Elucidating the roles of carbonic anhydrase on mesophyll
conductance to CO 2 and photosynthetic capacity in C3 plants

Photosynthesis is limited by the rate of carbon dioxide (CO 2 ) diffusion from the atmosphere to the sites of carboxylation in the chloroplast stroma. The stomatal conductance controls the rate of CO 2 into the leaf, and mesophyll conductance relates the ease of diffusion from the leaf air spaces to the chloroplast interior. More specifically, after entering through the stomata (gs), CO 2 diffuses through leaf intercellular air spaces, dissolves near the cell surface, where it may dissociate into bicarbonate or remain as CO2 , travel into (the liquid phase): cell walls, and travels through the cell membrane, cytosol, chloroplast envelopes, and stroma, where it finally reaches Ribulose-1,5 bisphosphate carboxylase/oxygenase (Rubisco).

 

The combined effect of all these resistors determines the total mesophyll conductance (gm). The diffusion of CO2 in the liquid phase is 10000 times slower than the gas phase. We aim to understand whether gm is controlled by the physics of the CO2 and HCO3 diffusion across effective path length in the liquid phase and/or controlled by a protein-facilitated process. We study the effect of carbonic anhydrase, which consists of a large family of proteins located in multiple sites within plant cells, these proteins facilitate the diffusion of CO 2 through the liquid phase by allowing it to diffuse faster in the form of HCO 3 . The reaction catalyzed by carbonic anhydrase is CO 2 + H 2 O --> HCO 3 - + H + which allows pH equilibrium along the cell compartments and facilitates a steady CO2 supply at the carboxylation site (Cc).

 

We are using model plants Arabidopsis and tomato to study the metabolic and anatomical factors that control and regulate g m under optimal and stress conditions. To identify the role of CA on gm, we are generating CRISPR/Cas9 mutants in tomato and use existing Arabidopsis CA mutants, including knockout overexpression and tissue-specific lines. By creating the transgenic lines, we expect to discover the subcellular localization functions of bicarbonate pH homeostasis and diffusion conductance inside the mesophyll compartments (cell wall, plasma membrane, cytosol, and chloroplast).