Role of nitrogen-metabolism genes expressed during pathogenicity of the alkalinizing Colletotrichum gloeosporioides and their differential expression in acidifying pathogens.

Pathogens can actively alter fruit pH around the infection site, signaling modulation of pathogenicity-factor expression, as found for alkalinizing (Colletotrichum and Alternaria spp.) and acidifying (Penicillium, Botrytis, and Sclerotinia spp.) fungi. The nitrogen-metabolism genes GDH2, GS1, GLT, and MEP genes are differentially expressed during colonization by Colletotrichum gloeosporioides, and a Δgdh2 strain reduces ammonia accumulation and pathogenicity. We analyzed the contribution of transporters GLT and MEPB to C. gloeosporiodes pathogenicity. Germinating spores of Δglt strains showed reduced appressorium formation; those of ΔmepB mutants showed rapid ammonia uptake and accumulation inside the hyphae, indicating deregulated uptake. Both mutants reduced pathogenicity, indicating that these transporters function during alkalinizing species pathogenicity. We compared the expressions of these genes in C. gloeosporioides and Sclerotinia sclerotiorum, and found five to 10-fold higher expression at the transcript level in the former. Interestingly, GLT and MEPB in the alkalinizing species showed no and very low sequence identity, respectively, with their counterparts in the acidifying species. Knockout analysis of GLT and MEPB and their differential transcript regulation in the alkalinizing and acidifying species suggest that the ammonia accumulation contributing to pathogenicity in the former is modulated by factors at the gene-regulation levels that are lacking in the acidifying species.