Antisense RNA Inhibition of RbcS Gene Expression Reduces Rubisco Level and Photosynthesis in the C4 Plant Flaveria bidentis

The C4 dicot Flaveria bidentis was genetically transformed with an antisense RNA construct targeted to the nuclear-encoded gene for the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; RbcS). RbcS mRNA levels in leaves of transformants were reduced by as much as 80% compared to wild-type levels, and extractable enzyme activity was reduced by up to 85%. There was no significant effect of transformation with the gene construct on levels of other photosynthetic enzymes. Antisense transformants with reduced Rubisco activity exhibited 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 C3 plants, Rubisco activity is a major determinant of photosynthetic flux in C4 plants under high light intensities and air levels of CO2.