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

Pectolytic enzymes produced by Didymella bryoniae in a liquid medium contai ning pectin as sole carbon source and in inoculated etiolated hypocotyls of 10 melon cultivars, as well as those constitutively expressed in spores, w ere 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 expre ssed in extracts from spores. The same PNL isoenzyme pattern was detected i n culture filtrates and in infected tissues of all the melon cultivars test ed. Polygalacturonase (PG) activity, represented by a single inducible acid ic band (pI 4.6) was detected only in culture filtrates. A single constitut ive basic pectin methylesterase (PME) isoenzyme (pI > 10.0) was also found in spores, culture filtrates and inoculated melon tissues. All cultivars we re susceptible at the seedling growth stage, but with differences in diseas e severity; cultivars Amarillo Oro and Juane Canari were, respectively, the least and most susceptible. Pectin lyase activity was highly correlated wi th disease severity. In rotted tissues and culture filtrates, an increase i n pH to values over 7.0 was recorded, values optimal for PNL activity. In t his plant-pathogen interaction, PNL activity represents the principal pecto lytic component and these isoenzymes were associated with the onset of dise ase, disease severity and an increase in pH of infected tissue.