Carbon metabolism and regulation in C3 plants: Isolation and characterization of high carbon dioxide responsive mutants of Arabidopsis thaliana

Given that atmospheric CO2 levels are expected to double during the 21st century and that crop and forest species account for two thirds of global photosynthesis, knowledge of mechanisms governing the acclimation response to high CO2 in plants is essential. The objective of this study was to better understand the mechanism of regulation affecting the response of higher plants to the availability of inorganic carbon. As genetic screens have proved useful in the understanding of physiology and signal transduction in several pathways, a genetic approach using the crucifer Arabidopsis thaliana was taken. Initial characterization of conditions that elicited a response from wildtype Arabidopsis plants was performed prior to screening mutagenized populations. Wildtype Arabidopsis plants exhibit a stress response when exposed to elevated CO2 (0.1--0.3%). Aspects of this stress response include anthocyanin accumulation, curling of leaves, necrosis, hyper-accumulation of foliar carbohydrates and down-regulation of photosynthetic gene expression. Characterization of wildtype facilitated the isolation of mutants which showed a response differing from that of wildtype plants when exposed to elevated CO2. Mutants that resulted from this screen were placed in two general categories; CO2 non-responsive (cnr) and CO2 hyper-responsive (chr). Initial characterization of representative mutants from the different groups was carried out. The largest group of mutants cnr was chosen for further study. These cnr mutants share varying degrees of a single combination of phenotypes which include reduced anthocyanin production, reduced or no curling of leaves, delayed senescence, and normal to vigorous growth under elevated CO2. Two non-allelic T-DNA tagged mutants, cnr 1-1 and cnr 2-1 were selected for genetic, molecular, biochemical and physiological analyses. cnr 1-1 is a dominant mutation and the gene affected by the T-DNA insert in this mutant is a leucine rich repeat receptor kinase. The CO2 insensitive and glucose insensitive phenotype of cnr 1-1 suggest a role for CNR1 in carbon metabolism and the molecular identity of CNR1 suggests a role in signal transduction. cnr 2-1 is a recessive mutation and the T-DNA insert disrupts a P450 monoxygenase gene. Physiological and biochemical analyses suggest that the P450 monoxygenase is involved in the abscisic acid pathway.