Genetic Responses to Drought
Plant scientists advance from genes to genomes in researching how plants deal with stress
Dave Amber / The Scientist 14[21]:18 30oct00

 

A consensus grass comparative map shows how grass genomes line up.

When the well's dry, we know the worth of water," wrote Benjamin Franklin under his Poor Richard alias. Not much has changed in 250 years as the United States, suffering one of the most devastating droughts in 90 years, painfully learns water's worth. The situation is gravest in the Southwest, which is enduring its fourth year of drought in five years. The U.S. Department of Agriculture continues to add more counties to its list of drought disaster areas eligible for government relief funds.

As farmers hear their echoes returned from empty wells, imagine the stress on plants. Probably no environmental factor limits plant productivity more than water. "Dealing with water limitation is one of the most fundamental adaptations of all living organisms on the terrestrial domain," says John Mullet, director of the Crop Biotechnology Center at Texas A&M University. Around the world, traditionally dry growing conditions challenge countries from Africa to Asia to develop hardy agricultural systems. An expanding human population forecast to reach 10 billion by mid-century makes this even more imperative.

Plant biologists have labored for centuries to develop hardier drought-resistant crops by crossbreeding species. These classical breeding efforts focus on the labor-intensive screening of populations for tolerance to drought, freezing, and salty conditions. However, some scientists say they have learned as much in the past three years as in the previous 100 about how plants resist stress. Recent advances come via gene sequencing technologies and the genome mapping of plants--rice, sorghum, and the weedy mustard plant, Arabidopsis thaliana, often considered the fruit fly of the plant world. New technologies hold promise for a vast research program to develop crops resistant to stresses such as drought, freezing, and salt stresses from the ground up, gene by gene.

The crude genomic techniques of complementary DNA cloning and gene expression experiments in the 1980s presaged recent advances in genomics that continue to foster more sophisticated technologies. The rice and Arabidopsis genome projects are bringing structural and functional genomics together to identify signal transduction pathways responsible for ways in which plants sense changes in their environment. "It's been a continuum of development of genomic technologies," says Michael Thomashow, a microbiologist at Michigan State University. Thomashow and others found and isolated the genes being turned on, and they use them to identify the key enzymatic players in stress responses.

Thomashow took a chilly approach to stress-resistance genetics, studying mechanisms for enhancing resistance to freezing. Much of freezing's injury is due to severe dehydration, implying a relationship between freezing and drought- tolerance mechanisms. Although the connection between freezing and drought tolerance had been accepted for decades, Thomashow wanted to make the genetic connection. Years ago, he found that isolated cold-regulated, or COR, genes were also being turned on by drought conditions. Overexpressing transcription factors affecting COR could enhance both the freezing and drought tolerance.

 

Bridging Plant Genomes In John Mullet's genetics lab at Texas A & M University, sorghum is king. For 15 years, Mullet's team has been researching the biology of sorghum, an important cereal grain highly resistant to dry conditions. Sorghum, he says, is the next logical species after rice to sequence in a series of concentric circles representing the genomes of increasingly more genetically complex grass species. The team is now beginning a large National Science Foundation-funded gene mapping and sequencing project with implications for other, more genetically complex crops, such as wheat and corn. An important cereal worldwide, sorghum is used for food in Africa and for feed in the United States--a minor crop compared to corn and wheat. But it is well-adapted to grow in Texas' dry climate. Drought tolerant and with a diverse germplasm collection from its African origins ("Africa is a continent that has been selecting for drought tolerance for a long time," notes Mullet), sorghum is a source of diverse genes key for general adaptation to the environment. Drought tolerance and adaptation to the environment are complex. Environmental conditions--soil moisture and temperature, for example--can vary dramatically within even a small geographical area. Although a field may look uniform, imaging commonly shows a great diversity of micro environments. Most crops are produced from few major "elite" genotypes used throughout vast regions. "It's not surprising that crops are not ideally adapted to their local environments, unlike native species that have grown up and been selected for each of the microenvironments within which they operate," says Mullet. Sorghum breeders across Texas are working with Mullet to create populations he can screen for genes to add to current elite lines to make them more drought tolerant. Crop improvement is a long-term business, he says, anticipating a decade-long project of tool building, gene mapping, testing and polishing to create an elite cultivar. But he says it makes a difference over the long term, if he is able to make improvements on crops for Texas and elsewhere. "But more important, if we can get to the genes that underlie that tolerance, they could be transferred to other crops," he says. --Dave Amber

Major advances in understanding these stress-resistance mechanisms came in the last three years through the work of Thomashow's group at Michigan State¹ and a team of researchers headed by Kazuo Shinozaki at Japan's Institute of Physical and Chemical Research.² Both teams looked at drought and freezing tolerance mechanisms involving multiple genes. They compared pathways from gene to gene expression and worked to find the most favorable process--the most effective chemical middleman--to translate the genetic information from DNA into the active drought and freeze-resistance proteins without harming the plant's growth. The two groups independently discovered a family of transcription signal activators--Thomashow called them CBF activators, Shinozaki, DRB activators--that act like a master switch controlling these batteries, or regulons, of genes. It has been through this work of the regulators--the CBF/DRB proteins--that a tie has been clearly demonstrated between freezing and drought tolerances.

