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Paenibacillus terrae NK3-4 is a plant growth-promoting rhizobacterium that may be useful for controlling plant diseases. We conducted a genomic analysis and identified the genes mediating antimicrobial functions. Additionally, an extracellular antifungal protein component was isolated and identified. The draft genome sequence was assembled into 54 contigs, with 5 458 568 bp and a G+C content of 47%. Moreover, 4 690 015 bp encoded 5090 proteins, 7 rRNAs, and 54 tRNAs. Forty-four genes involved in antimicrobial functions were detected. They mainly encode 19 non-ribosomal peptide synthetases (NRPSs); one polyketide synthase/NRPSs hybrid enzyme; four Zn-dependent metalloproteases; three antilisterial bacteriocin subtilosin biosynthesis proteins (AlbA); four serine proteases; five pectate lyases; three beta-glucanases; and four 1,4-beta-xylanases. These include four novel NRPSs that have not been found in any species of Paenibacillus. Furthermore, five proteins exhibiting antifungal activity were identified from the antifungal extracellular protein component based on MS/MS and the strain NK3-4 predicted protein library. On the basis of these features, we propose that strain NK3-4 represents a promising biocontrol agent for protecting plant from diseases. The draft genome sequence described herein may provide the genetic basis for the characterization of the molecular mechanisms underlying the biocontrol functions. It may also facilitate the development of rational strategies for improving the strain. Key words: Paenibacillus terrae, genome, antiomicrobial protein, biocontrol. Le Paenibacillus terrae NK3-4 est une rhizobacterie qui stimule la croissance des plantes et qui pourrait aider a diminuer les maladies. Les auteurs ont realise une analyse genomique et identifie des gènes conferant des fonctions antimicrobiennes. De plus, une composante proteique extracellulaire et antifongique a ete isolee et identifiee. L'ebauche d'un genome de reference a ete assemblee sous forme de 54 contigs totalisant 5 458 568 pb et une teneur en G+C de 47 %. De plus, 4 690 015 pb codaient pour 5090 proteines, 7 ARNr et 54 ARNt. Quarante-quatre gènes impliques dans des fonctions antimicrobiennes ont ete detectes. Ceux-ci codaient principalement pour 19 peptides synthetases non-ribosomiques (NRPS), une enzyme hybride polycetide synthase/NRPS, quatre metalloproteases dependantes du Zn, trois proteines impliquees dans la synthèse de bacteriocines anti-Listeria (AlbA), quatre proteases de type serine, cinq pectate lyases, trois bêta-glucanases et quatre 1,4-bêta-xylanases. Ceux-ci comprennent quatre NRPS inedits qui n'avaient encore ete rapportes chez aucune espèce de Paenibacillus.De plus, cinq proteines presentant une activite antifongique ont ete identifiees au sein de la fraction proteique extracellulaire au moyen de MS/MS et de la librairie de proteines predites pour la souche NK3-4. Sur la base de ces proprietes, les auteurs proposent que la souche NK3-4 constitue un agent de bio-contrôle prometteur pour proteger les plantes contre les maladies. L'ebauche du genome decrite dans ce travail pourrait fournir une assise genetique pour la caracterisation des mecanismes moleculaires impliques dans le bio-contrôle. Elle pourrait egalement faciliter le developpement de strategies rationnelles pour ameliorer la souche. [Traduit par la Redaction] Mots-cles : Paenibacillus terrae, genome, proteine antimicrobienne, bio-contrôle., Many microorganisms in the genus Paenibacillus activate soil nutrients, thereby promoting plant growth and controlling diseases, suggesting these species are potentially useful for the production of agriculturally important crops (Seldin [...]

Many plant pathogens, including those in the lineage of the Irish potato famine organism Phytophthora infestans, evolve by host jumps followed by specialization. However, how host jumps affect genome evolution remains largely unknown. To determine the patterns of sequence variation in the P. infestans lineage, we resequenced six genomes of four sister species. This revealed uneven evolutionary rates across genomes with genes in repeat-rich regions showing higher rates of structural polymorphisms and positive selection. These loci are enriched in genes induced in planta, implicating host adaptation in genome evolution. Unexpectedly, genes involved in epigenetic processes formed another class of rapidly evolving residents of the gene-sparse regions. These results demonstrate that dynamic repeat-rich genome compartments underpin accelerated gene evolution following host jumps in this pathogen lineage. 10.1126/science.1193070

