Cold Adaptation Strategies in Plants
An emerging role of epigenetics and antifreeze proteins to engineer cold resilient plants
Cold stress adversely affects plant growth, development, and yield. Also, the spatial and geographical distribution of plant species is influenced by low temperatures. Cold stress includes chilling and/or freezing temperatures, which trigger entirely different plant responses. Freezing tolerance is acquired via the cold acclimation process, which involves prior exposure to non-lethal low temperatures followed by profound alterations in cell membrane rigidity, transcriptome, compatible solutes, pigments and cold-responsive proteins such as antifreeze proteins. Moreover, epigenetic mechanisms such as DNA methylation, histone modifications, chromatin dynamics and small non-coding RNAs play a crucial role in cold stress adaptation. Here, we provide a recent update on cold-induced signaling and regulatory mechanisms. Emphasis is given to the role of epigenetic mechanisms and antifreeze proteins in imparting cold stress tolerance in plants. Lastly, we discuss genetic manipulation strategies to improve cold tolerance and develop cold-resistant plants.
Plants are sessile organisms constantly challenged by environmental stresses such as temperature extremes, UV radiation, salinity, drought, flooding, mineral toxicity, and pathogen attack. Among different environmental stresses, cold severely alters membrane fluidity, water and ionic balance, generates reactive oxygen species (ROS) that impair DNA, RNA, and protein stability, hamper photosynthetic efficiency and slow down biochemical reactions. These cellular and physiological changes reduce growth, development, and productivity and limit the geographical distribution of plants. Therefore, understanding the plant responses and adaptation processes is important for developing of cold resilient plants, which is critical for global food security. The last decades have witnessed tremendous efforts to understand cold adaptation mechanisms in plants.
Plants encounter two types of low-temperature regimes in their natural habitat. The temperature range between 0 and 15°C causes cold/chilling stress, while temperatures below 0°C cause freezing stress, and distinct adaptive mechanisms help plants to deal with these two cold stress types. Plants use avoidance and tolerance strategies to mitigate cold stress. Avoiding mechanism involves preventing the formation of ice crystals inside the cell and is primarily associated with structural aspects. However, cold tolerance involves acquiring tolerance to low non-freezing temperature through a process known as cold acclimation, which includes prior exposure to nonlethal low. Cold acclimation is mainly characterized by the regulation of gene expression and metabolic changes that lead to various morphological, biochemical, and physiological alterations in plants.
Cold acclimation involves plasma membrane rigidification that affects ion concentration and metabolite transport from apoplast, endomembranes, and organelles, which initiates downstream cold signaling. These signaling cascades ultimately regulate the expression of cold-responsive (COR) genes. COR genes are induced by C-repeat Binding Factors (CBFs), which are under the control of the Inducer of CBF Expression (ICE). CBFs are genes encoding transcriptional activators having important roles in plant cold adaptation. Further, the ICE-CBF-COR regulatory module is a central pathway affecting cold response in plants. Cold inducible genes regulate the synthesis of compatible solutes (soluble sugars and proline), pigments (xanthophylls and carotenoids), and cold-responsive proteins like antifreeze proteins (AFPs), late embryogenesis abundant (LEA) proteins, heat shock proteins (HSPs), cold shock proteins (CSPs), and dehydrins, which eventually impart cold tolerance.