Text by Lauri Laanisto
I like that this paper starts with a metaphor: “Chlorophyll (Chl) is a double-edged sword for plants.” Because “This molecule is capable of harvesting sunlight, initiating the process of photosynthesis, but it also involves an unavoidable risk of photooxidation.” Though I´m not sure whether this metaphor indeed holds water. The edges of this metaphoric sword seem to be awfully uneven. Especially because plants have developed pigments against it. And this paper tries to figure out which edge would cut the holder.
Another kind of curious aspect of this study is that while the whole Introduction part deals with vasular plants and how VCCPs (which are not defined in the paper so that I would actually understand what the acronym stands for) are stress regulated, the whole experiment was done with a lichen! The argument for that is: “We have chosen such a model and not a vascular plant to simplify the pathway between the thylakoids and the open atmosphere.” Is it really a substantial reason to make such a fundamental change in the study object? It´s a curious question…
Anyway, based on the results, the authors conclude that the presence of VCCPs could indicate the degradation process in caroteinoids. Whether the lichen was under heat stress or not. And in comparison with other volatiles, the emission rate of VCCPs had the lowest change. It went up just a little.
I kind of got the feeling that this paper represents the results of an experiment not going as expected. For example, the concluding paragraph reads as follows: “Overall, the present work shows that VCCPs are produced constitutively and released from the thalli of L. pulmonaria. However, although VCCP emissions from photosynthetic tissues might, potentially, serve similar functions as other well-recognized volatile infochemicals (LOX products, methyl salicylate, methyl jasmonate) that act as cues in biocommunication and plant signaling, low emission rates of VCCPs, and the absence of quantitative relationship with stress intensity, suggests that it is unlikely that these volatiles play such a role in the studied lichen species.”
Citation: García-Plazaola, J. I., Portillo-Estrada, M., Fernández-Marín, B., Kännaste, A., & Niinemets, Ü. (2017). Emissions of carotenoid cleavage products upon heat shock and mechanical wounding from a foliose lichen. Environmental and Experimental Botany, 133, 87-97. (link to full text)
Carotenoids constitute a major target of chloroplastic photooxidative reactions, leading to the formation of several oxidized derivatives and cleavage products, some of which are volatile (VCCPs). Among them, β-cyclocitral (β-CC), at least, is a retrograde signaling molecule that modulates the activity of many key physiological processes. In the present work, we aimed to study whether β-CC and other VCCPs are released into the atmosphere from photosynthetic tissues. To overcome stomatal limitations, the foliose chlorolichen Lobaria pulmonaria was used as the model system, and the emissions of biogenic volatiles, induced by heat and wounding stresses, were monitored by proton-transfer reaction time-of-flight mass-spectrometry (PTR-TOF-MS) and gas-chromatography (GC–MS). Prior to stress treatments, VCCPs were emitted constitutively, accounting for 1.3% of the total volatile release, with β-CC being the most abundant VCCP. Heat and wounding stresses induced a burst of volatile release, including VCCPs, and a loss of carotenoids. Under heat stress, the production of β-CC correlated positively with temperature. However the enhancement of production of VCCPs was the lowest among all the groups of volatiles analyzed. Given that the rates of carotenoid loss were three orders of magnitude higher than the release rates of VCCPs and that these compounds only represent a minor fraction in the blend of volatiles, it seems unlikely that VCCPs might represent a global stress signal capable of diffusing through the atmosphere to different neighboring individuals.