Pipecolic acid modulates physiological parameters and biotic stress resilience in Mesembryanthemum crystallinum in a photosynthetic mode-dependent manner

Pipecolic acid (Pip) functions as a precursor of key signalling metabolites involved in systemic acquired resistance; however, its broader role in modulating plant metabolism under biotic stress remains poorly understood. Here, we investigated how Pip influences metabolic and physiological responses to bacterial challenge in Mesembryanthemum crystallinum (ice plant), a semihalophytic species capable of shifting from C₃ photosynthesis to CAM through regulation of photosynthetic enzymes and stomatal opening rhythms under stress conditions. Prolonged exogenous pipecolic acid (Pip) supplementation induced several CAM-associated traits in C₃ plants, including nocturnal malate accumulation, diurnal fluctuations in cell-sap acidity, and increased phosphoenolpyruvate carboxylase (PEPC) levels, suggesting a partial shift toward CAM-like metabolism. In both C₃ and CAM plants, Pip treatment increased RuBisCO activity while reducing the abundance of the large RuBisCO subunit (LSU), indicating differential regulation of photosynthetic carbon assimilation. Gas-exchange measurements showed that Pip-treated plants maintained higher net photosynthetic rates (P N ) and improved water-use efficiency (WUE) under bacterial stress. The effects of Pip on stomatal morphology, photosynthetic pigment content, and photosynthesis-related proteins differed between C₃ and CAM species, highlighting photosynthetic type-dependent responses. Salicylic acid (SA) accumulation following Pip treatment was detected only in C₃ plants, whereas endogenous Pip levels increased in both photosynthetic types after bacterial challenge, with a more pronounced response in CAM plants. Our findings identify pipecolic acid as metabolic regulator that integrates defence signalling with photosynthetic plasticity, thereby coordinating plant responses to environmental stress. This highlights Pip as a promising target for strategies aimed at enhancing stress resilience while maintaining efficient carbon assimilation and photosynthetic performance.