Polygalacturonase-inhibiting proteins (PGIPs) are plant cell-wall proteins that specifically inhibit fungal endo-polygalacturonases (PGs) that contribute to the aggressive decomposition of susceptible plant tissues. The inhibition of fungal PGs by PGIPs suggests that PGIPs have a role in plant tolerance to fungal infections and this has been observed in transgenic plants expressing PGIPs. Xylella fastidiosa, the causal agent of Pierce's disease (PD) in grapevines, has genes that encode cell-wall-clegrading enzymes, including a putative PG. Therefore, we hypothesized that PGIP expression could confer tolerance against this bacterium as well as against the fungal pathogen Botrytis cinerea. To test this hypothesis, Vitis vinifera cvs.'Thompson Seedless' and 'Chardonnay' were transformed to express pear fruit PGIP-encoding gene (pPGIP) under the control of the CaMV 35S promoter. Substantial pear PGIP (pPGlP) activity was found in, crude extracts from leaves and in xylem exudate of transgenic lines obtained from independent transformation events, but not in untransformed controls. pPGlP activity was detected in xylem exudate of untransformed scions grafted on to transgenic' rootstocks expressing pPGIP. Leaves of transgenic plants infected with B. cinerea had reduced rates of lesion expansion. The development of PD was delayed in some transgenic lines with increased pPGlP activity. PD-tolerant transgenic lines had reduced leaf scorching, lower Xylella titres and better re-growth after pruning than the untransformed controls.