount of dry matter allocated to red clover inflorescences by 50 to 80%. Small broomrape dry matter accumulation was strongly related to total red clover–small broomrape dry matter accumulation. Small broomrape attachment number per red clover plant was a poor indicator of relative small broomrape dry weight accumulation. The results of this study indicated that small broomrape accumulated resources from red clover at the greatest expense to the economically important reproductive tissues.

Adenosine 3′,5′-cyclic monophosphate (cAMP) is known as a key second messenger in many living organisms, regulating a wide range of cellular responses. In higher plants the function of cAMP is poorly understood. In this study, we examined the role of cAMP in seed germination of the root parasitic plant Orobanche minor whose seeds require preincubation in warm moist environments for several days, termed conditioning, prior to exposure to germination stimulants released from roots of host plants. Accumulation of endogenous cAMP was observed in the conditioned O. minor seeds. When the seeds were exposed to light or supraoptimal temperature during the conditioning period, cAMP did not accumulate and the seeds showed low germination rates after stimulation with strigol, a germination stimulant. Addition of membrane-permeable cAMP to the medium restored the germination rates of the seeds treated with light or supraoptimal temperature during the conditioning period, suggesting that cAMP functions during the conditioning period. The endogenous cAMP levels of the seeds conditioned in the light or at a supraoptimal temperature were elevated by treatment of the seeds with gibberellin (GA) during the conditioning period. Uniconazole, a potent inhibitor of GA biosynthesis, blocked elevation of the cAMP level. Furthermore, a correlation between the endogenous cAMP level and GA level was observed during the conditioning period. These results suggest that GAs elevate the cAMP level, which is required for the germination of O. minor seeds.

Strigolactones released from plant roots induce hyphal branching of symbiotic arbuscular mycorrhizal (AM) fungi and germination of root parasitic weeds, Striga and Orobanche spp. We already demonstrated that, in red clover plants (Trifolium pratense L.), a host for both AM fungi and the root holoparasitic plant Orobanche minor Sm., reduced supply of phosphorus (P) but not of other elements examined (N, K, Ca, Mg) in the culture medium significantly promoted the secretion of a strigolactone, orobanchol, by the roots of this plant. Here we show that in the case of sorghum [Sorghum bicolor (L.) Moench], a host of both the root hemiparasitic plant Striga hermonthica and AM fungi, N deficiency as well as P deficiency markedly enhanced the secretion of a strigolactone, 5-deoxystrigol. The 5-deoxystrigol content in sorghum root tissues also increased under both N deficiency and P deficiency, comparable to the increase in the root exudates. These results suggest that strigolactones may be rapidly released after their production in the roots. Unlike the situation in the roots, neither N nor P deficiency affected the low content of 5-deoxystrigol in sorghum shoot tissues.

†Background and Aims Obligate root holoparasites of the genus Orobanche attack dicotyledonous crops and cause severe losses in many parts of the world. Chemical induction of plant defence systems such as systemic acquired resistance was proposed to be an available strategy to control the root parasite, but the detailed mechanisms involved have not been clarified. The aim of this study was to elucidate the effects of salicylic acid (SA), jasmonic acid (JA) and their analogues on resistance of red clover to Orobanche parasitism. †Methods Roots of red clover grown in plastic chambers were applied with SA, S-methyl benzo[1,2,3]thiadiazole-7carbothioate (BTH), methyl jasmonate (MeJA) and n-propyl dihydrojasmonate (PDJ), and then were inoculated with O. minor seeds. Attachments of the parasite were observed after 5 weeks. †Key Results SA and BTH, inducers of SA-mediated defences, significantly reduced the number of established parasites by more than 75 %. By contrast, MeJA and PDJ, inducers of JA-mediated defences, did not affect parasitism. The reduction in the number of established parasites by SA and BTH was due to the inhibited elongation of O. minor radicles and the activation of defence responses in the host root including lignification of the endodermis. †Conclusions These results suggest that SA-induced resistance, but not JA-induced resistance, is effective in inhibiting Orobanche parasitism and that the resistance is expressed by the host root both externally and internally.

