February 5, 2003

by Alessandro Pellegrineschi
Cell Biologist
International Maize and Wheat Improvement Center (CIMMYT)

Alessandro Pellegrineschi explores the scientific prospects for developing crops that are resistant to drought, and says that sceptics of GM technology must be won over to prevent future tragedies.

The recent catastrophic crop failure in southern Africa due to drought has brought on famine conditions of epic proportions. It also raises the question: what could genetic modification (GM) technology offer to poor farmers working marginal lands vulnerable to drought, including many of those in sub-Saharan Africa?

Several international agricultural research organisations have already devoted considerable effort to improving drought tolerance in the staple cereals that feed most of the world’s poor. Plant breeders and farmers are well aware that some plants cope with drought conditions much better than others; GM technology makes it possible to transfer genes conferring this drought tolerance to, and among, important food crops.

Yet the introduction of such crops, which have the potential to significantly enhance food production in drought-stricken parts of the world, has become the target of attacks by environmentalist groups. The result may be to prevent whole communities from gaining access to a technological development that could — literally — make the difference between life and death.

New technology on the horizon

One example of a promising new use of GM technology, which is sadly facing an uncertain future, is a technique for increasing drought tolerance being investigated at the International Maize and Wheat Improvement Centre (CIMMYT) in Mexico.

Wheat plants that have been genetically modified to withstand drought are now being tested in biosafety greenhouses at CIMMYT. Most of the plants produced have shown high tolerance to extreme low-water conditions.

This research project illustrates how recent advances in both molecular genetics and genetic engineering can be applied to enhance drought tolerance in plants. Progress has been slow and difficult, however, due to the complex effects of drought on plants.

Complex plant pathways

For example, at least four independent signalling pathways act in plants to switch on an array of genes in response to dehydration. Some of these genes code for proteins that help protect various parts of the plant cell during water loss while others detoxify harmful substances. ^{(1, 2)} Understanding how it would be best to utilise these genes is a lengthy process.

CIMMYT researchers have initially focused on incorporating a type of DREB gene (encoding a 'dehydration-responsive element binding' protein), which enables the wheat plants to withstand extreme water loss. Unfortunately, when this gene is continually "switched on", plants are smaller and produce much lower yields than unmodified varieties — significant disadvantages when it comes to plant breeding.

But the scientists then found that by fusing the DREB gene with the promoter region of another gene (rd29A), it is switched on only under stress conditions of dehydration or cold temperatures. The result is a plant that has a normal growth pattern and yield in good conditions, but is also much more resistant to drought, freezing, and high salinity. More work is now needed to fully characterise the function of the additional gene, and to dissect the complex process by which this gene is expressed.

A promising future?

The researchers at CIMMYT are optimistic that their technique offers a promising way to deal with the challenges of drought. Other approaches have also been investigated, such as:

• developing plants that stall seed development during periods of drought in order to conserve water, or that are better at taking up water (known as drought avoidance) ⁽³⁾;
• overexpression of a gene related to drought tolerance ⁽⁴⁾;
• accumulation of sugars and salts to protect against water loss ^{(5, 6)}; and
• further investigation, at a molecular level, of the physiological mechanisms by which plants adapt to extreme environments ⁽⁷⁾.

Such research, when combined, will lead to a much more complete understanding of drought tolerance in plants. And with the help of genetic engineering it will be possible to create plants with these traits, without the need for long and tedious breeding programmes. But taking the next critical step, namely moving these plants from the laboratory to the fields of resource-poor farmers in developing countries, will require a supportive public and the well-founded assent and collaboration of developing nation governments.

A concerted effort is now required to convince both decision-makers and environmentalist critics that the value of crops produced in this way — and the capability to alleviate to some extent the suffering faced by rural people in drought conditions — strongly outweighs any perceived health and environmental dangers. Failure to win over the sceptics could result in tragedies that are ultimately as much the responsibility of humans as of nature.

References:
 

1. Singh K et al (2002) Transcription factors in plant defense and stress responses. Curr. Opin. Plant Biol. 5(5):430-6
2. Xiong L et al (2002) Cell Signaling during Cold, Drought, and Salt Stress. Plant Cell. 14:165-183
3. Laporte MM et al (2002) Engineering for drought avoidance: expression of maize NADP-malic enzyme in tobacco results in altered stomatal function. J. Exp. Bot. 53(369):699-705
4. Qin X & Zeevaart JA (2002) Overexpression of a 9-cis-epoxycarotenoid dioxygenase gene in Nicotiana plumbaginifolia increases abscisic acid and phaseic acid levels and enhances drought tolerance. Plant Physiol. 128(2):544-51
5. Rontein D et al (2002) Metabolic engineering of osmoprotectant accumulation in plants. Metab. Eng. 4(1):49-56
6. Garg AK et al (2002) Trehalose accumulation in rice plants confers high tolerance levels to different abiotic stresses. Proc. Natl. Acad. Sci. 99(25):15898-15903
7. Bartels D & Salamini F (2001) Desiccation tolerance in the resurrection plant Craterostigma plantagineum. A contribution to the study of drought tolerance at the molecular level. Plant Physiol. 127:1346-1353

reprinted from SciDev.net