Identification of volatile organic compounds emitted by different grapevine genotypes in response to downy mildew infection

Vitis vinifera is susceptible to several pathogens including Plasmopara viticola, the causal agent of downy mildew. American grapevine species are resistant or tolerant to P. viticola and breeding programs have introduced resistance traits to susceptible cultivars. Although grapevine resistance to P. viticola has been widely characterized in resistant genotypes, the possible contribute of volatile organic compounds (VOCs) was not yet investigated. The aim of this work was the characterization of VOCs produced by resistant and susceptible genotypes in response to P. viticola inoculation, in order to identify VOCs associated to grapevine resistance. The susceptible V. vinifera cultivar Pinot noir, and the resistant genotypes Kober 5BB, SO4, BC4 and Solaris were grown under greenhouse conditions and they were subsequently inoculated with P. viticola. Leaves were harvested immediately before (0 dpi) and six days (6 dpi) after inoculation, and the lower disease severity in resistant genotypes as compared with Pinot noir was confirmed. A solid-phase microextraction-gas chromatography-mass spectrometry approach (SPME/GC-MS) was used to analyze VOCs emitted by the five genotypes studied. GC-MS chromatograms showed specific VOC emission profiles of the four resistant genotypes as compared with Pinot noir at 6 dpi. VOCs specifically emitted by resistant genotypes were then selected, and pure compounds were tested against P. viticola by leaf discs assays. Particularly, three sesquiterpenes, two aldehydes and one heterocyclic compound significantly reduced downy mildew severity on Pinot noir, demonstrating that VOCs could play an important role in the resistance against downy mildew by direct toxicity against P. viticola.