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

Abstract

Carbonic anhydrase (CA) catalyzes the first biochemical step of the carbon-concentrating mechanism of C₄ plants, and in C₄ 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 (pCO₂). However, growth and rates of photosynthesis under subambient pCO₂ are significantly impaired in the mutants. These findings suggest that, while CA is not limiting for C₄ photosynthesis in maize at current pCO₂, 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-pCO₂ conditions under which C₄ 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.