Application of glycerol as a foliar spray activates the defence response and enhances disease resistance of T heobroma cacao

Summary Previous work has implicated glycerol‐3‐phosphate ( G3P ) as a mobile inducer of systemic immunity in plants. We tested the hypothesis that the exogenous application of glycerol as a foliar spray might enhance the disease resistance of T heobroma cacao through the modulation of endogenous G3P levels. We found that exogenous application of glycerol to cacao leaves over a period of 4 days increased the endogenous level of G3P and decreased the level of oleic acid (18:1). Reactive oxygen species ( ROS ) were produced (a marker of defence activation) and the expression of many pathogenesis‐related genes was induced. Notably, the effects of glycerol application on G3P and 18:1 fatty acid content, and gene expression levels, in cacao leaves were dosage dependent. A 100 m m glycerol spray application was sufficient to stimulate the defence response without causing any observable damage, and resulted in a significantly decreased lesion formation by the cacao pathogen P hytophthora capsici ; however, a 500 m m glycerol treatment led to chlorosis and cell death. The effects of glycerol treatment on the level of 18:1 and ROS were constrained to the locally treated leaves without affecting distal tissues. The mechanism of the glycerol‐mediated defence response in cacao and its potential use as part of a sustainable farming system are discussed.