<i>Xanthomonas</i> coordinates type III-type II effector synergy by activating fruit-ripening pathway.

Abstract

Plant cell walls harbor vast carbohydrate reserves, yet how pathogens unlock them remains unclear. We show that the citrus canker pathogen <i>Xanthomonas citri</i> subsp. <i>citri</i> (<i>Xcc</i>) mobilizes cell wall sugars by hijacking a fruit-ripening program through the type III effector PthA4, which activates the ripening coordinator CsLOB1. CsLOB1 induces approximately 100 genes, many encoding enzymes involved in cell wall breakdown. In the nonfruiting species <i>Nicotiana benthamiana</i>, CsLOB1 likewise promotes <i>Xanthomonas</i> growth, showing that its activity is not strictly dependent on a ripening program. Transcriptomics and reporter assays revealed PthA4-dependent activation of the <i>Xcc</i> xylan CUT system, triggered by host-derived xylose and including a type II-secreted xylanase. Thus, PthA4-driven cell wall remodeling activates bacterial xylan use, establishing a TIII-TII effector feedforward loop that fuels <i>Xcc</i> proliferation.