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We have constructed a common bean genomic library enriched for microsatellite motifs (ATA), (CA), (CAC) and (GA). After screening, 60% of the clones selected from the library enriched for the (ATA) repeat contained microsatellites versus 21% of the clones from the library enriched for (GA) (CA) and (CAC) repeats. Fifteen primer pairs have been developed allowing for the amplification of SSR loci. We have evaluated the genetic diversity of these loci between 45 different bean lines belonging to nine various quality types. A total of 81 alleles were detected at the 15 microsatellite loci with an average of 5.3 alleles per locus. We have investigated the origin of allelic size polymorphism at the locus PvATA20 in which the number of repeats ranges from 24 to 85. We have related these large differences in repeat number to unequal crossing-over between repeated DNA regions. The diversity analysis revealed contrasted levels of variability according to the bean type. The lower level was evidenced for the very fine French bean, showing the effect of breeders intensive selection.

We have shown that the plant pathogen Alternaria brassicicola exhibited very high susceptibility to ambruticin VS4 and to a lesser extent to the phenylpyrrole fungicide fludioxonil. These compounds are both derived from natural bacterial metabolites with antifungal properties and are thought to exert their toxicity by interfering with osmoregulation in filamentous fungi. Disruption of the osmosensor group III histidine kinase gene AbNIK1 (for A. brassicola NIK1) resulted in high levels of resistance to ambruticin and fludioxonil, while a mutant isolate characterized by a single-amino-acid substitution in the HAMP domain of the kinase only exhibited moderate resistance. Moreover, the natural resistance of Saccharomyces cerevisiae to these antifungal molecules switched to sensitivity in strains expressing AbNIK1p. We also showed that exposure to fludioxonil and ambruticin resulted in abnormal phosphorylation of a Hog1-like mitogen-activated protein kinase (MAPK) in A. brassicicola. Parallel experiments carried out with wild-type and mutant isolates of Neurospora crassa revealed that, in this species, ambruticin susceptibility was dependent on the OS1-RRG1 branch of the phosphorelay pathway downstream of the OS2 MAPK cascade but independent of the yeast Skn7-like response regulator RRG2. These results show that the ability to synthesize a functional group III histidine kinase is a prerequisite for the expression of ambruticin and phenylpyrrole susceptibility in A. brassicicola and N. crassa and that, at least in the latter species, improper activation of the high-osmolarity glycerol-related pathway could explain their fungicidal properties.

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