REQUIREMENT FOR CARBONIC-ANHYDRASE ACTIVITY IN PROCESSES OTHER THAN PHOTOSYNTHETIC INORGANIC CARBON ASSIMILATION

A number of non-green plant tissues have high rates of HCO3--consuming reactions in the cytosol, i.e. C-4 dicarboxylic acid production preceding organic acid anion transport into dicarboxylate consuming compartments in N-2-fixing root nodules, in lipogenic tissues, and in thermogenic aroid spadices and, in the case of lipogenic tissues, in acetyl CoA incorporation Into lipid in plastid stroma. Since inorganic C supply to the cytosol or stroma by decarboxylation reactions, and by transmembrane fluxes, involves only CO2, the HCO3- consumed in the rapid metabolic processes must originate from hydration (hydroxylation) of CO2. Computations based on the first-order rate constant for uncatalysed conversion of CO2 to HCO and the most likely in vivo CO2 concentration show that the uncatalysed reaction is possibly adequate to supply the observed HCO3- requirement In the HCO3--consuming compartments. However, carbonic anhydrase activity is well established in legume root nodules, and also appears to occur In aroid spadices. In addition to coping with any heterogeneities in HCO3- consumption in the cytosol, the root nodule activity may be involved in optimizing haemoglobin function. Further work is needed on carbonic anhydrase expression is tissues with rapid HCO3- consumption, especially in view of reports of negligible carbonic anhydrase activity in some non-green plant tissues. Other possible roles of carbonic anhydrase in nongreen plant tissues are briefly discussed.