Two and a half decades of plant R-gene studies: A review

Resistance genes (R-Genes) are genes in plant genomes that convey disease resistance to plants against pathogens by producing R proteins. Food and Agriculture Organization, UN, defined R genes as “a class of plant genes conferring resistance to a specific strain of a particular pathogen primarily by sensing the presence of the pathogen and triggering the defense pathways in the plant”. Plant R-genes involved in gene-for-gene interactions with pathogens are expected to undergo co-evolutionary arms races in which plant specificity and pathogen virulence continually adapt in response to each other. Plants evolved disease resistance (R) proteins to specifically detect the presence of the pathogen effectors called avirulence factors (Avr) once recognized by R proteins and subsequently trigger a much stronger defense response to counter the suppression of the Microbe Associated Molecular Pattern (MAMP) triggered immunity (MTI). It has been more than 25 years of R gene isolation, cloning and characterization and rigorous reviews have revealed the functional mechanisms underlying resistance at molecular level but whatever is known till date is merely a tip of the iceberg.