ANTISENSE RNA INHIBITION OF RBCS GENE-EXPRESSION REDUCES RUBISCO LEVEL AND PHOTOSYNTHESIS IN THE C-4 PLANT FLAVERIA-BIDENTIS

The C-4 dicot Flaveria bidentis was genetically transformed with an an tisense RNA construct targeted to the nuclear-encoded gene for the sma ll subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco ; RbcS). RbcS mRNA levels in leaves of transformants were reduced by a s much as 80% compared to wild-type levels, and extractable enzyme act ivity was reduced by up to 85%. There was no significant effect of tra nsformation with the gene construct on levels of other photosynthetic enzymes. Antisense transformants with reduced Rubisco activity exhibit ed a stunted phenotype. Rates of photosynthesis were reduced in air at high light and over a range of CO2 concentrations but were unaffected at low light. From these results we conclude that, as is the case in C-3 plants, Rubisco activity is a major determinant of photosynthetic flux in C-4 plants under high light intensities and air levels of CO2.