Thomashow's 1998 paper in Science and Shinozaki's 1999 paper in Nature Biotechnology both explored this concept of regulon engineering. The ideas were fresh in that they involved manipulating complex pathways through key regulatory genes as opposed to the typical genetic engineering of single genes or a small number of genes to synthesize a particular compound. Instead of genetically engineering individual "end player" genes, scientists could manipulate the master switch types of genes to affect entire families of proteins functioning in stress resistance.  

"This is the change in the last three years from a purely gene-by-gene approach to a ground-up approach," comments University of Arizona biochemist Hans Bohnert. "We should let the plants tell us what is going on." For years, he and his collaborators at Purdue University and Oklahoma State University have been paving the way for other researchers by compiling and testing huge databases of plant genomic information. Bohnert uses a computer analogy to help describe how all plants contain the genetic hardware for stress tolerance.³ The difference between plants that can and cannot tolerate a stress such as drought, he says, is the software--how certain genetic programs are called up. Stress-tolerant plants recognize the stress earlier, regulate certain genes more strongly and persist in this regulation.

For instance, when researchers stress Arabidopsis--a plant with a relatively low tolerance to stresses--it exhibits a short rise in tolerance before dipping back to prestress levels. The reactions of stress-tolerant plants last longer. "How fast to react, how strongly to react, and how persistently to react seems to be the difference," Bohnert says. "That means the difference is not in all the biochemical pathways and the enzyme reactions, but how these enzymes are engaged."

When considering regulation mechanisms, however, overexpression of genes to help in survival must be balanced with agricultural productivity. Activating a process not normally activated--Thomashow calls it "cranking up the transcription factor"--may affect plant yield, creating dwarf plants. He says scientists would have to optimize expression level for the CBF genes to balance stress-tolerance enhancement and negative effects on productivity.

Thomashow holds a patent on the regulon engineering technology, which is licensed to 3-year-old Mendel Biotechnology, a Hayward, Calif., functional genomics company (www.mendelbio.com). Mendel will use the technology to develop more drought-resistant crops, beginning with the Brassica plant canola, a genetically altered form of rapeseed. Canola and Arabidopsis belong to the same family (Cruciferae). "We think the technologies we developed in Arabidopsis could almost be copied without modification to canola," says James Zhang, a senior scientist at Mendel, which is preparing for National Science Foundation-funded field trials next year.

Scientists concerned about peaking yields of the green revolution of the 1960s and 1970s urge a second biotechnology-based green revolution for agricultural production to keep pace with Earth's growing population.⁴ Fighting drought, to get the most from the land available, figures prominently in such plans. "I think these biotechnology approaches for enhancing yields and expanding areas of production are going to be a component that helps meet that goal," says Thomashow. "The potential for plant biotechnology is wide--from having food available to expanding to nonfood uses."

Every plant has a particular biology and basic mechanisms useful in particularly stressful situations. Some algae, for example, have adapted to life in high salt conditions by developing salt pumps for sequestering ions. Research into tolerance of drought and other stresses has never been more topical as scientists look for keys to transfer some of these mechanisms into crop plants. Bohnert calls this "plant genomics for the real world."

 Dave Amber (damber@nasw.org) is a freelance science writer in College Station, Texas.

 References

1. K.R. Jaglo-Ottosen et al., "Arabidopsis CBF1 overexpression induces COR genes and enhances freezing tolerance," Science, 280:104-6, 1998.

2. M. Kasuga et al., "Improving plant drought, salt, and freezing tolerance by gene transfer of a single stress-inducible transcription factor," Nature Biotechnology, 17: 287-91, 1999.

3. J.C. Cushman, H.J. Bohnert, "Genomic approaches to plant stress tolerance," Current Opinion in Plant Biology, 3:117-24, 2000.

4. S.P. Briggs, "Plant genomics: more than food for thought," Proceedings of the National Academy of Sciences, 95:1986-8, 1998.