EXPRESSION OF TOBACCO CARBONIC-ANHYDRASE IN THE C-4 DICOT FLAVERIA-BIDENTIS LEADS TO INCREASED LEAKINESS OF THE BUNDLE-SHEATH AND A DEFECTIVE CO2-CONCENTRATING MECHANISM

Flaveria bidentis (L.) Kuntze, a C-4 dicot, was genetically transforme d with a construct encoding the mature form of tobacco (Nicotiana taba com L.) carbonic anhydrase (CA) under the control of a strong constitu tive promoter. Expression of the tobacco CA was detected in transforma nt whole-leaf and bundle-sheath cell (bsc) extracts by immunoblot anal ysis. Whole-leaf extracts from two CA-transformed lines demonstrated 1 0% 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 pla nts with moderately reduced rates of CO2 assimilation (A) and an appre ciable increase in C isotope discrimination compared with the controls . With increasing O-2 concentrations up to 40% (v/v), a greater inhibi tion of A was found for transformants than for wild-type plants; howev er, the quantum yield of photosystem If did not differ appreciably bet ween these two groups over the O-2 levels tested. The quantum yield of photosystem Ii-to-A ratio suggested that at higher O-2 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 C-4 CO2-concentrating mechanism by incr easing the permeability of the bsc to inorganic C and, thereby, decrea sing the availability of CO2 for photosynthetic assimilation by ribulo se-1,5-bisphosphate carboxylase/oxygenase.