Genetic Basis of Drought Tolerance
Drought tolerance is an inherent ability of the plant to sustain growth through altering cellular and metabolic levels. It involves co-ordination of physiological and biochemical alterations such as turgor maintenance, protoplasmic resistance and dormancy (Beard and Sifers, 1997). Plants respond to drought conditions by shifting the expression of a complex array of genes and synthesis of molecular chaperons
- (Figure 3). Drought is interconnected with salinity, extreme temperatures, and oxidative stress and may induce similar cellular damage. For example, drought and/or salinization are manifested primarily as osmotic stress, resulting in the disruption of homeostasis and ion distribution in the cell (Serrano et al., 1999; Zhu, 2001). Oxidative stress along with high temperature, salinity, or drought stress, causes the denaturation of functional and structural proteins (Smirnoff, 1998) activating similar cell signaling pathways (Zhu 2001,2002) and cellular responses, such as the production of stress proteins, up-regulation of antioxidants and accumulation of compatible solutes (Wang et al, 2003b). As discussed earlier, plants adapt to drought conditions by regulating specific sets of genes which vary depending on factors such as the severity of drought conditions and other environmental factors, and the plant species (Wang et al, 2003b). These genes can be grouped into three major categories on the basis of their expression: 1) Genes involved in signal transduction pathways (STPs) and transcriptional control;
- 2) Genes with membrane and protein protection functions; and 3) Genes supporting water and ion uptake and transport (Vierling, 1991; Ingram and Bartels, 1996; Smirnoff, 1998; Shinozaki and Yamaguchi-Shinozaki, 2000). To date, success in genetic improvement of drought resistance have involved genetic manipulation using single or a few genes involved in signaling /regulatory pathways or that encode enzymes involved in the pathways (Wang et al, 2003b). The disadvantage of this is that there are numerous interacting genes involved, and efforts to improve drought tolerance through manipulation of one or a few of them is often associated with other, often undesirable, pleiotropic and phenotypic alterations (Wang et al, 2003b). These complex considerations, when coupled with the complexity of dr ought and the plant-environment interactions occurring at all levels of plant response to water deficit, demonstrate that the task plant researchers are faced with in engineering drought tolerant crop is darmtirrgly multi-faceted and extremely difficult. Genetic engineering using candidate genes for abiotic stress was found to be successful in model plants growing under controlled conditions and provided insights on the role of these genes in key physiological and biochemical processes (reviewed by Pardo, 2010; Vinocur and Altman, 2005). An exception was demonstrated by Rivero et al (2007) who manipulated a leaf senescence gene. Leaf senescence is an avoidance strategy and is accelerated in drought-sensitive plants to decrease canopy size. In crop plants, accelerated senescence is often associated with reduced yield and is thought to be the result of an inappropriately activated cell death program. Therefore, suppression of drought-induced leaf senescence in tobacco plants was investigated as a tool to enhance drought resistance. Other drought avoidance trait has been investigated include stay-green and cuticular biosynthesis. Stay-green is a variable and quantitative trait, which generally refers to delayed senescence which has not yet been used to successfully produce transgenic plants with increased drought tolerance in the field.
Fig. 3: The three aspects of drought and salt tolerance in plants (homeostasis, detoxification and growth control) and the pathways that interconnect them.
Salinity Stress: Complexity and its Impact on Agricultural Production
The negative effects of salt accumulation in agricultural soils have severely affected agricultural productivity in large swathes of arable land throughout the world and are affecting 45 million hectares of irrigated land which is expected to increase due to climate change and various irrigation practices as well (Roy et ah, 2014). In the majority, salinity conies from natural causes due to salt accumulation over long periods of time. High salinity causes both hyperionic and hyperosmotic stress and can lead to plant demise. Sea water contains approximately 3% of NaCl and in terms of molarity of different ions, Na+ is about 460 mM, Mg2+ is 50 mM and CT around 540 mM along with smaller quantities of other ions. Salinity in a given land area depends upon various factors like amount of evaporation (leading to increase in salt concentration), or the amount of precipitation (leading to decrease in salt concentration). Weathering of rocks also affects salt concentration. Inland deserts are marked by high salinity as the rate of evaporation far exceeds the rate of precipitation.
The significant portion of the cultivated land is fetching saline due to deforestation or excess irrigation and fertilization (Shannon, 1997). As drier areas in particular need intense irrigation, there is extensive water loss through a combination of both evaporation as well as transpiration. This process is known as evapotranspiration and as a result, the salt delivered along with the irrigation water gets concentrated, year-by-year in the soil. This leads to huge losses in terms of arable land and productivity as most of the economically important crop species are veiy sensitive to soil salinity. These salt sensitive plants, also known as glycophytes include rice (Oryza sativa), maize (Zea ways), soybean (Glycine max) and beans (Phaseolus vulgaris) which greatly affects the food supply.
