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Review
. 2015 Aug 20:6:641.
doi: 10.3389/fpls.2015.00641. eCollection 2015.

TAL effectors and the executor R genes

Junli Zhang  1 , Zhongchao Yin  2 , Frank White  3
Affiliations
Review

TAL effectors and the executor R genes

Junli Zhang et al. Front Plant Sci. .

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.

Keywords: R gene; TAL effectors; Xanthomonas.

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Figures

FIGURE 1
FIGURE 1
TAL effector and E gene interactions. (A) Schematic of the interaction between AvrXa27 and Xa27. Lower case r indicates the ineffective allele that lacks the AvrXa27 effector binding element. The r allele is missing three nucleotides and different in one nucleotide in the effector binding element of Xa27 and does not permit binding of AvrXa7 leading to a compatible interaction (Römer et al., 2010). R indicates the dominant and functional allele of Xa27. Xa27 expression leads to a resistance response and HR on leaves, indicated by the dark discoloring of the inoculation site (here, on a rice leaf). NLS, nuclear localization signal. AD, transcription activation domain of TAL effector. (B) Promoters of E genes and polymorphisms in dominant and recessive alleles. (i) Sequence alignment of a part of the promoters of Xa27 from the rice cultivar IRBB27 (gi 66735941 gb AY986491.1) and xa27 from the rice cultivar IR24 (gi 66735943 gb AY986492.1). (ii) Sequence of a part of the promoter of rice Xa10 from the rice cultivar IRBB10 (gi|448280729|gb|JX025645.1|). (iii) A part of the promoters of Xa23 from the rice cultivar CBB23 (gi|721363841|gb|KP123634.1|) and xa23 from the rice cultivar JG30 (gi|721363854|gb|KP123635.1|). (iv) A part of the promoters of Bs3 from Capsicum annuum L. cultivar ECW-30R (gi|158851516|gb|EU078684.1) and Bs3-E from C. annuum L. cultivar ECW (gi|158851512|gb|EU078683.1|). (v) A part of the promoters of Bs4C-R from pubescens cultivar PI 235047 (gi|414148024|gb|JX944826.1|) and Bs4C-S from Capsicum pubescens cultivar PI 585270 (gi|414148026|gb|JX944827.1|). The ATG start codon in each case is displayed in red letters. Nucleotides that are identical between the alleles are displayed as black letters. Predicted TATA boxes are underlined. Effector binding elements are highlighted in yellow with blue letters indicating differences between alleles. The TAL effectors are represented by the repeat regions using a single letter represents each RVD (I-NI; G-NG or HG; S-NS; D-HD or ND, *-N*, N-NN). * Represents no amino acid residue at what would otherwise be position 13.
FIGURE 2
FIGURE 2
The known E proteins. (A) A phylogenetic tree of BS3 related proteins. Proteins from closely related YUCCA proteins of C. annuum L. (Ca), A. thaliana (At), Tomato (To), Citrus sinensis (Cs) and the BS3 protein (Capana02g001306) were aligned. Names of proteins are given with the Phytozome ID or Pepper Genome Database ID (in parentheses). A monophyletic group that contains the predicted BS3 protein and tomato YUCCA-like proteins is boxed in red. Sequences were aligned with the online ClustalW server (http://www.ch.embnet.org/software/ClustalW.html) using the default values. MEGA6.0 was used for generating a tree on the basis of ClustalW output. Phylogenetic calculations are based on the maximum likelihood method, and Bootstrap analysis was used to evaluate the reliability of the nodes of the phylogenetic trees. Bootstrap values are based on 1000 replications. The branch lengths of the tree are proportional to divergence. The 0.1 scale represents 10% change. (B) Structural predictions for group 2 E proteins. (i) Bs4C-R; (ii) XA27; (iii) XA10 and XA23. Alignment of XA10 and XA23 was conducted using the online program ClustalW2 using the default parameters (http://www.ebi.ac.uk/Tools/msa/clustalw2/). Transmembrane helices predicted by the SOSUI program (http://bp.nuap.nagoya-u.ac.jp/sosui/sosui_submit.html) are highlighted in yellow. * Represents no amino acid residue at what would otherwise be position 13.
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