Abstract
Volatile organic compounds (VOCs) play a crucial role in the communication of plants with other organisms and are possible mediators of plant defence against phytopathogens. Although the role of non-volatile secondary metabolites has been largely characterised in resistant genotypes, the contribution of VOCs to grapevine defence mechanisms against downy mildew (caused by Plasmopara viticola) has not yet been investigated. In this study, more than 50 VOCs from grapevine leaves were annotated/identified by headspace-solid-phase microextraction gas chromatography-mass spectrometry analysis. Following P. viticola inoculation, the abundance of most of these VOCs was higher in resistant (BC4, Kober 5BB, SO4 and Solaris) than in susceptible (Pinot noir) genotypes. The post-inoculation mechanism included the accumulation of 2-ethylfuran, 2-phenylethanol, β-caryophyllene, β-cyclocitral, β-selinene and trans-2-pentenal, which all demonstrated inhibitory activities against downy mildew infections in water suspensions. Moreover, the development of downy mildew symptoms was reduced on leaf disks of susceptible grapevines exposed to air treated with 2-ethylfuran, 2-phenylethanol, β-cyclocitral or trans-2-pentenal, indicating the efficacy of these VOCs against P. viticola in receiver plant tissues. Our data suggest that VOCs contribute to the defence mechanisms of resistant grapevines and that they may inhibit the development of downy mildew symptoms on both emitting and receiving tissues.
Highlights
Plants are constantly exposed to environmental stressors and have evolved complex ways to defend themselves against pathogens, herbivorous arthropods, parasitic plants and neighbouring plant competitors[1]
41 Volatile organic compounds (VOCs) were annotated and 16 were found as unknown compounds according to the measured retention index (RI) of the HS-SPME/ GC-MS analysis (Supplementary Tables S1, S2 and S3)
VOC profiles of the tested grapevine genotypes were mainly consistent in the two experiments, and they differed according to the grapevine genotypes and time points (Fig. 2, Supplementary Tables S1 and S2)
Summary
Plants are constantly exposed to environmental stressors and have evolved complex ways to defend themselves against pathogens, herbivorous arthropods, parasitic plants and neighbouring plant competitors[1]. VOCs constitute approximately 1% of plant secondary metabolites[3] and are usually lipophilic molecules that can freely diffuse into the environment and pass biological membranes, thanks to their low molecular weight and high vapour pressure[4] Based on their structure and biosynthetic pathways, plant VOCs can be divided into four main classes: terpenoids, phenylpropanoids/benzenoids, fatty acid derivatives and those derived from non-aromatic amino acids[2,3]. The involvement of plant VOCs in resistance mechanisms against pathogens is supported by specific VOC emission profiles in resistant and susceptible genotypes of maize to Aspergillus flavus[23], citrus plants to ‘Candidatus Liberibacter asiaticus’[24] and grapevine plants to Plasmopara viticola[12] In the latter, the emission of the sesquiterpene and monoterpene classes was found to be more pronounced in downy mildew-resistant than in susceptible grapevine genotypes[12], and the emission of a sesquiterpene [(E,E)-α-farnesene] was associated with the resistance induced by a sulphated laminarin against downy mildew[25]. Due to the obligate biotrophic lifestyle of P. viticola, inhibitory effects of VOCs can be tested only in the presence of host tissues and the final goal was to better understand the contribution of grapevine VOCs to limit downy mildew development in susceptible leaves
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