More than 20% of the world’s irrigated land producing one third of world’s food supply is presently salt affected (Ghassemi et ah, 2006). With the expected increase in world population, the ultimate aim of the salt tolerance research is to enhance the ability of plants to maintain growth and productivity under saline condition relative to non-saline soils to minimize the effect of salinity on yield. Growth reduction under salinity is due to distinct processes related either to the accumulation of salts in the shoot or independent of shoots salt accumulation. Salinity reduces the ability of plants to absorb water, causing rapid reductions in growth rate, along with as array a suite of metabolic changes identical to those caused by water stress (Muuns, 2002). High salt concentration (Na+) in particular which deposit in the soil can alter the basic texture of the soil resulting in decreased soil porosity and consequently reducing the soil aeration and water conductance. The basic physiology of high salt stress and drought stress overlaps with each other. High salt depositions in the soil generate a low water potential zone in the soil making it increasingly difficult for the plant to acquire both water as well as nutrients.
Effect of Salinity Stress on Plant Cell
High salt concentration induces several deleterious consequences on plant cell. First, salt stress causes an ionic imbalance (Zhu eta!., 1997). When salinity results from an excess of NaCl, the most common type of salt stress, the increased intracellular concentration of Na+ and Cl2+ ions is deleterious to cellular systems (Serrano et a!., 1999). In addition, the homeostasis of not only Na+ and Cl2, but also K+ and Ca2+ ions is disturbed (Hasegawa et ah, 2000a, b; Rodriguez-Navarro, 2000). As a result, plant survival and growth depend on adaptations that re-establish ionic homeostasis, there-by reducing the duration of cellular exposure to ionic imbalance. Second, high concentrations of salt impose hyperosmotic shock by decreasing the chemical activity of w'ater and causing loss of cell turgor. This negative effect in the plant cell is thought to be similar to the effects caused by drought.
Third, salt-induced water stress reduction of chloroplast stromal volume and generation of reactive oxygen species (ROS) are also thought to play important roles in inhibiting photosynthesis (Price and Hendry, 1991). These can be summarized
- 1) Disruption of ionic equilibrium: Influx of Na“ dissipates the membrane potential and facilitates the uptake of CT down the chemical gradient.
- 2) Na+ is toxic to cell metabolism and has deleterious effect on the functioning of some of the enzymes (Niu et ah, 1995).
- 3) High concentrations of Na+ causes osmotic imbalance, membrane disorganization, reduction in gr owth, inhibition of cell division and expansion.
- 4) High Na+ levels also lead to reduction in photosynthesis and production of reactive oxygen species (Yeo, 1998).
The generic functions of K+, role of Ca2+ and SOS pathway in relation to imparting salt str ess tolerance, loss of w'ater due to salinity stress, role of osmolytes and DNA unwinding enzymes imparting stress tolerance are the various aspects of salinity stress which needs to be understood. Where sodium (Na+) is deleterious for plant growth, K+ is one of the essential elements and is required by the plant in large quantities.
Generic Role of K+
The K~ imparts three major functions i.e. 1) it is required for maintaining the osmotic balance; 2) K+ has a role in opening and closing of stomata, and 3) it acts as an essential co-factor for various enzymes like the pyruvate kinase, whereas Na+ is not.
Movement of salt into roots and shoots is a product of the transpirational flux required to maintain the water status of the plant (Flowers and Yeo 1992). As common proteins transport Na+ and K+, Na_ competes with K+ for intracellular influx (Amtmann and Sanders, 1999; Blumwald et ah, 2000). Many K+ transport systems have some affinity for Na+, i.e., Na+/K~ symporters. Thus external Na+ negatively impacts intracellular K“ influx. Most cells maintain relatively high K+ and low concentrations of Na+ in the cytosol. This is achieved through a coordinated regulation of transporters for FT, K+, Ca2+ and Na+.
The plasma membrane H+-ATPases serves as the primary pump that generates a proton motive force driving the transport of other solutes including Na+ and K+. Increased ATPase-mediated FT translocation across the plasma membrane is a component of the plant cell response to salt imposition (Watad et aJ., 1991). K+ and Na+ influx can be differentiated physiologically into two categories, one with high affinity for K+ over Na+ and the other for which there is lower KT/Na+ selectivity. The Na+/K+ transporter and K+ transporters with dual high and low affinity may contribute substantially to Na+ influx.
Role of Ca2+ in Relation to Salt Stress
For decades it has been shown that another ion, Ca2+ has role in providing salt tolerance to plant. Externally supplied Ca2+ reduces the toxic effects of NaCl, presumably by facilitating higher K~/Na+ selectivity (Liu and Zhu, 1998). High salinity results in increased cytosolic Ca2_ that is transported from the apoplast as well as the intracellular compartments (Knight et al, 1997). This transient increase in cytosolic Ca2+ initiates the stress signal transduction leading to salt adaptation.