Cloning and application of rice blast resistance genes: A success story

Plant diseases cause billion-dollars annual yield loss, and in some instances, these losses have severe consequences for human race leading to famine. The management of diseases in crops is still largely dominated by the use of chemical pesticides, but with the increase in the incidence of pesticide resistance and mounting concern for the environmental hazards resulting from excessive agrochemical use, the search for alternative, reliable methods of disease management is gaining momentum. One of the most convenient, inexpensive, and environmentally safe ways of plant disease management is to deploy disease-resistance varieties in different agro-climatic zones of India. The classical example of deployment of different resistance (R) genes in epidemiologically diverse regions of India is the management of wheat rusts. Disease resistance genes which detect pathogen infection at the preliminary stages of defense have been used extensively by the plant breeders by gene pyramiding and genetic engineering after their mapping and cloning. Till date about 101 blast resistance genes have been mapped from different rice lines. Out of these 19 genes resistant to rice blast pathogen Magnaporthe oryzae have been cloned during the past 10 years. Of these, one of the genes Pi-kʰ (Pi54) for rice blast resistance has been cloned and characterized at NRCPB, IARI, New Delhi. This gene was only the third one to be cloned in the series of rice blast resistance genes, after the cloning of Pib and Pita. The gene was renamed as Pi54, after it was further re-alienated to a slightly distant location from the Pik cluster of the genes. The predicted Pi54 protein contains a nucleotide binding site -leucine rich repeat (NBS-LRR) domain in addition to a small zinc finger domain. Functional complementation of the gene has confirmed its stable and high-level of resistance against geographically diverse strains of M. oryzae. The Pi54 gene is expressed constitutively at a basal level in both resistant transgenic as well as susceptible native lines upto 48 hpi. We further proved that, the single Pi54 gene regulates complex defense response mechanisms against M. oryzae in rice. Also, this gene is widely being used in pyramiding programme using marker assisted selection for broad spectrum resistance to blast in combination with other genes into Basmati and non Basmati type rice varieties in the country through many net work projects. The acquisition of new R-genes enables plants to resist more to the pathogen leading to the emergence of new and more virulent races of the pathogens. It is therefore important to understand how R-genes are regulated so that they can be introduced in to new plants for disease resistance without compromising their effects on pathogen evolution. Because of co-evolution of avirulence genes in the pathogen, most of the resistance genes succumb to pathogen attack and become ineffective within a few years of their release in the environment. Hence search for new R-genes or their alleles should be top priority of research in the country.