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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 CO 2 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 CO 2 availability is reduced. Current atmospheric CO 2 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 CO 2 was below 300 ppm (approximately 30.4 Pa). Thus, as CO 2 levels continue to rise, selective pressures for high levels of CA may be limited to arid climates where stomatal closure reduces CO 2 availability to the leaf.

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