1. Mechanisms of Freezing Adaptation in Plants
Plants continuously adapt to environmental fluctuations, among which low temperatures represent a major stress. Under subzero conditions, extracellular ice formation draws water out of the cells, leading to cellular dehydration, and the physical expansion of growing ice crystals causes mechanical damage to membrane structures. To avoid such freezing stress, many plants acquire freezing tolerance through a process known as cold acclimation.
While the accumulation of soluble sugars and changes in membrane composition have long been known to mitigate dehydration damage, our research has demonstrated that structural modifications of the cell wall also play a critical role in freezing tolerance. In addition to wall-associated polysaccharides, the accumulation and breakdown of storage polysaccharides such as starch and fructans contribute to osmotic adjustment and the maintenance of cellular structural stability. These polysaccharides, therefore, are not merely passive components but are central to the complex defense mechanisms against freezing and dehydration stress.
Furthermore, recent findings indicate that cold acclimation not only alters the biochemical composition of cells and tissues, but also induces morphological changes. These changes are closely associated with shifts in the mechanical properties of cells and are likely to contribute to enhanced tolerance to freezing and dehydration. We are currently investigating the biological significance and molecular mechanisms of these morphological changes, aiming to uncover an integrated understanding of plant strategies for low-temperature adaptation.
Cold exposure causes a wrinkled leaf surface in Komatsuna.
2. Mechanisms of Deacclimation in Plants
To adapt to cold environments, plants halt their growth and enhance their freezing tolerance through cold acclimation. However, when temperatures rise, they lose this tolerance and resume growth. This process, known as deacclimation, is a crucial mechanism that enables plants to adjust to seasonal and diurnal temperature fluctuations. It suggests that plants regulate a balance between tolerance and growth to optimize their adaptation to environmental changes.
We analyzed changes in sugars, particularly soluble sugars and cell wall components, during both cold acclimation and deacclimation. Our results showed that cold acclimation leads to an increase in soluble sugars and specific cell wall components, while deacclimation causes a decline in soluble sugars, with certain cell wall components remaining unchanged. Moreover, when deacclimated plants were subjected to cold acclimation again, they exhibited higher freezing tolerance than during the initial acclimation. These findings suggest that deacclimation is not merely the reverse of acclimation but rather an adaptive strategy that prepares plants for future freezing stress.
By further elucidating how these mechanisms influence freezing tolerance and plant growth, we aim to gain insights into adaptive strategies that help plants survive in fluctuating environments.
3. Secrets of Plant Desiccation Tolerance
Terrestrial plants are vulnerable to desiccation, which can lead to membrane damage, cell wall deformation, and ultimately cell death. However, resurrection plants can maintain their cellular structure even in extreme desiccation and rapidly resume metabolic activity upon rehydration.
One well-known factor contributing to desiccation tolerance is the accumulation of trehalose, which protects intracellular components. However, trehalose alone does not fully explain this phenomenon, suggesting the involvement of additional mechanisms. Selaginella tamariscina, a representative resurrection plant, can maintain its cells intact under extreme desiccation. In contrast, S. moellendorffii, a closely related species that accumulates trehalose, does not exhibit the same level of tolerance. This difference is likely influenced by variations in membrane composition, protective proteins, and other factors.
Our laboratory aims to elucidate the desiccation tolerance mechanisms of resurrection plants from a cell wall perspective by comparing S. tamariscina and S. moellendorffii. Understanding the unique strategies of resurrection plants will provide new insights into plant survival strategies from an evolutionary perspective.
S. tamariscina rapidly recovers upon rehydration, even after extreme desiccation.