Molecular Mechanisms of Freeze-Thaw Injury and Cold Acclimation in Herbaceous Plants: Merging Physiological and Genetic Approaches

Abstract

In nature several factors including ice nucleation, temperature, freezing (cooling) rate, duration of exposure to ice, thawing rate and post-thaw conditions contribute to the degree of injury caused by frost episodes. Our results show that an increase in cooling rate from 1 to 4 °C h-1 made the difference between survival and death. We have also found that in the critical temperature range where injury occurs, the thaw rate influenced the degree and type of injury. The plasma membrane is a key site of alteration by freeze-thaw stress and cold acclimation. Important properties in this respect include membrane lipids and proteins (plasma membrane ATPase, kinase, desaturase), and the concentration of membrane and cytosolic calcium. Plasma membrane ATPase appears to be an important site of cellular response to freeze-thaw stress and an alteration in the function of this enzyme is one of the earliest manifestations of stress. Our results provide evidence that these alterations could be mediated by perturbation of cellular Ca2+ and/or changes in membrane lipid composition. These results provide an insight into the mechanisms of incipient injury and recovery following injury. To understand the genetics of freezing stress resistance, we performed an interspecific hybridisation of two diploid potato species that vary in freezing tolerance and cold acclimation ability. The species were Solanum commersonii, which is freezing tolerant and able to cold-acclimate (double its freezing tolerance in 10 days at chilling temperatures) and Solanum cardiophyllum, which is freezing sensitive and unable to cold-acclimate. Analysis of the backcross progenies shows that non-acclimated freezing tolerance and acclimation ability are genetically distinct traits that segregate independently. Generation mean analysis revealed that cold-acclimation ability can be explained by a simple additive- dominance model. Our results indicate that the ability to cold-acclimate is genetically relatively simple and should be amenable to selection at the diploid level. We have recently performed lipid analysis of purified plasma membrane preparations obtained from the parents, F1 and the backcross progenies. Analysis shows that the relative increase in linoleic acid (18:2) in the plasma membrane is highly correlated to the cold acclimation ability. An increase in 18:2 co-segregated with the capacity to acclimate. Our results suggest that specific membrane lipids play a role in the genetic ability of the plant material to cold-acclimate.