In this article we will discuss about the effect of freezing temperature tolerance on plants.
Manifestation of cold acclamation is accomplished by increased freezing tolerance in several plants. Non-acclimatized plants are injured or killed by temperature above -10°C. Interestingly, certain freezing acclimated tree species can survive freezing temperature between -40 and -50°C, for example, non-acclimated wheat and rye plants are killed at temperature between -5 and -10°C, but, after hardening wheat can survive a temperature of about -15 to -20°C, suggesting disruption of plasma membrane, a primary cause of freezing injury in plants.
The membrane damage is primarily due to severe dehydration that occurs during freeze- thaw cycle. One form of cell or membrane injury is “expansion-induced lysis”, occurs mainly at freezing temperature at about -3 to -7°C.
As temperature drops below 0°C, cellular water begins to freeze, as a result, water activity steeply declines followed by elevated solute concentration in the extracellular spaces. In response, water moves out of the cell causing severe dehydration and cell shrinkage. When extracellular ice melts, the cell rehydrates and expands. At this juncture acclimatized cells can withstand freezing injury, whereas non-acclimated cells lyse.
Generally, ice formation begins within plant tissue at —1°C particularly in extra cytoplasmic space, where concentration of solute is lowest. The low vapour pressure of the ice in the extra cytoplasmic space withdraws water from the cytoplasm causing severe desiccation and shrinking cellular membranes.
Ice formation within the cytoplasm is lethal. If the temperature begins to rise before ice penetrates the cytoplasm, these cells are then consequently injured by complete destruction of the cell membrane as they recover from shrinkage and desiccated state. This type of “expansion induced by lysis” may be the primary cause of freezing injury.
Some cold hardy plants and fishes produce specific antifreeze proteins, which provide protection from frost injury by preventing growth of the ice crystals, for example, winter flounder (Pseudopleuronectus americanus) is protected from a freezing temperature of -1 to 5°C by antifreeze proteins rich in alanine in the blood. Its synthesis is restricted to liver and is season specific. Its level is highest during winter, declines rapidly in spring.
Flounder serum contains three distinct antifreeze proteins (AFP) of 12000, 8000 and 6000 Daltons. Cloning of cDNA for antifreeze proteins and their sequence was determined to study the structure and its regulation. AFP has been identified in other teleost fishes, terrestrial arthropods and vascular and non-vascular plants, including fungi and bacteria. The type I AFP has been characterized.
Sidebottom (2000) have discovered novel antifreeze protein in perennial rye grass, Lolium perenne. This protein is stable at 100°C and performs better at preventing ice crystallization. This property of the protein enables grasses to tolerate ice formation in their tissue. This grass-heat stable protein tolerant to boiling temperature has open reading frame encoding a protein of 118 hydrophobic amino acids, rich in asparagine, valine and serine.
Antifreeze protein molecule adsorbs to the crystal lattice of ice by hydrogen bonding via threonine residue. This prevents addition of water molecule and declines the rate of ice crystal growth. AFP also has an in vitro effect of stabilizing supercoiled liquid by reducing the efficiency of heterogenous ice nucleation sites and lowering the temperature at which ice formation begins.
In transgenic technology, a synthetic antifreeze gene, constructed based on the type I AFP of winter flounder fused to signal sequence of phytohemaglutinin (PHA) and directing it to the extracytoplasmic space where ice crystallization occurs first. Transgenic potato expressing AFP gene prevents growth of ice crystal and reduces electrolyte leakage and thereby provides high degree of tolerance to freezing.
Freezing tolerance has also been mainpulated in Arabidopsis thaliana by overexpressing the gene encoding phospholipase D δ (PLD δ), which is involved in membrane lipid hydrolysis and cell signalling. Overexpression of FLDδ, in freezing sensitive A. thaliana, increased freezing tolerance because of its involvement in the production of phosphatidic acid.