Rice stripe is a serious viral disease of japonica rice (Qryza sativa L.) cultivars in Asia. Resistance to this disease can be divided into that to rice stripe virus (RSV) and to the pest (small brown planthopper, SBPH). Here we have used a doubled-haploid (DH) population bred from the cross Zhaiyeqing8 (ZYQ8) x Jingxi17 (JX17) and four separate inoculation methods to genetically dissect the rice stripe resistance present in ZYQ8. Two linked RSV resistance quantitative trait loci (QTLs) were located on chromosome 1, and one on chromosome 11, jointly explaining 30 to 44% of the trait variance. The main-effect qSTV11 was mapped close to the established RSV resistance gene Stvb-i. For resistance to SBPH, three QTLs were located, mapping to chromosomes 1, 2, and 11, and jointly explaining 44 to 51% of the trait variance. The large-effect qSBPH1 appears to be a novel QTL conferring a degree of tolerance and antibiosis against SBPH. The positive alleles at all six QTLs were present in ZYQ8. Analysis of a set of chromosome segment substitution lines (CSSLs) in which chromosomal segments of various indica varieties were present in a japonica background showed that rice stripe resistance genes/QTLs are common on the long arm of chromosome 1. doi: 10.2135/cropsci2009.10.0591

Acid soils and associated aluminum toxicity could limit maize (Zea mays L.) production on more than 3950 million hectares worldwide. Callose formation in root tips is a reliable indicator of Al-induced cell damage. We studied the modes of gene action determining callose formation in root tips using two diallel mating experiments involving a total of 33 maize inbred lines grown in 25 [micro]M AI nutrient solution. Highly significant differences were found among parents, crosses, general combining ability (GCA), specific combining ability (SCA) effects, and for the contrast of parents vs. crosses in both diallel experiments. These results indicate that additive and nonadditive gene effects were important in the expression of callose content. doi: 10.2135/cropsci2009.11.0651

Rice tungro disease (RTD) is a significant yield constraint in rice-growing areas of South and Southeast Asia. Disease symptoms are caused largely by infection by the rice tungro bacilliform virus (RTBV). Two host transcription factors, RF2a and RF2b, regulate expression of the RTBV promoter and are important for plant development. Expression of a dominant negative mutant of these factors in transgenic rice resulted in phenotypes that mimic the symptoms of RTD, whereas overexpression of RF2a and RF2b had essentially no impact on plant development. Conversely, lines with elevated expression of RF2a or RF2b showed weak or no symptoms of infection after Agrobacterium inoculation of RTBV, whereas control plants showed severe stunting and leaf discoloration. Furthermore, transgenic plants exhibited reduced accumulation of RTBV RNA and viral DNA compared with nontransgenic plants. Similar results were obtained in studies after virus inoculation by green leafhoppers. Gaining disease resistance by elevating the expression of host regulators provides another strategy against RTD and may have implications for other pararetrovirus infections. basic leucine zipper | host factors | resistance

