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Activation of the phenylpropanoids pathway in date palm (Phoenix dactylifera L.) roots is a key defense mechanism against Fusarium oxysporum f. sp. albedinis (Foa), the causal agent of bayoud. In t...

To determine the beneficial effect of the combination of phosphite (Phi) and Streptomyces hygroscopicus (S11) on soybean plants infected by Phytophthora sojae, soybean seeds were pre-treated with a suspension of S11, the leaves were sprayed with Phi and the roots were subsequently inoculated with P. sojae zoospores. Seedling growth and colonization of root tissue by the pathogen were monitored and the Phi concentration in the roots was quantified. Finally, the effect of S11 and/or Phi on defense responses and SA and JA production were recorded. Both S11 and Phi inhibited the growth of P. sojae in dual V8 agar cultures, and soybean pre-treatment with S11 and/or Phi reduced further infection in planta. In response to P. sojae, defense genes were upregulated in both tolerant and susceptible soybean varieties, especially in the latter. SA and JA accumulated more in the susceptible variety in response to P. sojae, and the use of S11 and/or Phi prior to inoculation induced a decrease in their production. SA accumulated earlier than JA in soybean roots in response to P. sojae in the tolerant variety, whereas JA accumulated earlier than SA in the susceptible one. As such, S11 combined with Phi significantly reduced P. sojae infection in soybean by delaying and reducing the activation of stress hormones and defense-related genes, offering an efficient mean to manage soybean’s infection by P. sojae.

Metabolomics identify and analyze, in a comprehensive and high throughput manner, all the metabolites of an organism including amino acids, organic acids, sugars, fatty acids, lipids, steroids, small peptides, vitamins and phenolics. By measuring global sets of low molecular weight metabolites, metabolomics provide a snapshot readout of metabolic activity status in relation to the genetic makeup of the variety, its natural gene expression or its response to external stimuli encountered where it is grown. When the snapshot readout can be associated with the outcome phenotype (e.g., healthy vs diseased; high vs low fruit sugar content), more high-throughput analytical techniques and tools can be combined with metabolomics to provide a better understanding of the physiological processes occurring in the studied variety. In date palm, the metabolome representing the pool of small molecules chemically distinct through their structure (fingerprint) or biochemically involved in certain pathways (precursors, substrates, products) has traditionally been used to identify biomarkers for diagnosis and prediction. In recent years, metabolomics has become essential not only to the identification of simple biomarkers but as a technology for discovering actives that drive physiological processes, help create new cultivars with desired attributes, or add value to harvested dates.

Considering the high incidence of white mold caused by Sclerotinia sclerotiorum in a variety of field crops and vegetables, different control strategies are needed to keep the disease under economical threshold. This study assessed the effect of foliar application of a calcium formulation on disease symptoms, oxalic acid production, and on the oxidative stress metabolism in soybean plants inoculated with each of two isolates of the pathogen that have contrasting aggressiveness (HA, highly-aggressive versus WA, weakly-aggressive). Changes in reactive oxygen species (ROS) levels in soybean plants inoculated with S. sclerotiorum isolates were assessed at 6, 24, 48 and 72 h post inoculation (hpi). Generation of ROS including hydrogen peroxide (H2O2), anion superoxide (O2-) and hydroxyl radical (OH) was evaluated. Inoculation with the WA isolate resulted in more ROS accumulation compared to the HA isolate. Pre-treatment with the calcium formulation restored ROS production in plants inoculated with the HA isolate. We also noted a marked decrease in oxalic acid content in the leaves inoculated with the HA isolate in presence of calcium, which coincided with an increase in plant ROS production. The expression patterns of genes involved in ROS detoxification in response to the calcium treatments and/or inoculation with S. Sclerotiorum isolates were monitored by RT-qPCR. All of the tested genes showed a higher expression in response to inoculation with the WA isolate. The expression of most genes tested peaked at 6 hpi, which preceded ROS accumulation in the soybean leaves. Overall, these data suggest that foliar application of calcium contributes to a decrease in oxalic acid production and disease, arguably via modulation of the ROS metabolism.

