Fine-Tuning Immunity: Players and Regulators for Plant NLRs

NLR proteins are the major intracellular immune receptors in plants. Their transition between autoinhibited and activated states is fine-tuned by intra- and intermolecular interactions. NLR-interacting proteins play important roles in NLR-mediated immunity. Many NLR-interacting proteins have been identified; however, they have not been systematically classified. Plants have evolved a sophisticated innate immune system to defend against pathogen infection, and intracellular nucleotide-binding, leucine-rich repeat (NLR or NB-LRR) immune receptors are one of the main components of this system. NLR activity is fine-tuned by intra- and intermolecular interactions. We survey what is known about the conservation and diversity of NLR-interacting proteins, and divide them into seven major categories. We discuss the molecular mechanisms by which NLR activities are regulated and how understanding this regulation has potential to facilitate the engineering of NLRs for crop improvement. Plants have evolved a sophisticated innate immune system to defend against pathogen infection, and intracellular nucleotide-binding, leucine-rich repeat (NLR or NB-LRR) immune receptors are one of the main components of this system. NLR activity is fine-tuned by intra- and intermolecular interactions. We survey what is known about the conservation and diversity of NLR-interacting proteins, and divide them into seven major categories. We discuss the molecular mechanisms by which NLR activities are regulated and how understanding this regulation has potential to facilitate the engineering of NLRs for crop improvement. host proteins that act as guardees but where no clear biological, cellular, or physiological function has been identified so far except for pathogen recognition. proteins that mediate the interaction of E2 ubiquitin-conjugating enzyme that has been loaded with ubiquitin with the target protein to be ubiquitinated. Once ubiquitinated, the target proteins are often degraded by the proteasome pathway. E3 ubiquitin ligases (also known as E3 ligases) determine the protein specificity of the ubiquitin-mediated protein degradation pathway. They are divided into four major subfamilies: RING, HECT, CRL, and U-box. proteins that bind and hydrolyze GTP. They often act as molecular switches in signal transduction. the direct virulence targets of pathogen effectors. An NLR effectively monitors the status of its guardee and is activated upon modulation of the guardee by the effector. Guardees have specific cellular functions in addition to their roles in pathogen recognition, and usually function in the defense response pathway. rapid localized cell death at the point of pathogen penetration; HR is used by plants to restrict pathogen invasion. proteins that catalyze the transfer of phosphate groups to their substrate proteins. Kinases are key regulators of enzyme activity, protein–protein interactions, and subcellular localization. these act as intracellular immune receptors that recognize pathogen effectors and trigger innate immunity. proteins with strong homology to kinases but that lack kinase activity.