Pectolytic enzymes produced in vitro and during colonization of melon tissues by Didymella bryoniae

Pectolytic enzymes produced by Didymella bryoniae in a liquid medium containing pectin as sole carbon source and in inoculated etiolated hypocotyls of 10 melon cultivars, as well as those constitutively expressed in spores, were studied by isoelectric focusing, quantitatively and qualitatively. Five constitutive pectin lyase (PNL) isoenzymes differing in isoelectric joint (pI), one acidic (pI 3.9) and four basic (pI 8.4, 8.9, 9.3, 9.9) were expressed in extracts from spores. The same PNL isoenzyme pattern was detected in culture filtrates and in infected tissues of all the melon cultivars tested. Polygalacturonase (PG) activity, represented by a single inducible acidic band (pI 4.6) was detected only in culture filtrates. A single constitutive basic pectin methylesterase (PME) isoenzyme (pI > 10.0) was also found in spores, culture filtrates and inoculated melon tissues. All cultivars were susceptible at the seedling growth stage, but with differences in disease severity; cultivars Amarillo Oro and Juane Canari were, respectively, the least and most susceptible. Pectin lyase activity was highly correlated with disease severity. In rotted tissues and culture filtrates, an increase in pH to values over 7.0 was recorded, values optimal for PNL activity. In this plant–pathogen interaction, PNL activity represents the principal pectolytic component and these isoenzymes were associated with the onset of disease, disease severity and an increase in pH of infected tissue.