A multiphase flux balance model reveals flexibility of central carbon metabolism in guard cells of C3 plants.

X L Joshua Tan,C Y Maurice Cheung

doi:10.1111/tpj.15027

Abstract

Experimental research into 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 due to 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 stomatal opening and closing. Our model predicted metabolic flexibility in guard cells of C3 plants, showing that multiple metabolic processes can contribute to the synthesis and metabolism of malate and sucrose as osmolytes during stomatal opening and closing. Our model showed the possibility of guard cells adapting 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 inorganic pyrophosphate, 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.

Highlights

The guard cell’s role of regulating gas and water exchange between a plant and the atmosphere makes understanding guard cell function crucial for a holistic understanding of the whole plant (Lawson et al, 2014; Azoulay-Shemer et al, 2016)
What is the role of carbohydrate metabolism in guard cell function (Santelia and Lawson, 2016) and what is the origin of guard cell sugars (Lawson et al, 2014)? What is the rationale behind the choice of counter-ion? To what extent do photosynthetic products contribute to osmolyte production or energy for membrane transport (Santelia and Lawson, 2016)?
To model the metabolism of guard cells over a diel cycle, including the opening and closing of stoma, a four-phase modelling framework was developed as an extension of a diel modelling framework (Cheung et al, 2014) to represent four distinct phases of guard cell metabolism

Summary

Introduction

The guard cell’s role of regulating gas and water exchange between a plant and the atmosphere makes understanding guard cell function crucial for a holistic understanding of the whole plant (Lawson et al, 2014; Azoulay-Shemer et al, 2016). The uptake or synthesis of osmotically active metabolites (hereafter “osmolytes”) increases osmotic pressure within guard cells, causing the cell to become turgid and the stomatal pore to open (Outlaw, 2003). Questions about the interaction between metabolism and osmolyte synthesis/transport remain an open area of study. What is the role of carbohydrate metabolism in guard cell function (Santelia and Lawson, 2016) and what is the origin of guard cell sugars (Lawson et al, 2014)? What is the role of carbohydrate metabolism in guard cell function (Santelia and Lawson, 2016) and what is the origin of guard cell sugars (Lawson et al, 2014)? What is the rationale behind the choice of counter-ion? To what extent do photosynthetic products contribute to osmolyte production or energy for membrane transport (Santelia and Lawson, 2016)?

Methods

Results

Conclusion