Mechanism of freeze-thaw injury and recovery: A cool retrospective and warming up to new ideas

Understanding cellular mechanism(s) of freeze-thaw injury (FTI) is key to the efforts for improving plant freeze-tolerance by cultural methods or molecular/genetic approaches. However, not much work has been done in the last 25+ years to advance our understanding of the nature and cellular loci of FTI. Currently, two FTI lesions are predominantly implicated: 1) structural and functional perturbations in plasma membrane; 2) ROS-induced oxidative damage. While both have stood the test of time, many questions remain unresolved and other potentially significant lesions need to be investigated. Additionally, molecular mechanism of post-thaw recovery (PTR), a critical component of frost-survival, has not been well investigated. Mechanistic understanding of repair after reversible injury could expand the options for strategies to improve frost-hardiness. In this review, without claiming to be exhaustive, I have attempted to synthesize major discoveries from last several decades on the mechanisms of FTI and the relatively little research conducted thus far on PTR mechanisms. It is followed by proposing of hypotheses for mechanism(s) for irreversible FTI or PTR involving cytosolic calcium and ROS signaling. Perspective is presented on some unresolved questions and research on new ideas to fill the knowledge gaps and advance the field.