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Microbes can overcome the ability of plant resistance genes to confer protection against disease by mutating their corresponding avirulence genes. The fungus Leptosphaeria maculans causes blackleg disease on canola crops and subverts Brassica napus resistance genes through several DNA mutation mechanisms. One of these is repeat‐induced point (RIP) mutation, which can 'leak' into the avirulence genes from the adjacent repetitive sequences that the mutation process is targeting. Here, we identified populations of L. maculans in Australia that have extensive RIP mutations in the avirulence gene AvrLm2 and show that this has been triggered by a duplication of the gene and surrounding DNA that includes the distant (>55 kb in total) AvrLm6 gene. This finding provides another mechanism of mutation by which pathogens can overcome host resistance, and more broadly contributes to understanding the complex balance between gene duplication and genome defence. [ABSTRACT FROM AUTHOR]

Citrus canker is caused by two Xanthomonas species, Xanthomonas citri, which has become the primary pathogen where citrus canker occurs (type A citrus canker, Xc-A), and X. fuscans pv. aurantifolii (Xfa), which consists of strains B and C. The B strain is less pathogenic than the A strain, but produces symptoms in all citrus species. The C-type cankers only infect Key lime ( Citrus aurantifolia) and produce a hypersensitive reaction ( HR) in grapefruit ( Citrus paradisi) leaves. An avirulence gene, avrGf2, was identified in a C strain that was responsible for the HR in grapefruit. AvrGf2 is a member of Xop AG effector family and shares 45% identity at amino acid level with another member of the same family, AvrGf1 from strain Xc-Aw, which was previously shown to elicit an HR in grapefruit. AvrGf2 shares sequence identity with other Xop AG effectors present in Xanthomonas vasculorum, Xanthomonas campestris pv. musacearum and Pseudomonas syringae pv. tomato. Mutagenesis of avrGf2 in C strain resulted in a compatible reaction in grapefruit. There was no observable effect on virulence when Xc-A transconjugants containing either avirulence gene were inoculated on Key lime. Expression of avrGf1 or avrGf2 in Xc-A resulted in a similar phenotype following infiltration into grapefruit leaves, although the avrGf2 transconjugant elicited a faster HR and lower populations than the transconjugant containing avrGf1. Also, it was shown that all Xfa-B strains tested contain a transposon in avrGf2 that helps to explain the differences in host range between B and C strains. [ABSTRACT FROM AUTHOR]

Fusarium oxysporum f. sp. cubense (Foc), causal agent of fusarium wilt of banana, is among the most destructive pathogens of banana and plantain. The development of a molecular diagnostic capable of reliably distinguishing between the various races of the pathogen is of key importance to disease management. However, attempts to distinguish isolates using the standard molecular loci typically used for fungal phylogenetics have been complicated by a poor correlation between phylogeny and pathogenicity. Among the available alternative loci are several putative effector genes, known as SIX genes, which have been successfully used to differentiate the three races of F. oxysporum f. sp. lycopersici. In this study, an international collection of Foc isolates was screened for the presence of the putative effector SIX8. Using a PCR and sequencing approach, variation in Foc-SIX8 was identified which allowed race 4 to be differentiated from race 1 and 2 isolates, and tropical and subtropical race 4 isolates to be distinguished from one another. [ABSTRACT FROM AUTHOR]

Stem canker of oilseed rape (canola, Brassica napus) is associated with a species complex of two closely related fungal species, Leptosphaeria maculans and L. biglobosa. Of these, L. maculans is the most damaging and develops gene-for-gene relationships with the host . Here, a wide scale analysis of the L. maculans - L. biglobosa species complex was performed throughout the American continent (23 locations from Chile to Canada) plus several locations in Western Australia for comparison purposes, based on a collection of 1132 isolates from infected tissues of a susceptible cultivar. Fungal species were discriminated on the basis of morphological, phytopathological and molecular criteria and showed that L. biglobosa was closely associated with L. maculans in most of the locations. Multiple gene phylogeny using sequences of ITS, actin and β-tubulin confirmed the prevalence of the L. biglobosa‘canadensis’ sub-clade in Canada, whereas up to three different sub-clades of L. biglobosa were found in Georgia (USA). Race structure of L. maculans was investigated using a combination of pathogenicity tests and PCR amplification of avirulence alleles AvrLm1, AvrLm4 and AvrLm6. Three contrasting situations were observed: (i) race structure in Ontario, Chile and Georgia was related to that of European and Western Australian populations, with a low race diversity; (ii) only one race was found in Mexico, and not found outside of this country; (iii) a large diversity of races was observed in central Canada (Manitoba, Alberta and Saskatchewan) with very specific features including maintenance of avirulence alleles absent from Europe, absence of the AvrLm7 allele common in Europe (or eastern Canada) and wide location-to-location variability. [ABSTRACT FROM AUTHOR]

