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Drought is a meteorological event, which implies the absence of rainfall for a period of time, long enough to cause moisture depletion in the soil and water deficit with a decrease of water potential in the plant tissues.

Some plants have evolved physiological mechanisms to cope up with this adverse stressful situation. Each of these mechanisms involves one or a few biochemical steps in a metabolic pathway that ends in a final product. This final product equips the plant with the ability cope up with this stressful situation.

We know that each biochemical step in metabolic pathway is catalyzed by an enzyme and each enzyme’s synthesis r controlled by a gene by a mechanism known as gene expression. Only a few plants are equipped with these mechanisms. These plants are known as xerophytes,-v majority of the plants are not adapted to drought conditions.

Genetic engineers have been looking for genes, whose products may help the plant retain more water and withstand prolonged arid and drought situations. Such genes may be introduce, into isolated plant cells in culture and novel plants may be generated by the tissue culture technique.

The new plant generated will be known as a transgenic plant, which expresses the gene and hence, will acquire the mechanism to retain more water and grow in arid conditions? Although, a satisfactory progress has not been made in this area of biotechnology, it bears a significant promise in raising such plants in near future, which will help increase the productivity.

Most of the early work has been conducted on the model plant, Arabidopsis thaliana. Currently, research is underway and several genes and their molecular mechanisms associated with the retention of water have been discovered. Trial is underway to produce drought resistant plants. In the following section a few such genes and their molecular mechanisms are discussed.

Drought Resistance Strategy

Drought resistance is a complex character. Its action depends on an interaction of several morphological, physiological and biochemical characters. In the course of evolution, xerophytes have developed these characters, which equip them with the ability to withstand water deficit at least over a certain period. The strategies fail under three classes: morphological, physiological, biochemical, and genetic.

Morphological

1. Deep root system enables the plant to reach a rich source of underground water.

2. Reduced leaf area minimizes the aerial loss of water by transpiration.

3. Reduced absorption of sunlight by leaf rolling or folding results in accumulation of carbon dioxide by respiration and high carbon dioxide concentration stimulates the closure of the stomata. Further, influx of potassium ions from the adjoining cells into the guard cells is preceded by the absorption of water. This increases the turgor pressure of the guard cells.

4. Thick cuticle on the outer surface with secreted wax prevents excessive loss of water by transpiration.

5. In addition to reducing water loss, some xerophytes store water in juicy organs such as roots, stems or leaves. Such xerophytes are called succulents.

6. Dropping off leaves before the onset of the summer season.

Physiological

1. Reduced transpiration due to the presence of a few stomata and closure of the stomata in response to the reduced absorption of sun light and accumulation of excess carbon dioxide.

2. High water-use efficiency.

3. Stomatal closure due to an increase in carbon dioxide concentration and influx of potassium ions into the guard cells.

4. Osmotic adjustment and osmo-protection by shipping sodium ions into vacuoles of the guard cells.

5. Osmotic stress leads to an overproduction of reactive oxygen species (nascent oxygen), causing extensive cellular damage and inhibition of photosynthesis. This phenomenon has been termed as oxidative stress. Plant cells contain anti­oxidant enzymes such as peroxidases and superoxide dismutases, which convert the nascent oxygen intermediates, thus preventing damage.

Biochemical

1. Accumulation of metabolic by-products such as glycine betaine, mannitol, proline, fructan, trehalose and polyamine, etc. These accumulated products function as osmoprotectants, which adjust the osmotic pressure of the stomatal guard cells.

2. Increased nitrate reductase activity.

3. Increased storage of carbohydrates.

Genetic

The osmoprotectants enumerated above are the products of biochemical pathways. Each biochemical pathway consists of one or several biochemical reaction steps.

Each step is catalyzed by an enzyme and each enzyme is the product of a structural gene. Gene products induced during drought stress are classified into two groups: Functional proteins and regulatory proteins.