Stripe rust, caused by Puccinia striiormis Westend f. sp. tritici, is one of the most important foliar diseases of wheat (Triticum aestivum L.) worldwide. Stripe rust resistance genes Yr27, Yr31, YrSp, YrV23, and YrCN19 on chromosome 2BS confer resistance to some or all Chinese P. striiormis f. sp. tritici races CYR31, CYR32, SY11-4, and SY11-14 in the greenhouse. To screen microsatellite (SSR) markers linked with YrCN19, Fl, FZ, and F3 populations derived from cross Ch377/CN19 were screened with race CYR32 and 35 SSR primer pairs. Linkage analysis indicated that the single dominant gene YrCN19 in cultivar CN19 was linked with SSR markers Xgwm410, Xgwm374, Xwmc477, and Xgwm382 on chromosome 2BS with genetic distances of 0.3, 7.9, 12.3, and 21.2 cM, respectively. Crosses of CN19 with wheat lines carrying other genes on chromosome 2B showed that all were located at different loci. YrCN19 is thus different from the other reported Yr genes in chromosomal location and resistance response and was therefore named Yr41. Prospects and strategies of using Yr41 and other Yr genes in wheat improvement for stripe rust resistance are discussed. Key words: microsatellite (SSR), stripe rust, wheat, gene mapping. La rouille jaune, causee par le Puccinia striiformis f. sp. tritici (PST), est l'une des plus importantes maladies foliaires chez le ble (Triticum aestivum L.) dans le monde. Les genes de resistance a la rouille jaune Yr27, Yr31, YrSp, YrV23 et YrCN19 sur le chromosome 2BS conferent la resistance a certaines ou toutes les races chinoises du PST (CYR31, CYR32, SY11-4 et SY11-14) en serre. Pour identifier des marqueurs microsatellites (SSR) lies au gene YrCN19, des populations Fl, FZ et F3 derivees du croisement Ch377/CN19 ont ete inoculees avec la race CYR32 et genotypees avec 35 paires d'amorces SSR. Une analyse de liaison genetique a revele que, chez le cultivar CN19, le gene dominant YrCN19 est lie aux marqueurs Xgwm410, Xgwm374, Xwmc477 et Xgwm382 sur le chromosome 2BS, respectivement a 0,3, 7,9, 12,3 et 21,2 cM. Des croisements entre CN19 et des lignees de ble portant d'autres genes de resistance sur le chromosome 2B ont montre que ces genes etaient situes a d'autres locus. Le gene YrCN19 est ainsi different des autres genes Yr connus tant pour ce qui est de sa position chromosomique que pour sa resistance. Pour ces raisons, il a ete nomme Yr41. Les auteurs discutent des perspectives et strategies en vue de l'exploitation du gene Yr41 et d'autres genes Yr en amelioration genetique du ble pour la resistance a la rouille jaune. Mots-cles : microsatellite (SSR), rouille jaune, ble, cartographie genetique. [Traduit par la Redaction], Introduction Wheat stripe rust, caused by Puccinia striiformis Westend f. sp. tritici (PST), is one of the most devastating diseases of wheat (Triticum aestivum L.) worldwide, especially in wheat-growing areas [...]

We isolated the barley stem rust resistance genes Rpg5 and rpg4 by map-based cloning. These genes are colocalized on a 70-kb genomic region that was delimited by recombination. The Rpg5 gene consists of an unusual structure encoding three typical plant disease resistance protein domains: nucleotide-binding site, leucine-rich repeat, and serine threonine protein kinase. The predicted RPG5 protein has two putative transmembrane sites possibly involved in membrane binding. The gene is expressed at low but detectable levels. Posttranscriptional gene silencing using VIGS resulted in a compatible reaction with a normally incompatible stem rust pathogen. Allele sequencing also validated the candidate Rpg5 gene. Allele and recombinant sequencing suggested that the probable rpg4 gene encoded an actin depolymerizing factor-like protein. Involvement of actin depolymerizing factor genes in nonhost resistance has been documented, but discovery of their role in gene-for-gene interaction would be novel and needs to be further substantiated. actin depolymerizing factor | barley | disease resistance domains | map-based cloning

Rhizobia can infect roots of host legume plants and induce new organs called nodules, in which they fix atmospheric nitrogen. Infection generally starts with root hair curling, then proceeds inside newly formed, intracellular tubular structures called infection threads. A successful symbiotic interaction relies on infection threads advancing rapidly at their tips by polar growth through successive cell layers of the root toward developing nodule primordia. To identify a plant component that controls this tip growth process, we characterized a symbiotic mutant of Medicago truncatula, called rpg for rhizobium-directed polar growth. In this mutant, nitrogen-fixing nodules were rarely formed due to abnormally thick and slowly progressing infection threads. Root hair curling was also abnormal, indicating that the RPG gene fulfils an essential function in the process whereby rhizobia manage to dominate the process of induced tip growth for root hair infection. Map-based cloning of RPG revealed a member of a previously unknown plant-specific gene family encoding putative Iong coiled-coil proteins we have called RRPs (RPG-related proteins) and characterized by an 'RRP domain' specific to this family. RPG expression was strongly associated with rhizobial infection, and the RPG protein showed a nuclear localization, indicating that this symbiotic gene constitutes an important component of symbiotic signaling. genetics | symbiosis | coiled-coil

