Low Bundle Sheath Carbonic Anhydrase Is Apparently Essential for Effective C₄ Pathway Operation

Abstract

The concentrating of inorganic carbon in bundle sheath cells is the key function of the unique reactions of the C4 pathway (13). The prime purpose of this process is to favor the carboxylation reaction catalyzed by Rubiscol at the expense of oxygenase activity, thereby eliminating photorespiration. Since CO2 as such is the substrate for Rubisco (5), and since it is the ratio of CO2 to 02 that determines the relative rates of carboxylase and oxygenase activities (1), it is the concentration of the CO2 component of the inorganic carbon which is critical. Recently, we reported that CO2 is the product of each of the three decarboxylase reactions operating in C4 plants (15). Based on the assumption that CO2 is both the product of decarboxylation in bundle sheath cells and the substrate for Rubisco a model was developed which predicted that the inorganic carbon pool generated during steady state photosynthesis would exist predominantly as CO2 (8). That is, a steady state situation could exist where the relative concentrations of CO2 and HCO3-remain far removed from thermodynamic equilibrium.In this model another key assumption for such a steady state where CO2 is the predominant inorganic carbon species was that bundle sheath cells contain little or no carbonic anhydrase. In fact, there is some evidence that the carbonic anhydrase activity in leaves of C4 plants is at least largely associated with mesophyll cells (9,16). Carbonic anhydrase would be essential in mesophyll cells to sustain rapid conversion of CO2 entering the cell to bicarbonate, the substrate for PEP carboxylase (15). However, I Abbreviations: Rubisco, ribulose 1,5-bisphosphate carboxylase-oxygenase; PEP, phosphoenolpyruvate.

Carbonic Anhydrase Activity in Leaves and Its Role in the First Step of C4 Photosynthesis

“…The values recorded here are much higher than those reported previously (4), due primarily to a consistent calculation error made in the earlier study. This did not affect the major conclusion of the earlier investigation, which was to show that bundle sheath cells contained a very low or negligible proportion of the CA activity in C4 leaves.…”

“…To sustain this process, atmospheric CO2 entering mesophyll cells must be rapidly converted to HCO3-and this reaction should rightly be regarded as the first step in C4 photosynthesis. Consistent with this concept is the fact that the CA of C4 leaves is very largely or exclusively confined to the cytosol of mesophyll cells (4,6,8), where PEP carboxylase is also located. In spite of this apparently critical role of CA in C4 photosynthesis, the quantitative aspects ofthe operation ofthis enzyme have been largely neglected.…”

Biochemical and Molecular Inhibition of Plastidial Carbonic Anhydrase Reduces the Incorporation of Acetate into Lipids in Cotton Embryos and Tobacco Cell Suspensions and Leaves

“…Also, Maeda et al (2000) identified a gene (CmpA) that encodes a substrate-binding protein that can specifically bind to HCO 3 Ϫ in cyanobacteria, which may further aid in the diffusion of HCO 3 Ϫ and elevation of CO 2 in the carboxysome. In C 4 plants, CA is localized to the cytosol of mesophyll cells, where it supplies HCO 3 Ϫ to phosphoenolpyruvate carboxylase (Burnell and Hatch, 1988;Hatch and Burnell, 1990), and calculations show that without CA, the rate of photosynthesis is reduced. In fact, a recent study with transgenic Flaveria bidentis clearly supported a role for CA in the assimilation of CO 2 in C 4 plants (Ludwig et al, 1998).…”

Mechanism of C4 Photosynthesis

“…Bundle sheath cells contain little or no carbonic anhydrase, certainly less than 1% of total leaf activity (2). From this evidence and consideration of the earlier model (8), it was proposed that low bundle sheath cell carbonic anhydrase activity may be essential for effective operation of C4 pathway photosynthesis (2).…”

“…The size and composition of such inorganic carbon pools would be affected by the pH of the various subcompartments and their permeabilities to bicarbonate. Uncertainty about the level and location of carbonic anhydrase is another complicating factor (2).…”