Verticillium dahliae Secretory Aspartyl Protease VdSAP Targets Cotton GhARP to Modulate Plant Defence and Defoliation

Verticillium dahliae isolates causing devastating vascular wilt in cotton plants can be divided into defoliating and nondefoliating pathotypes. The mechanisms underlying how V. dahliae uses secretory proteins to manipulate plant physiological processes and suppress immunity in cotton plants have received renewed research focus over the past several years. Here, we describe the role of a secretory protein named VdSAP (secretory aspartyl protease) from the defoliating V. dahliae strain XJ592 in virulence and defoliation. Deletion of VdSAP in strain XJ592 led to significantly reduced virulence and cotton plant defoliation. VdSAP functioned as a protease that targeted cotton plant GhARP (auxin-repressed protein), and VdSAP negatively regulated GhARP content in plants. Transient expression of GhARP in Nicotiana benthamiana enhanced the expression of plant defence signals and inhibited plant abscission signals. Furthermore, GhARP negatively regulated the ethylene (ET) signal and positively regulated the salicylic acid (SA) signal. In addition, GhARP interacted with 1-aminocyclopropane carboxylate oxidases (GhACOs), suggesting that it might function through the ET signal during V. dahliae-cotton plant interactions. These results suggest that GhARP is a molecular link between plant defence and abscission signals, and that VdSAP decreases accumulation of GhARP and enhances the virulence and defoliation caused by V. dahliae.