Submitted to: Proceedings of the National Academy of Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/11/2005
Publication Date: 11/1/2005
Citation: Tosa, Y., Osue, J., Eto, Y., Nakayashiki, H., Mayama, S., Leong, S.A. 2005. Evolution of an arivulence gene avr1-co39 concomitant with the evolution and differentiation of magnaporthe oryzae. Proceedings of the National Academy of Sciences. 18(11):1148-1160. Interpretive Summary: Blast disease of grasses is a widespread problem throughout the world with rice blast disease being one of the most devastating in terms of food security and economic sustainability of farming systems. Recently, grey leaf spot disease of perennial rye has become an important concern of the turf industry and golf course managers in the U.S. and Japan. We have cloned a family of genes all related to the gene AVR1-CO39 from the fungus Magnaporthe that causes rice blast and grey leaf spot. Interestingly, all strains that attack cereals such as wheat, millet and rye have functional copies of this gene, while strains that attack rice lack a functional copy of this gene. The mutations observed in the rice strains appear to have occurred long ago likely during the first appearance of cultivated rice in agriculture. This may mean that AVR1-CO39 plays an important and yet to be determined role in the disease cycle and evolutionary success of these strains that still carry this gene. The rice resistance gene Pi-CO39, that has been previously identified, may be useful if transferred to rye and finger millet since the pathogen carries the functional AVR1-CO39 gene. This resistance would protect these grasses from infection.
Technical Abstract: Significance of AVR1-CO39, an avirulence gene of the blast fungus corresponding to Pi-CO39(t) in rice cultivars, during the evolution and differentiation of the blast fungus was evaluated from its function and distribution in Pyricularia spp. When the presence/absence of AVR1-CO39 was plotted on a dendrogram constructed from rDNA sequences, a perfect parallelism was observed between its distribution and the phylogeny of Pyricularia isolates. AVR1-CO39 homologs were exclusively present in one species, P. oryzae, suggesting that AVR1-CO39 appeared during the early stage of evolution of P. oryzae. Transformation assays showed that all the cloned homologs tested function as an avirulence gene suggesting that there has been a selection pressure to maintain their function. Nevertheless, Oryza isolates (isolates virulent on Oryza spp.) in P. oryzae were exceptionally non-carriers of AVR1-CO39. All Oryza isolates suffered from one of the two types of rearrangements at the AVR1-CO39 locus, i.e., G type and J type. (I thought we had found another haplotype in US lines in last paper) These types were parallel to lineages of Oryza isolates from Japan ?? determined by MGR586 and MAGGY. These results suggest that AVR1-CO39 was lost during the early stage of evolution of the Oryza-specific subgroup of P. oryzae. Interestingly, its corresponding resistance gene, Pi-CO39(t) is not widely distributed in Oryza spp. This may be explained by assuming that the acquisition of the virulence on rice occurred recently in a confined area where indigenous rice strains or population(s) carried Pi-CO39(t) at a high frequency.