Induction of different pathogenesis-related cDNAs in grapevine infected with powdery mildew and treated with ethephon

Uncinula necator, the causal agent of grapevine powdery mildew, was found t o elevate the activity of the pathogenesis-related (PR) proteins, chitinase and beta-1,3-glucanase in leaves and berries of a number of susceptible gr apevine (Vitis vinifera) cultivars. The increase in hydrolytic activity was directly related to the severity of powdery mildew infection on both leave s and berries but no systemic induction was observed in uninfected tissues. A number of cDNA clones encoding various PR proteins, including chitinases (PR-2), beta-1,3-glucanases (PR-3) and thaumatin-like (TL) proteins (PR-5) , were isolated and their expression in response to powdery mildew infectio n examined. PR genes encoding extracellular proteins were most strongly ind uced in infected leaves, including an acidic class III chitinase (VvChi3), a basic class I glucanase (VvGlub) and a thaumatin-like protein (VvTl2). A basic class I chitinase (VvChi1b) was also detected, but only in severely i nfected leaves. A similar pattern of PR gene induction was observed in mild ew-infected preveraison berries, with the exception of VvChi1b, which was n ot detected. Chitinase and beta-1,3-glucanase enzyme activities were also f ound to be highly induced in leaves and preveraison berries by ethephon tre atment. The pattern of chitinase gene induction by ethephon, however, was m arkedly different for mildew infection, with no apparent induction of VvChi 3 but a marked increase in the transcript level of an acidic class TV chiti nase, VvChi4. These results demonstrate that the leaves and preveraison ber ries of grapevine respond to environmental and stress-related stimuli by th e expression of specific PR genes but that the induction of these genes dur ing pathogen invasion does not afford complete protection of these tissues against Uncinula necator. The following abbreviations have been used: Tris: 2-amino-2-(hydroxymethyl) -1,3-propanediol; EDTA: ethylenediaminetetra-acetic acid; PVPP: polyvinylpo lypyrrolidone. The GenBank accession numbers for VvChi1b and VvGlub are AF0 53341 and AF053750, respectively.