A non-mycoparasitic restriction enzyme-mediated DNA integration (REMI) mutant of Coniothyrium minitans (R2427) contains two tandem plasmid copies integrated towards the 3' end of an ORF. The predicted polypeptide (845 aa) exhibits high similarity with DNA-helicase proteins from other filamentous fungi and yeasts that play a role in mitochondrial DNA maintenance and repair. Disruption of the C. minitans PIF1 DNA helicase gene results in altered morphology, reduced growth rates and a concomitant loss in ability to mycoparasitize sclerotia of Sclerotinia sclerotiorum. In infection bioassays, R2427 exhibited sparse mycelial growth on the surface of live sclerotia, but no mycelia were detected inside the sclerotia. Conversely, R2427 readily colonized autoclaved sclerotia. Complementation of the mutant with wild-type PIF1 restored normal mycelial growth and mycoparasitic capability, confirming a functional role in the host-pathogen interaction. The C. minitans PIF1 DNA helicase may maintain mitochondrial stability in response to reactive oxygen species, either produced endogenously within the mycoparasite, or exogenously from the sclerotial host.

Xanthomonas axonopodis pv. citri (Xac) causes citrus canker and the completion of the Xac genome sequence has opened up the possibility of investigating basic cellular mechanisms at the genomic level. Copper compounds have been extensively used in agriculture to control plant diseases. The copA and copB genes, identified by annotation of the Xac genome, encode homologues of proteins involved in copper resistance. A gene expression assay by Northern blotting revealed that copA and copB are expressed as a unique transcript specifically induced by copper. Synthesis of the gene products was also induced by copper, reaching a maximum level at 4 h after addition of copper to the culture medium. CopA was a cytosolic protein and CopB was detected in the cytoplasmic membrane. The gene encoding CopA was disrupted by the insertion of a transposon, leading to mutant strains that were unable to grow in culture medium containing copper, even at the lowest CuS[O.sub.4] concentration tested (0.25 mM), whereas the wild-type strain was able to grow in the presence of 1 mM copper. Cell suspensions of the wild-type and mutant strains in different copper concentrations were inoculated in lemon leaves to analyse their ability to induce citrus canker symptoms. Cells of mutant strains showed higher sensitivity than the wild-type strain in the presence of copper, i.e. they were not able to induce citrus canker symptoms at high copper concentrations and exhibited a more retarded growth in planta.

Gray leaf spot disease (GLS; caused by Cercospora zeae-maydis Tehon and Daniels) is among the major maize (Zea mays L.) production constraints in southern Africa. Maize is predominantly grown by small-scale farmers without fungicides; hence, there is need to develop GLS resistant hybrids. There is limited information about the mode of inheritance for GLS resistance in regionally adapted germplasm. This study was initiated to determine gene action controlling GLS resistance. Seventy-two hybrids were generated by mating 27 inbred lines in a North Carolina design II scheme. Experimental and check hybrids were evaluated in an 8 by 12 [alpha]-lattice design with two replications at three locations, during the 2004-2005 season. There was significant variation among the hybrids for GLS resistance and yield. Inbreds L13, L15, L18, L19, and L24, from A, N3, B, K, and SC heterotic groups, respectively, contributed high levels of resistance to hybrids. Both general combining ability (GCA) and specific combining ability (SCA) effects were highly significant (P < 0.01), but the predominance of GCA for GLS (86%) and yield (74%) indicated that additive effects were more important than nonadditive gene action in controlling both traits. Hybrids ranked similarly for GLS across environments, suggesting that few significant crossover genotype by environment interactions, which would cause problems in hybrid selection, were observed. Overall, results indicated that it would be readily possible to develop inbred lines with high GLS resistance from this germplasm.

The purB and purH mutants of Mesorhizobium loti exhibited purine auxotrophy and nodulation deficiency on Lotus japonicus. In the presence of adenine, only the purH mutant induced nodule formation and the purB mutant produced few infection threads, suggesting that 5-aminoimidazole-4-carboxamide ribonucleotide biosynthesis catalyzed by PurB is required for the establishment of symbiosis.

Abstract: Populations of the ascomycete pathogen Sclerotinia sclerotiorum sampled from a canola field were analysed using microsatellite markers. Fifty isolates were collected from ascospore-infested canola petals and, later in the [...]

