Sequence domains required for the activity of avirulence genes avrB and avrC from Pseudomonas syringae pv. glycinea.

avrB and avrC from Pseudomonas syringae pv. glycinea share significant amino acid homology but interact with different soybean resistance genes to elicit the hypersensitive defense reaction. Recombinant genes constructed between avrB and avrC revealed that the central regions were required for avirulence gene activity but the 5' and 3' termini were interchangeable. Recombinants involving the central regions did not yield any detectable avirulence gene activity, and no new avirulence phenotypes were observed from any of the chimeric genes. These results suggest that the protein products of avrB and avrC possess catalytic properties that are required for the avirulence phenotypes.     

Gene-for-gene interactions: bacterial avirulence proteins specify plant disease resistance

Resistance of plants to bacterial pathogens is often controlled by corresponding genes for resistance and avirulence in host and pathogen, respectively. Fifty years after discovery of the genetic basis of gene-for-gene interactions, several avirulence and plant resistance genes have been isolated and are being studied on the molecular level. Tremendous progress has been made due to a better understanding of type III secretion systems that are required for bacterial pathogenicity. We are beginning to grasp how the plant actually recognizes bacterial avirulence determinants. The current view is that the bacterium translocates avirulence proteins into the host cell by the Hrp type III secretion system and that recognition occurs in the plant cell.     

Plant disease resistance triggered by pathogen-derived molecules: refined models of specific recognition

Plant disease resistance is often determined by complementary pairs of resistance genes (from the plant) and avirulence genes (from the invading pathogen). This gene-for-gene interaction has generally been interpreted as a receptor-ligand model in which the avirulence protein binds to the corresponding resistance protein, which in turn initiates plant defense. Recent studies indicate that co-localization of avirulence and resistance proteins is essential for their function. However, there is also accumulating evidence that resistance proteins are not the primary receptor for the avirulence protein. We review advances in our understanding of gene-for-gene resistance and examine validity of the receptor-ligand model and other proposed models in the context of the most recent findings.     

Prospects for understanding avirulence gene function

Avirulence genes are originally defined by their negative impact on the ability of a pathogen to infect their host plant. Many avirulence genes are now known to represent a subset of virulence factors involved in the mediation of the host-pathogen interaction. Characterization of avirulence genes has revealed that they encode an amazing assortment of proteins and belong to several gene families. Although the biochemical functions of the avirulence gene products are unknown, studies are beginning to reveal the features and interesting relationships between the avirulence and virulence activities of the proteins. Identification of critical virulence factors and elucidation of their functions promises to provide insight into plant defense mechanisms, and new and improved strategies for the control of plant disease.     

Intragenic recombination of a single plant pathogen gene provides a mechanism for the evolution of new host specificities.

Gene pthA is required for virulence of Xanthomonas citri on citrus plants and has pleiotropic pathogenicity and avirulence functions when transferred to many different xanthomonads. DNA sequencing revealed that pthA belongs to a family of Xanthomonas avirulence/pathogenicity genes characterized by nearly identical 102-bp tandem repeats in the central region. By inserting an nptI-sac cartridge into the tandemly repeated region of pthA as a selective marker, intragenic recombination among homologous repeats was observed in both Xanthomonas spp. and Escherichia coli. Intragenic recombination within pthA created new genes with novel host specificities and altered pathogenicity and/or avirulence phenotypes. Many pthA recombinants gained or lost avirulence function in pathogenicity assays on bean, citrus, and cotton cultivars. Although the ability to induce cell division (hyperplastic cankers) on citrus could be lost, this ability was not acquired on cotton or bean plants. Intragenic recombination therefore provides a genetic mechanism for the generation of multiple, different, and gratuitous avirulence genes from a single, required, host-specific pathogenicity gene.     

A gene from Xanthomonas campestris pv. vesicatoria that determines avirulence in tomato is related to avrBs3.

Strains of Xanthomonas campestris pv. vesicatoria that were avirulent in tomato leaves but virulent in pepper leaves were identified. A cloned gene, avrBsP, from one of the strains, Xv 87-7, converted a virulent strain in tomato to avirulent in tomato. A 1.7-kb subclone containing the avirulence gene cross-hybridized with the avirulence gene, which determines race 1 within the pepper group of strains (avrBs3). However, the two avirulence genes differ in their biological activity. The base sequences of the two avirulence genes were almost identical through the 1.7-kb segment of avrBsP, with significant differences only in some bases in the repeat region.     

