Advancement of research on plant NLRs evolution, biochemical activity, structural association, and engineering

In this review, we have included evolution of plant intracellular immune receptors, oligomeric complex formation, enzymatic action, engineering, and mechanisms of immune inspection for appropriate defense outcomes. NLR (Nucleotide binding oligomerization domain containing leucine-rich repeat) proteins are the intracellular immune receptors that recognize pathogen-derived virulence factors to confer effector-triggered immunity (ETI). Activation of plant defense by the NLRs are often conveyed through N-terminal Toll-like/ IL-1 receptor (TIR) or non-TIR (coiled-coils or CC) domains. Homodimerization or self-association property of CC/ TIR domains of plant NLRs contribute to their auto-activity and induction of in planta ectopic cell death. High resolution crystal structures of Arabidopsis thaliana RPS4^TIR, L6^TIR, SNC1^TIR, RPP1^TIR and Muscadinia rotundifolia RPV1^TIR showed that interaction is mediated through one or two distinct interfaces i.e., αA and αE helices comprise AE interface and αD and αE helices were found to form DE interface. By contrast, conserved helical regions were determined for CC domains of plant NLRs. Evolutionary history of NLRs diversification has shown that paired forms were originated from NLR singletons. Plant TIRs executed NAD⁺ hydrolysis activity for cell death promotion. Plant NLRs were found to form large oligomeric complexes as observed in animal inflammasomes. We have also discussed different protein engineering methods includes domain shuffling, and decoy modification that increase effector recognition spectrum of plant NLRs. In summary, our review highlights structural basis of perception of the virulence factors by NLRs or NLR pairs to design novel classes of plant immune receptors.