The complexity of sensory perception in insect: study of an aphid-plant-fungal system

This study is framed in the context of an innovative approach on the search of natural compounds of fungal origin interfering with the insect pest processes of host plant selection. Aphids (Hemiptera Aphidoidea) are important crop pest; they are sap sucking insects able to induce heavy damage directly by nutrient drain or indirectly via several mechanisms, including virus transmissionPrevious studies revealed that the fungal isolates Trichoderma citrinoviride Bisset ITEM 4484 and Trichoderma harzianum ITEM 908 can influence feeding preference of two different species of aphids restraining specimens from feeding on treated leaves. The studied species are Schizaphis graminum (Rondani), widespread and dangerous for various cereals of agricultural interest, and Myzus persicae (Sulzer), a cosmopolitan species that attacks plants in the field, readily infests vegetables and ornamental plants in greenhouses. A multidisciplinary study, aiming to identify the metabolites of T. citrinoviride and T. harzianum responsible of the effects on aphid feeding, have been carried out. At the moment, the purification of the bioactive metabolites from organic extracts of T. citrinoviride culture carried out with chromatographic techniques combined with behavioral tests has lead to obtain and characterize six metabolites, belonged to three distinct chemical family: a symmetrical disubstituted hexa-1,3-dienyl ester of acetic acid and a tetrasubstituted cyclohexane-1,3-diol, respectively, and four bisorbicillinoids. Four of the six metabolites influence aphid feeding preference. These metabolites could constitute the base for the development of new active principles for crop protection from aphids. Purification of the extract of T. harzianum was carried out by a combination of column chromatography and thin layer chromatography using direct and reverse phase. Preliminary results showed that the fungus produces several different bioactive metabolites some of which could be identical to those isolated from T. citrinoviride. In terms of the aphid sensory organs involved in the perception of the metabolites, electroantennographic studies carried out with the two aphid species showed that the volatile compounds of the behaviourally active suspensions and extracts of fungal cultures or identified metabolites were not able to stimulate the antennal olfactory sensilla of both aphid species. Using a Single Cell Recording technique we demonstrated that taste cells located on the aphid tarsomeres are involved in the perception of fungal metabolites. Moreover single-cell responses to some fungal metabolites are dose-dependent and consistent with behavioral results. Studies also pointed out a different sensitivity across the winged and wingless morphs of S. graminum towards the fungal cultures and some of the characterised metabolites. Indeed, some metabolites influence winged but not wingless morph feeding preference. It has been argued that the different sensitivity of the two aphid morphs might be due to the different distribution and number of adequate gustatory receptors. This hypothesis is largely supported both by the lower percentage of taste cells that evoke spike activity in wingless morphs and by the lack of responses to the tested compounds. SEM and TEM studies will be carried out to describe the morphological features of the implicated sensorial structures in aphids. In particular, we aim to functionally and morphologically define the sensory structures located on the tarsi which remain unknown to date.