Sclerotinia sclerotiorum (Lib.) de Bary is a fungal plant that causes serious losses in important crops. This study examined the expression of selected defense-related genes in the isoflavonoids pathway after soybean inoculation with two Sclerotinia isolates possessing different levels of aggressiveness (SSPetri, highly aggressive and SS18, weakly aggressive) and the effects of pre-treatment with calcium on disease symptoms. PAL1 displayed an early expression after inoculation regardless of the isolate used. Expression of IFS2 and IFR2 were more pronounced in leaves inoculated with the weakly aggressive isolate SS18 than with its highly aggressive counterpart. Oppositely, CHS3, CHS5 and CHI1B1–CHI1B2 had higher transcript abundance in presence of the highly aggressive isolate SSPetri. Pre-treatment with calcium enhanced the expression of CHS3, CHS5, CHI1B1–CHI1B2 IFS2 and IFR2 in the leaves inoculated with SSPetri. An increase in isoflavone aglycones and their malonyl and acetyl conjugates was recorded in the inoculated leaves, with higher contents in response to SSPetri. Pre-treatment with calcium improved the accumulation of daidzein, genistein and acetyl glycitin, especially in the leaves inoculated with SSPetri. De novo synthesis of glyceollins occurred in response to inoculation, especially in response to the highly aggressive isolate SSPetri.

Plant-fungi and plant-oomycete interactions have been studied at the proteomic level for many decades. However, it is only in the last few years, with the development of new approaches, combined with bioinformatics data mining tools, gel staining, and analytical instruments, such as 2D-PAGE/nanoflow-LC-MS/MS, that proteomic approaches thrived. They allow screening and analysis, at the sub-cellular level, of peptides and proteins resulting from plants, pathogens, and their interactions. They also highlight post-translational modifications to proteins, e.g., glycosylation, phosphorylation or cleavage. However, many challenges are encountered during in planta studies aimed at stressing details of host defenses and fungal and oomycete pathogenicity determinants during interactions. Dissecting the mechanisms of such host-pathogen systems, including pathogen counter-defenses, will ensure a step ahead towards understanding current outcomes of interactions from a co-evolutionary point of view, and eventually move a step forward in building more durable strategies for management of diseases caused by fungi and oomycetes. Unraveling intricacies of more complex proteomic interactions that involve additional microbes, i.e., PGPRs and symbiotic fungi, which strengthen plant defenses will generate valuable information on how pathosystems actually function in nature, and thereby provide clues to solving disease problems that engender major losses in crops every year.

Verticillium dahliae is the main pathogen causing Verticillium wilt in potato. Management of this vascular disease is very challenging due to the soilborne nature of the pathogen. A better understanding of the molecular host–pathogen interactions is important for the development of novel strategies to control Verticillium wilt. In this pathosystem, the disease cycle starts with stimulation and germination of the V. dahliae microsclerotia through host root exudates. The present study reports on the use of potato root extracts derived from a susceptible (Kennebec) and a moderately resistant (Ranger Russet) cultivar to elicit pathogenicity-related genes in highly and weakly aggressive isolates of V. dahliae. Using a combined approach of subtractive hybridization and cDNA-AFLP, 573 transcripts differentially accumulated in one or the other isolate in response to root extracts were detected. Sixteen primer combinations representing EcoRI/MseI AFLP primers + A, T, C, or G were used to provide a complete coverage of the subtractive hybridization products. The detected differentially expressed transcripts in the highly and weakly aggressive isolates were 301 and 272, respectively. Among the amplified transcripts, 185 were recovered from the PAGE gel then re-amplified by PCR and further sequenced. BLAST search against the NCBI, the Broad Institute V. dahliae genome, and V. dahliae ESTs collection COGEME databases showed that some of the differentially expressed transcripts matched with known sequences, with assigned functions in V. dahliae such as polygalacturonases or with conserved hypothetical proteins. The remaining sequences had no match in these databases. The results are discussed based on the potential involvement of these genes in V. dahliae's pathogenesis.

