Learning from facultative CAM plant Mesembryanthemum Crystallinum: Molecular mechanisms for climate-smart agriculture.

Global population growth and frequent episodes of drought pose significant challenges to agricultural sustainability, necessitating the development of crops with enhanced stress adaptability and water-use efficiency (WUE). Crassulacean acid metabolism (CAM) is a specialized photosynthetic pathway that improves plant drought tolerance and WUE by diurnal stomatal closure and nocturnal CO2 assimilation. The facultative CAM species Mesembryanthemum crystallinum (common ice plant) has become a key model for studying the reversible transition from C3 photosynthesis to CAM under salt or drought stress. The C3-to-CAM transition is a highly plastic and coordinated process. It is governed by the integration of environmental cues, circadian regulation, hormonal signaling, and metabolic reprogramming, resulting in extensive remodeling of carbon metabolism, vacuolar transport, and cellular energetics. In this review, we synthesize current understanding of the C3-to-CAM transition in M. crystallinum, with an emphasis on regulatory mechanisms and recent omics-driven insights. We also highlight emerging research directions, including phosphoproteomics, single-cell approaches, and functional genomics, which are essential for advancing CAM biology and equipping C3 crops with CAM plasticity to improve drought resilience and recovery.