Wax discovery surprises
Unexpectedly, plants use a lipid transporter like those in mammalian cells
to transport wax
 By Graciela Flores

Plants export wax from epidermal cells to the surface of their aerial parts
through a lipid transporter similar to those present in mammalian cells,
researchers report in Science this week. This is the first component of the
plant lipid export system to be characterized functionally.

"Up until now, we knew that plants produce this waxy coating on their
cuticle, which is essential for water conservation, and for their ecology in
general, but no one knew how these highly hydrophobic molecules that are
made in the cells get out of the cells," author Lacey Samuels, of the
University of British Columbia, told The Scientist.

Analyzing the export of wax precursors biochemically is an extremely
difficult task, so the researchers chose the genetic approach. While
searching for an Arabidopsis thaliana plant defective in lipid transport,
they came across cer5, a mutant with a glossy, bright green stem phenotype
caused by a reduced wax layer. "It had a very unusual cell structure," said
Samuels, "with cytoplasmatic lipid inclusions I had never seen before."
Samuels was surprised to discover that the curious structures were identical
to those present in patients with the genetic disease adrenoleukodystrophy
(ALD). The fact that ALD patients have a defect in an ATP-binding cassette
(ABC) lipid transporter put them in the right direction.

The CER5 gene, they later discovered, indeed encodes an ABC transporter,
actually a "half transporter" that probably forms a dimeric pore.
"It really
fit well with our mutant phenotype and with both the chemical and structural
analysis," Samuels explained.

Philip Rea, of the University of Pennsylvania, who did not participate in
the research, thinks this study is "a beautiful juxtaposition of detailed
microscopy, histochemical GUS analyses, and work with GFP [green fluorescent
protein] fusions to demonstrate localization of the transporter, and of the
waxy inclusions."

Rea, however, was left with uncertainties. "Although the simplest hypothesis
is that CER5 is a lipid transporter, it cannot be discounted that it acts
more indirectly by regulating other transporters," he said. "Does CER5 have
a broad substrate specificity, or does it transport a single species, whose
transport is needed for the transport and/or stabilization of a wider range
of waxy substances? CER5 might even transport a protein that complexes waxy
materials for ejection across the plasma membrane."

Traditionally, wax precursors were thought to be exported by a vesicular
pathway from their site of synthesis to their destination at the plant
surface. The identification of the CER5 transporter does not absolutely rule
out the vesicular hypothesis. Because of the limited resolution of
fluorescence imaging, it remains possible that CER5 is localized in a
compartment involved in secretion, according to Wolf Frommer of Stanford
University, an author of an accompanying Perspective article. However, "this
study advances a very interesting mechanism that provides the handle to
study this in more detail." The Perspective article develops a number of
alternative hypotheses to try to explain how this system could be working.

"The simplest hypothesis, advanced by the authors, is that the transporter
located in the plasma membrane has a pore that goes from the inside to the
outside through which lipids go out. It could also have a side pore, similar
to the bacterial MsbA ABC transporter, through which the lipids enter or
exit the transporter. A third possibility is that the transporter is not a
pore but a flippase that flip-flops fatty acids from the inner to the outer
leaflet of the plant cell plasma membrane," Frommer suggests.

For Frommer, these results could be a boon for agriculture. "By making
different types of wax layer you could probably change the plant
permeability to water, or the properties related to the recognition of the
plants by other organisms such as pathogens. We could get more from the
potential the plant has."

Links for this article
J.A. Pighin et al., "Plant cuticular lipid export requires an ABC
transporter," Science, 306:702-4, October 22, 2004.
http://www.sciencemag.org/

Lacey Samuels
http://www.botany.ubc.ca/samuels.html

Philip A. Rea
http://www.bio.upenn.edu/faculty/rea/

Wolf Frommer
http://www-ciwdpb.stanford.edu/research/research_frommer.php

B. Schulz, W.B. Frommer, "A plant ABC transporter takes the lotus seat,"
Science, 306, October 22, 2004.
http://www.sciencemag.org/

>From The Scientist.com
http://www.biomedcentral.com/news/20041022/01

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