Signal Transduction of R Gene-Mediated Plant Resistance against Pathogens

Plants have developed efficient defense responses to counterbiotic stress factors that cause major losses in productivity. These defense responses are triggered upon the recognition of specific factors released by the invading pathogens by the corresponding elements of the host plants. They are encoded by the avirulence (Avr) and resistance (R) genes, respectively. R genes are crucial for providing resistance to pathogens in plant hosts by recognizing the cognate pathogenic Avr-dependent effectors, either by direct or indirect means. Following recognition of a pathogen, R gene-encoded products or R proteins mediate signaling cascade(s) to coordinate specific defense responses that terminate in hypersensitive responses. Nucleotide-binding leucine-rich repeat proteins (NLRs) form the largest class of R proteins. Effector recognition is followed by the activation of NLRs and their oligomerization into homo- and heteromeric complexes. While some of the NLRs are capable of inducing signaling by themselves, others work in complementary pairs with other NLRs (NLR–NLR pairs) and some others rely on ‘helper’ NLRs. Downstream NLR signaling requires other non-NLR proteins, such as Enhanced Disease Sensitivity 1 (EDS1) and Non-race-specific Disease Resistance 1 (NDR1), and may also involve MAPKs and transcription factors (TFs) for positive regulation of defense-related genes, and salicylic acid signaling for the activation of systemic acquired resistance (SAR). Recently, it has been shown that some NLRs function through the formation of higher-order structures termed as ‘resistosomes’, which get inserted into the plasma membrane and trigger Ca²⁺ influx by serving as Ca²⁺-permeable channels. The Ca²⁺ influx, in turn, induces an oxidative burst through ROS production required for cell death. Despite considerable advancement in our understanding of R gene-mediated signaling, many missing links still exist which need to be investigated. As further progress is made in the area of R gene-mediated signaling, many promising opportunities will be unraveled for resistance introduction in the host plants to facilitate effective disease management.