Mechanisms by which convergently evolved innate immune receptors function in plants

Abstract

Some nucleotide-binding and leucine-rich repeat receptors (NLRs) indirectly detect pathogen effectors by monitoring perturbations on host proteins targeted by the effectors. In Arabidopsis thaliana (hereafter, Arabidopsis), RPM1 INTERACTING PROTEIN 4 (RIN4) is targeted by multiple sequence- unrelated effectors and activates immune responses mediated by Arabidopsis NLRs including RESISTANCE TO PSEUDOMONAS SYRINGAE PV. MACULICOLA 1 (RPM1) and RESISTANCE TO P. SYRINGAE 2 (RPS2). Arabidopsis RIN4-modifying effectors trigger cell death in Nicotiana benthamiana, but the corresponding NLRs have yet not been identified. To identify N. benthamiana NLRs (NbNLRs) that recognize Arabidopsis RIN4-modifying effectors, we conducted a rapid reverse genetic screen using an NbNLR virus-induced gene silencing (VIGS) library. We identified that the N. benthamiana homolog of Pseudomonas tomato race 1 (NbPtr1) recognizes the Pseudomonas effectors AvrRpt2, AvrRpm1, and AvrB. We demonstrated that recognition of the Xanthomonas effector AvrBsT and the Pseudomonas effector HopZ5 in N. benthamiana is conferred independently by the N. benthamiana homolog of Ptr1 and ZAR1. We further 20212708 investigated the expanded recognition of NbPtr1 and searched for host proteins that NbPtr1 monitors. N. benthamiana RIN4 homologs and nitrate- induced domain-containing proteins (NOI proteins) were tested for the ability to suppress NbPtr1 autoactivity. NbNOI1, NbNOI4, NbNOI6, NbNOI8, and NbNOI9 suppressed cell death induced by overexpression of NbPtr1, and silencing of the five NbNOIs caused stunted growth phenotype which was dependent on NbPtr1 activity. Moreover, additional expression of effectors eliminated NbNOI suppression on NbPtr1. This suggests that the five NbNOIs negatively regulate NbPtr1, and NbPtr1 indirectly detects pathogen effectors by monitoring NbNOIs. The diversified NOI proteins are the key drivers of convergently evolved effector recognition in N. benthamiana and Arabidopsis and contribute to the expansion of NbPtr1 effector recognition specificities.