Time of day and network reprogramming during drought induced CAM photosynthesis in Sedum album

Plants with facultative crassulacean acid metabolism (CAM) maximize performance through utilizing C3 or C4 photosynthesis under ideal conditions while temporally switching to CAM under water stress (drought). While genome-scale analyses of constitutive CAM plants suggest that time of day networks are shifted, or phased to the evening compared to C3, little is known for how the shift from C3 to CAM networks is modulated in drought induced CAM. Here we generate a draft genome for the drought-induced CAM-cycling species Sedum album. Through parallel sampling in well-watered (C3) and drought (CAM) conditions, we uncover a massive rewiring of time of day expression and a CAM and stress-specific network. The core circadian genes are expanded in S. album and under CAM induction, core clock genes either change phase or amplitude. While the core clock cis-elements are conserved in S. album, we uncover a set of novel CAM and stress specific cis-elements consistent with our finding of rewired co-expression networks. We identified shared elements between constitutive CAM and CAM-cycling species and expression patterns unique to CAM-cycling S. album. Together these results demonstrate that drought induced CAM-cycling photosynthesis evolved through the mobilization of a stress-specific, time of day network, and not solely the phasing of existing C3 networks. These results will inform efforts to engineer water use efficiency into crop plants for growth on marginal land.