Identification of a <i>COMT</i> Gene Involved in the Biosynthesis of Melatonin Which Mediates Resistance to Citrus Canker

Citrus canker, caused by Xanthomonas citri subsp citri (Xcc), represents a severe threat to the citrus industry. The conventional control measures for citrus canker primarily rely on chemical bactericide. However, overuse of bactericide will cause environmental and food security concerns. To address this problem, efforts are being made to develop environmentally friendly bio-bactericide alternatives. In this study, we identified a caffeic acid O-methyltransferase gene, AbCOMT1, from Atalantia buxifolia, a Citrus-related species exhibiting high resistance to citrus canker. AbCOMT1 encodes a key enzyme involved in melatonin biosynthesis, and its overexpression in sweet orange significantly enhances resistance to citrus canker. We found elevated melatonin levels in the AbCOMT1 overexpressing sweet orange lines and demonstrated that the AbCOMT1 overexpression not only directly inhibited Xcc proliferation but also activated citrus immune responses. To further improve the inhibitory efficacy of melatonin, we tested several melatonin derivatives, achieving a tenfold increase in inhibitory activity. Notably, the melatonin derivative MT-3 exhibited outstanding efficacy in controlling citrus canker under field conditions. Our results revealed AbCOMT1 as a promising resistance gene and identified the highly efficient melatonin derivatives for citrus canker disease control.