Search

Citrus bacterial canker (CBC) is a disease that poses a major threat to global citrus production and is caused by infection with Xanthomonas citri subsp. citri (Xcc). Wall-associated receptor-like kinase (WAKL) proteins play an important role in shaping plant resistance to various bacterial and fungal pathogens. In a previous report, CsWAKL01 was identified as a candidate Xcc -inducible gene found to be up-regulated in CBC-resistant citrus plants. However, the functional role of CsWAKL01 and the mechanisms whereby it may influence resistance to CBC have yet to be clarified. Here, CsWAKL01 was found to localize to the plasma membrane, and the overexpression of the corresponding gene in transgenic sweet oranges resulted in pronounced enhancement of CBC resistance, whereas its knockdown had the opposite effect. Mechanistically, the effect of CsWAKL01 was linked to its ability to reprogram jasmonic acid, salicylic acid, and abscisic acid signaling activity. CsWRKY53 was further identified as a transcription factor capable of directly binding to the CsWAKL01 promoter and inducing its transcriptional up-regulation. CsWRKY53 silencing conferred greater CBC susceptibility to infected plants. Overall, these data support a model wherein CsWRKY53 functions as a positive regulator of CsWAKL01 to enhance resistance to CBC via the reprogramming of phytohormone signaling. Together these results offer new insights into the mechanisms whereby WAKLs shape phytopathogen resistance while underscoring the potential value of targeting the CsWRKY53–CsWAKL01 axis when seeking to breed CBC-resistant citrus plant varieties. [ABSTRACT FROM AUTHOR]

SUMMARY: Citrus bacterial canker (CBC) is a serious bacterial disease caused by Xanthomonas citri subsp. citri (Xcc) that adversely impacts the global citrus industry. In a previous study, we demonstrated that overexpression of an Xcc‐inducible apetala 2/ethylene response factor encoded by Citrus sinensis, CsAP2‐09, enhances CBC resistance. The mechanism responsible for this effect, however, is not known. In the present study, we showed that CsAP2‐09 targeted the promoter of the Xcc‐inducible WRKY transcription factor coding gene CsWRKY25 directly, activating its transcription. CsWRKY25 was found to localize to the nucleus and to activate transcriptional activity. Plants overexpressing CsWRKY25 were more resistant to CBC and showed higher expression of the respiratory burst oxidase homolog (RBOH) CsRBOH2, in addition to exhibiting increased RBOH activity. Transient overexpression assays in citrus confirmed that CsWRKY25 and CsRBOH2 participated in the generation of reactive oxygen species (ROS) bursts, which were able to restore the ROS degradation caused by CsAP2‐09 knockdown. Moreover, CsWRKY25 was found to bind directly to W‐box elements within the CsRBOH2 promoter. Notably, CsRBOH2 knockdown had been reported previously to reduce the CBC resistance, while demonstrated in this study, CsRBOH2 transient overexpression can enhance the CBC resistance. Overall, our results outline a pathway through which CsAP2‐09‐CsWRKY25 transcriptionally reprograms CsRBOH2‐mediated ROS homeostasis in a manner conducive to CBC resistance. These data offer new insight into the mechanisms and regulatory pathways through which CsAP2‐09 regulates CBC resistance, highlighting its potential utility as a target for the breeding of CBC‐resistant citrus varieties. Significant Statement: In the present study, we identify a pathway in which CsAP2‐09 and CsWRKY25 form a transcription factor cascade, transcriptionally reprogramming CsRBOH2‐mediated reactive oxygen species homeostasis and citrus bacterial canker (CBC) resistance. Our study explores new insights into the mechanistic role of CsAP2‐09 and the associated regulatory network, extending the potential value of CsAP2‐09 in the breeding of CBC‐tolerant citrus plants. [ABSTRACT FROM AUTHOR]