Water is becoming increasingly scarce. Groundwater supplies are being used up faster than they are being replenished and overpumping may now be the biggest threat to food production -levels are falling by 700mm each year in Pakistan. By 2050, there will be around four billion people living in countries facing serious water problems.
If we cannot produce more water, can we produce crops which are more resistant to drought? Professor Julian Schroeder of the University of California is investigating this possibility.
Plants have microscopic pores called stomata on the surface of their leaves which allow them to breathe in carbon dioxide and replenish the atmosphere with the oxygen produced by photosynthesis. These little pores also allow evaporation of water and special pairs of guard cells control their closing to prevent wilting and to fine-tune the balance between maximising carbon dioxide gain and minimising water loss.
When water is scarce, the plant hormone abscisic acid (ABA) triggers a chemical cascade that closes the pores and reduces water loss. Schroeder told the recent Agbiotech conference in London that his team has identified specific genes that respond to this hormone. The movement of ions, small charged particles, in and out of special channels in the membrane of the guard cells, affects the amount of water in the cells causing them to swell or shrink. ABA activates the ion channels in the membrane which leads to pore closure.
The ERA1 gene produces a protein that acts as the "molecular handbrake" in the pathway to pore closing. Mutant plants lacking this gene are hypersensitive to ABA and close their stomata more quickly. Mutant and control plants were watered for 21 days and then left for 12 days without water.
The control plants quickly wilted and shrivelled. The plants lacking the ERA1 gene however, looked green and healthy with firm leaves full of water. The rate of water flow, transpiration, from the soil through the plant to the air is decreased in these plants and the aperture of their pores is smaller. Engineering crops to use water more efficiently could have profound affects on productivity in drought-stricken environments.
Soil stress also limits agricultural productivity. In acidic soil, aluminium, which is normally tightly bound in mineral deposits, exists as ions which poison plant root systems causing stunted growth and poor yields. Conventional crossing of sensitive plants with those resistant to these toxic conditions is a long process. Genetic engineer ing allows the transfer of a "natural response to environmental stress" to a wider spectrum of species than conventional breeding allows.
Acidic soils make up about 40% of the world's arable land and it is a significant problem in central and south America. Some plants tolerate acidic, aluminium-rich soils by excreting organic acids such as citrate to bind the metal in a complex outside the root and prevent it entering the cells.
Dr Herrera-Estrella of the plant genetic engineering department in Iraputato, Mexico, has developed transgenic tobacco plants that overproduce organic acids and tolerate normally toxic levels of aluminium. These plants also take up phosphate more efficiently from soils containing low quantities of this essential nutrient. Researchers are now experimenting with rice, corn and papaya.
Citrate-excreting crops could benefit developing countries because land previously unsuitable for agriculture could be used to grow crops that require fewer inputs, such as phosphorus fertilisers.
There are approximately 800 million chronically malnourished people around the world and vitamin A and iron deficiencies are major concerns: 24% of the world population have insufficient iron in their diet and 60% of women in South Asia are anaemic.
Hundreds of millions of children suffer from vitamin A deficiency, the most important cause of blindness in the developing world. They are seriously at risk from diarrhoeal and respiratory diseases and measles is often fatal in these children.
People in countries where rice is the main food are particularly vulnerable to vitamin A and iron deficiencies since rice not only has a low iron content, but also contains large quantities of a compound called phytate which prevents iron uptake from the diet.
Professor Ingo Potrykus leads a team at the Swiss Federal Institute of Technology which set about tackling this global health problem by genetically engineering rice to contain higher levels of vitamin A and iron.
Genes from daffodils have been engineered into rice to produce b-carotene, a molecule that our bodies turn into vitamin A. This golden-coloured rice will rule out vitamin A deficiency if you eat 300g of rice a day.
Vitamin A deficiency is also associated with the availability of iron from the diet, so researchers have crossed these rice plants with rice containing genes to produce high levels of iron and allow more efficient iron uptake from the diet.
This work is financed by public money and the Rockefeller Foundation and not industry. Furthermore, the material will be freely available for non-commer cial use in developing countries.
Collaborations with rice breeders in the major rice growing countries of Asia, Africa and Latin America, have already been established. The next step is to cross-breed these traits into the most successful locally grown varieties of rice.
Crops with improved nutrient content could make a crucial contribution to the health and prosperity of the developing world.
How many people the earth can support depends on many factors, but we cannot feed today's population with yesterday's agriculture. Engineering crops that are shaped to the environment to counter stresses such as drought, salinity and metal toxicity will bring marginal lands into agricultural use, boosting local economy and self esteem of farmers.
The rate of increase in food production cannot keep pace with the population growth rate and the total area available to grow crops is shrinking as top soil erosion and drought reduces the viability of land for farming.
Agriculture faces tough choices as it struggles to supply sufficient food and a balanced diet for a population predicted to reach 11 billion by the middle of the next century.
Dr Claire Cockcroft is a molecular biologist at the Institute of Biotechnology in Cambridge
