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In Charles Darwin’s book The Power of Movement in Plants, published
in 1881, he described an experiment he had carried out in which plants bent
in response to blue light. More than a century later, two molecular biologists
have finally found a gene which they believe may code for a blue light photoreceptor
molecule.

Plant photoreceptors that respond to red light – proteins known as phytochromes
– were first discovered in the 1950s. But the search for a molecule in
plants that responds to blue light has proven much more arduous. Now, Margaret
Ahmad and Anthony Cashmore of the University of Pennsylvania report that
they have used molecular genetics to find what may be the first known example
of a blue light receptor (Nature, 11 November, p 162).

Darwin’s experiment showed that blue light affects ‘phototropism’. Å®ÉúСÊÓƵs
later learnt that blue light influences other responses such as inhibiting
stem growth and opening pores on leaves. Ahmad and Cashmore searched for
a blue photo-receptor gene in thale cress (Arabidopsis thaliana), a plant
popular with geneticists because of its simple genome, quick growth, and
small size. ‘We can screen tens of thousands of these seedlings in a small
room,’ says Cashmore. ‘If I was working with corn, I’d need half of New
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The researchers used a mutant plant whose stem growth is not inhibited
by blue light and so grows taller than normal plants under blue light. This
mutant was a transgenic plant tagged with a stretch of known DNA introduced
by bacteria that naturally infect plants and insert their own genetic sequences
into their host. By fishing out the sequences of DNA beside the tagged site,
the researchers identified an allele, or version of a gene, which seemed
to be responsible for the loss of sensitivity to blue light.

They bolstered their finding by identifying different mutations in the
same gene present in several other mutant plants which also grew taller
than average under blue light. But they could not be sure that the gene
they had found coded for the photoreceptor protein. It could have coded
for proteins responsible for synthesising the light-absorbing molecule that
binds to the photoreceptor protein, or for some other protein involved with
passing on the signal from the photoreceptor. ‘We hoped it was the photoreceptor,’
says Cashmore, ‘but we knew we had to entertain the possibility that it
could be something else.’

Ahmad and Cashmore sequenced the gene and found that it bore a ‘striking’
resemblance to the gene for a class of enzymes in bacteria which catalyse
the repair of damaged DNA only when stimulated by blue light. These enzymes
are a class of flavoproteins – which catalyse oxidation in cells – called
photolyases, and plant researchers had speculated for some time that the
blue light receptor in plants might be a flavoprotein. The similarity of
the receptor’s chemical be-haviour to that of the bacterial enzyme made
Cashmore ‘almost completely confident’ that they had found a blue photoreceptor,
but admits that ‘it’s going to be quite difficult to prove’. Researchers
also believe the resemblance between the plant and bacterial genes raises
interesting evolutionary questions.

According to Winslow Biggs of the Carnegie Institution of Washington,
a plant biochemist who studies blue light phototropism, the new study is
‘very elegant’. ‘Right now, all they have is the gene sequence, but the
chances are extremely good that they have the photoreceptor. It’s a pretty
darn strong hypothesis.