Plant Physiol, October 2001, Vol. 127, pp. 607-614

Insertional Mutants of Chlamydomonas reinhardtii That Require Elevated CO₂ for Survival¹

Interdepartmental Plant Physiology Major (K.V., Y.W., M.H.S.) and Department of Botany (K.V., Y.W., Y.N., M.H.S.), 353 Bessey Hall, Iowa State University, Ames, Iowa 50011

Aquatic photosynthetic organisms live in quite variable conditions of CO₂ availability. To survive in limiting CO₂ conditions, Chlamydomonas reinhardtii and other microalgae show adaptive changes, such as induction of a CO₂-concentrating mechanism, changes in cell organization, increased photorespiratory enzyme activity, induction of periplasmic carbonic anhydrase and specific polypeptides (mitochondrial carbonic anhydrases and putative chloroplast carrier proteins), and transient down-regulation in the synthesis of Rubisco. The signal for acclimation to limiting CO₂ in C. reinhardtii is unidentified, and it is not known how they sense a change of CO₂ level. The limiting CO₂ signals must be transduced into the changes in gene expression observed during acclimation, so mutational analyses should be helpful for investigating the signal transduction pathway for low CO₂ acclimation. Eight independently isolated mutants of C. reinhardtii that require high CO₂ for photoautotrophic growth were tested by complementation group analysis. These mutants are likely to be defective in some aspects of the acclimation to low CO₂ because they differ from wild type in their growth and in the expression patterns of five low CO₂-inducible genes (Cah1, Mca1, Mca2, Ccp1, and Ccp2). Two of the new mutants formed a single complementation group along with the previously described mutant cia-5, which appears to be defective in the signal transduction pathway for low CO₂ acclimation. The other mutations represent six additional, independent complementation groups.