BIOTIC AND ABIOTIC STRESSES IN PLANTS

Cellular Changes during Cold Acclimation in Plants
Cold acclimation refers the phenomena whereby there is an increase in the freezing tolerance of some plants that resulting from exposure to low temperatures. Freezing tolerance on the other hand is a freezing injury in plant tissues that results from severe dehydration of the cells. This occurs due to ice formation in the cellular membrane which is usually the primary site of the freeze induced injury. The morphological symptom of a plant tissue after a freeze injury is a water soaked appearance. Tester and Munns (2008, p. 652), note that there are varied cellular changes that do occur in plants during cold acclimation and these include

Changes in sugar content. The levels of soluble sugars that include sucrose, glucose and fructose increase gradually on exposure to non freezing temperatures. Therefore the sugar content in plants is positively correlated to the degree of freezing tolerance. Exposure of plants to low temperatures induces freezing tolerances. The acquisition of freezing tolerance leads to an accumulation of sugars. These changes are reversed rapidly when the plants are exposed to normal temperatures. This is because when the freezing tolerance is lost through deacclimation at controlled temperatures, there is usually a large reduction in the sugar content.

Changes in enzyme activity. The thermostability of hormones increase after cold acclimation.
Ultra structure-parenchyma cells changes. The Plasma membrane seems extensively turn over and the endocytotic vesicles become budding.

Cell membrane invaginations occur. There is also modification in the cell membrane structure. The cell membrane loses compartmentalization and this leads to leakage of organic solutes and ions. Therefore the plant is unable to regains turgor after recovery. The invagination causes loss of surface area of the cell membrane and a reduction in the cell volume.

There is sequestering of membrane materials.
There is a progressive global change in metabolite profiles and in gene expression.
There is an accumulation of amino acids, an increase of antioxidants and a reduced water content.
There is a transient increase in the levels of calcium and the activity of plasma membrane calcium increases significantly. Calcium elevation usually occurs on the free cytosolic concentration of calcium. The increase in Ca2 is usually due to rapid cold shock.
There is an alteration in the hormonal balance.
Reorganization and stabilization of the cytoskeleton.
The energy balance is altered and the active transport system is affected.
The membrane fluidity and viscosity is affected due to the rigidity of the membrane that results from cold acclimation.

The process of cold acclimation is usually accompanied by altered genes expression. These genes are either up regulated or down regulated. The Cold Regulated Genes (COR) contain sequence elements whose main purpose is to mediate the genes self induction (Tester  Munns 2008, p. 667). Some of these elements include the dehydration responsive element (DRE) and ABA responsive elements whose main purpose is to mediate ABA responsiveness of the genes. The ABA independent and ABA dependent pathways are the main regulators of COR genes expression. The mechanism that initiates the response to cold resistance constitutes of three steps detection of external changes transduction of these signals to the nucleus and the activation of genes (Tester Munns 2008, p. 678). Response to cold stress is basically a multigenic trait. Membrane receptors are responsible for detecting changes in the external environment. This is followed by ligand binding which prompts conformational changes within the receptors thus initiating the kinase activity. This leads to the subsequent transduction of the signal through kinase cascades. Gene activation then follows the induction of transduction pathways. Gene expression following the activation process increases the tolerance of plants to frost. However, not much is know about the mechanism of the CROs.

Ion Homeostasis and Osmotic Stress Management
Plants respond to high salinity stress either through toleration or by avoiding the stress. This implies that under high salt stress, the plants are either dormant or undergo a cellular adjustment to enable them tolerate the stress. Salt tolerance mechanisms are classified into those that control salinity stress by minimizing osmotic stress ion disequilibrium or those that alleviate the consequent impacts of the stress.

The high saline content contains a chemical potential that causes an imbalance of the water potential between the simplest and the apoplast leading to a decrease in turgor pressure. The cells respond to turgor reduction through osmotic adjustment. The organelle and cystolic systems for halophytes and glycophytes are equally sensitive to both chloride (Cl2 ) and sodium (Na ) ions, and therefore osmotic adjustment usually occurs in these compartments through an accumulation of compatible osmoprotectants and osmolytes (Sarhan et al 1997, p. 468). Osmotic adjustment necessitates that both the organelles and the cytosol contain an accumulation of compatible osmolytes which encompass of essential ions though the majority are organic solutes such as polyso, sugars, amino acids, sulphonium, quaternary ammonium and tertiary compounds. However the compatible solutes are varied among different plants species. The accumulation of compatible osmolytes is a metabolic response to salt stress and once synthesized, these mediate the adjustment of osmotic stress. Osmolytes do not only protect the sub-cellular elements but also reduce the oxidative damage induced by the free radicals produced following the high salinity. Following a high salt stress, the usual intermediary process of metabolism is diverted into specific biochemical reactions. Higher plants for instance produce Glycine betaine (GB) using choline and through two, betaine aldehyde dehydrogenase (BADH) and choline mono-oxygenase (CMO) catalyzed metabolic processes (Sarhan et al 1997, p. 470). GB-a quaternary ammonium product, for instance acts as a stabilizing osmolyte thus protecting macromolecules in instances of dehydration stress caused by salt stress. GB also preserves the integrity of both the plasma membrane and thykaloid under conditions of high saline stress.                                                  

Homeostasis is the maintenance of a stable internal environment through a homeostatic control system. The response of the cell to salt stress usually involves osmolyte synthesis and an increase in the uptake of calcium and potassium ions (Sarhan et al 1997, p. 472). In a typical cell there is usually a flux of ions in and out of the cell. An ionic homeostasis is achieved through an adjustment of the net flux and this is important for cellular requirements. In saline conditions, the concentration of NaCl is usually high and hence there is a disturbance of the kinetic steady states of the ions. This leads to an ionic imbalance and toxicity. Osmotic stress induces oxidative stress. Osmotic stress induces uptake of Na and K ions. However, little is known about the mechanism involved in the Cl- homeostasis regulation or Cl- transport. According to Sarhan et al (1997, p. 473), halophytic cells generally respond to high salinity through an increased plasma membranes Na efflux and a through a high sodium accumulation in the vacuoles. Compartmentalization of sodium is only achieved when the permeability of the tonoplast to these ions is maintained as low as possible and when there is active transportation of both Cl- and Na ions into the vacuole. Intracellular salt tolerance and sodium ions homeostasis are facilitated by both Ca2 and a high sodium ions concentration (Sarhan et al 1997, p. 473). Sodium ions compete with potassium ions for uptake. The uptake takes place through the common transport systems and the competition is usually effective since sodium concentration in the highly saline environment is greater than that of potassium. Ca2 is responsible for enhancing selective intracellular accumulation of either K or Na. Research into the mediation of K and Na homeostasis has identified Ca2 as having a vital role in the control of transports systems. The stress signaling pathway (SOS) is very useful in salt tolerance and ion homeostasis.

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