Review

Plant NBS-LRR proteins: adaptable guards

Leah McHale, Xiaoping Tan, Patrice Koehl and Richard W Michelmore

Address: The Genome Center, University of California, Davis, CA 95616, USA.Correspondence: Richard W Michelmore. Email: rwmichelmore@http://www.doczj.com/doc/0e45723de87101f69e31953e.html

Published: 26 April 2006

Genome Biology 2006, 7:212(doi:10.1186/gb-2006-7-4-212)The electronic version of this article is the complete one and can be found online at http://www.doczj.com/doc/0e45723de87101f69e31953e.html/2006/7/4/212? 2006 BioMed Central Ltd

Most of the disease resistance genes (R genes) in plants cloned to date encode nucleotide-binding site leucine-rich repeat (NBS-LRR) proteins characterized by nucleotide-binding site (NBS) and leucine-rich repeat (LRR) domains as well as variable amino- and carboxy-terminal domains (Figure 1). These large, abundant, proteins are involved in the detection of diverse pathogens, including bacteria,viruses, fungi, nematodes, insects and oomycetes. There have been numerous extensive reviews since the first NBS-LRR-encoding genes were cloned from plants in 1994 (for example [1-5]). This article aims to provide a current overview of the structure and function of this protein family as well as to highlight recent advances.

Plant NBS-LRR proteins are similar in sequence to members of the mammalian nucleotide-binding oligomerization domain (NOD)-LRR protein family (also called ‘CARD, tran-scription enhancer, R (purine)-binding, pyrin, lots of leucine repeats’ (CATERP I LLER) proteins), which function in inflammatory and immune responses [6]. But although mammalian NOD-LRR proteins have the same tripartite domain organization as plant NBS-LRR proteins, including a nucleotide-binding domain and a LRR domain, the func-tional similarities between NBS-LRR and mammalian NOD proteins are probably the result of convergent evolution [7].There are no NOD-related proteins in Caenorhabditis elegans or Drosop hila melanogaster and the downstream

partners of the two families differ [7,8]. The human NOD protein apoptotic protease activating factor 1 (APAF-1) has an NBS domain with greater protein-sequence similarity to plant NBS-LRR proteins than to other mammalian NOD proteins; however, it shares neither the amino-terminal nor the carboxy-terminal LRR domains characteristic of plant NBS-LRR proteins.

Evolution and genome organization

Plant NBS-LRR proteins are numerous and ancient in origin. They are encoded by one of the largest gene families known in plants. There are approximately 150 NBS-LRR-encoding genes in Arabidopsis thaliana,over 400 in Oryza sativa [3,9,10], and probably considerably more in larger plant genomes that have yet to be fully sequenced. Many NBS-encoding sequences have now been amplified from a diverse array of plant species using PCR with degenerate primers based on conserved sequences within the NBS domain and there are currently over 1,600 NBS sequences in public databases (Additional data file 1). They are found in non-vascular plants and gymnosperms as well as in angiosperms; orthologous relationships are difficult to determine, however, owing to lineage-specific gene duplica-tions and losses [11,12]. In several lineages, NBS-LRR-encod-ing genes have become amplified, resulting in family-specific subfamilies (Figure 2; Additional data file 2) [13].Of the 150