Elucidation of the second-phase freezing acclimation mechanism: plants accumulate sugars via photosynthesis under frozen conditions

Overwintering plants increase their tolerance to freezing in two phases: the first-phase, known as cold acclimation (CA), occurs at chilling temperatures, followed by second-phase acclimation at sub-zero temperatures. We previously showed that second-phase freezing acclimation (2PFA), in which cold-acclimated Arabidopsis is exposed to -2{degrees}C with extracellular ice formation under light, confers the highest freezing tolerance (LT50 below -20{degrees}C) through a pathway requiring photosynthetic electron transport, possibly independent of the CBF-COR pathway. However, how light enhances freezing tolerance in frozen plants remains unclear. Here, we developed a system to measure chlorophyll fluorescence in intact plants maintained in a frozen state, and found that PSII retained substantial photochemical capacity (Fv/Fm) in frozen leaves. Comparative transcriptomics revealed that 2PFA under light induced genes involved in starch, sucrose and galactose metabolism. Consistently, soluble sugars, mainly sucrose, glucose and fructose, accumulated to levels approximately 8-fold higher than those in CA plants, and total sugar content was strongly correlated with freezing tolerance. This accumulation was suppressed by the photosynthetic electron transport inhibitor DCMU. These results suggest that photosynthetic activity is maintained in frozen plants and supplies sugars that may contribute to the freezing tolerance acquired during second-phase acclimation, beyond the level achieved by first-phase CA, unveiling a previously unrecognized mechanism in the winter survival strategy of plants.