pH regulation of pectate lyase secretion modulates the attack of Colletotrichum gloeosporioides on avocado fruits

Growth of Colletotrichum gloeosporioides in pectolytic enzyme-inducing medi um (PEIM) increased the pH of the medium from 3.8 to 6.5. Pectate lyase (PL ) secretion was detected when the pH reached 5.8, and the level of secretio n increased up to pH 6.5. PL gene (pel) transcript production began at pH 5 .0 and increased up to pH 5.7. PL secretion was never detected when the pH of the inducing medium was lower than 5.8 or when C. gloeosporioides hyphae were transferred from PL-secreting conditions at pH 6.5 to pH 3.8. This be havior differed from that of polygalacturonase (PG), where pg transcripts a nd protein secretion were detected at pH 5.0 and continued up to 5.7. Under in vivo conditions, the pH of unripe pericarp of freshly harvested avocado (Persea americana cv, Fuerte) fruits, resistant to C. gloeosporioides atta ck, was 5.2, whereas in ripe fruits, when decay symptoms were expressed, th e pericarp pH had increased to 6.3. Two avocado cultivars, Ardit and Etting er, which are resistant to C. gloeosporioides attack, had pericarp pHs of l ess than 5.5, which did not increase during ripening. The present results s uggest that host pH regulates the secretion of PL and may affect C,gloeospo rioides pathogenicity. The mechanism found in avocado may have equivalents in other postharvest pathosystems and suggests new approaches for breeding against and controlling postharvest diseases.