RESUMO
Flaveria bidentis (L.) Kuntze, a C4 dicot, was genetically transformed with a construct encoding the mature form of tobacco (Nicotiana tabacum L.) carbonic anhydrase (CA) under the control of a strong constitutive promoter. Expression of the tobacco CA was detected in transformant whole-leaf and bundle-sheath cell (bsc) extracts by immunoblot analysis. Whole-leaf extracts from two CA-transformed lines demonstrated 10% to 50% more CA activity on a ribulose-1,5-bisphosphate carboxylase/oxygenase-site basis than the extracts from transformed, nonexpressing control plants, whereas 3 to 5 times more activity was measured in CA transformant bsc extracts. This increased CA activity resulted in plants with moderately reduced rates of CO2 assimilation (A) and an appreciable increase in C isotope discrimination compared with the controls. With increasing O2 concentrations up to 40% (v/v), a greater inhibition of A was found for transformants than for wild-type plants; however, the quantum yield of photosystem II did not differ appreciably between these two groups over the O2 levels tested. The quantum yield of photosystem II-to-A ratio suggested that at higher O2 concentrations, the transformants had increased rates of photorespiration. Thus, the expression of active tobacco CA in the cytosol of F. bidentis bsc and mesophyll cells perturbed the C4 CO2-concentrating mechanism by increasing the permeability of the bsc to inorganic C and, thereby, decreasing the availability of CO2 for photosynthetic assimilation by ribulose-1,5-bisphosphate carboxylase/oxygenase.
RESUMO
The availability of a complete genome database for the cyanobacterium Synechocystis sp. PCC6803 (glucose-tolerant strain) has raised expectations that this organism would become a reference strain for work aimed at understanding the CO(2)-concentrating mechanism (CCM) in cyanobacteria. However, the amount of physiological data available has been relatively limited. In this report we provide data on the relative contributions of net HCO(3) (-) uptake and CO(2) uptake under steady state photosynthetic conditions. Cells were compared after growth at high CO(2) (2% v/v in air) or limiting CO(2) conditions (20 ppm CO(2)). Synechocystis has a very high dependence on net HCO(3) (-) uptake at low to medium concentrations of inorganic carbon (Ci). At high Ci concentrations net CO(2) uptake became more important but did not contribute more than 40% to the rate of photosynthetic O(2) evolution. The data also confirm that high Ci cells of Synechocystis sp. PCC6803 possess a strong capacity for net HCO(3) (-) uptake under steady state photosynthetic conditions. Time course experiments show that induction of maximal Ci uptake capacity on a shift from high CO(2) to low CO(2) conditions was near completion by four hours. By contrast, relaxation of the induced state on return of cells to high CO(2), takes in excess of 230 h. Experiments were conducted to determine if Synechocystis sp. PCC6803 is able to exhibit a 'fast induction' response under severe Ci limitation and whether glucose was capable of causing a rapid inactivation in Ci uptake capacity. Clear evidence for either response was not found.