We have cloned the PGL1 gene from Saccharomyces cerevisiae, an poly-α-1,4 galacturonide glycanohydrolase (EC 3.2.1.15) commonly named endopolygalacturonase (endoPG), with a cis-acting regulatory region. This enzyme is known as pectin hydrolase, pectin being one of the main constituents of primary cell walls and middle lamella of higher plant cells. These pectins constitute a structural barrier to microbial invasion. The construct pRPGL1-2 has been introduced into the haploid receiver strain of S. cerevisiae, devoided of endoPG activity, to study the potential role of polygalacturonases in pathogenicity towards Vitis vinifera and in pseudohyphae formation. The latter corresponds to a switch from a single-cell form to the formation of long-branched chains of elongated cells, a phenomenon mainly reported for diploid yeast. We observed in the isogenic S. cerevisiae strains that endoPG, encoded by PGL1 gene, is required for pseudohyphae development and is involved in plant pathogenesis. The direct penetration of the host plant has been confirmed histologically for the transformed haploid receiver.