A Limited Role for Carbonic Anhydrase in C4 Photosynthesis as Revealed by a ca1ca2 Double Mutant in Maize

Carbonic anhydrase (CA) catalyzes the first biochemical step of the carbon-concentrating mechanism of C-4 plants, and in C-4 monocots it has been suggested that CA activity is near limiting for photosynthesis. Here, we test this hypothesis through the characterization of transposon-induced mutant alleles of Ca1 and Ca2 in maize (Zea mays). These two isoforms account for more than 85% of the CA transcript pool. A significant change in isotopic discrimination is observed in mutant plants, which have as little as 3% of wild-type CA activity, but surprisingly, photosynthesis is not reduced under current or elevated CO2 partial pressure (pCO(2)). However, growth and rates of photosynthesis under subambient pCO(2) are significantly impaired in the mutants. These findings suggest that, while CA is not limiting for C-4 photosynthesis in maize at current pCO(2), it likely maintains high rates of photosynthesis when CO2 availability is reduced. Current atmospheric CO2 levels now exceed 400 ppm (approximately 40.53 Pa) and contrast with the low-pCO(2) conditions under which C-4 plants expanded their range approximately 10 million years ago, when the global atmospheric CO2 was below 300 ppm (approximately 30.4 Pa). Thus, as CO2 levels continue to rise, selective pressures for high levels of CA may be limited to arid climates where stomatal closure reduces CO2 availability to the leaf.