Entomosporium leaf and berry spot represent the most important disease of saskatoons (Amelanchier alnifolia Nutt.). This disease, caused by the ascomycete Entomosporium mespili (D.C.) Sacc., is difficult to control via conventional methods and no known saskatoon cultivar carries resistance to this pathogen. The aim of the present study was to enhance existing and induced defense responses of saskatoons using two host defense inducers, namely jasmonic acid and an extract from Canada milkvetch on two saskatoon cultivars, Smoky and Martin. Both inducers exhibited an ability to differentially induce the synthesis/accumulation of defense-related genes including those encoding for PR-1, PR-2, PR-5, LOX and PAL. Pre-treatment of saskatoon leaves with these inducers reduced the disease levels especially when applied to the moderately susceptible cultivar Martin. An accumulation of various hydroxycinnamic acid and proanthocyanidin derivatives also correlated with the disease levels recorded on both cultivars. Results are discussed on the basis of the induced pathways in an attempt to lay the ground for a better understanding of this hostpathogen interaction. In the meantime,wehave shown that the use of defense enhancers may be useful in integrated management strategies to control entomosporium leaf and berry spot in saskatoons spot in saskatoons. Fil: Wolski, Erika Alejandra. Universidad Nacional de Mar del Plata. Facultad de Ciencias Agrarias; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Mar del Plata; Argentina Fil: Henriquez, Maria A.. University Of Manitoba; Canadá Fil: Adam, Lorne R.. University Of Manitoba; Canadá Fil: Badawi, Mohamed. University Of Manitoba; Canadá Fil: Andreu, Adriana Balbina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Mar del Plata. Instituto de Investigaciones Biológicas; Argentina. Universidad Nacional de Mar del Plata; Argentina Fil: Hadrami, Abdelbasset El . University Of Manitoba; Canadá Fil: Daayf, Fouad. University Of Manitoba; Canadá

Chitin and chitosan are naturally-occurring compounds that have potential in agriculture with regard to controlling plant diseases. These molecules were shown to display toxicity and inhibit fungal growth and development. They were reported to be active against viruses, bacteria and other pests. Fragments from chitin and chitosan are known to have eliciting activities leading to a variety of defense responses in host plants in response to microbial infections, including the accumulation of phytoalexins, pathogen-related (PR) proteins and proteinase inhibitors, lignin synthesis, and callose formation. Based on these and other proprieties that help strengthen host plant defenses, interest has been growing in using them in agricultural systems to reduce the negative impact of diseases on yield and quality of crops. This review recapitulates the properties and uses of chitin, chitosan, and their derivatives, and will focus on their applications and mechanisms of action during plant-pathogen interactions.

Blackleg (Phoma leaf spot), caused by the ascomycete Leptosphaeria spp. (anamorph: Phoma lingam), is a serious yield-limiting factor in canola. Leptosphaeria spp. populations are highly diverse and display differential interactions with canola cultivars, ranging from hypersensitive-like reactions to partial resistance and susceptibility. Leptosphaeria biglobosa encompasses weakly aggressive isolates and can be found late in the season towards maturity stages of the crop while other isolates from Leptosphaeria maculans are highly aggressive and can be detected throughout the season. In an earlier study, we showed the primary involvement of lignin in the containment of the progress of Phoma leaf spot symptoms around the infection sites by comparing the reaction of three different cultivars with a set of isolates with various levels of aggressiveness. The present investigation reports on further characterization of the host responses to L. biglobosa and L. maculans isolates used individually or subsequently ...

Blackleg (phoma stem canker), caused by Leptosphaeria spp., is an important disease of canola (oilseed rape, Brassica napus). Control strategies rely on the use of resistant cultivars, chemical and disease-reducing cropping practices. In Canada, the pathogen population is represented by L. maculans and L. biglobosa, which are considered to be highly and weakly aggressive, respectively. It is largely admitted that L. biglobosa isolates are not able to cause a significant amount of stem canker and develop on the plant only when it becomes senescent, late in the season. The prevalence of L. maculans over L. biglobosa has been considered to be linked to the low aggressiveness of the latter. However, in this study, we show that L. biglobosa isolates could become highly aggressive in terms of lesion appearance on cotyledons, if the right conditions of temperature and relative humidity (RH) are provided. Percent germination of inoculated pycnidiospores was not affected by the RH regimes tested. This is the first study to show the importance of RH as a factor conditioning the pathogenicity of L. biglobosa isolates on canola cotyledons. Concurrent changes in the host defence mechanisms against L. biglobosa isolates in response to variations in the RH were also investigated. Under high RH, the increase in disease caused by the weakly aggressive isolates coincided with a reduced accumulation of lignin at the early stages of infection.

