The avirulence gene AVR2-MARA of the pathogenicfungus Magnaporthe grisea

Abstract

'The ascomycetal fungus Magnaporthe grisea causes the most damaging fungal disease of rice, blast. Resistant rice cultivars are typically dependent on the presence of one, or a few, major genes that are effective only toward particular M. grisea isolates. These isolates have avirulence genes that correspond to specific rice resistance genes, in a gene-for-gene relationship. Results presented here show the genetic identification of AVR2-MARA, a M. grisea a gene that confers avirulence toward the rice cultivar Maratelli. Two techniques were used for determining the chromosomal location of AVR2-MARA, restriction fragment length polymorphism analysis (RFLP), and bulked segregant analysis. Through RFLP analysis AVR2-MARA was mapped to Chromosome 7, between markers cos196 and cos209. Bacterial Artificial Chromosome (BAC) clones, as well as cosmids, were utilized in chromosome walking to the locus. Walks were initiated from markers on both sides of the locus, allowing the identification of sequences that were inseparable from AVR2-MARA. However, I was unable to clone the complete locus. In contrast, avr2-MARA , the virulent locus, was isolated from the virulent parental strain O-135. Sequence analysis of markers inseparable from the AVR2-MARA locus showed a higher AT content than typically observed in the M. grisea genome. Sequence analysis of a fragment inseparable from the AVR2-MARA locus also revealed a putative open-reading-frame (ORF) with significant homology to AVR-Pita. During this study I found that the segregation of avirulence on Maratelli corresponded with the segregation of avirulence towards the rice cultivars M-103 and M-201. AVR-M201, conferring avirulence toward the rice cultivar M-201, was identified several years ago (Valent et al. 1991), but never mapped. In our mapping cross, consisting of 65 progeny, AVR2-MARA and AVR-M201 are inseparable. Furthermore, we isolated two virulent mutants of AVR2-MARA via UV mutagenesis and both had also gained virulence on cultivars M-103 and M-201. This suggests that AVR2-MARA, AVR-M201 (and AVR-M103) are the same avirulence gene. Because AVR2-MARA originated in a finger millet isolate, and confers avirulence toward several different rice cultivars, it may represent a host-species-specificity factor. Such rice cultivars may be guarded from infection by finger millet isolates due to recognition of the AVR2-MARA gene product.