Blackleg disease (phoma stem canker) of Brassica napus (canola, oilseed rape) is caused by the fungus Leptosphaeria maculans. In some regions of Australia, resistance in oilseed rape cultivars derived from B. rapa subs . sylvestris (e.g. cv. Surpass 400) became ineffective within three years of commercial release. The genetic control of avirulence in L. maculans towards cv. Surpass 400 is described. When Australian field isolates were screened on this cultivar, three phenotypic classes were observed; virulent, intermediate and avirulent. Analysis of crosses between fungal isolates varying in their ability to infect cv. Surpass 400 demonstrated the presence of two unlinked avirulence genes, AvrLm1 and AvrLmS. Complementation of isolates (genotype avrLm1) with a functional copy of AvrLm1, and genotyping of field isolates using a molecular marker for AvrLm1 showed that virulence towards Rlm1 is necessary, but not sufficient, for expression of a virulent phenotype on cv. Surpass 400. Taken together, these data strongly suggest that cv. Surpass 400, with ‘ sylvestris-derived’ resistance, contains at least two resistance genes, one of which is Rlm1. [ABSTRACT FROM AUTHOR]

The interactions between Xanthomonas oryzae pv. oryzae (Xoo) and rice are controlled in a gene-for-gene manner. In this study, a 359 bp DNA fragment of the avrXa3 gene containing three nuclear localization signal (NLS) motifs present in all members of the avrBs3/pthA family was used as a probe to screen a genomic library of the JXOIII strain of Xoo. The results demonstrated that diverse members of the family exist in the pathogen genome. The avrBs3/pthA genes occurred at isolated individual portions or in clusters. The positive avr gene clones were transferred into the virulent recipient PXO99A. Pathogenicity tests in near isogenic lines of rice confirmed that four resistance (R) genes ( Xa2, Xa3, xa5 and xa8) matched the four avr genes ( avrXa2, avrXa3, avrxa5 and avrxa8) in the genome of Xoo strain JXOIII. The avrBs3/PthA-like gene (1·7 kb) present in cosmid p54, may specifically interact with the Xa3 gene present in IRBB3, and is designated avr/pthA3. Sequencing indicated that there are only 1·5 copies of the 102 bp repeat unit in avr/pthA3. Alignment of the twelfth and thirteenth amino acids in the repetitive units encoded by this gene with those in other representatives of the AvrBs3 family revealed a unique repeat arrangement which might contribute to variation in the avirulence genes in Xoo. The parental rice line IR24 was found to contain several R genes for resistance to Xoo bacterial blight. [ABSTRACT FROM AUTHOR]

Aerial populations of Blumeria graminis f.sp. hordei were studied in two French and two Danish regions from 1991 to 1999, at a time of year when only winter barley was present. A high frequency of genotypes not able to grow on the spring-sown crop of the previous growing season (denoted ‘spring-avirulent’) was observed in most years and regions. This frequency increased with increasing proportion of winter barley; it was highest in France and decreased in general over the 8-year period. Most of the spring-avirulent genotypes possessed the V a 22 virulence gene, matching a resistance that has never been present in barley cultivars grown in Europe. A hypothetical cropping system, including winter- and spring-sown crops with three resistance genes altogether, was constructed to mimic the utilization of host cultivars in the four regions. Results from a mathematical model simulating changes in the composition of the pathogen population in this system, demonstrated that selection solely due to host resistance genes, i.e. without assuming any cost of virulence, might lead to such results as those observed. The changes in frequency of spring-avirulent genotypes and the frequency of unnecessary virulence genes may be predicted from the proportion of the barley area sown with winter barley, the use of resistance genes in the cultivars, the initial composition of the pathogen population, and hitch-hiking due to gametic disequilibria. [ABSTRACT FROM AUTHOR]