Twenty-one isolates of microorganisms, including Bacillus spp., Rhizobium spp., Ulocladium atrum, Candida guilliermondii, Pseudomonas sp., Rahnella aquatilis and other bacteria not yet identified, were tested to determine their effects on the mycelial growth and the sporulation of Fusarium oxysporum f.sp. albedinis (Foa), the causal agent of bayoud on date palm. The potential of these antagonists in the induction of defense reactions in date palm seedlings was also studied. Four bacteria, B. pumilus W1, R. aquatilis W2, B. cereus X16 and n.d. S1, have exhibited a high inhibition toward mycelial growth of Foa (70–77%), and its sporulation (80–95% of the control). Moreover, cytological alterations have been detected in the Foa mycelium grown in the inhibition zone. Application of these antagonists into date palm seedlings has led to trigger defense reactions with an accumulation of non-constitutive hydroxycinnamic acid derivatives, such as the sinapic derivative I2, known to play a crucial role in resistance of date palm to Foa. This reaction was more pronounced in resistant cultivar (BSTN) than in susceptible (JHL). The combined effects of direct and indirect actions of Foa antagonists are discussed in the hope of providing a biocontrol strategy against bayoud.

The time-course and the spatial accumulation of PR-proteins pr-1 and pr-5 gene transcripts were investigated in two potato cultivars differing in their levels of susceptibility to late blight, caused by Phytophthora infestans (Mont.) de Bary. Cultivars Russet Burbank (RB, susceptible) and Kennebec (KB, moderately tolerant) were inoculated with either P. infestans genotype US-1 (old lineage) or US-8 (new lineage). A strong induction of both genes was detected in both cultivars inoculated with either P. infestans genotype, as compared to the healthy-controls. The accumulation of transcripts from both genes occurred earlier in KB than in RB leaflets. By comparing the two P. infestans isolates tested, a stronger and earlier induction of both PR genes was recorded in response to US-1 as compared to US-8. The spatio-temporal profiling of pr-1 and pr-5 genes expression showed a strong and early accumulation of transcripts at the local infection site, a late and intermediate level of induction at the proximal site, and no or very weak induction at a distal site remote from the infection site. These results show that pr-1 and pr-5 genes both are related to the defense mechanisms of potato to late blight, and that the higher infection success of P. infestans US-8 as compared to US-1 might be due to the late and/or the weak induction of these defense genes.

Late blight (LB) caused by Phytophthora infestans (Mont.) de Bary is a major constraint to potato production worldwide. In North America, this disease has re-emerged as a major concern to potato producers due to an important shift in its population structure. To design sustainable control strategies, it is necessary to have a better understanding of differential interactions that potato has developed in response to the currently pre-dominant versus the old populations of the pathogen. In this study, isolates from two P. infestans genotypes, US-1 (old, mildly aggressive) and US-8 (currently predominant, highly aggressive), were used to inoculate two potato cultivars, Russet Burbank (susceptible) and Kennebec (moderately resistant). Induction patterns of genes encoding three PR proteins (pr-2, pr-3 and pr-9) in response to isolates from those two genotypes were studied using northern blot analysis in three leaf strata (Local: inoculated leaflets, Proximal: un-inoculated leaflets in the same leaf inoculated with P. infestans, and Distal: un-inoculated leaflets from the leaf adjacent to the inoculated leaf) and different times after inoculation. Our results indicated that these pr genes were activated earlier in both cultivars when inoculated with P. infestans US-1 as compared to US-8 genotype. In addition, the induction of these genes occurred earlier in Kennebec than in Russet Burbank in response to both genotypes. Furthermore, pr-2 and pr-3 were found to be induced first locally, then systemically while pr-9 transcripts could be detected only locally at the site of inoculation and only in Kennebec inoculated with P. infestans US-1.