Genetic exchange of avirulence determinants and extensive karyotype rearrangements in parasexual recombinants of Fusarium oxysporum

In order to genetically map and eventually isolate avirulence genes, parasexual crosses between different races of Fusarium oxysporum f. sp. lycopersici were performed by means of protoplast fusion. Two wild-type strains, race 1 Fol004 (A1a2a3) and race 3 Fol029 (a1a2A3), were transformed with phleomycin and hygromycin resistance genes, respectively. In total 32 fusion products were selected by screening for the presence of both marker genes. The presence of either avirulence gene A1 or A3 in the fusion products was determined by plant bioassays. Segregation of avirulence revealed a bias for the presence of A1. Two recombinants for the avirulence phenotype were observed, each with a new association of avirulence genes never observed to exist in the wild. Electrophoretic karyotype analysis revealed that chromosome patterns were different for all fusion products. Hybridization patterns using various probes indicated that chromosome rearrangements and recombination had occurred. Karyotype analysis of the two avirulence recombinants revealed hybrid karyotypes resulting from a massive exchange of parental DNA. This indicates that the present population of recombinants can be used for gene mapping in the asexual fungus F. oxysporum f. sp. lycopersici.     

稻瘟菌无毒基因研究进展

无毒基因编码的产物激发病原物与植物特异性相互作用。水稻与稻瘟菌之间的特异互作符合“基因对基因”关系。从研究稻瘟菌无毒基因的意义、已鉴定和克隆的稻瘟菌无毒基因、稻瘟菌无毒基因与其抗病基因的互作特点等几个方面,对稻瘟菌无毒基因研究进展作了简要评述 。     

Recognition of bacterial avirulence proteins occurs inside the plant cell: a general phenomenon in resistance to bacterial diseases?

One of the recent exciting developments in the research area of plant-microbe interactions is a breakthrough in understanding part of the initial signalling between avirulent Gram-negative bacteria and resistant plants. For resistance to occur, both interacting organisms need to express matching genes, the plant resistance gene and the bacterial avirulence gene. The biochemical function of bacterial avirulence genes and the nature of the signal molecules recognized by the plant have been a mystery for a long time. Recently, several laboratories have shown that bacterial avirulence proteins function as elicitors that are perceived within the plant cell.     

Identification of Pseudomonas syringae pathogens of Arabidopsis and a bacterial locus determining avirulence on both Arabidopsis and soybean.

To develop a model system for molecular genetic analysis of plant-pathogen interactions, we studied the interaction between Arabidopsis thaliana and the bacterial pathogen Pseudomonas syringae pv tomato (Pst). Pst strains were found to be virulent or avirulent on specific Arabidopsis ecotypes, and single ecotypes were resistant to some Pst strains and susceptible to others. In many plant-pathogen interactions, disease resistance is controlled by the simultaneous presence of single plant resistance genes and single pathogen avirulence genes. Therefore, we tested whether avirulence genes in Pst controlled induction of resistance in Arabidopsis. Cosmids that determine avirulence were isolated from Pst genomic libraries, and the Pst avirulence locus avrRpt2 was defined. This allowed us to construct pathogens that differed only by the presence or absence of a single putative avirulence gene. We found that Arabidopsis ecotype Col-0 was susceptible to Pst strain DC3000 but resistant to the same strain carrying avrRpt2, suggesting that a single locus in Col-0 determines resistance. As a first step toward genetically mapping the postulated resistance locus, an ecotype susceptible to infection by DC3000 carrying avrRpt2 was identified. The avrRpt2 locus from Pst was also moved into virulent strains of the soybean pathogen P. syringae pv glycinea to test whether this locus could determine avirulence on soybean. The resulting strains induced a resistant response in a cultivar-specific manner, suggesting that similar resistance mechanisms may function in Arabidopsis and soybean.     

