From plant resistance response to the discovery of antimicrobial compounds: The role of volatile organic compounds (VOCs) in grapevine downy mildew infection

The discovery of new mechanisms of resistance and natural bioactive molecules could be two of the possible ways to reduce fungicide use in vineyard and assure an acceptable and sustainable protection against Plasmopara viticola, the grapevine downy mildew agent. Emission of volatile organic compounds (VOCs), such as terpenes, norisoprenoids, alcohols and aldehydes, is frequently induced in plants in response to attack by pathogens, such as P. viticola, that is known to cause a VOCs increment in cultivars harboring American resistance traits. In this study, the role of leaf VOCs in the resistance mechanism of two resistant cultivars (Mgaloblishvili, a pure Vitis vinifera cultivar, and Bianca, an interspecific hybrid) and the direct antimicrobial activity of four selected VOCs have been investigated. The leaf VOCs profiles, analyzed through solid-phase microextraction gas chromatography-mass spectrometry analysis, as well as the expression of six terpene synthases (TPSs), were determined upon pathogen inoculation. In both cultivars, the expression pattern of six TPSs increased soon after pathogen inoculation and an increment of nine VOCs has been detected. While in Mgaloblishvili VOCs were synthesized early after P. viticola inoculation, they constituted a late response to pathogen in Bianca. All the four terpenes (farnesene, nerolidol, ocimene and valencene), chosen according to the VOC profiles and gene expression analysis, caused a significant reduction (53-100%) in P. viticola sporulation. These results support the role of VOCs into defense mechanisms of both cultivars and suggest their potential role as a natural and eco-friendly solution to protect grapevine from P. viticola.