Requirement for either a host- or pectin-induced pectate lyase for infection of Pisum sativum by Nectria hematococca

Fungal pathogens usually have multiple genes that encode extracellular hydr olytic enzymes that may degrade the physical barriers in their hosts during the invasion process. Nectria hematococca, a plant pathogen, has two induc ible pectate lyase (PL) genes (pel) encoding PL that can help degrade the c arbohydrate barrier in the host. pelA is induced by pectin, whereas pelD is induced only in planta. We show that the disruption of either the pelA or pelD genes alone causes no detectable decrease in virulence. Disruption of both pelA and pelD drastically reduces virulence. Complementation of the do uble disruptant with pelD gene, or supplementation of the infection droplet s of the double disruptant with either purified enzyme, PLA, or PLD, caused a recovery in virulence. These results show that PL is a virulence factor. Thus, we demonstrate that disruption of all functionally redundant genes i s required to demonstrate the role of host barrier-degrading enzymes in pat hogenesis and that dismissal of the role of such enzymes based on the effec ts of single-gene disruption may be premature.