Plant derived sesquiterpene strigolactones, which have previously been characterized as germination stimulants for root parasitic plants, have recently been identified as the branching factors which induce hyphal branching morphogenesis, a critical step in host recognition by arbuscular mycorrhizal (AM) fungi. We show here that, in red clover plants (Trifolium pratense L.), which is known as a host for both AM fungi and the root holoparasitic plant Orobanche minor Sm., reduced supply of phosphorus (P) but not of other elements examined (N, K, Mg, Ca) in the culture medium significantly promotes the release of a strigolactone, orobanchol, by the roots of this plant. In red clover plants, the level of orobanchol exudation appeared to be regulated by P availability and was in good agreement with germination stimulation activity of the root exudates. This implies that under P deficiency, plant roots attract not only symbiotic fungi but also root parasitic plants through the release of strigolactones. This is the first report demonstrating that nutrient availability influences both symbiotic and parasitic interactions in the rhizosphere.

Total RNA samples were extracted from the non-photosynthetic root parasitic plants Orobanche minor and O. ramosa and used to synthesize cDNAs. The cDNA fragments encoding ethylene biosynthesis enzymes, 1-amino-1-cyclopropanecarboxylic acid (ACC) synthase and ACC oxidase, were amplified and characterized. ACC synthases (OmACS and OrACS) and ACC oxidases (OmACO and OrACO) were found to have high homologies with those in other plants, in particular, with SHACS2 and SHACO1 of the root hemiparasite Striga hermonthica. The expression of these genes after germination stimulation by (+)-strigol, a natural germination stimulant, was slightly increased in O. ramosa but not in O. minor, indicating the possible involvement of ethylene biosynthesis in the germination of O. ramosa. In these root parasites, however, inhibitors of ethylene biosynthesis and action did not affect seed germination elicited by (+)-strigol except for the ethylene action inhibitor silver thiosulfate in O. ramosa.

Orobanche and Phelipanche species (the broomrapes) are root parasitic plants, some of which represent serious weed problems causing heavy yield losses on important crops. Current control relies on the use of certain agronomic practices, resistant crop varieties, and herbicides, albeit success has been marginal. Agronomic practices such as the use of allelopathic species in intercropping or cover crops, or the use of direct seedling over residues of allelopathic species incorporate the principle of allelopathy exerted by molecules exuded from roots or released by crop residues to control broomrapes. In addition, the isolation of natural substances from root exudates of plants with potential to inhibit broomrape development opens the door to the design of new herbicides based on natural and benign sources. Ryecyanatines A and B and ryecarbonitrilines A and B, the first new substituted cyanatophenol, substituted cyanatobenzo[1,3]dioxole, and the latter two new substituted benzo[1,3]dioxolecarbonitriles were isolated from rye (Secale cereale L.) root exudates. They were characterized as 4-cyanato-2-methoxyphenol, 2-cyanato-benzo[1,3]dioxole, 2-methoxybenzo[1,3]dioxole-5-carbonitrile and benzo[1,3]dioxole-2-carbonitrile by spectroscopic (essentially NMR and HRESI MS spectra) methods. These compounds were investigated for allelopathic activity on Orobanche germination and development. Ryecarbonitriline A induced germination of Orobanche cumana seeds, and this germination can be considered as suicidal because O. cumana does not parasite rye roots and cannot survive without host resources beyond germination stage. In addition, ryecyanatine A promotes a rapid cessation of O. cumana, Orobanche crenata and Orobanche minor radicle growth with the promotion of a layer of papillae at the radicle tip in O. cumana and O. crenata hampering the contact of the parasite to the host. Ryecarbonitriline B also displayed the same activity although being less active than ryecyanatine A and mainly restricted to O. cumana., This research was carried out in part in the frame of Programme STAR, financially supported by UNINA – Italy and Compagnia di San Paolo – Italy and in part by project FP7-ARIMNet-MEDILEG. This work was carried out within the COST ACTION FA1206 Strigolactones: biological roles and applications. Mónica Fernández-Aparicio has received the support of the Japan Society for the Promotion of Science Postdoctoral Fellowship ID noP 13390 and from the European Union, in the framework of the Marie-Curie FP7 COFUND People Programme, through the award of an AgreenSkills’ fellowship (under grant agreement noP COFUND-GA-2010-267196).

Alectrol was first isolated from root exudates of cowpea (Vigna unguiculata), a genuine host of a root parasitic weed Striga gesnerioides, as a germination stimulant for seeds of the parasite. The proposed structure, an isomer of strigol, was disproven by chemical synthesis. Recently, another structure, namely orobanchyl acetate, was proposed. Surprisingly, however, the synthetic compound having this proposed structure for alectrol was not active in inducing germination of S. gesnerioides seeds although it was active toward seeds of other root parasitic weeds such as S. hermonthica and Orobanche minor. Detailed studies on 1H NMR, mass and CD spectra of naturally occurring alectrol, re-isolated from cowpea root exudates, revealed that the genuine structure of the germination stimulant is not orobanchyl acetate but its stereoisomer ent-2′-epi-orobanchyl acetate. Accordingly, the structure of natural orobanchol was revised to ent-2′-epi-orobanchol 12 years after a tentative structure of orobanchol was proposed. Strict stereochemical requirements of strigolactones for germination induction of S. gesnerioides seeds, authentic samples of synthetic strigolactones and advanced analytical instruments made the structural assignment possible, thus ending a 20 years controversy concerning the true structure of alectrol.

