Molecular mechanism of Plant Freezing Tolerance
Authors: Rakesh Kumar Prajapat1, Ashish Marathe2, Deepak Pawar1
1 Ph.D scholar, NRCPB, IARI, New Delhi-110012
2 Ph.D scholar, Division of Biochemistry, IARI, New Delhi-110012

Plants have different potential to survive under cold environment. At one extreme are plants from tropical regions that have virtually no capacity to survive even the slightest freeze. In contrast, herbaceous plants from temperate regions generally survive freezing temperatures ranging from -5°C to -30°C, depending on the species, whereas perennials in the boreal forests routinely survive winter temperatures below -30°C. Significantly, the maximum freezing tolerance of plants is not "constitutive" but is induced in response to low, nonfreezing temperatures (below approximately 10°C), a phenomenon known as "cold acclimation." Wheat plants grown at normal warm temperatures, for instance, are killed by freezing at about 25°C, but after cold acclimation, they can survive freezing temperatures as low as -20°C.

The molecular basis of cold acclimation and freezing tolerance in plants, mainly in Arabidopsis and winter cereals, has been extensively studied. To adapt to cold stress during cold acclimation, gene expression is reprogrammed and the metabolism is also modified. Cold response is a very complex trait involving many different metabolic pathways, gene regulations and cell compartments. Plants may sense low temperature through membrane and immediately alteration in membrane fluidity occurs during cold stress.

Plasma membrane rigidification raised by a membrane rigidifier eg: dimethyl sulfoxide (DMSO), can induce the expression of COR (cold-responsive) genes, at normal temperatures, whereas the application of a membrane fluidizer eg: benzyl alcohol, prevents the induction of COR gene expression at low temperatures.

Causes Of Freezing Injury:

As temperatures decrease below 0°C, ice typically forms in the intercellular spaces of plant tissues because the intercellular fluid generally has a higher freezing point than the intracellular fluid and presence of ice nucleating agents. The accumulation of ice in the intercellular spaces can potentially result in the physical disruption of cells and tissues caused in part by the formation of adhesions between the intercellular ice and the cell walls and membranes. However, most of the injury results from the severe cellular dehydration that occurs with freezing. At a given subzero temperature, the chemical potential of ice is less than that of liquid water. Thus, when ice forms intercellularly, there is a decrease in water potential outside the cell. Consequently, there is movement of unfrozen water down the chemical potential gradient from inside the cell to the intercellular spaces. Freeze-induced dehydration could have a number of effects that result in cellular damage, such as the denaturation of proteins and precipitation of various molecules.

Cold-Responsive Gene Regulation

The expression of COR genes is critical in plants for both chilling tolerance and cold acclimation. COR78/RD29A, COR47, COR15A and COR6.6 in Arabidopsis and other plants encode dehydrins, which is known as group 2 LEA (LEA II) proteins and are induced by cold stress. LEA proteins are thought to be important for membrane stabilization and prevent protein aggregation. The cold-inducible dehydrins ERD10 (early response to dehydration10) and ERD14 function as chaperones and interact with phospholipid vesicles through electrostatic forces. In addition, HSP (heat shock protein) expression is also induced by cold in plants. These HSPs function in membrane protection, in the refolding of denatured proteins and in preventing protein aggregation. Some PR (pathogen-related) proteins, such as PR1, PR2 (Beta-1,3-glucanase) and PR5 (thaumatin-like proteins), are induced by cold treatment in Arabidopsis. PR10 (Bet v-1 homologues), PR11 (chitinases) and PR14 (lipid transfer proteins) are also cold-inducible in several species. The antifreeze activity of Beta-1,3-glucanase, chitinases and thaumatin-like proteins inhibits the recrystallization of intercellular ice in the apoplastic space and prevents intracellular ice formation, as cell dehydration is promoted by extracellular freezing. In addition to these proteins, many enzymes are involved in the cold response machinery, such as detoxification and antioxidant cascades, photosynthesis, lignin metabolism, secondary metabolism, cell wall polysaccharide remodeling, starch metabolism, sterol biosynthesis and oligosaccharide synthesis.

Post-Transcriptional Regulation

Post-transcriptional mechanisms based on alternative splicing, pre-mRNA processing, RNA stability, RNA silencing and export from the nucleus play critical roles in cold acclimation and cold tolerance. Pre-mRNA processing and export are important processes for the regulation of gene expression in eukaryotes. Plants regulate the stress-dependent export of mRNA from the nucleus and the selective translation of stress-associated genes and increase the stability of related transcripts. Because, at low temperatures, misfolded RNA molecules become over-stabilized, RNA binding proteins function as RNA chaperones that help RNA achieve their native conformation. Glycine-rich protein GRP7 plays a role in the export of mRNA from the nucleus to the cytoplasm under cold stress conditions. The RNA helicase LOS4 (low expression of osmotically responsive gene4) is important for nuclear mRNA export, particularly in response to temperature stress. mRNA export is inhibited by the los4-1 mutation, leading to the reduced expression of CBF and sensitivity to chilling stress. A null mutation in the AtNUP160 gene, which encodes a nucleoporin protein involved in mRNA export, causes the decreased induction of CBFs and some CBF targets in response to cold. These results suggest that mRNA export plays an important role in the regulation of CBF expression.

Post-Translational Regulation

The ubiquitylation of a protein leads to its degradation by the 26S proteasome. The ubiquitin-proteasome pathway plays important roles in many biological functions, including abiotic stress responses. Arabidopsis HOS1 (high expression of osmotically responsive gene1) is an ubiquitin E3 ligase that exerts a negative control on cold response and degrades ICE1. The hos1 mutant exhibits upregulation of CBF/DREB1s and several cold-regulated genes with cold treatment. HOS1 is shuttled from the cytoplasm to the nucleus during cold acclimation for the poly-ubiquitylation of ICE1. The substitution of serine 403 of ICE1 to alanine promotes the stabilization of ICE1. The ubiquitylation of ICE1(S403A) is inhibited, and the overexpression of ICE1(S403A) enhances cold tolerance more than the overexpression of wild-type ICE1.

Cold Response and Plant Hormones

Low temperature affects several aspects of plant adaptation, e.g., freezing tolerance, plant growth, abiotic resistance and senescence. Among phytohormones, ABA, auxin, gibberellic acid (GA), salicylic acid (SA) and ethylene are related to the cold responses positively or negatively

About Author / Additional Info:
I am a Ph. D. Research Scholar at IARI, New Delhi