CitPH4 Confers Resistance to Citrus Canker by Activating Papain-Like Cysteine Protease

Citrus canker, a devastating disease caused by Xanthomonas citri subsp. citri (Xcc), poses a significant threat to global citrus production due to the high susceptibility of nearly all commercial citrus cultivars to it. Although transcription factor Citrus PH4 (CitPH4) is well known for regulating fruit acidity, its potential role in plant immunity remains elusive. Here, we demonstrate that CitPH4 positively regulates citrus defence against Xcc. Transcriptomic and biochemical analyses indicate that CitPH4 binds to the promoter of the responsive to dehydration 21a (CsRD21a), a gene encoding a papain-like cysteine protease (PLCP), thus modulating its expression. Functional characterisation further reveals that CsRD21a is essential for CitPH4-mediated resistance to Xcc. Mechanistically, CsRD21a interacts with pathogenesis-related protein 5 (CsPR5) to promote salicylic acid (SA) accumulation. In addition, reciprocal activation of PLCP activity and SA signalling enhances immune defences against Xcc. Collectively, our findings reveal a CitPH4-mediated citrus immunity mechanism in which the CitPH4-CsRD21a-CsPR5 module constitutes a defence loop by integrating reciprocal activation effects of PLCP activity and SA signalling. This study provides a novel insight into the regulatory network of citrus resistance against Xcc, offering potential targets for breeding citrus cultivars with enhanced disease resistance and superior fruit quality.