Closely related facultative and constitutive CAM phenotypes activate different transcriptional responses upon water withholding in the subgenus Tillandsia

Crassulacean acid metabolism (CAM) is a water-efficient photosynthetic strategy involving a coordinated suite of complex traits including metabolic, anatomical and regulatory aspects that shift across the diel cycle. While CAM has evolved repeatedly in land plants, the evolutionary routes enabling this convergence remain elusive. Whereas the same core CAM (de)carboxylation genes are consistently involved, a key question is whether distinct CAM phenotypes also depend on a shared set of auxiliary genes, reflecting a quantitative continuum of expression, or whether they can instead emerge through divergent or redundant peripheral solutions. The bromeliad subgenus Tillandsia, with diverse photosynthetic strategies, offers an ideal system to explore this question. Using physiological and transcriptomic analyses of well-watered and water-limited accessions of two closely related species, we characterized facultative and constitutive CAM. By comparing orthologous gene expression and orthogroup recruitment, we found that while both species performed CAM upon water-withholding, transcriptional shifts in pathways related to stomatal movement, sugar/malate transport, aquaporins and starch metabolism showed minimal overlap. Core enzymes involved in the CAM (de)carboxylation cycle exhibited broadly shared expression patterns, yet the facultative CAM species uniquely up-regulated PPC2 at night instead of the canonical CAM-related PEPC ortholog PPC1. Our study reveals that, while the expression of certain core CAM enzymes is conserved, the surrounding transcriptional architecture can differ substantially even between closely related species. This supports a model in which CAM evolves through a mosaic recruitment of functionally equivalent, yet non-orthologous genes - underscoring its flexible and modular genetic architecture. These insights advance our understanding of the mechanisms enabling the repeated evolution of CAM and its capacity to facilitate adaptive diversification.