Differential expression of pal1 and hmgr2 was investigated using northern blot analysis in two potato cultivars (Russet Burbank (RB), susceptible and Kennebec (KB), moderately tolerant) after inoculation with two Phytophthora infestans isolates from the formerly (US-1) and currently predominant genotypes (US-8). The accumulation of pal1 transcripts was weaker in response to US-8 as compared to US-1 and occurred earlier in KB than in RB. The stronger expression of pal1 in response to US-1, as compared to US-8, is suggested to be due to defense gene suppression by the latter. No apparent strong accumulation of hmgr2 transcripts was recorded in RB as compared to KB inoculated with either US-1 or US-8. The induction of pal1 and hmgr2 was first observed in un-inoculated (proximal) close to the inoculated leaflets, then in un-inoculated (distal) leaflets of leaves adjacent to the inoculated leaf, and finally in local inoculated leaflets. The stronger expression of the two genes in proximal and distal leaflets, as compared to the local site of inoculation suggests the translocation of signal(s) from this site to healthy parts of the plant.

Resume Trois populations naturelles d'Atriplex halimus, situees dans trois contextes climatiques differents, ont ete analysees a l'aide des caracteristiques foliaires (surface et rapport foliaires, largeur moyenne des feuilles et rapport feuilles sur rameaux) et des marqueurs isoenzymatiques. L'etude a montre l'existence d'une variabilite phenotypique hautement significative, qui est d'autant plus importante que les populations sont eloignees geographiquement et situees dans des climats differents. Le caractere graduel de cette variabilite morphologique, le long d'un gradient climatique, indique qu'il s'agit d'une differenciation clinale. D'autre part, des clones obtenus a partir de boutures semi-ligneuses prelevees au niveau de chaque population et places dans une parcelle commune ont maintenu les memes caracteristiques foliaires que les populations sources, suggerant l'existence d'une base genetique a cette variabilite. L'etude de quatre systemes isoenzymatiques confirme l'existence de cette variabilite. Ainsi, le pourcentage de locus polymorphes (P), le taux d'heterozygote theorique (He) et le nombre moyen d'alleles par locus (A) ont ete estimes respectivement a 77,52 %, 0,319 et 1,99. L'indice de diversite genetique (Fst) obtenu est de 0,089. Pour citer cet article : A. Abbad et al., C. R. Biologies 327 (2004).

Plants have evolved sophisticated mechanisms to sense and respond to pathogen attacks. Resistance against necrotrophic pathogens generally requires the activation of the jasmonic acid (JA) signaling pathway, whereas the salicylic acid (SA) signaling pathway is mainly activated against biotrophic pathogens. SA can antagonize JA signaling and vice versa. Here, we report that the necrotrophic pathogen Botrytis cinerea exploits this antagonism as a strategy to cause disease development. We show that B. cinerea produces an exopolysaccharide, which acts as an elicitor of the SA pathway. In turn, the SA pathway antagonizes the JA signaling pathway, thereby allowing the fungus to develop its disease in tomato (Solanum lycopersicum). SA-promoted disease development occurs through Nonexpressed Pathogen Related1. We also show that the JA signaling pathway required for tomato resistance against B. cinerea is mediated by the systemin elicitor. These data highlight a new strategy used by B. cinerea to overcome the plant's defense system and to spread within the host. © 2011 American Society of Plant Biologists. All rights reserved. Fil:Rigano, L. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina. Fil:Rodriguez, M.C. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina. Fil:Vojnov, A. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.

