Role of auxin and gibberellin in citrus canker development and in the transcriptional control of cell-wall remodeling genes modulated by Xanthomonas axonopodis pv. citri

Xanthomonas axonopodis pv. citri (Xac) is the causal agent of citrus canker, a disease characterized by hyperplastic lesions on the host surface. We have shown previously that Xac strongly increases transcription of sweet orange (Citrus sinensis) genes encoding cell-wall remodeling enzymes and proteins associated with cell division and expansion. In addition, a number of Xac-regulated genes are implicated in the synthesis, mobilization and signaling of auxin and gibberellin. Since auxin and gibberellin control cell division and elongation and are known to modulate the expression of cell-wall hydrolytic enzymes, we examined their ability to modulate the transcription of the cell-wall remodeling genes altered by Xac. Indeed, the expression of various citrus cellulases, pectinesterases, expansin and galactosyltransferase was rapidly and similarly affected by auxin and gibberellin. Moreover, the changes in transcript levels of the cell-wall remodeling genes mediated by auxin and gibberellin correlated with changes in the expression of genes involved in gibberellin synthesis, auxin signaling and mobilization, as observed during Xac infection. Most significantly, auxin promoted canker development whereas an inhibitor of auxin transport reduced pustule formation. Interestingly, an inhibitor of gibberellin synthesis significantly inhibited the auxin-induced transcription, as well as the appearance of canker lesions, thus suggesting that a crosstalk between auxin and gibberellin controls the transcription of orange genes involved in cell division and expansion trigged by Xac.