Current status on mapping of genes for resistance to leaf- and neck-blast disease in rice.

Blast disease caused by fungal pathogen Pyricularia oryzae is a major threat to rice productivity worldwide. The rice-blast pathogen can infect both leaves and panicle neck nodes. Nearly, 118 genes for resistance to leaf blast have been identified and 25 of these have been molecularly characterized. A great majority of these genes encode nucleotide-binding site-leucine-rich repeat (NBS-LRR) proteins and are organized into clusters as allelic or tightly linked genes. Compared to ever expanding list of leaf-blast-resistance genes, a few major genes mediating protection to neck blast have been identified. A great majority of the genetic studies conducted with the genotypes differing in the degree of susceptibility/resistance to neck blast have suggested quantitative inheritance for the trait. Several reports on co-localization of gene/QTLs for leaf- and neck-blast resistance in rice genome have suggested the existence of common genes for resistance to both phases of the disease albeit inconsistencies in the genomic positions leaf- and neck-blast-resistance genes in some instances have presented the contrasting scenario. There is a strong evidence to suggest that developmentally regulated expression of many blast-resistance genes is a key determinant deciding their effectiveness against leaf or neck blast. Testing of currently characterized leaf-blast-resistance genes for their reaction to neck blast is required to expand the existing repertoire resistance genes against neck blast. Current developments in the understanding of molecular basis of host-pathogen interactions in rice-blast pathosystem offer novel possibilities for achieving durable resistance to blast through exploitation of natural or genetically engineered loss-of-function alleles of host susceptibility genes.