Fusarium head blight (FHB), incited by Fusarium graminearum Schwabe, has caused devastating losses in both yield and quality of barley (Hordeum vulgate L.) produced in the northern Great Plains from 1993 to 2003. Thirty-five barley germplasm lines with partial resistance to FHB have been identified in exotic and unadapted germplasm lines. Little is known about their agronomic characteristics, malt quality, and reaction to other diseases as compared to adapted cultivars. This information is needed so barley breeders can make informed decisions when planning crosses involving the resistant germplasm lines. The objective of this study was to compare the agronomic performance, malt quality, and disease reaction of barley germplasm lines with partial FHB resistance to cultivars grown in the northern Great Plains. Agronomic and malting data were collected on the 35 germplasm lines and five check cultivars grown in five environments in North Dakota from 1998 to 2000. Data for FHB severity and deoxynivalenol (DON, a mycotoxin produced by F. graminearum) accumulation were obtained for the same 40 entries grown in FHB-epidemic nurseries in North Dakota from 1997 to 1999. Seedling responses to foliar pathogens common in the northern Great Plains were determined in the greenhouse during fall 1997. None of the FHB-resistant barley germplasm lines had acceptable malt quality for all traits. Kernel plumpness, grain protein concentration, and malt extract were the traits impacted most severely. The FHB-resistant barley germplasm lines headed significantly later than the adapted barley cultivars. Most FHB-resistant germplasm lines were susceptible to the common foliar diseases of the northern Great Plains. At least four cycles of breeding will probably be necessary to develop FHB-resistant germplasm lines acceptable to producers and the malting and brewing industry. Abbreviations: ASBC, American Society of Brewing Chemists; DON, deoxynivalenol; FHB, Fusarium head blight; RCBD, randomized complete block design., FUSARIUM HEAD BLIGHT has caused devastating losses in both the yield and quality of barley produced in the northern Great Plains from 1993 to 2003. From 1993 to 1997, total [...]

Many soybean [Glycine max (L.) Merr.] genotypes that carry resistance to soybean cyst nematode (Heteroderaglycines Ichinohe) (SCN) from plant introduction (PI) 88788 also carry resistance to brown stem rot (BSR) caused by the soilborne fungus Phialophora gregata (Allington & Chamberlain) W. Gams f. sp. sojae Kobayashi, Yamamoto, Negishi, and Ogoshi. The objectives of our research were to map and localize BSR resistance quantitative trait loci (QTL) from Bell, a BSR resistant cultivar with SCN resistance from PI 88788. Initial mapping was done with a population of 93 [F.sub.4]-derived lines developed from a cross between Bell and the SCN and BSR susceptible cultivar, Colfax. Lines were evaluated for BSR resistance in two field environments, a greenhouse, and with genetic markers from linkage group (LG) J. To confirm and further localize a resistance QTL, three near isogenic line (NIL) populations were created using an [F.sub.4]-derived line from the Bell x Colfax population. In the [F.sub.4] population, markers on LG J were significantly (P < 0.001) associated with BSR resistance in both the field ([R.sup.2] = 45%) and greenhouse ([R.sup.2] = 51%). Data from the NIL populations indicates the QTL is near the closely linked markers 21E22.sp1, 21E22.sp2, and 35E22.sp1, which is the same genetic region where Rbs1, Rbs2, and Rbs3, the three named BSR resistance genes, were previously mapped. In addition, the genetic region is tightly linked to a previously identified SCN resistance QTL from PI 88788. These results explain why many SCN resistant cultivars also carry BSR resistance and should assist breeders when selecting for resistance to both diseases. Abbreviations: BSR, brown stem rot; CAPS, cleaved amplified polymorphic sequence; cM, centimorgans; LG, molecular linkage group; NIL, near isogenic line; PCR, polymerase chain reaction; PI, plant introduction; QTL, quantitative trait locus/loci; RIL, recombinant inbred line; SCN, soybean cyst nematode; SSR, simple sequence repeat, BROWN STEM ROT is an important soybean disease in northern U.S. soybean production regions (Gray and Grau, 1999) and is caused by P. gregata f. sp. sojae (Kobayashi et al., [...]

