Arabidopsis stromal carbonic anhydrases exhibit non-overlapping roles in photosynthetic efficiency and development.

ABSTRACT

Carbonic anhydrases (CAs) are ubiquitous enzymes that accelerate the reversible conversion of CO2 to HCO3 - . The Arabidopsis genome encodes members of the α-, ß- and γ-CA families, and it has been hypothesized that ßCA activity has a role in photosynthesis. In this work, we tested this hypothesis by characterizing the two plastidial ßCAs, ßCA1 and ßCA5, in physiological conditions of growth. We conclusively established that both proteins are localized in the chloroplast stroma and that the loss of ßCA5 induced the expression of ßCA1, supporting the existence of regulatory mechanisms to control the expression of stromal ßCAs. We also established that ßCA1 and ßCA5 have markedly different enzymatic kinetics and physiological relevance. Specifically, we found that ßCA5 had a first-order rate constant ~10-fold lower than ßCA1, and that the loss of ßCA5 is detrimental to growth and could be rescued by high CO2 . Furthermore, we established that, while a ßCA1 mutation showed near wild-type growth and no significant impact on photosynthetic efficiency, the loss of ßCA5 markedly disrupted photosynthetic efficiency and light-harvesting capacity at ambient CO2 . Therefore, we conclude that in physiological autotrophic growth, the loss of the more highly expressed ßCA1 does not compensate for the loss of a less active ßCA5, which in turn is involved in growth and photosynthesis at ambient CO2 levels. These results lend support to the hypothesis that, in Arabidopsis,ßCAs have non-overlapping roles in photosynthesis and identify a critical activity of stromal ßCA5 and a dispensable role for ßCA1.