Constraints on evolution of virus avirulence factors predict the durability of corresponding plant resistances

Understanding the factors driving pathogen emergence and re-emergence is a major challenge, particularly in agriculture, where the use of resistant plant cultivars imposes strong selective pressures on plant pathogen populations and leads frequently to 'resistance breakdown'. Presently, durable resistances are only identified after a long period of large-scale cultivation of resistant cultivars. We propose a new predictor of the durability of plant resistance. Because resistance breakdown involves modifications in the avirulence factors of pathogens, we tested for correlations between the evolutionary constraints acting on avirulence factors or their diversity and the durability of the corresponding resistance genes in the case of plant–virus interactions. An analysis performed on 20 virus species–resistance gene combinations revealed that the selective constraints applied on amino acid substitutions in virus avirulence factors correlate with the observed durability of the corresponding resistance genes. On the basis of this result, a model predicting the potential durability of resistance genes as a function of the selective constraints applied on the corresponding avirulence factors is proposed to help breeders to select the most durable resistance genes.     

Soybean Resistance Genes Specific for Different Pseudomonas Syringae Avirulence Genes Are Allelic, or Closely Linked, at the Rpg1 Locus

RPG1 and RPM1 are disease resistance genes in soybean and Arabidopsis, respectively, that confer resistance to Pseudomonas syringae strains expressing the avirulence gene avrB. RPM1 has recently been demonstrated to have a second specificity, also conferring resistance to P. syringae strains expressing avrRpm1. Here we show that alleles, or closely linked genes, exist at the RPG1 locus in soybean that are specific for either avrB or avrRpm1 and thus can distinguish between these two avirulence genes.     

Marker-based cloning of the region containing the UhAvr1 avirulence gene from the basidiomycete barley pathogen Ustilago hordei

Race-cultivar specialization during the interaction of the basidiomycete smut pathogen Ustilago hordei with its barley host was described in the 1940s. Subsequent genetic analyses revealed the presence of dominant avirulence genes in the pathogen that conform to the gene-for-gene theory. This pathosystem therefore presents an opportunity for the molecular genetic characterization of fungal genes controlling avirulence. We performed a cross between U. hordei strains to obtain 54 progeny segregating for three dominant avirulence genes on three differential barley cultivars. Bulked segregant analysis was used to identify RAPD and AFLP markers tightly linked to the avirulence gene UhAvr1. The UhAvr1 gene is located in an area containing repetitive DNA and this region is undetectable in cosmid libraries prepared from the avirulent parental strain. PCR and hybridization probes developed from the linked markers were therefore used to identify cosmid clones from the virulent (Uhavr1) parent. By walking on Uhavr1-linked cosmid clones, a nonrepetitive, nearby probe was found that recognized five overlapping BAC clones spanning 170 kb from the UhAvr1 parent. A contig of the clones in the UhAvr1 region was constructed and selected probes were used for RFLP analysis of the segregating population. This approach genetically defined an approximately 80-kb region that carries the UhAvr1 gene and provided cloned sequences for subsequent genetic analysis. UhAvr1 represents the first avirulence gene cloned from a basidiomycete plant pathogen.     

稻瘟病菌无毒基因研究进展

水稻与稻瘟病菌之间的特异互作符合基因对基因假说。本文将从稻瘟病菌与水稻抗病基因间的互作特点、稻瘟病菌的分子标记、已克隆的稻瘟病菌无毒基因三个方面对稻瘟病菌无毒基因研究进展作简要介绍     

Protein-protein interactions in pathogen recognition by plants

Protein-protein interactions have emerged as key determinants of whether plant encounters with pathogens result in disease or successful plant defense. Genetic interactions between plant resistance genes and pathogen avirulence genes enable pathogen recognition by plants and activate plant defense. These gene-for-gene interactions in some cases have been shown to involve direct interactions of the products of the genes, and have indicated plant intracellular localization for certain avirulence proteins. Incomplete specificity of some of the interactions in laboratory assays suggests that additional proteins might be required to confer specificity in the plant. In many cases, resistance and avirulence protein interactions have not been demonstrable, and in some cases, other plant components that interact with avirulence proteins have been found. Investigation to date has relied heavily on biochemical and cytological methods including in vitrobinding assays and immunoprecipitation, as well as genetic tools such as the yeast two-hybrid system. Observations so far, however, point to the likely requirement for multiple, interdependent protein associations in pathogen recognition, for which these techniques can be insufficient. This article reviews the protein-protein interactions that have been described in pathogen recognition by plants, and provides examples of how rapid future progress will hinge on the adoption of new and developing technologies.     

