My entire career has been dedicated to understanding the impacts of environmental stresses on plant performances, particularly when plants are submitted to temperature changes, drought, variable light regimes or nutrient limitations/excess.
My graduate work on the interaction between respiration, photosynthesis and nitrogen assimilation proved that we still lack a metabolic understanding of how plants take advantage of their environment. Contrary to what was thought before, this work suggested that nitrogen and carbon assimilations are decoupled and that respiration is the cornerstone of their interaction (Gauthier et al 2010). In other words, plants need to experience darkness to be able to use the carbon fixed by photosynthesis to assimilate nitrogen. These results allowed us to present a new. This model was the first metabolic demonstration that respiration is inhibited in the light. Nevertheless, a lot remains to be done before total acceptation of the phenomenon, as there is still some controversy on what control such inhibition.
Straight after my PhD, I moved to The Australian national University, Australia where I examined the effects of elevated temperature, elevated CO2 and drought on the temperature response of dark respiration in Eucalyptus globulus (Gauthier et al 2014). These results highlighted the need to exercise caution when assuming a constant respiration vs photosynthesis ratio in predictive models. They also highlighted the dynamic nature of the temperature dependence of dark respiration in plants experiencing future climate change scenarios, particularly with respect to drought and elevated CO2. If these findings can be generalized to many plants, this would have substantial implications for terrestrial C storage in a future, warmer world. Building on this idea, I was part of a global effort to investigate plants traits that leads to the creation of a global dataset of plant respiration (GlobResp, Atkin et al 2015). Nevertheless, a lot remains to be done to constrain our understanding of the effect of drought on respiration (and more generally plant performances). I believe that understanding cell metabolic pathways allows doing so.
As part of my research in Princeton University, I designed and built the first gas exchange system that allows direct, continuous and long-term measurement of gross photosynthesis in entire leaves (Gauthier et al. 2016, in press) using oxygen isotopes. This method involves measuring net O2 production from the change in O2 concentration, and gross production from the rate at which 18O-labeled O2 is generated from 18O labeled water. This system takes advantage of the uptake of water by a leaf through the transpiration stream to raise the leaf water isotopic composition up to ~ +10,000‰. The O2 produced by photosynthetic water splitting is then registered as a change in d18O of O2. The change of d18O of O2, and O2/N2, are measured to high precision, allowing net and gross production to be accurately assessed at irradiances as low as a few tens of mmol m-2 s-1. This system constitutes an incredible opportunity to study fundamental metabolic pathways and link them to leaf gas exchanges.
Initial work using this system led me to study the origin of the inhibition of respiration in the light in Sunflower leaves (Gauthier et al., in prep). Using glucose 13C labelled in specific carbon positions, we found that the origin of the empirical observation of this inhibition known as the Kok effect was led mostly by the inhibition of Pyruvate dehydrogenase. This finding brings opportunities to better understand the variability of this inhibition across species. This study has culminated recently in securing funds awarded by the Cooperative Institute for Climate Science to creating the first Respiration model and including it into NOAA-GFDL terrestrial Ecosystem Model (LM3).
Finally, I modified the gas exchange system to allow oxygen measurements in the field. Over Summer 2015, I travelled to the Arctic with a group of students to studying the impact of 24h sunlight on the rate of respiration in the light. For this study, I secured funding from Princeton Environmental Institute to support my student expenses in Abisko, Sweden. We found that under continuous light, leaf photosynthetic parameters were considerably affected and that current models are irrelevant for simulating Carbon balance in Arctic ecosystems (Gauthier et al., in prep).
All these different achievements in my career constitute a foundation of my current research that aims to understand how variations in photosynthesis and respiration under climate change-driven environmental stresses affect ecosystem sustainability and food security; and what are the metabolic and physiological origins of these variations.