Molecular Pathogens 2024, Vol.15, No.5, 246-254 http://microbescipublisher.com/index.php/mp 247 type III secretion system. These effectors are characterized by their unique modular structure, which includes a central domain composed of tandem repeats of 33~35 amino acids. Each repeat contains a pair of repeat variable diresidues (RVDs) that determine the specificity of DNA binding by interacting with specific nucleotides in the host genome (Booher et al., 2015; Read et al., 2016). The N-terminal region of TAL effectors is crucial for their DNA-binding activity, while the C-terminal region contains nuclear localization signals (NLS) and an acidic activation domain (AD) that facilitate their transport into the plant cell nucleus and subsequent activation of target genes. Interestingly, some TAL effectors, known as truncTALEs, lack significant portions of the N- and C-terminal regions but still play roles in modulating host responses, such as suppressing resistance mechanisms. 2.2 Mechanism of TAL effector-dna interactions The interaction between TAL effectors and DNA is highly specific and is mediated by the central repeat domain of the effector proteins. Each RVD within the repeats binds to a specific nucleotide, allowing the TAL effector to recognize and bind to precise DNA sequences known as effector binding elements (EBEs) in the promoters of target genes (Wilkins et al., 2015). This modularity and specificity make TAL effectors powerful tools for manipulating host gene expression. Upon binding to their target EBEs, TAL effectors activate the transcription of downstream genes, which can include both susceptibility (S) genes that facilitate bacterial infection and resistance (R) genes that trigger defense responses (Wang et al., 2017; Xu et al., 2017). The ability of TAL effectors to drive transcription bidirectionally from EBEs on either strand of the DNA further expands their functional versatility and potential impact on host gene regulation. The specificity of TAL effector-DNA interactions is not only determined by the RVDs but also by the overall sequence context of the target DNA. Small changes in the RVDs can lead to significant differences in target gene activation, as demonstrated by the differential activation of cassava genes by closely related TAL effectors from different Xanthomonas strains (Cohn et al., 2016). This fine-tuned specificity underscores the importance of understanding the precise mechanisms of TAL effector-DNA interactions for developing effective disease control strategies. 3 Role of TAL Effectors inXanthomonas Pathogenicity 3.1 Activation of host susceptibility (S) genes 3.1.1 Mechanism of S-gene activation by TAL effectors TAL effectors are secreted by Xanthomonas species through the type III secretion system and enter the host cell nucleus, where they bind to specific DNA sequences in the promoters of host genes, activating their transcription. This binding is mediated by a central domain in the TAL effectors, which contains repeat variable diresidues (RVDs) that determine the sequence specificity of DNA binding. The activation of these host genes often leads to increased susceptibility to bacterial infection, facilitating pathogen growth and symptom development (Cohn et al., 2016). 3.1.2 Case studies of S-gene activation in legumes In cassava, the TAL effector TAL14Xam668 fromXanthomonas axonopodis pv. manihotis activates over 50 host genes, promoting bacterial growth and symptom formation. Interestingly, even a single RVD difference between TAL effectors can result in differential activation of target genes, as seen with TAL14CIO151, which complements the TAL14Xam668 mutant defect, highlighting the importance of specific S-genes in disease susceptibility. In rice, the TAL effectors Tal2b and Tal2c from Xanthomonas oryzae pv. oryzicola target homologous genes OsF3H03g and OsF3H04g, respectively, to promote infection by reducing salicylic acid production, a key component of plant defense (Wu et al., 2021; Huang, 2024). 3.2 Suppression of plant defense responses Xanthomonas species also employ non-TAL effectors to suppress plant immune responses. For instance, XopD fromXanthomonas euvesicatoria represses host transcription through its SUMO protease activity, which is crucial
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