Conservation and Diversity of Plant R Genes

Since the isolation of the first plant R gene, Hm1 in maize in 1992 (Johal and Briggs 1992), over 60 plant R genes controlling resistance against pathogens ranging from viruses, bacteria, fungi to nematodes have been isolated from different plant species

(Xiao 2006). Most isolated R genes seem to activate common or overlapping sets of defense programs in local areas infected by pathogens. Those defense responses include transcriptional induction of pathogenesis-related (PR) genes, production of reactive oxygen species, fortification of the cell wall, synthesis of antimicrobial compounds and, in many cases, a hypersensitive response (HR) which is a form of plant programmed cell death analogous to animal apoptosis (Hammond-Kosack and Jones 1997; Dangl and Jones 2001; Nurnberger et al. 2004). The primary local resistance triggered by R genes may also lead to activation of a secondary defense termed systemic acquired resistance in the uninfected tissues, which is a more long-lasting immune response throughout the whole plant against a broad range of pathogens (Durrant and Dong 2004).

Based on features of the deduced domain structures and/or biochemical functions, R proteins can be divided into three classes (Table 1). The largest class contains a nucleotide-binding site (NBS) and leucine-rich-repeat (LRR) motifs (Hammond-Kosack and Jones 1997; Dangl and Jones 2001). These R proteins confer resistance to various pathogens and can be further subdivided into two groups, based on their N-terminal features. The first group contain an N-terminal domain resembling the cytoplasmic signaling domain of the Drosophila toll and human interleukin-1 receptors (TIR) and are called TIR-NBS-LRRs (Whitham et al. 1994; Lawrence et al. 1995). The second group contain (in most cases) a coiled-coil (CC) domain and thus often are referred to as CC-NBS-LRRs (Bent et al. 1994; Grant et al. 1995). An exceptional case in the TIR-NBS-LRR group is the Arabidopsis RRS1-R protein that has a WRKY domain attached to the LRR at the C-terminus (Deslandes et al. 2002). The WRKY domain is found in a group of transcription factors implicated in the signal transduction of R genes (Eulgem 2005). The structural feature of RRS1-R implies a direct link between Avr-recognition and the transcriptional activation of defense genes (Deslandes et al. 2003).

The second class of R proteins comprise cell surface receptor-like transmembrane proteins (RLP) and receptor-like kinases (RLK) (Table 1). The common feature of these proteins is that they possess an extracellular LRR (eLRR) domain. Representatives of RLP R proteins are tomato Cf proteins conferring resistance to the tomato fungal pathogen Cladosporium fulvum (Jones et al. 1994; Hammond-Kosack and Jones 1997) and Arabidopsis RPP27 conferring resistance to the oomycete Hyaloperonospora parasitica (Tor et al. 2004). RLK R proteins are represented by rice Xa21 and Xa26, both of which confer resistance to multiple strains of Xanthomonas oryzae pv. oryzae (Song et al. 1995; Sun et al. 2004).

The remaining R genes encode proteins that either resemble the overall structure or a domain of the above two classes with some degree of structural variation, or have a novel protein structure that does not show significant homology to any other R proteins (Table 1). Therefore, they are atypical R genes in comparison with the LRR-encoding R genes. For example, tomato Pto and Arabidopsis PBS1 encode members of a conserved protein kinase family (Martin et al. 1993; Swiderski and Innes 2001) that resemble the cytoplasmic protein kinase domain of RLK R proteins. The broad-spectrum powdery mildew R gene RPW8 from Arabidopsis encodes a small protein containing an N-terminal transmembrane domain and a CC

Table 1 Conservation and diversity of plant R proteins. TIR Toll and interleukin-1 receptor. NBS nucleotide binding site. (e)LRR (extracellular) leucine rich repeats. CC coiled coil. Kin kinase. TM transmembrane helix predicted by TMpred and TMHMM

R protein class Schematic domain structure

Predicted function

Examples

References

NBS-LRR

eLRR

Atypical

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