On 19 February at 14.15 Kaspar Koolmeister will defend his doctoral thesis "Stomatal CO2 regulation pathway and its application for modulating tomato plants" for obtaining the degree of Doctor of Philosophy (in Environmental Engineering).
Supervisors:
Professor Hannes Kollist, University of Tartu
Associate Professor Hanna Hõrak, University of Tartu
Opponent:
Professor Matthew Gilliham, University of Adelaide
Summary: Plants are the foundation for life on Earth, but stress caused by changing environmental conditions poses major challenges for crop production. Higher temperatures and lower air humidity increase plant water use and lower yields, making it necessary to breed crops with reduced water usage and improved drought resilience. Stomata enable plants to take up carbon dioxide for photosynthesis, but water is lost in the process. A stoma consists of two guard cells and the pore between them, the aperture of this pore is adjusted according to environmental conditions: stomata open in the light, under low CO₂ and high air humidity, and close in darkness, under high CO₂ and in response to low air humidity. In this doctoral thesis, guard-cell CO₂ signalling and its function in Arabidopsis thaliana were studied across different CO₂ concentration ranges and under normal and low air humidity. A key role for the mitogen-activated protein kinases MPK12 and MPK4 in stomatal CO₂ regulation was identified. The kinetics of stomatal responses depended on the CO₂ range, at low to ambient concentrations, responses were slower and of greater amplitude, whereas at ambient to high CO₂, closure was faster and smaller in amplitude. Previous knowledge of stomatal CO₂ signalling was applied in tomato (Solanum lycopersicum), where HT1 homologues were inactivated using genetic editing. Stomata of the resulting slht1 slht2 double mutant line were insensitive to changes in CO₂, but still responded to low air humidity and abscisic acid (ABA). The mutants had lower stomatal conductance, reduced water use, and improved drought resilience, while maintaining biomass accumulation and fruit traits comparable to wild type. This work describes how stomatal CO₂ regulation functions across distinct CO₂ ranges and how it is affected by air humidity. The results also support the use of HT1 as a target for reducing plant water use.
