Introduction

Agriculture is the largest water consumer in the world. In developing countries about 90% of water is used for Agriculture. These vast amounts of water are still not enough as most developing countries are dry and thus the agriculture industry in these countries is highly restricted by water. Global warming also poses another threat of drought in the world, as most parts of the world will become dry according to researchers looking into global warming. Conventional breeding methods target plant species that are drought tolerance and the tolerant lines, after being identified, are used continuously and even interbred with close species to produce somewhat drought resistant or tolerant plants. Conventional breeding methods though successful to a certain extend, are very slow and thus it takes time to identify the drought tolerant lines, by which time there has been severe loses to the crop yield. Genetic engineering then offers a fast and efficient tool to produce drought resistant and tolerant plants and thus efficient water uptake, use and retention by plants.

Engineering Drought Tolerant Plants

In order to genetically manipulate plants to be drought tolerant or resistance, genes from the plants that are tolerant or even from other organisms can be used. To make plants to be drought resistant, researchers have discovered different genes, molecules and compounds that can be genetically altered to make the plants to be drought tolerant and resistance, without the yields of the plants being reduced. These genes, molecules or compound can be grouped into three drought tolerance engineering strategies. These are the engineering of functional proteins, manipulating the expression of transcription factors and the regulation of signalling pathways involved in drought tolerance.

Functional proteins are involved in the efficient water uptake and prevent water loss by plants, are referred to as osmoprotectants. They act as osmolytes to prevent water loss by making the plant to increase water uptake and its retention once in the plant. Osmoprotectants also act to protect plant cells from damage due to water stress thus making the plant to be drought tolerant by preventing cell death. The most common of these osmoprotectants are glycine betaine and proline.

Glycine betaine accumulates in plants naturally during water stress and in other organisms such as bacteria. Some plants such as rice and potato do not accumulate glycine betaine. Thus if genes from plants that do or from other organism that are not necessarily plants genes, that are involved in the biosynthesis of glycine betaine can be expressed in these plants they will be able to synthesise and accumulate glycine betaine and thus become drought tolerant and resistant.

Proline has been shown to be an osmolyte that counteracts the effects of osmotic stress thus making plants to be drought tolerant. Its biosynthesis in water stressed plants can be regulated by either activating its biosynthesis or by inactivating its degradation. The biosynthesis of this protein can be manipulated genetically in order to make plants to be drought tolerant. This can be done by activating the production of the enzymes that are important in its biosynthesis which have been identified as delta(1)-pyrolline-5-carboxylase synthase or delta(1)-pyrolline-5-carboxylase reductase, or by inhibiting the action of the two enzymes that degrade it which have been identified which are proline dehydrogenase and delta(1)-prolline-5-carboxylase dehydrogenase. Thus these genes can be used for genetic transformation by up-regulating the production of the biosynthesis enzymes and down-regulating the synthesis of the degrading enzymes.

Other functional proteins or genes important that can be genetically engineered or manipulated in plants are the ones responsible for the synthesis and degradation of antioxidants. Antioxidants harvest reactive oxygen species that if left uncontrolled in the cells, can result in loss of function by the cells and eventually cell death. Genes that are involved in the synthesis of the plant hormone abscisic acid (ABA) can also be activated as this plant hormone has been shown to trigger stomatal closure in plants thus a reduction in water loss and thus tolerating drought stress.

Expression of transcription factors that have been shown to be important in drought stress tolerance or resistance in plants can be either up regulated or down-regulated depending on their function. Dehydration-responsive element binding (DREB) transcription factors have been shown by research to control the expression of genes involved in plant drought tolerance. Thus over-expressing the DREB transcription factor responsible for the activation of these genes will result in drought tolerant and resistant plants. Some transcription factors such as AtMYB60 have to be down regulated in plants as it encourages the opening of the stomata pores, thus when down-regulated the pores will not open that much thus reduced water loss by plants.

Regulation of signalling pathways involved in drought tolerance involve second messenger molecules such as nitric oxide (NO) and cyclic guanosine monophosphate (cGMP). NO has been found to regulate detoxification of superoxides and enhances the activity of the enzyme that degrades the hydrogen peroxide which is produced when superoxides are degraded. Thus NO prevents cells from loss of function and thus cell death, making the plant to survive in osmotic stress conditions.

Conclusion

Genetic engineering of the above mentioned genes, molecules and compounds responses in plants can thus result in the plant being able to grow well in water deficient environments. This will lead to reduction in water used for irrigation in agriculture and also an increase in crop yields in dry areas of the world, thus alleviating poverty.

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