Broomrapes (Orobanche spp.) are chlorophyll-lacking root parasites of many dicotyledonous species and cause severe damage to vegetable and field crops from several botanic families such as Fabaceae, Solanaceae, Compositae, and Umbelliferae. In Oregon, small broomrape has been identified as a parasite of red clover. In Oregon field studies, small broomrape control was excellent when imazamox was applied postemergence to red clover but preemergence to small broomrape. Temperature is one of the main factors that affect broomrape development. The objective of this study was to optimize small broomrape chemical control in red clover based on growing degree days (GDD). The study was conducted in controlled temperature conditions. Red clover plants were grown in soil artificially infested with small broomrape seeds. Imazamox was applied at 800, 1,000, 1,200, and 1,400 GDD. There was no injury to red clover from any imazamox treatment at any of the application timings. Small broomrape shoot emergence was reduced where imazamox was applied compared to the untreated control. Early imazamox applications reduced small broomrape biomass more than later applications. Control was greatest when imazamox was applied at 20 g ai ha−1 at 1,000 GDD. This application controlled small broomrape for 800 GDD after initial treatment. However, season-long control would require an additional treatment. This model predicts the optimal timing and rate of imazamox application for small broomrape control in red clover.

Summary Orobanche minor is a parasitic weed that attaches to theroots of red clover (Trifolium pratense) and a numberof other broad-leaved plant species in the PacificNorthwest USA. Orobanche minor seed must bestimulated by host plant exudates for germinationand attachment to occur. However, plant species calledfalse-hosts can stimulate parasitic seed germinationwithout attachment. These species could be utilized astrap crops to reduce the amount of parasitic seed ininfested soil. Wheat (Triticum aestivum), was found tobe a false-host of O. minor; therefore, growth chamber,glasshouse and field soil experiments were conducted toevaluate the effect of six soft white winter wheats(T. aestivum), one durum wheat (Triticum turgidum),and one triticale (Triticale hexaploide)onO. minorgermination. In growth chamber experiments, wheatand triticale induced 20–70% of O. minor seeds togerminate. In glasshouse studies, O. minor attachmentwas minimal on red clover plants grown in potspreviously planted to wheat or triticale. In pots thatdid not receive a false-host treatment, red clover plantsaveraged 4.2 O. minor attachments per plant. Redclover plants also had fewer O. minor attachmentswhen grown in field soil taken from the plots wherewheat or triticale were grown compared with plantsgrown in soil where no wheat or triticale werepreviously grown. Our results demonstrate that wheatmay have the potential to be effectively integrated intoan O. minor management system.Keywords: parasitic plant, integrated weed manage-ment, wheat, red clover, Orobanche minor.L

Small broomrape is an annual holoparasitic weed that was recently discovered in red clover production fields in Oregon. Imidazolinone herbicides such as imazamox control small broomrape; however, the mechanism of uptake by the parasite is largely unknown. Studies were conducted to determine the imazamox route of uptake by small broomrape in red clover, and to determine the potential for imazamox to be exuded from red clover and the subsequent effect on small broomrape. Small broomrape control was best at 90% when imazamox was foliar-applied, and worst at 42% or less when imazamox was soil-applied. The presence of activated charcoal to adsorb imazamox at the soil surface did not affect efficacy of broadcast foliar treatment. Small broomrape control was also evaluated when a foliar-treated red clover plant was grown in the same pot as a nontreated, parasitized red clover plant that was bagged during herbicide application. Activated charcoal was spread on the soil surface to adsorb imazamox, thus limiting herbicide uptake routes to the foliage of one of two red clover plants in the pot. Small broomrape attachment decreased on nontreated red clover when the other red clover plant in the pot was treated, suggesting roots exuded the herbicide or an active metabolite.