Date palm, Phoenix dactylifera L., is a perennial long-lived dioecious monocotyledon of great socio-economic importance especially in North Africa and the Middle East. These countries grow 62 million of the 105 million trees available worldwide on an area of over a million hectares (Fig. 6.1; Table 6.1). These ‘trees’ are cultivated not only for their valuable fruits (dates), but also for producing fuel, fibre and as shelter for ground crops. Production of dates is of approximately 6.5 million metric tons around the world (Table 6.2) and generates an important commercial activity. Countries such as Egypt, Islamic Republic of Iran and Saudi Arabia represent the top three producers worldwide. Furthermore, in the areas where it is cultivated, the date palm contributes to the creation of a micro-climate that enables agricultural development of other species.

Date palm (Phoenix dactylifera L.) is qualified as a 'tree' of great ecological and socio-economical importance in desert oases. Unfortunately, it is being decimated, especially in Morocco and Algeria, by a fusariosis wilt called bayoud and caused by Fusarium oxysporum f. sp. albedinis (Fao). Controlling this disease requires the implementation of an integrated management program. Breeding for resistance is one of the most promising component strategies of this program. Few naturally resistant cultivars with a mediocre fruit quality (dates) are known. Conventional and non-conventional methods are under development and have to use the simplest and easiest methods to screen for resistant individuals. The use of pathogen toxins as selective agents at the tissue culture step might be a source of variability that can lead to the selection of individuals with suitable levels of resistance to the toxin and/or to the pathogen among the genetic material available. Foa produces toxins such as fusaric, succinic, 3-phenyl lactic acids and their derivatives, marasmins and peptidic toxins. These toxins can be used bulked or separately as selective agents. The aim of this contribution was to give a brief overview on toxins and their use as a mean to select resistant lines and to initiate a discussion about the potential use of this approach for the date palm-Foa pathosystem. This review does not pretend to be comprehensive or exhaustive and was prepared mainly to highlight the potential use of Foa toxins for selecting date palm individuals with a suitable resistance level to bayoud using toxin-based selective media.

Three natural populations of Atriplex halimus, located in three different climatic contexts, were studied using leaf characteristics (leaf area, leaf length-to-maximum-width ratio, average width of the leaves and leave-to-branch ratio) and isoenzymatic markers. The study showed the existence of a highly significant phenotypical variability. This variability is all the more significant, as populations are geographically distant and located in different climates. The gradual character of this morphological variability, along a climatic gradient, indicates that it is almost a clinical differentiation. Clones obtained from semi-woody cuttings taken on the level of each population and placed in a common parcel have maintained the same leaf characteristics as the population sources, suggesting the genetic origin of this variability. The study of four isoenzymatic systems confirms the existence of this variability. Thus, the percentage of polymorphic loci (P), the expected heterozygosity (He) and the mean number of alleles per locus (A) are of 77.52%, 0.319, and 1.99, respectively. The genetic diversity index (Fst) obtained is 0.089.

Verticillium dahliae Kleb. is a soilborne fungal pathogen of many crops. In potato, it is the major causal agent of Early Dying. In Manitoba, potato fields planted with cv. Russet Burbank are infested with highly pathogenic V. dahliae isolates, which can produce up to 90 % disease severity. The objective of the study was to evaluate selected compost, green manure, and seed-meal treatments, in comparison with the soil fumigant Vapam, for their ability to reduce propagule density of V. dahliae in soil and decrease disease, and to enhance potato yield. Select green manure crops (oriental and white mustard, Canada milk vetch, sorghum-sudangrass, rye, alfalfa, oat/pea mixture), organic amendments (composted cattle manure and mustard seed-meal), and Vapam, and crop sequences that contribute to the suppression of Verticillium, or the improvement of potato yield were used in a 3-year field study initiated in 2006. Survival in soil of microsclerotia was evaluated as a measure of treatments’ success in potentially reducing Early Dying. Compost and seed-meal treatments, compared to an untreated control, reduced incidence to 30 and 40 %, respectively, but only seed-meal reduced V. dahliae propagule density. Overall, green manures over 1 or 2-years were ineffective in reducing propagule density or improving potato yield. Vapam was partially effective in reducing the propagule density only at the beginning of the potato season, but it did not reduce disease incidence compared to the control. Compost and seed-meal are promising as alternative control of V. dahliae. Only compost reduced disease and increased potato yield, which was associated with improved nutrient availability (phosphorus and sulfate) in soil.