The diverse roles of NB-LRR proteins in plants

Plant innate immunity relies on specialised immune receptors that can detect and defend against a wide variety of microbes. The first group of receptors comprises the transmembrane pathogen- or pattern-recognition receptors (PRRs), which respond to slowly evolving pathogen- or microbe-associated molecular patterns (PAMPs/MAMPs). The second group of immune receptors is formed by the polymorphic disease resistance (R) proteins that detect microbe-derived effector proteins. Most R proteins are members of the nucleotide binding leucine-rich repeat (NB-LRR) class. Although this class comprises one of the biggest protein families in plants, relatively few have been functionally characterised to date. The question rises whether all NB-LRRs function as immune receptors, or that they might have alternative functions. The answer is: yes, they do have alternative functions that are different from the immune receptor function. This review summarises the current knowledge about non-immune receptor signal transduction functions of NB-LRRs in plants.