Ammonium Secretion by Colletotrichum coccodes Activates Host NADPH Oxidase Activity Enhancing Host Cell Death and Fungal Virulence in Tomato Fruits

Colletotrichum pathogens of fruit and leaves are known ammonium secretors. Here, we show that Colletotrichum coccodes virulence, as measured by tomato (Solanum lycopersicum cv. Motelle) fruit tissue necrosis, correlates with the amount of ammonium secreted. Ammonium application to fruit tissue induced hydrogen peroxide (H2O2) accumulation. To examine whether the tomato NADPH oxidase, SlRBOH, is a source for the ammonium-induced H2O2, wild-type and antisense lines abrogated for SlRBOH (SlRBOH-AS) were examined. Wild-type lines produced 7.5-fold more reactive oxygen species when exposed to exogenous ammonium than did SlRBOH-AS lines. C. coccodes colonization of wild-type tomato lines resulted in higher H2O2 production and faster fungal growth rate compared with colonization in the SlRBOH-AS mutant, although the amount of ammonium secreted by the fungi was similar in both cases. Enhanced ion leakage and cell death of fruit tissue were correlated with H2O2 accumulation, and treatment with the reactive oxygen scavenger N-acetyl-l-cysteine decreased H2O2 production, ion leakage, and cell death. Importantly, the activation of reactive oxygen species production by ammonium was positively affected by an extracellular pH increase from 4 to 9, implying that ammonium exerts its control via membrane penetration. Our results show that C. coccodes activates host reactive oxygen species and H2O2 production through ammonium secretion. The resultant enhancement in host tissue decay is an important step in the activation of the necrotrophic process needed for colonization.