Genetic mapping of Pyrenophora teres f. teres genes conferring avirulence on barley

A Pyrenophora teres f. teres cross between isolates 0-1 and 15A was used to evaluate the genetics of avirulence associated with barley lines Canadian Lake Shore (CLS), Tifang, and Prato. 15A is avirulent on Tifang and CLS, but virulent on Prato. Conversely, 0-1 is avirulent on Prato, but virulent on Tifang and CLS. Avirulence:virulence on Tifang and CLS segregated 1:1, whereas avirulence:virulence on Prato segregated 3:1. An AFLP-based linkage map was constructed and used to identify a single locus derived from 15A (AvrHar) conferring avirulence to Tifang and CLS. Virulence on Prato was conferred by two epistatic genes (AvrPra1 and AvrPra2). AvrPra2 co-segregated with AvrHar, but the two genes from opposite parents conferred opposite reactions. This work provides the foundation for the isolation of these avirulence genes.     

Genetic control of virulence of Pyrenophora teres drechs, the causative agent of net blotch in barley

The genetic control of virulence was studied in four isolates of the fungus Pyrenophora teres f. teres, originating from various geographic regions in experiments with nine barley accessions, possessing known resistance genes. Experiments were performed with the ascospore progeny of two crosses. The results of segregation for virulence in the progeny of direct crosses were confirmed by analysis of backcrosses and sib crosses. One to four genes for avirulence toward various barley genotypes were found in the isolates under study. It is suggested that dominant suppressor genes are involved in the genetic control of avirulence toward four barley genotypes.     

Amino acid substitution in P3 of Soybean mosaic virus to convert avirulence to virulence on Rsv4‐genotype soybean is influenced by the genetic composition of P3

The modification of avirulence factors of plant viruses by one or more amino acid substitutions converts avirulence to virulence on hosts containing resistance genes. Limited experimental studies have been conducted on avirulence/virulence factors of plant viruses, in particular those of potyviruses, to determine whether avirulence/virulence sites are conserved among strains. In this study, the Soybean mosaic virus (SMV)–Rsv4 pathosystem was exploited to determine whether: (i) avirulence/virulence determinants of SMV reside exclusively on P3 regardless of virus strain; and (ii) the sites residing on P3 and crucial for avirulence/virulence of isolates belonging to strain G2 are also involved in virulence of avirulent isolates belonging to strain G7. The results confirm that avirulence/virulence determinants of SMV on Rsv4‐genotype soybean reside exclusively on P3. Furthermore, the data show that sites involved in the virulence of SMV on Rsv4‐genotype soybean vary among strains, with the genetic composition of P3 playing a crucial role.     

RPS2, an Arabidopsis disease resistance locus specifying recognition of Pseudomonas syringae strains expressing the avirulence gene avrRpt2.

A molecular genetic approach was used to identify and characterize plant genes that control bacterial disease resistance in Arabidopsis. A screen for mutants with altered resistance to the bacterial pathogen Pseudomonas syringae pv. tomato (Pst) expressing the avirulence gene avrRpt2 resulted in the isolation of four susceptible rps (resistance to P. syringae) mutants. The rps mutants lost resistance specifically to bacterial strains expressing avrRpt2 as they retained resistance to Pst strains expressing the avirulence genes avrB or avrRpm1. Genetic analysis indicated that in each of the four rps mutants, susceptibility was due to a single mutation mapping to the same locus on chromosome 4. Identification of a resistance locus with specificity for a single bacterial avirulence gene suggests that this locus, designated RPS2, controls specific recognition of bacteria expressing the avirulence gene avrRpt2. Ecotype Wü-0, a naturally occurring line that is susceptible to Pst strains expressing avrRpt2, appears to lack a functional allele at RPS2, demonstrating that there is natural variation at the RPS2 locus among wild populations of Arabidopsis.     

Determinants of pathogenicity and avirulence in plant pathogenic bacteria

Many plant pathogenic bacteria possess a conserved protein secretion system that is thought to transfer Avr (avirulence) proteins, with potential activities in both parasitism and defense elicitation, into plant cells. avr genes may be acquired horizontally by these bacteria, and avr gene compositions are highly variable. In the past year, heterologous expression experiments have revealed that the products of avr genes can be interchanged among different genera of bacteria with retention of secretion, pathogenicity, and avirulence activities, suggesting mechanisms for rapid coevolution of these parasites with changing plant hosts.