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.