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A throttle for plant growth
Newly discovered protein tells seedlings when they’ve reached light
BY TIEN LEE
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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