Plant NBS-LRR proteins in pathogen sensing and host defense

Abstract

Plant proteins belonging to the nucleotide-binding site-leucine-rich repeat (NBS-LRR) family are used for pathogen detection. Like the mammalian Nod-LRR protein 'sensors' that detect intracellular conserved pathogen-associated molecular patterns, plant NBS-LRR proteins detect pathogen-associated proteins, most often the effector molecules of pathogens responsible for virulence. Many virulence proteins are detected indirectly by plant NBS-LRR proteins from modifications the virulence proteins inflict on host target proteins. However, some NBS-LRR proteins directly bind pathogen proteins. Association with either a modified host protein or a pathogen protein leads to conformational changes in the amino-terminal and LRR domains of plant NBS-LRR proteins. Such conformational alterations are thought to promote the exchange of ADP for ATP by the NBS domain, which activates 'downstream' signaling, by an unknown mechanism, leading to pathogen resistance.Plants lack the adaptive immunity that vertebrates rely on to respond to pathogens. To successfully detect and ward off pathogens, plants must rely solely on genes stably encoded in the genome. Although the exact mechanisms of pathogen detection differ, plants, like animals, use two distinct defense 'systems' to recognize and respond to pathogen challenge 1 . Pathogen-associated molecular patterns (PAMPs), such as bacterial flagellin, lipopolysaccharides and fungal-oomycete cellulose-binding elicitor proteins, are recognized by plant transmembrane receptors that activate basal defense, a first line of defense against pathogens that is reminiscent of innate immunity in vertebrates 2, 3. In both plants and animals, it is hypothesized that a biological 'arms race' is occurring, in which pathogens have acquired mechanisms to evade PAMP-triggered immunity by evolving effector molecules that modify the state of the host cell, thereby bypassing or disrupting the first line of defense. Plant evolution has countered with proteins that detect specific effector molecules, a mechanism called 'effector-triggered immunity'1 that amounts to a second line of defense. Plant effector-triggered immunity is more akin to mammalian adaptive immunity in that pathogen effectors, rather than conserved elements such as PAMPs, are specifically recognized. However, unlike the situation in mammalian adaptive immunity, the plant host specificity determinants of effector-triggered immunity are encoded in every cell of an organism.The genes encoding the specificity determinants of effector-triggered immunity are known as resistance (R) genes. Most R genes encode proteins that contain a nucleotide-binding site (NBS) and leucine-rich repeats (LRRs). NBS-LRR proteins are involved in the recognition of specialized pathogen effectors (also called avirulence (Avr) proteins) that are thought to provide virulence function in the absence of the cognate R gene 1 . NBS-LRR proteins are also important in animal innate immune systems; however, in animals they seem to be involved in PAMP recognition rather th...

NLRX1 is a regulator of mitochondrial antiviral immunity

“…Rather than directly engaging a pathogen-derived product, NLRX1 seems to function as a modulator of PAMP receptors (MOPR) by affecting the interaction of the pathogen receptor RIG-I (and possibly MDA-5) with MAVS. This mechanism is more in-line with the Guard hypothesis proposed for the indirect detection of pathogen products by plant R proteins 21,22 . Because both NLRX1 and RIG-I interact with the CARD domain of MAVS, a probable scenario is that NLRX1 and RIG-I compete for binding with the CARD domain of MAVS, with opposing outcomes.…”

The Fractionated Orthology of Bs2 and Rx/Gpa2 Supports Shared Synteny of Disease Resistance in the Solanaceae

“…While some motifs have been found in these variable regions, the major feature ascribed to these proteins, a coiled-coil domain, has never been demonstrated, only computationally predicted. Since oligomerization of NB-LRRassociated coiled-coil domains has not been reported, and cellular proteins that interact with this domain show no common structures, it would seem that the existence of a coiled-coil structure either has a role in protein conformation or is simply an artifact of the prediction programs (Deyoung and Innes 2006).…”

Binding Properties of the N-Acetylglucosamine and High-Mannose N-Glycan PP2-A1 Phloem Lectin in Arabidopsis

“…LRR domains are present in proteins with diverse functions, providing a framework for the formation of protein-protein interactions. In plants, they have been implicated in innate immunity signaling pathways (DeYoung and Innes, 2006). Several aphid resistance genes, conferring phloem-specific resistance, such as the mi gene in tomato (Solanum lycopersicum) and the vat gene in C. melo, encode nucleotide-binding site-LRR proteins (Kaloshian et al, 2000;Dogimont et al, 2008), suggesting a potential role, in sieve elements, in interactions with phloem-feeding insects.…”