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The quest for drought tolerance

Water shortage is probably the single most significant challenge that will confront the world's farmers in the coming years. Droughts are increasingly common, and research funded by the National Science Foundation in the US recently found that drought areas have more than doubled in the last 30 years, with climate change implicated as at least part of the cause. Drought tolerance is a feature of some crop plants, and for many years farmers and, more recently, plant breeders have taken advantage of these tolerant varieties in traditional breeding. Can biotechnology now accelerate this effort to develop urgently needed drought-tolerant crops?

Sowing transgenic drought-tolerant wheat for screenhouse trial (CIMMYT)
Sowing transgenic drought-tolerant wheat for screenhouse trial

Alessandro Pellegrineschi, a cell biologist at the Mexico-based International Maize and Wheat Improvement Center (CIMMYT), is among those who believe that it can. He is working to produce transgenic wheat with drought tolerance and recently had significant success in a trial under field-like conditions. "I believe we are in the same situation as in the 1980s when we were developing insect-resistant plants," he says. "We need more public research and more donors who believe in our work. With these, I think we could have drought-tolerant transgenic wheat varieties available within five years."

Successful trials

Pellegrineschi's team took a gene from Arabidopsis thaliana called DREB1A, which is known to confer tolerance to drought, low temperatures and salinity in that plant, and transferred it to wheat. After preliminary trials in a biosafety greenhouse looked promising, a larger trial was indicated under more realistic conditions. The screenhouse trial was the first transgenic crop trial approved by the Mexican government since it lifted a moratorium on such trials. Biosafety was paramount for all concerned, and the trial adhered to strict biosafety rules.

Transgenic wheat plants from the CIMMYT trial show drought tolerance compared with control plants on the right (CIMMYT)
Transgenic wheat plants from the CIMMYT trial show drought tolerance compared with control plants on the right

After ten days of drought stress, control plants began to show drought symptoms, while transgenic plants remained healthy for five more days before symptoms appeared. The transgenic plants also responded better to watering after the period of drought, quickly returning to their unstressed state. The researchers see this as a major step in the right direction, though problems remain. While the transgenic plants performed well under drought conditions, they performed worse than control plants under normal, non-stressed conditions. The answer, according to Pellegrineschi, is to find a promoter gene that can efficiently switch on the responses to drought when needed and switch them off again when not. Work continues in the lab, and a more extensive field trial is planned.

Other groups of researchers around the world are pursuing the same drought tolerance goal. In October 2004, scientists in Egypt announced successful results of a field trial of transgenic wheat containing the HVAI1 gene from barley. The wheat thrived on just one irrigation when non-transgenic plants required eight to survive. Some groups have looked at the remarkable "resurrection plants", desert species that appear to shrivel up and die during long periods of extreme drought and recover fully when water becomes available. Researchers at Cornell University have produced drought-tolerant rice by inserting a bacterial gene for a sugar - trehalose - which was found to be key in this response. Still others are looking at different crops and other genes and gene sequences.

Non-transgenic alternatives

Many believe, however, that there is no need for transgenics as enough drought-tolerant crop varieties already exist. Screening the world's genebanks would likely reveal many more candidates. Use of new technologies such as molecular markers to identify desired traits has accelerated traditional breeding efforts, and varieties with pronounced tolerance to drought have been produced in recent years. Examples include the maize variety ZM521, which yields up to 50 per cent more than traditional varieties under drought conditions; Drysdale semi-dwarf wheat, which is capable of ten per cent greater yield; and short-duration chickpea varieties that avoid the drought period by maturing early.

It remains to be seen whether genetic engineering will contribute significantly in the coming years to the struggle to produce sufficient food in increasingly dry areas of the world. Certainly, as climate change raises temperatures and predicted water deficits are exacerbated, more crops with the ability to tolerate drought will be essential. As an extremely complex phenomenon controlled by multiple genes and regulatory pathways, drought tolerance has proved much more difficult to engineer into plants than more simply inherited traits governed by single genes. Molecular biologists are persevering nonetheless and are optimistic that they are drawing closer to a transgenic solution to this global problem.

Date published: March 2005


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