Protective Strategies of Haberlea rhodopensis for Acquisition of Freezing Tolerance: Interaction between Dehydration and Low Temperature

Resurrection plants are able to deal with complete dehydration of their leaves and thenrecover normal metabolic activity after rehydration. Only a few resurrection species are exposed tofreezing temperatures in their natural environments, making them interesting models to study thekey metabolic adjustments of freezing tolerances. Here, we investigate the effect of cold and freezingtemperatures on physiological and biochemical changes in the leaves of Haberlea rhodopensis undernatural and controlled environmental conditions. Our data shows that leaf water content affects itsthermodynamical properties during vitrification under low temperatures. The changes in membranelipid composition, accumulation of sugars, and synthesis of stress-induced proteins were significantlyactivated during the adaptation of H. rhodopensis to both cold and freezing temperatures. In particular,the freezing tolerance of H. rhodopensis relies on a sucrose/hexoses ratio in favor of hexoses duringcold acclimation, while there is a shift in favor of sucrose upon exposure to freezing temperatures,especially evident when leaf desiccation is relevant. This pattern was paralleled by an elevatedratio of unsaturated/saturated fatty acids and significant quantitative and compositional changes instress-induced proteins, namely dehydrins and early light-induced proteins (ELIPs). Taken together,our data indicate that common responses of H. rhodopensis plants to low temperature and desiccationinvolve the accumulation of sugars and upregulation of dehydrins/ELIP protein expression. Furtherstudies on the molecular mechanisms underlying freezing tolerance (genes and genetic regulatorymechanisms) may help breeders to improve the resistance of crop plants.