A high-quality genome of the Atacama Desert plant Cistanthe cachinalensis and its photosynthetic behavior related to drought and life history.

Crassulacean acid metabolism (CAM) photosynthesis has independently evolved many times in arid-adapted plant lineages. Cistanthe cachinalensis (Montiaceae), a desert annual, can upregulate CAM facultatively upon stress such as drought. Few studies, however, consider life history stages when measuring CAM activity or its facultative onset. To test the effect of drought and flowering on photosynthetic activity, we assayed Cistanthe individuals in fully watered and drought conditions, as well as fully watered individuals at pre-flowering and flowering life stages. We assembled and annotated a chromosome-scale genome of C. cachinalensis and compared it with the genome of Portulaca amilis and analyzed differential gene expression. Results show significantly upregulated CAM in drought conditions as compared to fully watered conditions; furthermore, flowering individuals showed slightly higher CAM activity as compared to pre-flowering plants, even when fully watered. Differential gene expression analyses provide preliminary support for the possible coregulation of CAM expression and reproduction. We emphasize the potentially missed significance of life history in the CAM literature and consider how the CAM biochemical module could become co-opted into other plant behaviors and responses, such as the shift to reproduction or flowering in annuals.