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A throttle for plant growth
Newly discovered protein tells seedlings when they’ve reached light
Take two genetically identical plants, grow them on two different windowsills, and they could look absolutely nothing like each other. “In order to optimize their growth, plants must continuously monitor the intensity, duration and direction of light,” says Peter Hare, a research associate in Nam-Hai Chua’s Laboratory of Plant Molecular Biology.
But since plants don’t have brains — or muscles or eyes or gut feelings — the mechanics of that adaptation take place exclusively at a molecular level. More specifically, plants rely on phytochromes, a family of
receptors that monitor specific wavelengths of light. One such receptor, phytochrome A (phyA), is the key in sensing when a newly germinated seedling has emerged from underground and can begin devoting its resources to photosynthesis rather than upward growth.
It’s a critical transition, yet the specifics of how it occurs are still largely mysterious.
Now, experiments conducted by Chua, who is Rockefeller’s Andrew W. Mellon Professor, and Hare, along with Simon Møller and Li-Fang Huang, show
that loss of a protein called LAF3 causes partial blindness — leading to plants that keep growing towards the light even when they have already reached it. Normal Arabidopsis plants grow about 4.2 millimeters during four days under conditions that specifically activate phyA. Mutants lacking LAF3 grew 9.7 millimeters during that time. Mutants lacking phyA, meanwhile, grew 12 millimeters.
Hare proposes that LAF3 is involved in the activation of only a few of the many genes that get switched
on or off when a seedling first encounters light. One of those genes is XTR7, which codes for an enzyme that breaks down cell walls to allow seedling stems to elongate. Like phyA mutants, LAF3 mutants express the XTR7 gene at a much higher level than normal seedlings.
But since LAF3 is just one of roughly a dozen proteins involved in the signaling process, the Rockefeller scientists’ next step is to compile a scheme detailing how the individual components interact as a whole to control plant development. “At least for now, we’ve got enough bits and pieces in the puzzle. Our next challenge is try to fit these together and describe the relationships between them,” says Hare.
“One of the most interesting things about LAF3 is its location around the rim of the cell’s nucleus,” says Hare. LAF3 is the first phyA-specific signaling protein known to accumulate in this area. Physical separation of molecules within cells is often key to regulating signaling pathways, and there is substantial evidence indicating that plants use this approach in responding to sunlight.
“We now want to expose the biochemical function of LAF3. The sequence of the protein offers us no compelling clues, but if we mislocalize it, will it still execute its function? Although classical genetics is a great way to uncover the components of the light signaling pathway, it cannot fully reveal how the pathway works,” says Hare.

February 27, 2004



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