A “protector” model for membrane trafficking-regulated and NLR-mediated plant immunity

Plants are constantly exposed to a vast diversity of micro-organisms. They have accordingly evolved a sophisticated innate immunity system, mainly consisting of pattern-triggered immunity (PTI) and effector-triggered immunity (ETI) ( Jones and Dangl, 2006 Jones J.D.G. Dangl J.L. The plant immune system. Nature. 2006; 444: 323-329 Crossref PubMed Scopus (8775) Google Scholar ) to combat microbial pathogens. Pattern-recognition receptors (PRRs) on the cell membrane sense immunogenic molecular patterns derived from microbes (pathogen-associated molecular patterns, PAMPs) and the host itself (damage-associated molecular patterns, DAMPs) to activate PTI ( Zhou and Zhang, 2020 Zhou J.-M. Zhang Y. Plant immunity: danger perception and signaling. Cell. 2020; 181: 978-989 Abstract Full Text Full Text PDF PubMed Scopus (339) Google Scholar ; Zhao et al., 2022 Zhao Y. Zhu X. Chen X. Zhou J.-M. From plant immunity to crop disease resistance. J Genet Genomics. 2022; 49: 693-703 Crossref PubMed Scopus (10) Google Scholar ). Adapted pathogens secrete virulence proteins called effectors into the host cell for successful invasion. Resistant plants deploy intracellular receptors to monitor effectors secreted by pathogens and activate ETI, a more potent immune response. The intracellular receptors are usually nucleotide-binding domain leucine-rich repeat containing (NLR) proteins, which are commonly termed as resistance (R) proteins. The effector monitored by a certain R protein is usually referred as avirulent (Avr) protein ( Zhou and Zhang, 2020 Zhou J.-M. Zhang Y. Plant immunity: danger perception and signaling. Cell. 2020; 181: 978-989 Abstract Full Text Full Text PDF PubMed Scopus (339) Google Scholar ; Zhao et al., 2022 Zhao Y. Zhu X. Chen X. Zhou J.-M. From plant immunity to crop disease resistance. J Genet Genomics. 2022; 49: 693-703 Crossref PubMed Scopus (10) Google Scholar ). R protein can monitor Avr protein either by direct binding or indirect recognition through recruiting other host proteins, like that described in the guardee and decoy models ( Zhou and Zhang, 2020 Zhou J.-M. Zhang Y. Plant immunity: danger perception and signaling. Cell. 2020; 181: 978-989 Abstract Full Text Full Text PDF PubMed Scopus (339) Google Scholar ; Zhao et al., 2022 Zhao Y. Zhu X. Chen X. Zhou J.-M. From plant immunity to crop disease resistance. J Genet Genomics. 2022; 49: 693-703 Crossref PubMed Scopus (10) Google Scholar ). Upon recognition of Avr protein, R protein is activated, and ETI is elicited in a manner such as the formation of a resistome on the plasma membrane (PM) to conduct calcium ion ( Zhao et al., 2022 Zhao Y. Zhu X. Chen X. Zhou J.-M. From plant immunity to crop disease resistance. J Genet Genomics. 2022; 49: 693-703 Crossref PubMed Scopus (10) Google Scholar ). However, the R protein family contains a large number of members with high diversity on both sequence and structure. It remains mysterious where and how these R proteins are activated.