An [F.sub.1] mapping population of the septoria tritici blotch pathogen of wheat, Mycosphaerella graminicola, was generated by crossing the two Dutch field isolates IPO323 and IPO94269. AFLP and RAPD marker data sets were combined to produce a high-density genetic linkage map. The final map contained 223 AFLP and 57 RAPD markers, plus the biological traits mating type and avirulence, in 23 linkage groups spanning 1216 cM. Many AFLPs and some RAPD markers were clustered. When markers were reduced to 1 per cluster, 229 unique positions were mapped, with an average distance of 5.3 cM between markers. Because M. graminicola probably has 17 or 18 chromosomes, at least 5 of the 23 linkage groups probably will need to be combined with others once additional markers are added to the map. This was confirmed by pulsed-field gel analysis; probes derived from 2 of the smallest linkage groups hybridized to two of the largest chromosome-sized bands, revealing a discrepancy between physical and genetic distance. The utility of the map was demonstrated by identifying molecular markers tightly linked to two genes of biological interest, mating type and avirulence. Bulked segregant analysis was used to identify additional molecular markers closely linked to these traits. This is the first genetic linkage map for any species in the genus Mycosphaerella or the family Mycosphaerellaceae.

Hypersensitive response (HR) is a programmed cell death that is commonly associated with disease resistance in plants. Among the different HR-related early induced genes, the AtMYB30 gene is specifically, rapidly, and transiently expressed during incompatible interactions between Arabidopsis and bacterial pathogens. Its expression was also shown to be deregulated in Arabidopsis mutants affected in the control of cell death initiation. Here, we demonstrate that overexpression in Arabidopsis and tobacco of AtMYB30 (i) accelerates and intensifies the appearance of the HR in response to different avirulent bacterial pathogens, (ii) causes HR-like responses to virulent strains, and (iii) increases resistance against different bacterial pathogens, and a virulent biotrophic fungal pathogen, Cercospora nicotianae. In antisense AtMYB30 Arabidopsis lines, HR cell death is strongly decreased or suppressed in response to avirulent bacterial strains, resistance against different bacterial pathogens decreased, and the expression of HR- and defense-related genes was altered. Taken together, these results strongly suggest that AtMYB30 is a positive regulator of hypersensitive cell death.

Animal SGT1 is a component of Skp1-Cullin-F-box protein (SCF) ubiquitin ligases that target regulatory proteins for degradation. Mutations in one (SGT1b) of two highly homologous Arabidopsis SGT1 genes disable early plant defenses conferred by multiple resistance (R) genes. Loss of SGT1b function in resistance is not compensated for by SGT1a. R genes differ in their requirements for SGT1b and a second resistance signaling gene, RAR1, that was previously implicated as an SGT1 interactor. Moreover, SGT1b and RAR1 contribute additively to RPP5-mediated pathogen recognition. These data imply both operationally distinct and cooperative functions of SGT1 and RAR1 in plant disease resistance., In plants, a major form of resistance to disease caused by microbial pathogens is by expression of complementary gene pairs in the plant and pathogen, known respectively as resistance (R) [...]

Interactions between Cauliflower mosaic virus (CaMV) and its aphid vector are regulated by the viral protein P2, which binds to the aphid stylets, and protein P3, which bridges P2 and virions. By using baculovirus expression of P2 and P3, electron microscopy, surface plasmon resonance, affinity chromatography, and transmission assays, we demonstrate that P3 must be previously bound to virions in order that attachment to P2 will allow aphid transmission of CaMV. We also show that a P2:P3 complex exists in the absence of virions but is nonfunctional in transmission. Hence, unlike P2, P3 and virions cannot be sequentially acquired by the vector. Immunogold labeling revealed the predominance of spatially separated P2:P3 and P3:virion complexes in infected plant cells. This specific distribution indicates that the transmissible complex, P2:P3:virion, does not form primarily in infected plants but in aphids. A model, describing the regulating role of P3 in the formation of the transmissible CaMV complex in planta and during acquisition by aphids, is presented, and its consequences are discussed.

Research has been conducted on the pathogenic dormant-like viable but nonculturable condition bacteria. The effect of these bacteria on the plants has been investigated and characterized via the use of the plant-pathogenic bacterium Ralstonia solanacearum, and the results are discussed.

Four two-way mixtures of wheat genotypes susceptible to different races of Paccinia striiformis, were planted at different frequencies in both the presence and absence of disease. Disease was not able to reverse the relative ranking of the two genotypes based on competition and to create the negative frequency dependence on both genotypes in a mixture that is needed for the maintenance of genetic polymorphism.