Demands for plant growth regulators (chemicals that control plant growth) are increasing globally, especially in developing countries. Both positive and negative plant growth regulators are widely used to enhance crop production and to suppress unwanted shoot growth, respectively. Strigolactones (SLs) are multifunctional molecules that function as phytohormones, inhibiting shoot branching and also functioning in the rhizospheric communication with symbiotic fungi and parasitic weeds. Therefore, it is anticipated that chemicals that regulate the functions of SLs will be widely used in agricultural applications. Although the SL biosynthetic pathway is not fully understood, it has been demonstrated that beta-carotene isomerases, carotenoid cleavage dioxygenases (CCDs), and a cytochrome P450 monooxygenase are involved in strigolactone biosynthesis. A CCD inhibitor, abamine, which is also an inhibitor of abscisic acid biosynthesis, reduces the levels of SL in several plant species and reduces the germination rate of Orobanche minor seeds grown with tobacco. On the basis of the structure of abamine, several chemicals have been designed to specifically inhibit CCDs during SL synthesis. Cytochrome P450 monooxygenase is another target enzyme in the development of SL biosynthesis inhibitors, and the triazole-derived TIS series of chemicals is known to include SL biosynthesis inhibitors, although their target enzyme has not been identified. Recently, DWARF14 (D14) has been shown to be a receptor for SLs, and the D-ring moiety of SL is essential for its recognition by D14. A variety of SL agonists are currently under development and most agonists commonly contain the D-ring or a D-ring-like moiety. Several research groups have also resolved the crystal structure of D14 in the last two years. It is expected that this information on the D14 structure will be invaluable not only for developing SL agonists with novel structures but also in the design of inhibitors of SL receptors.

Orobanche minor is a non-photosynthetic root holoparasitic plant. Although it is known that photosynthesis-related genes are inactivated or have been eliminated from the plastid genomes of holoparasites, little is known about the alterations in their genes involved in the signaling networks by which light regulates photosynthesis. Cryptochromes (crys), which are blue-light receptors, appear to control both photosynthesis-related and non-photosynthetic responses to light in higher plants. Because we are interested in to what extent a cry-mediated light signaling network remains in the holoparasites, we cloned CRY homologous cDNA from O. minor (OmCRY1) and used real-time RT-PCR to compare its expression under natural daylight and darkness. We found that the OmCRY1 has a high degree of homology with CRY1 s from photosynthetic plants. Expression of the OmCRY1 gene was higher in plants grown in the dark than that in the plants grown under natural daylight. This is the first report of the gene expression of a blue-light receptor in non-photosynthetic plants.

Weeds of the genus Orobanche parasitize many dicotyledonous species, causing severe damage to vegetable and field crops worldwide. In Oregon, the number of red clover fields contaminated with small broomrape has increased in recent years. Small broomrape parasitism in red clover is temperature related. In this study, the temperature-dependent relationship was developed into a predictive model based on growing degree-days (GDD) for small broomrape parasitism in red clover. The model was developed in greenhouse studies and validated in the field during three growing seasons. A strong relationship between GDD and parasite size allowed for the creation of a simple predictive model for tubercle number based on GDD. The proposed model is based on a temperature range realistic to western Oregon climatic conditions and predicts lag, log, and maximum phases for four parasitism sizes in relation to GDD. Small broomrape parasitism in red clover began at about 400 GDD, but red clover biomass accumulation was...

The root-parasitic broomrape species cause severe damage to field and vegetable crops worldwide. This study evaluated the relationship between small broomrape development and temperature with red clover as a host plant. Red clover plants were grown in soil artificially infested with small broomrape seed in temperature-controlled growth chambers. Parasite development was quantified at 48 different accumulated growing degree days (GDD). Small broomrape parasitism and temperature were strongly related. Small broomrape tubercle initiation was delayed by low temperature. Tubercle development initiated at about 750 GDD and peaked at about 1,100 GDD. Small broomrape biomass accumulation correlated with the increase in tubercle number over time. Parasitism stages were divided into lag, log, and maximum phases that were strongly related to GDD. Development of a predictive system for parasitism growth stage is needed to allow precise herbicide application for effective control before small broomrape shoot ...

Summary There is an increasing interest in the legume species Medicago truncatula as a model plant for structural and functional genomic studies that can be used to identify agronomically important genes in legumes. Field screening has shown very high levels of resistance to Orobanche crenata in M. truncatula. However, in vitro studies with O. crenata, Orobanche foetida, Orobanche ramosa and Orobanche minor showed useful variation among accessions at early stages of the parasite–host interaction. Significant differences were observed in the levels of germination of O. crenata and O. foetida seeds induced by different accessions of M. truncatula. Only limited germination was observed on accession SA-4327. All accessions induced little O. ramosa and O. minor germination. Accessions also varied in the number of O. crenata and O. foetida attachments supported, with few developing on accession SA-27774. The variation observed for induction of germination and of subsequent attachment will be useful to isolating and characterizing genes involved in the early stages of Orobanche–host plant interaction and for the study of the biosynthetic pathways of production for germination stimulants.

