Frost-resistant plants

During the bitterly cold weather that enveloped most of the UK over the Easter holidays my wife and I struggled every night to protect the tender blossoms on the apple trees in our cottage garden in Northumberland by covering them with plastic bags. Fortunately we only have four trees and the biggest is less than four feet tall, so it was a manageable task, if a cold one. The same was not true for fruit growers in the Southern United States last month. They suffered millions of dollars worth of damage to peach trees, strawberries, blueberries and other crops as a cold front swept across the country.

Frost is always more damaging in the middle of warm weather. Most plants that are resistant to frost need several days in the fridge before they can tolerate a freezer: temperatures between 5 & 10 degrees  prime the mechanisms that protect them from frost damage. According to Gary Warren of Imperial College London, global warming makes it more important than ever to understand how plants become resistant to cold and how to make them resist sudden changes. Global warming will make the weather more variable, Warren says, so sudden frosts in the middle of warm weather will be more common.

This month, a group led by Michael Thomashow of Michigan State University, have taken a step forward by creating a strain of the plant Arabidopsis [Latin species name should have capital initial letter & italics] that resists a sudden freeze to minus 5 degrees C from a growing temperature of 22 degrees. Arabidopsis is not a food plant – it is the guinea-pig of plant science – but most of the mechanisms that exist in Arabidopsis have counterparts in food plants. So the creation of a frost hardy strain could be very important indeed.

Previous work had shown that several different genes were involved in the development of frost-hardiness during cold acclimation. The genes are present in the plant all the time, but they are inactive in warm weather. As the weather gets cold the genes gradually switch on, making the plant produce proteins that make it frost-resistant in ways that are not yet fully understood.

All the genes – there are about 25 of them – are switched on together during cold acclimation and no single gene is enough to make the plant frost-resistant.  Creating artificially frost-hardy plants by arranging that all these genes are permanently switched on would be extremely difficult if each gene had to be modified individually.

However Thomashow recently discovered a commander gene: when it is activated it makes a protein that switches on several of the cold-resistance genes. The permanently frost-hardy plant was created by giving the plant a copy of the commander gene that is permanently active, instead of being activated gradually over several days when the temperature falls towards freezing.

If the same commander gene controls frost acclimation in other plants then frost hardy strains could be created in the same way, by permanently activating the commander gene. Warren, who studies how individual genes contribute to frost-tolerance, doubts whether the technique will always work, especially in plants that are only distantly related to Arabidopsis or in plants that have no frost-resistance genes. Having the commander issuing orders is no good if there is nobody listening and able to obey them. Mutations in individual genes that contribute to frost tolerance can block the whole process. “I can show you eight different mutations in Arabidopsis that would prevent this approach from working”, he says.

Monica Hughes, of Newcastle University, studies frost-tolerance in winter barley, which is an important food crop. Winter barley does not usually suffer frost damage: after acclimation it can withstand temperatures of -20 degrees C. However if a tiny fraction of the frost-resistance of barley could be transferred to maize – which cannot withstand the fridge, let alone the freezer – it would reduce its susceptibility to bad weather.

It is  more difficult to manipulate genes in barley than in Arabidopsis. However it would be easier to apply results from barley to improve the hardiness of maize because Arabidopsis is a very distant relative.

So far Hughes’s results suggest that the commander gene approach will not work in cereals. So far she has identified four different genes that affect frost tolerance in barley. Not only do the genes have different commands to activate them, in half of them the commands work in a completely different way. Normally a gene is activated by creating more copies of its message to make a particular protein. However in some cases identified by Hughes, the message simply stays around for longer, to make more copies of its protein.

With this intricate variety of ways of activating frost-tolerance genes, it is unlikely that a single commander will control all of them. It looks as if it may be a little while before we can grow maize in our cottage garden!