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

¹Department of Postharvest Science of Fresh Produce, Agricultural Research Organization, the Volcani Center, Bet Dagan 50250, Israel; ²Department of Plant Sciences, Weizmann Institute of Science, Rehovot, Israel; ³Department of Dryland Biotechnologies, Jacob Blaustein Institutes for Desert Research, Ben-Gurion University, Sede Boqer Campus, Midreshet Ben-Gurion 84990, Israel

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.