General and species-specific transcriptional responses to downy mildew infection in a susceptible (Vitis vinifera) and a resistant (V. riparia) grapevine species.

Background: Downy mildew is a destructive grapevine disease caused by Plasmopara viticola (Berk. and Curt.) Berl.and de Toni, which can only be controlled by intensive fungicide treatments. Natural sources of resistance fromwild grapevine (Vitis) species are used in conventional breeding approaches, but the signals and effectors involvedin resistance in this important crop species are not well understood.Results: Early transcriptional changes associated with P. viticola infection in susceptible V. vinifera and resistant V.riparia plants were analyzed using the Combimatrix microarray platform. Transcript levels were measured 12 and 24h post-inoculation, reflecting the time points immediately preceding the onset of resistance in V. riparia, asdetermined by microscopic analysis. Our data indicate that resistance in V. riparia is induced after infection, and isnot based on differences in basal gene expression between the two species. The strong and rapid transcriptionalreprogramming involves the induction of pathogenesis-related proteins and enzymes required for the synthesis ofphenylpropanoid-derived compounds, many of which are also induced, albeit to a lesser extent, in V. vinifera. Moreinterestingly, resistance in V. riparia also involves the specific modulation of numerous transcripts encodingcomponents of signal transduction cascades, hypersensitive reaction markers and genes involved in jasmonatebiosynthesis. The limited transcriptional modulation in V. vinifera represents a weak attempted defense responserather than the activation of compatibility-specific pathways.Conclusions: Several candidate resistance genes were identified that could be exploited in future biotechnologicalapproaches to increase disease resistance in susceptible grapevine species. Measurements of jasmonic acid andmethyl jasmonate in infected leaves suggest that this hormone may also be involved in V. riparia resistance to P.viticola.