UV-B radiation affects plant volatile emissions and shade avoidance responses

Abstract

Plants detect and integrate an assortment of signals from their environment, and use these signals to maximise their performance by adjusting their growth and development as well as their secondary metabolite production. In this thesis, we investigated how plants integrate visual and olfactory signals. First, a system to measure plant volatile organic compounds (VOCs) was set up (chapter 2). Second, the effect of UV-B radiation on tomato and Arabidopsis thaliana VOC emissions was described and its regulation via TERPENE SYNTHASE gene expression and the UV-B photoreceptor UVR8 examined (chapter 3 and 4). Third, the interaction between UV-B and low red to far-red (R:FR) responses was investigated at the physiological and signal transduction level (chapter 5). In tomato, UV-B induced VOC emissions with differential timing: a first class of VOCs increased immediately upon UV-B exposure, a second class of compounds had a delayed response and a third class increased only after UV-B exposure stopped. This third class included monoterpenes and methyl salicylate (MeSA), compounds known to function as signals in ecological interactions. We identified six different monoterpenes that were induced by UV-B, but found no correlation between their induced emission and up-regulation of their TERPENE SYNTHASE (TPS) genes, except for β-phellandrene and TPS4 expression in leaves. Using Arabidopsis thaliana, we demonstrated that UV-B induces VOC emissions independent of the UV-B photoreceptor UVR8. These are the first data showing that some secondary metabolites are regulated by UV-B exposure in a UVR8-independent manner. In addition, comparing UV-B-mediated VOC emissions of tomato and A. thaliana revealed that (i) the timing of UV-B-induced monoterpene emissions and (ii) UV-B-induced MeSA emissions are species specific. By exposing A. thaliana to UV-B and low R:FR simultaneously, we demonstrated that UV-B represses low R:FR responses via the UVR8 pathway and that this interaction involves several components of the R:FR signalling network. Mutant studies indicated involvement in the UV-FR interaction of the early UVR8-signalling component HY5, growth-repressing DELLA proteins, central shade avoidance regulators PIF4 and PIF5, but possibly also PIF1 and PIF3, auxin-related and PIF-inhibiting PAR1, and photoreceptor-interacting PKS2. In addition, gene expression studies suggested a role for the PAR1-binding and BR-inducible transcription factor PRE1, as well as for HFR1, which inhibits PIF4 and PIF5 action. Concerning the involvement of hormones in the UV-FR interaction, we found that BR might be important, whereas we found no such evidence for auxin and GA. This thesis thus demonstrates that plant responses to distinct environmental cues interact, and that there is cross-talk between the intrinsic signalling pathways a plant uses to assess and respond to light quality changes. This is especially relevant when considering plant responses in patchy environments.