Why leaves change color

Autumn starts officially tomorrow and we'll soon be able to look at all the marvelous colors of autumn leaves, at least if we live in a place where autumn means something. And now we know that a single protein is responsible for the splendor of the season.

Autumn starts officially tomorrow and we'll soon be able to look at all the marvelous colors of autumn leaves, at least if we live in a place where autumn means something. And I'm sure you know why tree leaves change color in the fall. It's because chlorophyll is disappearing from the leaves. While it's true, there is a responsible for the fading of the green pigment bound to a protein called LHCII. RedNova News reports that European researchers from Sweden and Poland say that a "single protein triggers the glories of the season." This protein, or more exactly, the protease FtsH6, is at the origin of the degradation of LHCII, revealing the other red, yellow or gold pigments which give to autumn leaves their wonderful colors.

Here is the introduction from the Red Nova article.

Autumn's joyous pageant of red, yellow and gold relies on a single protein, new research reveals.
The protein -- with the less-than-poetic name of FtsH6 -- degrades a second compound that spends most of the year holding tight to the green chlorophyll in leaves. As this compound (called LHCII) slips away, hidden pigments of red and yellow are revealed, explain researchers at Umeå Plant Science Centre in Sweden.

Before going further, below is a picture of a falling autumn leaf. It was taken by Fodd who uploaded it to Flickr and you can find this picture in different sizes here (Credit: Fodd).

Autumn leaves

Now, let's look at this news release from the Swedish Research Council (September 5, 2005) for more details.

The different pigments in a leaf are bound to different proteins. Most of the chlorophyll, which lends plants their green color, is bound to a protein called LHCII. Every individual protein is incredibly small (nearly a million times smaller than the human eye can perceive), but it is possible see them if there are many of them together. LHCII is probably the most commonly prevalent membrane protein on earth.
In the tropics there is no autumn, but in our climate deciduous trees and other perennials lose their chlorophyll in the fall. The reason for this is that the proteins in the leaves contain amino acids that the plant needs to recycle. The leaves' proteins are therefore degraded and the amino acids are stored in the trunk, branches, and roots until next year, when they are used as building blocks for new leaves. Other proteins, so-called proteases, have the task of degrading these proteins.

Let's return to RedNova News for the last details.

[The researchers] used genetically modified plants in which various FtsH proteases had already been removed to conduct their study. One plant variation lacking a key protease, FtsH6, was largely unable to break down LCHII. That suggests FtsH6 is crucial to seasonal chlorophyll removal, the researchers say.

For more information about this research, you can check two other pages about scientists of the Umeå Plant Science Centre.

Stefan Jansson describes his research about "the photosynthetic antenna proteins and functional genomics of aspen leaves" while Christiane Funk writes about "chlorophyll transport during biosynthesis of the photosynthetic apparatus and proteases involved in senescence of aspen."

Their latest work has been published by the Proceedings of the National Academy of Sciences (PNAS) under the title "AtFtsH6 is involved in the degradation of the light-harvesting complex II during high-light acclimation and senescence" (September 20, 2005, vol. 102, no. 38, pages 13699-13704). Here is a link to the abstract of the paper.

Finally, if you're more interested by the splendor of autumn than by the protease FtsH6, you should read "Why Leaves Change Color published by the College of Environmental Science and Forestry at the State University of New York (SUNY).

Sources: RedNova News, September 17, 2005; and various web sites

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