Role of ammonia secretion and pH modulation on pathogenicity of Colletotrichum coccodes on tomato fruit

Colletotrichum coccodes was found to alkalinize the decaying tissue of tomato fruit via accumulation and secretion of ammonia. Alkalinization dynamics caused by ammonia secretion from growing hyphae was examined microscopically using the pH-sensitive fluorescent dye 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein. Values of pH of 7.9 observed in the host tissue close to the hyphal tips declined to pH 6.0 at 10 mm away from the hyphal tip, which was a value that was still higher than that detected in the healthy tissue, pH 4.2. Ammonia accumulation at the infection site depended on the initial environmental pH. Treatments with low (4.0) pH buffer at the infection site resulted in high levels of ammonia secretion and increased virulence of C. coccodes compared with similar treatments with buffer at pH 7.0. Significantly, mutants of C. coccodes defective in nitrogen utilization, nit-, and areA- were impaired in ammonia secretion and showed reduced decay development. The reduced infection rate of nit- mutants could be complemented by adding glutamine at the infection site. Thus, ammonia accumulation is a critical factor contributing to C. coccodes pathogenicity on tomato fruit. The results show that the initial acidic pH of the fruit is conducive to ammonia secretion and the subsequent alkalinization of the infection site, and facilitates fungal virulence and the transformation from the quiescent-biotrophic to active-necrotrophic state.