Small broomrape, a holoparasitic weed, was recently introduced to the Pacific Northwest and contaminates a limited number of red clover fields in Oregon. Greenhouse and field studies were conducted to evaluate small broomrape response to common crop and weed species in the Pacific Northwest, and to evaluate a quick-screening method for plant species response to the parasite. In greenhouse studies, plants were grown in a hydroponic polyethylene bag system to allow for continuous visibility of their roots and monitoring of small broomrape seed germination and tubercle development. Results of the greenhouse study were validated in a field contaminated with small broomrape. In the greenhouse polyethylene bag study, small broomrape germinated and tubercles developed on alfalfa, arrowleaf clover, carrot, celery, crimson clover, lettuce, prickly lettuce, red clover, spotted catsear, subterranean clover, white clover, and wild carrot. Small broomrape germinated but did not develop tubercles when grown with barley, birdsfoot trefoil, common vetch, creeping bentgrass, cucumber, field corn, red fescue, flax, Italian ryegrass, nasturtium, oats, orchardgrass, perennial ryegrass, snap bean, sugar pea, sunflower, sweet corn, tall fescue, tomato, and wheat. In the field study, tubercles developed on alfalfa, arrowleaf clover, common vetch, crimson clover, red clover, subterranean clover, and white clover, but small broomrape flower stalks did not emerge in arrowleaf clover or crimson clover. Common vetch was a host for small broomrape in the field study but not in the hydroponic polyethylene bag system, thus, caution is warranted in using the quick screening method to elucidate potential small broomrape host species.

The genus Orobanche includes chlorophyll-lacking root parasites that parasitize many dicotyledonous species and cause severe damage to vegetable and field crops worldwide. In the United States, small broomrape is a federally listed noxious weed that has been found in red clover seed production fields in Oregon. In this study, we classified the susceptibility of red clover, white clover, and crimson clover to small broomrape infection under controlled conditions. Red, white, and crimson clover seeds were sown in pots containing small broomrape–inoculated soil. Differences in susceptibility to infection were observed among clover species; red clover was severely infected, white clover was slightly infected, and crimson clover was not infected. Nomenclature: Small broomrape, Orobanche minor J. E. Smith. # ORAMI; crimson clover, Trifolium incarnatum L. # TRFIN; red clover, Trifolium pratense L. # TRFPR; white clover, Trifolium repens L. # TRFRE.

Root exudates of sunflower (Helianthus annuus L.) line 2607A induced germination of seeds of root parasitic weeds Striga hermonthica, Orobanche cumana, Orobanche minor, Orobanche crenata, and Phelipanche aegyptiaca. Bioassay-guided purification led to the isolation of a germination stimulant designated as heliolactone. FT-MS analysis indicated a molecular formula of C20H24O6. Detailed NMR spectroscopic studies established a methylfuranone group, a common structural component of strigolactones connected to a methyl ester of a C14 carboxylic acid via an enol ether bridge. The cyclohexenone ring is identical to that of 3-oxo-α-ionol and the other part of the molecule corresponds to an oxidized carlactone at C-19. It is a carlactone-type molecule and functions as a germination stimulant for seeds of root parasitic weeds. Heliolactone induced seed germination of the above mentioned root parasitic weeds, while dehydrocostus lactone and costunolide, sesquiterpene lactones isolated from sunflower root exudates, were effective only on O. cumana and O. minor. Heliolactone production in aquacultures increased when sunflower seedlings were grown hydroponically in tap water and decreased on supplementation of the culture with either phosphorus or nitrogen. Costunolide, on the other hand, was detected at a higher concentration in well-nourished medium as opposed to nutrient-deficient media, thus suggesting a contrasting contribution of heliolactone and the sesquiterpene lactone to the germination of O. cumana under different soil fertility levels.

The beta-glucosidase of a root parasitic angiosperm, Orobanche minor Sm., was purified and characterized. The optimum pH and temperature for activity of the enzyme were 5.0 and 50 degrees C. The beta-glucosidase was stable at up to 50 degrees C at pH 4.0-10.0. The M(r) was estimated to be 33 kD by SDS-PAGE. The enzyme hydrolyzed p-nitrophenyl-beta-D-glucopyranoside and salicin, but not the cell wall of O. minor or cellohexaose.