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In this article we will discuss about Development of Drought Resistant Plants.
Adverse environmental factors, of which water scarcity represents the most severe constraint to agriculture, account for about 70% of potential yield losses worldwide. Global warming is also predicted to affect most severely developing countries, where agricultural systems are most vulnerable to climatic conditions and where small increases in temperature are very detrimental to productivity.
Water becomes an increasingly scarce and precious commodity. It is thus essential to improve water use efficiency in agriculture. The development of crop varieties with increased tolerance to drought, both by conventional breeding methods and by genetic engineering, is an important strategy to meet global food demands with less water.
Generation of Drought Tolerant Crops:
Conventional breeding requires the identification of genetic variability to drought among crop varieties, or among sexually compatible species, and introducing this tolerance into lines with suitable agronomic characteristics.
Although conventional breeding for drought tolerance has and continues to have some success, it is a slow process that is limited by the availability of suitable genes for breeding. The development of tolerant crops by genetic engineering, on the other hand, requires the identification of key genetic determinants underlying stress tolerance in plants, and introducing these genes into crops. Drought triggers a wide array of physiological responses in plants, and affects the activity of a large number of genes.
Natural Mechanisms Followed by Plants for Drought Tolerance:
The physiological responses of plants to a deficit of water include leaf wilting, a reduction in leaf area, leaf abscission, and the stimulation of root growth by directing nutrients to the underground parts of the plants.
Plants are more susceptible to drought during flowering and seed development, as plant’s resources are deviated to support root growth. In addition, abscisic acid (ABA), a plant stress hormone, induces the closure of leaf stomata, thereby reducing water loss through transpiration, and decreasing the rate of photosynthesis. These responses improve the water-use efficiency of the plant on the short term.
Genetic Engineering Drought Tolerant Plants:
Although not a crop plant, Arabidopsis has played a vital role in the understanding of the basic processes underlying stress tolerance, and the knowledge obtained has been transferred to a certain degree to important food plants.
Many of the genes known to be involved in stress tolerance have been isolated initially in Arabidopsis. The introduction of several stress-inducible genes into plants by genetic engineering has resulted to increased tolerance of transgenic to drought, cold and salinity stresses.
Some examples are reviewed as follows:
1. Genetic Manipulation of the Stress Response to Abscisic Acid (ABA):
ABA levels in the plant greatly increase in response to water stress, resulting in the closure of stomata thereby reducing the level of water loss through transpiration from leaves and activate stress response genes. The reaction is reversible: once water becomes available again, the level of ABA drops, and stomata re-opens.
Increasing the plant’s sensitivity to ABA has therefore been a very important target for improving drought tolerance. ERA1, a gene identified in Arabidopsis, encodes the β-sub- unit of a farnesyl-transferase, and is involved in ABA signalling.
Plants lacking ERA1 activity have increased tolerance to drought. However, they are also severely compromised in yield. In order to have a conditional, reversible down-regulation of ABA, a group of Canadian researchers used a drought-inducible promoter to drive the antisense expression of ERA1, in both Arabidopsis and canola plants.
Transgenic plants performed significantly better under water stress, with consistently higher yields over conventional varieties. It is important to note that there was no substantial difference between transgenic and controls (wild plant) in conditions of sufficient water. This demonstrates that the technology has no much final benefits.
2. ABA-Independent Gene Regulation to Drought Stress:
The transcription factors DREB1 and DREB2, are important in the ABA-independent drought tolerant pathways, that induce the expression of stress response genes. Over-expression of the native form of DREB 1, and of a constitutively active form of DREB2, increases the tolerance of transgenic Arabidopsis plants to drought, high salinity and cold.
Although these genes were initially identified in Arabidopsis plants, their presence and role in stress tolerance have been reported in many other important crops, such as rice, tomato, barley, canola, maize, soybean, rye, wheat and maize, indicating that this is a conserved, universal stress defense mechanism in plants.
This functional conservation makes the DREB genes important targets for crop improvement for drought tolerance through genetic engineering.