Abstract
Transcription activator-like (TAL) effectors are bacterial type III secretion proteins that function as transcription factors in plants during Xanthomonas/plant interactions, conditioning either host susceptibility and/or host resistance. Three types of TAL effector associated resistance (R) genes have been characterized—recessive, dominant non-transcriptional, and dominant TAL effector-dependent transcriptional based resistance. Here, we discuss the last type of R genes, whose functions are dependent on direct TAL effector binding to discrete effector binding elements in the promoters. Only five of the so-called executor R genes have been cloned, and commonalities are not clear. We have placed the protein products in two groups for conceptual purposes. Group 1 consists solely of the protein from pepper, BS3, which is predicted to have catalytic function on the basis of homology to a large conserved protein family. Group 2 consists of BS4C-R, XA27, XA10, and XA23, all of which are relatively short proteins from pepper or rice with multiple potential transmembrane domains. Group 2 members have low sequence similarity to proteins of unknown function in closely related species. Firm predictions await further experimentation on these interesting new members to the R gene repertoire, which have potential broad application in new strategies for disease resistance.
Highlights
Xanthomonas infects monocotyledonous and dicotyledonous plant species, and the pathogenicity of many species depends in part on the effector proteins secreted by a type III secretion (T3S) system (Leyns et al, 1984; Tampakaki et al, 2004)
Thirteen R genes have been cloned for resistance to Xanthomonas diseases—all coming from rice, pepper, or tomato
In addition to Bs4, are representatives of the two major classes of R genes, the receptor linked kinases (RLK) and nucleotide binding site leucine rich repeat (NBS-LRR) genes which are represented by Xa21 (RLK, rice), Xa26 (RLK, rice), Xa1 (NBS-LRR, rice), and Bs2 (NBS-LRR, pepper; Yoshimura et al, 1998; Tai et al, 1999; Zhang and Wang, 2013)
Summary
Xanthomonas infects monocotyledonous and dicotyledonous plant species, and the pathogenicity of many species depends in part on the effector proteins secreted by a type III secretion (T3S) system (Leyns et al, 1984; Tampakaki et al, 2004). The transcription activator-like (TAL) effector family is a distinct family of type III effectors, which includes members with cognate susceptibility (S) and/or resistance (R) genes. TAL effectors function as host gene specific transcription factors that can target both S and R genes, leading to enhanced expression and consequential phenotypic effects (Gu et al, 2005; Yang et al, 2006; Kay et al, 2007; Römer et al, 2007). Three types of TAL effector associated R genes have been reported-recessive, dominant nontranscriptional (classical) and dominant TAL effector-dependent transcriptional based resistance. The TAL effector AvrXa7 may target an as yet uncharacterized E gene Xa7 due to the requirements for the effector nuclear localization signals (NLSs) and the transcription acidic activator domain in Xa7-dependent resistance (Hopkins et al, 1992; Yang et al, 2000)
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