1th quarter 2006

Drought and salt tolerance: Fundamentals and present status of genetic engineering

Water shortage and soil salination are already today restricting agricultural production in many regions of the earth. In future, water will be the most important factor that limits the increase in food production. In the face of climate change and a persistently increasing mean global temperature, it is to be expected that dry, hot summers will become more frequent and that drought stress will cause crop losses in the agriculture.

Efforts are therefore being made in many countries to develop drought and salt-tolerant plants. At this time, however, little is known about the complex mechanisms. Many researchers are assuming that salt and drought tolerance are based on the interaction of a large number of genes. Introducing individual genes does not appear to hold much promise. There are nevertheless numerous research approaches that are studying the effect of individual genes and have produced quite promising results.

Natural protective mechanisms against drought and high salt content

Drought and salt-tolerant plants are characterised by certain morphological and/or physiological characteristics:

Protective morphological mechanisms: Deep root system; reinforcing the cuticle (layer of wax on the surface) to reduce evaporation; reducing the transpiration surface (e.g. small leaves, shedding leaves in drought); water storage in the tissue (e.g. succulents with fleshy leaves); salt excretion through special glands

Protective physiological mechanisms: Importing and exporting certain ions; forming substances which increase the stability of the membrane; enzymes which break down harmful intermediate products; functional and regulative proteins

Where does research stand today?

To date, most approaches to research in genetic engineering have concentrated on physiological mechanisms and on decoding the molecular basis of abiotic stress tolerance.

Some research projects concentrate on osmoprotectors, i.e. materials, which are concentrated in the cell and promote water retention, thus preventing the cell from drying out. Another approach deals with special proteins of the cell membrane, which act as channelling and transporting proteins and thus regulate the water pressure in plant cells. Still other projects work with detoxification systems or with regulative proteins which are involved in controlling the activity of genes.

A number of other examples show the intensive work being done on this topic and that researchers again and again succeed in transferring individual genes. Although different mechanisms have been found for increasing stress tolerance under controlled conditions, and individual transgenic drought and salt-tolerant plants have already been developed, we are still far from producing cultivable varieties. To date, most experiments have been conducted in greenhouses under artifical environmental conditions and only few have been tested in the open field. It is often yield and other important characteristics of the plant that suffer. Since many factors and genes interact in the presence of abiotic stress, developing tolerant plants is difficult and time-consuming. It will be years before the first genetically engineered drought and/or salt-tolerant plants are approved.