Minisymposium 8: Photosynthesis: Carbon

Abs # 18004: Isolation and analysis of CAM-defective mutants in the common ice plant, Mesembryanthemum crystallinum.

Presenter:	Cushman, John C, jcushman@unr.edu
Authors	Cushman, John C (A)   Branco, Josh B (A)   Agarie, Sakae  (B)   Elliott, Stewart  (C)   Borland, Anne  M (C)
Affiliations:	(A): Department of Biochemistry, University of Nevada (B): Faculty of Agriculture, Saga University (C): School of Biology, University of Newcastle

Crassulacean acid metabolism (CAM), an important adaptation of photosynthetic carbon fixation to limited water or CO₂ availability, is present in approximately 7% of vascular plant species (1). By conducting the bulk of atmospheric CO₂ uptake and assimilation using phosphoenolpyruvate carboxylase at night when evapotranspiration rates are low, water use efficiency is improved giving CAM plants a competitive edge in environments with transient or long-term water deficits. However, the mechanisms that regulate this alternative photosynthetic carbon fixation pathway are poorly understood. In contrast to C₃ and C₄ plants for which well-studied genetic models are available, no genetic model for CAM plants currently exists. To overcome this deficiency, we have established large mutant collections using fast neutron mutagenesis in the common ice plant, Mesembryanthemum crystallinum, as well as a facile screening method for the isolation of CAM-defective mutants. To date, two CAM-defective mutants have been identified and characterized that fail to conduct significant nocturnal CO₂ fixation or reciprocal diel fluctuations in C₄ acids and starch. The mutants display increased daytime accumulation of soluble sugars (e.g., glucose and fructose). The mutants also show reduced seed set and seed weight and display C₃-like carbon isotope ratios in their seeds. The mutants confirm that a failure to maintain a transitory starch pool within leaves has a direct impact upon the capacity for nocturnal PEP synthesis, CO₂ fixation, and C₄ acid production during Phase I of CAM (2). The availability of CAM-defective mutants should provide novel insights into the complex interplay between metabolites and the circadian clock that regulates CAM. Efforts are underway to characterize the molecular genetic defects of these mutants using representational difference and oligonucleotide-based microarray analyses. This work is supported by NSF (IBN-0196070). (1) Cushman JC (2001) Crassulacean Acid metabolism (CAM): A plastic photosynthetic adaptation to arid environments. Plant Physiol. 127: 1439-1448. (2) Dodd AN, Griffiths H, Taybi T, Cushman JC, Borland AM (2003) Integrating diel starch metabolism with the circadian and environmental regulation of Crassulacean acid metabolism in Mesembryanthemum crystallinum. Planta 216: 789-797.