Imperative Role of R-Genes and Associated Molecular Mechanisms in Plant Disease Resistance

Plant’s survival is under constant threat due to attacks by several pests and pathogens. Being sessile, plants cannot evade such attacks but only protect themselves against these invaders. The defence can be either constitutive through pre-formed physical and chemical barriers or inducible through the expression of genes related to plant defense. The latter is called the secondary line of defense, which can further occur in layers. The first layer, Pathogen Triggered Immunity (PTI), is triggered by the recognition of certain pathogen associated molecular patterns by the pattern recognition receptors on the host cell membrane. It usually entails physiological changes, production of phytoalexins and reactive oxygen species, and manifestation of hypersensitive response. Pathogens have developed strategies to overcome PTI, and they directly secrete effectors into host cells, concurrent to which plants have evolved Effector Triggered Immunity (ETI), which is the second layer of defense and involves the expression of resistance genes (R-genes). The first ever R-gene was cloned from Zea mays following where several others were cloned from model and crop plants which have revealed the occurrence of several conserved domains in the resistance proteins facilitating identification of resistance gene analogues (RGAs) using various tools and techniques. Further, the mechanisms by which products of R-genes confer resistance have been divided into nine types, broadly, through perception of pathogen effectors or due to loss-of-susceptibility. The chapter discusses the structure of R-proteins and how these conserved domains can be used for genome wide identification of RGAs. We also discuss, with examples from different plant-pathogen systems, the nine mechanisms of disease resistance in detail.