Functional Genomics of Crassulacean Acid Metabolism

To ensure their survival in arid environments, approximately 6% of all vascular plant species in 33 families and 328 genera engage in a water-conserving photosynthetic pathway known as Crassulacean acid metabolism (CAM), making it the second most common form of photosynthesis. CAM plants conduct the bulk of atmospheric CO2 uptake at night, achieving water use efficiencies that are three- to six-fold higher than C4 and C3 plants. This ability depends on several important mechanisms including circadian clock orchestration of a complex set of genes, intraorganellar transport of large, reciprocal diel fluxes in C4 acids and storage carbohydrates, and metabolic control sustaining the temporal separation of carboxylation processes during the diel cycle. Despite several decades of intensive work on the biochemical sequence of metabolic reactions that underpin the CAM cycle, very little is known about the molecular mechanisms responsible for circadian regulation, transport processes, and metabolic feedback controls essential to the performance of CAM are relatively uncharitable. The long-term goal of our research is to develop integrated functional genomic tools to identify and functionally test key regulatory components that control the unique metabolic demands of CAM. Towards this goal, preliminary comparative analyses of mRNA, protein and metabolite expression patterns over a diel time course in the common ice plant, Mesembryanthemum crystallinum, a model in which CAM is inducible by abiotic stress, have identified pronounced differences between C3 and CAM modes of photosynthesis. These findings represent a unique framework from which to test key hypotheses regarding the intricate regulatory controls required to conduct this important photosynthetic adaptation to water-limitation.

The primary objectives of our research are to: 1) provide a comprehensive description of mRNA expression profiles over a diel time course in plants performing C3 photosynthesis and following induction of CAM using large-scale EST sequencing and oligonucleotide microarray-based expression monitoring, 2) identify changes in protein expression profiles in leaves and specifically identify low abundance proteins that play critical roles in CAM using state-of-the-art proteomics methodologies, 3) conduct comparative metabolite profiling in ice plant leaves to identify differences between C3 and CAM modes of photosynthesis, 4) identify the vacuolar malate channel responsible for nocturnal vacuolar acidification and investigate the mechanisms by which the influx and efflux of C4 acids through acidic vacuoles are regulated and 5) provide “hands-on” training opportunities in various aspects of integrative functional genomics CAM research between UNR and Western Oregon University, increase public awareness of CAM plants through an educational module and experimental kit, and a public display about CAM plants.