Physiological importance of subsidiary cells in the succulent model plant Kalanchoë laxiflora

Stomata are “breathing” pores in plants that regulate gas exchange (uptake of CO2 for photosynthesis and release of water vapor by transpiration) between the interior of the leaf and the environment. Specialized cells called guard cells (GCs) surround a pore that can be more or less open in response to environmental factors and internal signals. Some plant species recruit subsidiary cells (SCs) to the central GCs. In grasses, the SCs make stomatal movements faster resulting in more water-use-efficient gas exchange. Their function in succulents such as Kalanchoë laxiflora, however, remains elusive. Succulents employ an alternative, extremely water-use-efficient photosynthetic lifestyle (CAM metabolism), where stomata only open at night. Thus, we speculate that SCs in CAM plants are not linked to fast movement but rather be involved in efficiently closing stomata during the daytime to prevent water loss. Here, we want to elucidate the function of SCs in the succulent model plant Kalanchoë laxiflora. First, we will perform comparative physiological analyses of species with diverse stomatal morphologies (with and without SCs) and different photosynthetic lifestyles. We will use leaf-level gas exchange measurements, microscopy, pharmacological treatments affecting stomatal aperture and carbon isotopes analysis. Second, we will assess stomatal physiology in K. laxiflora mutants with changed stomatal morphology and attempt to induce succulent-like SC traits in Arabidopsis thaliana, which lacks SCs. Together, this will determine the physiological relevance of SCs in succulents.