Mechanisms of the Evolutionary Origins of Crassulacean Acid Metabolism in Tropical Orchids

The water-conserving photosynthetic pathway known as Crassulacean acid metabolism (CAM) has evolved multiple times in 33 families and 328 genera comprising more than 6% of all vascular plant species making it the second most common mode of photosynthesis among vascular plants.Although the basic metabolic reactions required for CAM are well known, the molecular mechanisms responsible for the evolution of this important photosynthetic adaptation to water-limitation are completely uncharacterized. Current estimates indicate that approximately 50% of the 20,000 species within the Orchidaceae, the largest family of vascular plants, exhibit CAM photosynthesis. However, this estimate is based on surveys of less than approximately 3% of all species. Foliar carbon isotopic composition (d13C) measurements and nocturnal acid accumulation measurements within this family show that CAM exhibits a bimodal distribution with the majority of species having d13C values around –28 ‰ diagnostic of C3 photosynthesis and weak CAM, and -15‰ diagnostic of strong CAM. The long-term goal of our research is to test the hypothesis that at least three major changes have occurred during evolution to adapt the CAM progenitor genes for functions in CAM: 1) CAM isoform genes have evolved highly expressed mRNA expression patterns in plants performing CAM, 2) CAM isoform genes have evolved leaf-specific or leaf-preferential expression patterns in plants performing CAM, and 3) CAM isoform genes have evolved expression patterns that are under circadian clock control. We will test these hypotheses by mapping the occurrence of CAM within Oncidiinae, a subtribe within Orchidaceae.

The primary objectives of our research are 1) to map the occurrence of both weak and strong CAM species upon a well-established phylogeny within the Oncidiinae, a subtribe within Orchidaceae. by conducting simultaneous molecular phylogenetic analysis using DNA sequence data, foliar carbon isotopic composition, and nocturnal acid accumulation measurements; 2) to develop novel molecular markers to trace the evolutionary progression of CAM and correlate these data with traditional, quantitative diagnostic indicators of CAM and existing molecular phylogenies based on rapidly evolving plastid loci; 3) to reveal the full spectrum of gene expression changes that are associated with the C3 to CAM evolutionary progression using oligonucleotide microarrays in closely related species that perform C3 photosynthesis, weak CAM and strong CAM; and 4) to elucidate the cis-acting elements that are diagnostic of CAM evolution. 5) provide unique “hands-on” training opportunities in integrative molecular systematic and ecophysiology at the University of Nevada (UNR) and University of Florida and develop an exhibit of tropical orchids for public outreach and integration of research and education on the importance of ecophysiological diversity of terrestrial and epiphytic orchids in the neotropics and the larger role that tropical rain forests play in global climate change.

This research is a collaborative effort among investigators at the University of Nevada, Reno, University of Florida, and the Smithsonian Tropical Research Institute (STRI).