A multi-phase flux balance model reveals flexibility of guard cell central carbon metabolism
Experimental research in guard cell metabolism has revealed the roles of the accumulation of various metabolites in guard cell function, but a comprehensive understanding of their metabolism over the diel cycle is still incomplete, given the limitations of current experimental methods. In this study, we constructed a four-phase flux balance model of guard cell metabolism to investigate the changes in guard cell metabolism over the diel cycle, including the day and night and stomata opening and closing. Our model demonstrated the metabolic flexibility in guard cells, showing that multiple metabolic processes can contribute to the synthesis and metabolism of malate and sucrose as osmolytes during stomatal opening and closing. We showed that guard cells can adapt to varying light availability and sucrose uptake from the apoplast during the day by operating in a mixotrophic mode with a switch between sucrose synthesis via the Calvin-Benson cycle and sucrose degradation via the oxidative pentose phosphate pathway. During stomatal opening, our model predicted an alternative flux mode of the Calvin-Benson cycle with all dephosphorylating steps diverted to diphosphate--fructose-6-phosphate 1-phosphotransferase to produce PPi, which is used to pump protons across the tonoplast for the accumulation of osmolytes. An analysis of the energetics of the use of different osmolytes in guard cells showed that malate and Cl- are similarly efficient as the counterion of K+ during stomatal opening. Significance statementThis work presents the first four-phase metabolic model for predicting guard cell metabolism over the diel cycle, which predicted an alternative flux model of the Calvin-Benson cycle that maximises the production of PPi during stomatal opening. While multiple metabolic processes were shown be important in synthesising and metabolising osmolytes in guard cells of different experimental systems, our model demonstrated that these processes can operate simultaneously and at different rates depending on conditions.