Developing drought-tolerant plants: Scientists challenged
This article first appeared in the St. Louis Beacon, May 1, 2009 - Gardeners in this area are all too aware that failing to water during our long, hot summers may turn colorful flower beds into studies in brown. And our vegetable gardens won’t give us much produce without adequate water.
On a global scale, growing enough food to support a projected population of 9 billion by 2050 (currently 6.8 billion) will seemingly require that much more water at a time when weather appears to be increasingly erratic.
Developing plant crops to produce more with less water would be a step toward food sufficiency. Plant scientists here in St. Louis are heavily involved in efforts to develop such crops, called drought tolerant or drought resistant.
How do these scientists approach the problem?
As it turns out, many promising avenues exist that may all lead to crops suited to 21st century conditions. To get an idea of the problem's complexity, it seems worthwhile to travel a few of these highways.
Corn, Not a Cactus
It's not that plants haven't evolved survival techniques for extreme dryness. Anyone who has seen a desert in bloom after spring rains can appreciate that some plants have learned how to exploit their ecological niche for survival by maximizing the good times. When the desert blooms in the southern California desert, more than 750 plants can be seen - and identified by botanists.
Unfortunately, in those very long periods between rains, most desert plants simply shut down and become dormant.
If a crop plant such as corn or soybean shuts down during a period of drought, explains Mark Running of the Donald Danforth Plant Science Center, it is likely to miss the harvest.
The goal of current drought-tolerance research is to keep plants growing during dry times, to keep them on track with respect to seed production and to optimize seed production.
To reach those goals, as Liming Xiong of the Danforth Center points out, drought resistant crops must develop mechanisms to do three things:
The Arabidopsis Model
Most genetic experimentation in plants starts with a little member of the mustard family called Arabidopsis. This flowering plant has many advantages for the researcher.
- Conserve water they get
- Mitigate the effects of stress
- Take up more water
- It makes many seeds and is easily cultivated.
- Its life cycle is only six weeks from seed germination to seed production. Thus the effect of a single manipulation can be quickly studied in all stages of a plant's life.
- Its genome has been completely sequenced, and all 5 chromosomes have been mapped.
- It can be transformed efficiently. Genes can be put in or removed.
Arabidopsis is the model plant used all over the world, both in university-type laboratories and in industry. The technology of changing metabolic pathways will usually be studied extensively in Arabidopsis before it is tried in plants with long life cycles such as corn.
Of course, response to drought has been well characterized in Arabidopsis.
A Shot of Plant Adrenalin
As mammals respond to danger by releasing adrenalin to the bloodstream, so plants release a stress hormone, abscisic acid. ABA initiates a cascade of metabolic events to deal with water stress.
ABA can reduce water loss by making the stomata close down. Stomata are mouthlike structures found mostly on the underside of leaves. Stomata open to allow the exchange of oxygen and carbon dioxide. But when they are open, water vapor also escapes. Closing the stomata conserves water.
ABA can also induce plants to grow longer and deeper roots to seek water at deeper levels. Other major ABA responses include slowing growth and increasing the dormancy of buds.
Plants biologists can apply ABA to plants in the laboratory and study the response under controlled conditions. Within three minutes of application to Arabidopsis, stomata close. After about 30 minutes, gene expression begins to change. Signals are sent, transcription factors cause genes to be activated or inactivated, and eventually the protein composition is changed.
Jo Seltzer is a freelance writer with more than 30 years on the research faculty at the Washington University School of Medicine and seven years teaching tech writing at WU's engineering school.