The research team's findings are being published in the scientific journal PNAS, Proceedings of the National Academy of Sciences.
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. There is so much of it, in fact, that it is visible from space--in satellite images of the earth the tropical and temperate forest areas are green.
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, and there is extensive research under way in this field. For example, the 2004 Nobel Prize went to three scientists who work with proteases. Proteases are extremely important for all living organisms, but the proteases that break down chlorophyll-binding proteins are the only ones whose activities can be observed from space.
Working with the model plant mouse-ear cress (Arabidopsis thaliana), a research team at Umea Plant Science Center (UPS C), in association with a Polish scientist, has identified a protease that degrades LHCII. The researchers assumed that the protease belonged to a certain family of proteases, the so-called FtsH proteases, and they used genetically modified mouse-ear cress plants in which various FtsH proteases had been removed. One of these plants had a severely impaired ability to degrade LHCII. This led the researchers to conclude that the protease FtsH6 helps degrade LHCII.
The article is titled "AtFtsH6 is involved in the degradation of the light harvesting complex II during high light acclimation and senescence." The authors are Agnieszka Zelisko, Maribel Garcia-Lorenzo, Grzegorz Jackowski, Stefan Jansson, and Christiane Funk. Grzegorz Jackowski works at Adam Mickiewicz University in Poznan; among the UPSC scientists, Stefan Jansson works at the Department of Plant Physiology and the others at the Department of Biochemistry. The article is being published this week in the Early Edition articles of Proceedings of the National Academy of Sciences of the USA ( http://www.pnas.org/papbyrecent.shtml).
Ume?Plant Science Center, UPSC, was established in 1999 in collaboration between the Department of Plant Physiology at Umea University and the Department of Forest Genetics and Plant Physiology at the Swedish University of Agricultural Sciences (SLU) in Umea.