In the last decade plant-to-plant communication has received an increasing attention, particularly for the role of Volatile Organic Compounds as possible elicitors of plant defense. The role of β-ocimene as an interspecific elicitor of plant defense has been recently assessed in multitrophic systems including different plant species (Solanaceae, Poaceae, legumes) and different pest species including chewer insects and phytophagous mites. Both chewer insects and phytophagous mites are known to elicit specific plant defensive pathways which are different (at least in part) from those elicited by sap feeders. The aim of this research was to fill this gap of knowledge and to assess the role of β-ocimene as an elicitor of plant defense against aphid pests, which are sap feeders. For this purpose we used as transgenic tobacco plant releasing an odour plume enriched in this compound as emitter and a tomato plant as receiver. We selected the aphid Macrosiphum euphorbiae and its natural enemy, the parasitoid Aphidius ervi, as the targets of plant induced defense. Tomato plant defense induced by β-ocimene was assessed by characterizing the aphid performance in terms of fixing behaviour, development and reproduction (direct plant defense) and the parasitoid performance in terms of attraction towards tomato plants (indirect plant defense). The characterization of tomato response to β-ocimene was completed by the identification of Volatile Organic Compounds as released by conditioned tomato plants. Tomato plants that were exposed to the volatiles of transgenic tobacco enriched in β-ocimene resulted in less suitable for the aphids in respect to control ones (direct defense). On tomato plants "elicited" by β-ocimene we recorded: a significant lower number of aphids settled; a significant lower number newborn nymphs; a significant lower weight of aphids feeding. In addition, tomato plants "elicited" by β-ocimene resulted became more attractive towards the parasitoid A. ervi than control ones. These results could be explained at least in part by examining the composition of the Volatile Organic Compounds released by tomato plants "elicited" by β-ocimene. Indeed, we found a significantly higher release of several compounds including methyl salicylate and cis-3-hexen-1-ol. These two compounds have been demonstrated to impair aphid development and reproduction and to be involved in the attraction of the aphid parasitoid A. ervi. By considering the ubiquity of β-ocimene and its ability to regulate the communication of plants belonging 30 to different species (if not families), we concluded that this compound is an ideal candidate for new 31 strategies of sustainable control of agricultural pests.
Keywords: Aphidius ervi; Macrosyphum euphorbiae; Methyl salicylate; cis-3-Hexen-1-ol.
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