The exbD2 gene of Xanthomonas capestris pv. campestris is required for induction of a hypersensitive response on the nonhost pepper (Capsicum annuum). The ferric-iron-uptake genes exbB, tonB and exbD1 are also necessary. Mutational analysis has been carried out. The induction of typical black rot symptoms on the host plant Brassica oleracea, cauliflower, has also been studied. The ferric-iron-uptake genes are required for induction of an HR response on it as well.

A study of the immunodominant membrane protein (IMP) of the apple proliferation phytoplasma (AP) from AP-infected periwinkle plants resulted in the isolation of the gene encoding the P-318B IMP. The isolated gene was expressed in Escherichia coli and the resulting protein used in the immunization of rabbits. The study resulted in the development of a novel approach to generating immunogens for the production of specific antibodies for the characterization and detection of non-culturable phytoplasmas.

Disease resistance in plants is commonly specified by genetically paired plant resistance (R) and pathogen avirulence (Avr) proteins. Recent molecular analyses of several R-Avr protein interactions provide compelling evidence for a receptor-ligand mechanism in which specific binding of the Avr elicitor by its matching R protein leads to activation of the plant defence response. Direct R-Avr protein recognition precedes the function of an expanding battery of plant resistance signalling components.

The Pto protein kinase perceives and transduces signals from avr Pto to form multiple disease resistance loci within tiny genetic intervals through a number of repeat sequences and clustering of members of the gene family at the Pto locus. The Pto kinase interacts with a low molecular weight effector conditioned by avr Pto to sense the bacterial signal.

The occurrence of a wheat stem rust (Puccinia graminis Pers.:Pers. f. sp. tritici Eriks. & E. Henn.) pathotype (Pgt-QCC) with virulence for the Rpg1 gene in barley (Hordeum vulgare L.) necessitated the search for resistant barley germplasm. From preliminary screenings of over 18 000 barley accessions, 13 lines were identified as possessing resistance to pathotype QCC: 'Diamond', 'Hietpas 5', Q21861, PC 11, PC 84, PC 249, PC 250, CI 5541, PI 452406, PI 452421, PI 477843, PI 477854, and PI 477860. This study was conducted to further characterize the reaction of the selected lines to pathotype QCC. The reaction was assessed by evaluating infection types at the seedling stage and infection responses at the adult plant stage in the greenhouse, and by evaluating disease severity and infection responses at the adult plant stage in the field compared to susceptible cultivars. Most lines exhibited low to intermediate infection types at the seedlings stage and moderately resistant to moderately susceptible infection responses at the adult plant stage in the greenhouse experiments. Among the selected lines, Q21861 exhibited the highest level of resistance at both the seedling and adult plant stages. These lines may provide an adequate level of resistance to pathotype QCC for cultivar development.

Barley yellow dwarf (BYD) is the most economically important and widespread virus disease of small grain cereals in the world. The North American bread wheat (Triticum aestivum L. emend. Thell.) cultivar Anza and several other International Maize and Wheat improvement Center (CIMMYT) wheats have shown tolerance to BYD. Anza and nine other tolerant wheats were studied to understand the genetic basis of tolerance. The tolerant wheats were intercrossed and crossed with the susceptible wheats |Bobwhite' and/or |Bagula'. Parents and the [F.sub.1], and [F.sub.2] generations were tested during 1990 in the field near Toluca (State of Mexico) with the MAV-Mex serotype of barley yellow dwarf virus. Seventy-two individual [F.sub.2]-plant-derived [F.sub.3] lines from each cross were classified in the field during 1991, together with the parents and [F.sub.1]'s, with the same serotype. The intercrosses among tolerant parents failed to segregate, and the observed distribution of [F.sub.3] lines in the crosses of tolerant and susceptible wheats was in ac. cordance with a monogenic segregation ratio. We conclude that the tolerance of Anza and the other nine wheats is due to a common, partially effective and partially dominant gene, designated as Bdv1. Bdv1 could have originated in the Brazilian cultivar Frontana and should be considered a durable source, because of its deployment in numerous CIMMYT wheats worldwide.

The utilization of genus, species and pathovar levels of rRNA gene restriction patterns for the characterization of Xanthomonas campestri is discussed. Bacterial strains were subjected to DNA extraction, gel electrophoresis, hybridization, immunodetection of rRNA-rDNA duplexes and pathogenicity tests. Results indicate that pathogenicity patterns can be used to characterize Xanthomonas strains.