Shared expression of crassulacean acid metabolism (CAM) genes pre-dates the origin of CAM in the genus Yucca

Crassulacean acid metabolism (CAM) is a carbon-concentrating mechanism that has evolved numerous times across flowering plants and is thought to be an adaptation to water-limited environments. CAM has been investigated from physiological and biochemical perspectives, but little is known about how plants evolve from C(3) to CAM at the genetic or metabolic level. Here we take a comparative approach in analyzing time-course data of C(3), CAM, and C(3)+CAM intermediate Yucca (Asparagaceae) species. RNA samples were collected over a 24 h period from both well-watered and drought-stressed plants, and were clustered based on time-dependent expression patterns. Metabolomic data reveal differences in carbohydrate metabolism and antioxidant response between the CAM and C(3) species, suggesting that changes to metabolic pathways are important for CAM evolution and function. However, all three species share expression profiles of canonical CAM pathway genes, regardless of photosynthetic pathway. Despite differences in transcript and metabolite profiles between the C(3) and CAM species, shared time-structured expression of CAM genes in both CAM and C(3)Yucca species suggests that ancestral expression patterns required for CAM may have pre-dated its origin in Yucca.