Expression of Tobacco Carbonic Anhydrase in the C4
Dicot Flaveria bidentis Leads to Increased Leakiness of the
Bundle Sheath and a Defective CO2-Concentrating Mechanism
Martha Ludwig*, 1,
Susanne von Caemmerer,
G. Dean Price,
Murray R. Badger, and
Robert T. Furbank
Molecular Plant Physiology Group, Research School of Biological
Sciences, Australian National University, G.P.O. Box 475, Canberra
2601, Australia (M.L., S.v.C., G.D.P., M.R.B.); and Commonwealth
Scientific and Industrial Research Organization, Division of Plant
Industry, G.P.O. Box 1600, Canberra 2601, Australia (R.T.F.)
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.
1
Present address: Division of Biochemistry and
Molecular Biology, School of Life Sciences, Australian National
University, Canberra, ACT 0200, Australia.
*
Corresponding author; e-mail martha.ludwig{at}anu.edu.au; fax
61-2-62490313.
Plant Physiol. (1998) 117: 1071-1081
Copyright Clearance Center: 0032-0889/98/117/1071/11
© 1998 American Society of Plant Physiologists
