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TAL effectors and the executor R genes.

Zhang J, Yin Z, White F - Front Plant Sci (2015)

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Plant Pathology, Kansas State University , Manhattan, KS, USA.

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.

No MeSH data available.


Related in: MedlinePlus

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.
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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.

Mentions: E genes are unique in the panoply of R genes in that specificity is not in the R gene coding sequence but in the expression of the R gene in the presence of the effector (Gu et al., 2005; Römer et al., 2007). The TAL effector, itself, contains two notable regions—the central repetitive region and a C-terminal region with NLS motifs and a potent transcription activation domain (AD). The NLS and AD were shown to be required for E gene function in the case of Bs3, Xa10, Xa7, and Xa27 (Van den Ackerveken et al., 1996; Zhu et al., 1998, 1999; Yang et al., 2000; Szurek et al., 2001). TAL effector specificity is determined by the central repetitive region (Herbers et al., 1992; Yang and Gabriel, 1995; Yang and White, 2004; Yang et al., 2005), and is the structural basis for the TAL effector code, where each repeat specifies the probability of accommodating individual nucleotides (Boch et al., 2009; Moscou and Bogdanove, 2009; Deng et al., 2012; Gao et al., 2012; Mak et al., 2012). The repetitive domain consists of 33–35 amino acid repeats that are polymorphic at amino acid residues 12 and 13, which are referred to as the repeat-variable di-residues (RVDs), each of which can be represented by amino acid residue 13 and corresponds to one DNA base in the effector binding element. Proximal to the N-terminal portion of the repetitive domain are non-canonical repeats that mediate pairing with an initial 5′ thymine (Boch et al., 2009; Moscou and Bogdanove, 2009). E gene expression occurs upon cognate effector binding to a compatible effector binding element in the respective promoter (Figure 1A). The known E genes, with the exception of Xa10, have dominant and recessive alleles that differ in DNA sequence polymorphisms in the promoter region (Figure 1B). AvrBs3, for example, fails to induce Bs3-E, an allele of Bs3 with a 13-bp insertion in the effector binding element in the promoter (Figure 1B, iv; Römer et al., 2007, 2009b; Kay et al., 2009). E genes, S genes, and TAL effector genes, therefore, reflect selective pressures in the evolution of the host and pathogen interaction. In this regard, it is important to note that naturally occurring TAL effectors are not necessarily optimized for the cognate promoters simply in terms of the binding requirements. Natural TAL effector configurations may reflect adaptive responses to other factors, including the level of target gene expression and frequency of binding sites within a genome.


TAL effectors and the executor R genes.

Zhang J, Yin Z, White F - Front Plant Sci (2015)

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.
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Related In: Results  -  Collection

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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.
Mentions: E genes are unique in the panoply of R genes in that specificity is not in the R gene coding sequence but in the expression of the R gene in the presence of the effector (Gu et al., 2005; Römer et al., 2007). The TAL effector, itself, contains two notable regions—the central repetitive region and a C-terminal region with NLS motifs and a potent transcription activation domain (AD). The NLS and AD were shown to be required for E gene function in the case of Bs3, Xa10, Xa7, and Xa27 (Van den Ackerveken et al., 1996; Zhu et al., 1998, 1999; Yang et al., 2000; Szurek et al., 2001). TAL effector specificity is determined by the central repetitive region (Herbers et al., 1992; Yang and Gabriel, 1995; Yang and White, 2004; Yang et al., 2005), and is the structural basis for the TAL effector code, where each repeat specifies the probability of accommodating individual nucleotides (Boch et al., 2009; Moscou and Bogdanove, 2009; Deng et al., 2012; Gao et al., 2012; Mak et al., 2012). The repetitive domain consists of 33–35 amino acid repeats that are polymorphic at amino acid residues 12 and 13, which are referred to as the repeat-variable di-residues (RVDs), each of which can be represented by amino acid residue 13 and corresponds to one DNA base in the effector binding element. Proximal to the N-terminal portion of the repetitive domain are non-canonical repeats that mediate pairing with an initial 5′ thymine (Boch et al., 2009; Moscou and Bogdanove, 2009). E gene expression occurs upon cognate effector binding to a compatible effector binding element in the respective promoter (Figure 1A). The known E genes, with the exception of Xa10, have dominant and recessive alleles that differ in DNA sequence polymorphisms in the promoter region (Figure 1B). AvrBs3, for example, fails to induce Bs3-E, an allele of Bs3 with a 13-bp insertion in the effector binding element in the promoter (Figure 1B, iv; Römer et al., 2007, 2009b; Kay et al., 2009). E genes, S genes, and TAL effector genes, therefore, reflect selective pressures in the evolution of the host and pathogen interaction. In this regard, it is important to note that naturally occurring TAL effectors are not necessarily optimized for the cognate promoters simply in terms of the binding requirements. Natural TAL effector configurations may reflect adaptive responses to other factors, including the level of target gene expression and frequency of binding sites within a genome.

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Plant Pathology, Kansas State University , Manhattan, KS, USA.

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.

No MeSH data available.


Related in: MedlinePlus