Tuesday, October 29, 2013

Genetic basis of low-temperature stress tolerance



In spite of various adaptations to cold, plants may be injured through exposure to cold temperatures in a variety of ways, depending on the temperature range.
 One type of injury, called chilling injury, occurs at exposures to temperature of between 20 and 0 _C. Some injuries are irreversible. Common chilling injuries
include interruption of normal germination, flowering,and fruit development, which eventually adversely impact crop yield. Stored products may also suffer chilling injury. A more severe low-temperature injury is freezing injury, which occurs when temperatures drop below the freezing point of water. Sometimes, ice crystals form in the protoplasm of cells,resulting in cell death and possibly plant death.
Plants may be classified into three groups according to the tolerance to low temperatures. Frost tender plants are intolerant of ice in their tissues, and are hence sensitive to chilling injury. The plant can be killed when temperatures fall just below 0 _C. Frost resistant plants can tolerate some ice in their cells and can survive cold temperatures of up to _40 _C. Cold-hardy plants are predominantly temperate woody species. Most crops that originate in the tropics and subtropics are sensitive to chilling temperatures. However,some temperate fruits are also susceptible to chilling injury. The temperature at which chilling injury starts varies among species and depends on where they originate. Temperate fruits exhibit chilling injury starting at 0–4 _C, whereas the starting temperature is 8 _C for subtropical fruits and 12 _C for tropical fruits. Grains such as corn and rice suffer chilling injury at temperatures below 10 _C. When chilling temperatures occur at the seedling stage, susceptible
crops suffer stand loss. Also, crop maturity is delayed while yield is reduced.

The capacity of a genotype to tolerate low temperature has been extensively studied. It is agreed that low-temperature tolerance is a complex trait that may be influenced by several mechanisms. Reports indicate recessive, additive, partial dominance,and overdominance as the modes of action that occur in nature for cold stress. The diversity in the results is partly blamed on the way research is often conducted. Some workers use controlled-freeze tests while others use field tests. Furthermore, various reports indicate a role of cytoplasmic factors and nonadditive
gene effects, even though such effects are generally believed to be minor. Genes that condition varying levels of low-temperature tolerance occur within and among species. This genetic variability has been exploited to a degree in cultivar development within production regions.
A large amount of low-temperature tolerance research has been conducted in wheat. Lowtemperature tolerance in cereals depends on a highly integrated system of structural, regulatory, and developmental genes. Several vernalization genes have
been identified.The vrn1 is homeoallelic to the locus Sh2 in barley and Sp1 in rye. These two genes have been linked to genetic differences in low-temperature tolerance. Winter cereals also produce several proteins in response to low-temperature stress, for example, the dehydrin families of genes.

Mechanisms of resistance to low temperature

Like drought resistance, certain physiological or morphological adaptations can make plants either avoid or tolerate stress due to low temperatures. Plants are described as cold-hardy when they have the capacity to withstand freezing temperatures. On the other hand, winter-hardy species are able to avoid or tolerate a variety of weather-related effects associated with winter.The mechanisms of low-temperature resistance may be grouped into two.
 Chilling resistance. The factors that confer resistance to chilling are believed to operate at the cell membrane level where they influence membrane fluidity. Chilling resistant seeds are known to imbibe moisture slowly. The presence of phenols in the seed coat of legumes is implicated in conferring chilling resistance.
 Freezing resistance. Several mechanisms are used by plants to resist freezing, including:
_ Escape. Like drought, cultural practices may be adopted by producers to prevent the vulnerable stage of growth coinciding with the presence of the stress factor.
_ Avoidance. One of the injuries of low temperature results from the intracellular formation of ice following nucleation of ice in the tissue. Water may remain super cooled without forming ice crystals. Certain compounds that are capable of promoting ice nucleation are active at low temperatures. Bacteria such
as Pseudomonas syringae are capable of producing ice nucleating proteins. The first field test of a bioengineered organism was the testing of “ice minus”, a microbe genetically engineered to be incapable of producing the bacterial protein that causes ice nucleation. This was intended to be an approach to help frost-sensitive plants survive frost.

_ Tolerance. Freezing tolerance occurs when a plant is able to withstand both intracellular and extracellular ice formation. A group of genes called the cold-response genes is believed to play a significant role in freezing tolerance. These genes are activated when the plant is exposed for a period to low but non-freezing temperatures conferring hardening on the plant. It is the hardening that makes plants survive freezing temperatures.

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