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Genes of autumn leaves mapped

February 28, 2003

It is not known what genes turn leaves yellow in the fall. However, scientists at Ume'å Plant Science Center, Ume'å University, and the Royal Institute of Technology in Stockholm (KTH) have managed to identify more than 2,400 genes that take part in the process.

One of the most magnificent pageants of nature every year is when trees take on their autumn colors. Scientists have long known that these colors appear when the green pigment chlorophyll is broken down at the same time as the yellow carotenoids remain and red antocyanines are formed. This process is steered, like all others in a living organism, by genes. Certain genes see to it that the process starts at the right time, others make sure that the chlorophyll and a number of other constituents of the leaf are degraded and others are created, such as antocyanines. Even though this is a process that has fascinated people throughout the ages, molecular biologists have previously not paid any attention whatsoever to the process, and until now no one has known anything about a single gene that takes part in the process.

Now scientists at Ume'å University and the Royal Institute of Technology in Stockholm who have been studying aspens have stepped forward, presenting more than 2,400 genes that are expressed in autumn leaves. With the help of so-called EST sequencing, they distinguished 5,128 pieces of gene sequences that proved to belong to 2,407 different genes, all of which were expressed on September 14, 1999 in the leaves of an aspen growing on the Ume'å University campus.

It may seem odd that no one has previously studied the genes in autumn leaves when so many other processes have been studied in the greatest detail. "We thought so too," says Stefan Jansson, professor at Ume'å University, "and therefore we decided to do something about it. Even though there are tens of thousands of plant molecular biologists in the world, nearly all of them are studying annual plants like rice, potatoes, tomatoes, maize, rapeseed, and above all thale cress, the darling of plant scientists. Annual plants do not go through the same processes in the fall since the whole point of the process is to recycle nutrients in the leaves in order to use them again in the coming year's leaves, and annuals, as we know, live only one year." These Swedish researchers, however, have been pioneers in gene research on trees and are therefore well suited to take on this question.

Many of the 2,407 identified genes that were all expressed on September 14, 1999, in the leaves of an aspen on the Ume'å University campus naturally have a function during other times of year and in other parts of the tree. However, 35 of the genes are not expressed anywhere other than in huge quantities in autumn leaves.

Recycling the constituents of the leaf must take place before severe frost kills the cells of the leaf, after which nothing else can happen apart from the leaf falling off. Of course, plants cannot know what the temperature is going to be the following week and therefore consult the almanac about when the process needs to start. They quite simply sense how long the nights are, and when the length of the night exceeds a critical value, the process starts. This works fine for the most part, but certain years, like the fall of 2002, in fresh memory, with snow and cold weather coming unusually early, the process does not have time to play out. No doubt many people saw how some of Ume'å's birches stood with green leaves in the snow that came the first weekend in October. The inner calendar differs somewhat among the birches that naturally occur in the Ume'å area; this can be seen by studying "wild" birches in the woods. But those that have had an especially hard time are those that come from farther south. They are genetically programmed for autumn to arrive much later. Above all in the 1970s many birches were planted in Ume'å, the City of Birches, that had been purchased in Poland. Most of these imported birches have by now succumbed to the cold; some of them have survived, however, but they look like mountain birches, crooked and gnarled, a result of frost damage.

Even though these scientists have identified 2,407 genes that all have some function in autumn leaves, they still do not know which of these genes actually steer the process. "Many of these genes naturally have a function during other times of year and in other parts of the tree. However, 35 of the genes are not expressed anywhere other than in huge quantities in autumn leaves," says Stefan Jansson. "The aim of this research is of course to understand how it all works, and to do this we are now studying when all of these genes are turned on and off during the autumn. Can this research, besides providing an understanding of a fascinating phenomenon, have any practical value? Yes, if we grasp what determines when autumn leaves turn yellow, we can identify those trees that have an optimal "calendar" for various climatic zones, that is, those that do not turn color too late and therefore lose nutrients and suffer frost damage, or turn too early and therefore stop growing too early in the fall. In the future it should be possible to apply genetic technology to the creation of trees that have another inner calendar than the "natural" one, but whether we can modify the genes of peach trees so they can survive in the Ume'å area is another matter entirely."

Vetenskapsr�det (The Swedish Research Council)