Shnaiderman, C., Department of Postharvest Science of Fresh Produce, ARO, Bet Dagan, Israel, Robert H. Smith Faculty of Agriculture, Food and Environment, Hebrew University of Jerusalem, Rehovot, Israel
Miyara, I., Department of Postharvest Science of Fresh Produce, ARO, Bet Dagan, Israel
Kobiler, I., Department of Postharvest Science of Fresh Produce, ARO, Bet Dagan, Israel
Sherman, A., Genomic Unit, Plant Sciences Institute, ARO, Bet Dagan, Israel
Prusky, D., Department of Postharvest Science of Fresh Produce, ARO, Bet Dagan, Israel

Ammonium secreted by the post-harvest pathogen Colletotrichum gloeosporioides during host colonization accumulates in the host environment due to enhanced fungal nitrogen metabolism. Two types of ammonium transporterencoding genes, AMET and MEP, are expressed during pathogenicity. Gene disruption of AMET, a gene modulating ammonia secretion, showed twofold reduced ammonia secretion and 45% less colonization on avocado fruit, suggesting a contribution to pathogenicity. MEPB, a gene modulating ammonium transport, is expressed by C. gloeosporioides during pathogenicity and starvation conditions in culture. Gene disruption of MEPB, the most highly expressed gene of the MEP family, resulted in twofold overexpression of MEPA and MEPC but reduced colonization, suggesting MEPB expression's contribution to pathogenicity. Analysis of internal and external ammonia accumulation by Δmep B strains in mycelia and germinated spores showed rapid uptake and accumulation, and reduced secretion of ammonia in the mutant versus wild-type (WT) strains. Ammonia uptake by the WT germinating spores but not by the Δmep B strain with compromised ammonium transport activated cAMP and transcription of PKA subunits PKAR and PKA2. Δmep B mutants showed 75% less appressorium formation and colonization than the WT, which was partially restored by 10 mM exogenous ammonia. Thus, whereas both AMET and MEPB genes modulate ammonia secretion, only MEPB contributes to ammonia accumulation by mycelia and germinating spores that activate the cAMP pathways, inducing the morphogenetic processes contributing to C. gloeosporioides pathogenicity. © 2013 The American Phytopathological Society.