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<p>Citrus bacterial canker (CBC), caused by <i>Xanthomonas citri</i> subsp. <i>citri</i> (<i>Xcc</i>), poses a significant threat to the citrus industry. <i>Xcc</i> employs the transcription activator-like effector (TALE) PthA4 to target the major susceptibility (<i>S</i>) gene <i>CsLOB1</i> in citrus, promoting host susceptibility to bacterial canker. However, the contribution of other <i>Xcc</i> TALEs, aside from PthA4, to virulence remains underexplored. In this study, we characterized two PthA1 variants, designated PthA5 and PthA6, which facilitate <i>Xcc</i> infection in susceptible citrus species&#xa0;by promoting the formation of hypertrophy and hyperplasia symptoms. Both PthA5 and PthA6 bind directly to effector-binding elements (EBEs) in the promoter of <i>CsGRAS9</i>, suppressing its expression. <i>CsGRAS9</i> negatively regulates <i>Xcc</i> growth in citrus and contributes to CBC resistance. Notably, natural variations in the EBEs of the <i>FhGRAS9</i> promoter, a homolog of <i>CsGRAS9</i> in Hong Kong kumquat, prevent <i>Xcc</i> from affecting <i>FhGRAS9</i> expression. Using the PTG/Cas9 system, we generated <i>proCsGRAS9</i>-edited sweet orange lines #18–2 and #23, which contain 86-bp and 62-bp deletions in the EBE regions of the <i>CsGRAS9</i> promoter. These mutant lines showed enhanced <i>CsGRAS9</i> expression and increased resistance to CBC during <i>Xcc</i> infection. Several GA-related genes and <i>CsTAC1</i>, regulated by <i>CsGRAS9</i>, were also identified. This is the first report that TALEs act as repressors of a resistance gene to confer host susceptibility.</p>

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Two TAL effectors of Xanthomonas citri promote pustule formation by directly repressing the expression of GRAS transcription factor in citrus

  • 期刊论文

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  • 发表日期：

    2025年3月14日

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  • Yichao Yan,
  • Xiaomei Tang,
  • Zhongfeng Zhu,
  • Ke Yin,
  • Yikun Zhang,
  • Zhengyin Xu,
  • Qiang Xu,
  • Lifang Zou,
  • Gongyou Chen

摘要

Citrus bacterial canker (CBC), caused by Xanthomonas citri subsp. citri (Xcc), poses a significant threat to the citrus industry. Xcc employs the transcription activator-like effector (TALE) PthA4 to target the major susceptibility (S) gene CsLOB1 in citrus, promoting host susceptibility to bacterial canker. However, the contribution of other Xcc TALEs, aside from PthA4, to virulence remains underexplored. In this study, we characterized two PthA1 variants, designated PthA5 and PthA6, which facilitate Xcc infection in susceptible citrus species by promoting the formation of hypertrophy and hyperplasia symptoms. Both PthA5 and PthA6 bind directly to effector-binding elements (EBEs) in the promoter of CsGRAS9, suppressing its expression. CsGRAS9 negatively regulates Xcc growth in citrus and contributes to CBC resistance. Notably, natural variations in the EBEs of the FhGRAS9 promoter, a homolog of CsGRAS9 in Hong Kong kumquat, prevent Xcc from affecting FhGRAS9 expression. Using the PTG/Cas9 system, we generated proCsGRAS9-edited sweet orange lines #18–2 and #23, which contain 86-bp and 62-bp deletions in the EBE regions of the CsGRAS9 promoter. These mutant lines showed enhanced CsGRAS9 expression and increased resistance to CBC during Xcc infection. Several GA-related genes and CsTAC1, regulated by CsGRAS9, were also identified. This is the first report that TALEs act as repressors of a resistance gene to confer host susceptibility.