PULLMAN, Wash.–If you think your outdoor plants are just sitting around waiting to freeze in the fall while you’re rooting around in you cedar chest for another sweater, think again. Plants also respond to colder temperatures in ways that will help them survive the coming winter.
John Browse’s laboratory at WSU’s Institute of Biological Chemistry has been investigating how plants survive and grow in cold or freezing temperatures. Browse and co-workers study Arabidopsis thaliana, a small weed that’s been the favorite laboratory tool of plant geneticists for many years.
Browse’s lab started with the knowledge that if you grow an Arabidopsis at 72 degrees Fahrenheit, then switch it to a colder temperature, it will die if that temperature is lower than 24. However, if you grow the plant first at 72, then for two or three days at 39, it can survive temperatures as low as 10 degrees.
“In other words,” says Browse, “the plant adapts to the idea it will freeze.” It not only senses the change in temperature but alerts its cells to make the changes the plant will need to survive in the cold to come.
Arabidopsis is not the only plant with this ability. Wheat, barley and other plants also sense when winter’s coming, although many species need a week or more at 39 degrees to adapt to the cold.
Post doctoral associate Zhanguo Xin is interested in how the Arabidopsis plant senses the lower temperature and then makes the changes it needs to survive freezing temperatures. In the past, researchers have addressed this question by looking at differences between plants growing at normal temperatures and those growing at colder temperatures, such as differences in what types of sugars or fats or proteins they contain. However, there are a large number of these differences, and it’s difficult if not impossible to separate those that cause changes from those that are responses to them.
Xin is taking a different approach. He’s spent the last year screening over 800,000 mutant Arabidopsis plants, plants whose DNA has been chemically altered. Because he was looking for negative regulators of the freeze tolerance process, he looked for plants that were able to survive freezing temperatures without the 39 degree conditioning period.
Negative regulators function somewhat like cancer suppressers. When present, each prevents something from happening. Cancer suppressers keep a cell from becoming cancerous. In freeze tolerant plants, negative regulators keep the plant from wasting energy-producing molecules necessary for tolerating freezing temperatures unless it’s cold. If the negative regulator is not present, then the plant goes ahead and develops the ability to tolerate freezes regardless of the temperature.
The year of screening plants has paid off with 20 mutants that meet his criteria, plants whose genes that encode negative regulators have presumably been mutated. Currently he’s concentrating on one of these plants, trying to isolate its negative regulator gene.
It’s a long process. First Xin maps the mutation to the smallest possible piece of the mutant Arabidopsis chromosome, rather like finding which block on a long street the mutation occupies. Then he’ll isolate the same piece from a normal Arabidopsis chromosome, which will contain the normal gene for the negative regulator. The piece of normal chromosome will be chopped into sections that are about one gene long and put into mutant plants. The section containing the normal gene will reverse the mutation, and plants containing that section will grow normally.
Once he’s found the gene and verified its function, then what?
“Once you understand the whole process you can develop ways to improve it,” says Xin. Maybe plants can be developed in which the negative regulator can be controlled by the grower. With such a system it would be possible to protect the plants from unseasonable frosts. Since it’s probable that a similar gene exists in plants with agricultural value, it might be possible to produce genetically altered crops that are more tolerant of freezing temperatures.
Post doctoral associate Jim Tokuhisa is looking at another type of cold tolerance in plants, the ability to grow well at low but non-freezing temperatures. It’s something that normal Arabidopsis do well. They’ll even grow in the refrigerator.
Tokuhisa’s approach also is genetic and started with the screening of large numbers of mutant plants. However, he looked for plants that grew well at 72, but not 39 degrees, plants that presumably had lost the tools needed to grow well in the cold.
He’s found a variety of plants that meet these criteria. Some die. Others are small or yellow or purple when put in the cold. “This implies there is more than one set of tools involved in the ability to grow at low temperatures,” says Tokuhisa.
At this time he also is concentrating on one mutant. Although it continues to grow when switched to 39 degrees, all of the new growth is yellow rather than green. “This happens outside the laboratory in plants such as corn, sorghum, and sugar cane, so it may well be relevant to agriculture,” says Tokuhisa.
He has isolated the gene that mutated to cause the changed growth. By comparing the gene’s DNA sequence to previously characterized DNA sequences from a variety of living organisms, he discovered that the gene is similar to one involved in protein synthesis in both yeast and bacteria. Yeast without the gene not only are unable to make protein properly, but also die. However, bacteria without the gene grow normally.
Tokuhisa is currently working on experiments designed to determine the gene’s function in Arabidopsis. It appears that the gene product locates to chloroplasts, the parts of the plant cell that are responsible for the plant’s green color. This is interesting not only because the mutant plants produce yellow growth in the cold, but also because chloroplasts are thought to have evolved from bacteria.
Like Xin’s work, Tokuhisa’s is what’s termed “basic science.” However, it is clear the work of both could be useful outside the laboratory, in agriculture or a home garden.
We might envision plants that do better at lower, even below-freezing, temperatures–plants that could survive many frosts or grow in areas now too cold to allow for successful harvests. It’s something to think about–harvesting tomatoes for Christmas dinner.