Plant leaves can emit large amounts of volatiles into the air. When attacked by insects, the composition of these blends changes markedly. It is well known that these changes affect not only the behavior of insects interacting with the plant but also the metabolism of the plant itself as well as its nearby competitors. However, how plants perceive these volatiles and generate a functional response is not known.
My research activities have been dedicated to a group of plant volatiles emitted the earliest upon herbivory, the so-called green leaf volatiles (GLVs). We discovered a class of enzymes, present in plants and insects, that profoundly affect ecological interactions by converting the highly abundant GLV Z-3-hexenal into E-2-hexenal. These two compounds, as well as their derivatives, among which Z-3- and E-2-hexenyl acetate, have distinct effects on the behavior of herbivorous and predacious insects as well as on the metabolism of plants.
During my PhD I investigated how the GLVs of wild tobacco (Nicotiana attenuata) modulate the behaviour of herbivorous and carnivorous arthropods. We discovered two important new facts. Firstly we found that especially the ratio between Z-3- and E-2-isomers determines how insects respond. Secondly, we discovered that the conversion of Z-3-hexenal to E-2-hexenal (thereby changing their ratio) was not mediated by the plant (as was always assumed) but by the feeding caterpillar (Manduca sexta). This unexpected observation called for a revised paradigm on the adaptiveness of induced volatiles in plant-herbivore interactions. Hence we continued with elucidating the nature and occurrence of this puzzling trait. Recently we have identified the caterpillar enzyme responsible for this phenomenon and characterized it. Interestingly, also some plant species (but not wild tobacco) are capable of performing the same conversion. We are now interested in understanding the importance of hexenal isomerisation on the development of insect larvae and host-plant selection by adults.
Plants display distinct reproducible responses after perception of GLVs. These responses include the transcriptional upregulation of specific subsets of defense-related genes and accumulation of downstream metabolites. Besides immediate activation, plant defenses can also be primed by GLV exposure: this enables a plant to upregulate defenses more rapidly and/or more effectively at the actual moment of attack. Our knowledge of the molecular basis of odour perception by insects has grown enormously in recent years. However, far less progress has been made in our understanding of odour perception in plants. Previous work by my colleague Rob Schuurink has led to the identification of the first Arabidopsis mutants that are resistant to E-2-hexenal (her mutants) and they have mapped the her1 mutation to discover a role for GABA in this process as well as in the defense against Pseudomonas. They also mapped and cloned the her2 mutant and determined that several WRKY transcription factors may regulate the E-2-hexenal response in Arabidopsis. Moreover, in the Arabidopsis-Pseudomonas interaction, the microbe seems to hijack the C6 volatiles to enhance jasmonate biosynthesis and susceptibility. The research on E-2-hexenal signaling will be continued by my group using transcriptomics and proteomics approaches.
Inhibition of root growth in Arabidopsis seedlings by E-2-hexenal exposure: Arabidopsis seedlings normally respond to E-2-hexenal exposure with the inhibition of root growth. This root inhibition assay has been used to identify mutants which are unable to respond to E-2-hexenal.