PG-inhibiting proteins'. (PGIPs) are leucine rich repeat plant cell wall proteins that specifically inhibit fungal polygalacturonases (PGs). Their role in plant defense response suggests that they, may be useful for genetic engineering to obtain transgenic plants with increased tolerance to fungal infection. In addition, the fact that Xylella fastidiosa, the causal agent of Pierce's Disease (PD) in grapevines, has genes putatively encoding PG and other cell wall-degrading enzymes led us to the hypothesis that PGIP could confer resistance against this bacterium. In order to test this hypothesis, proembryogenic calli originating from anthers of Vitis vinifera cvs. Thompson Seedless and Chardonnay were co-cultivated with Agrobacterium tumefaciens strain EHA 101 harboring binary plasmid pDU94.0928 that contains the pear pgip gene under the control of the CaMV 35S promoter. Plants from 51 independently isolated lines have been transferred to the greenhouse. Putative transgenic lines growing in the greenhouse were analyzed by PCR, Western blotting and PGIP activity assays: Western blot analysis demonstrated the presence of the protein in roots, leaves and young stems of the transgenic plants but not in untransformed controls. High levels of enzyme activity have been found in crude extracts from leaves of transgenic lines obtained from independent transformation events but not in untransformed controls. Preliminary results have shown that the development of PD in the transgenic lines analyzed so far is delayed. We also present results regarding the disease response of greenhouse-grown plants after inoculation with Botrytis cinerea.