THE
SPIRIT OF PLANTS - SUBTERRANEAN VISION
By
this interesting article published in 2016 in the digital magazine New
Scientist, Alice Klein gives us a surprising information about unsuspected
capacities of plants.
“Plants
‘see’ underground by channelling light to their roots
A light-bulb moment? Plants seem to
pipe sunlight directly down into underground roots to help them grow.
Light receptors in stems, leaves
and flowers have long been known to regulate plant growth. Roots also have
these receptors, but it has been unclear how they sense light deep in dark
soil.
Sensing light in the dark
Dr John Runions/Science Photo
Library
Hyo-Jun Lee at Seoul National
University in South Korea and his colleagues used Arabidopsis thaliana – a
small flowering plant from the mustard family – as a model to study this phenomenon.
They found that the plant stem acts
like a fibre-optic cable, conducting light down to receptors in the roots known
as phytochromes. These trigger the production of a protein called HY5, which
promotes healthy root growth.
When the plants were engineered to
have phytochrome mutations, HY5 production declined. And when they had HY5
mutations, their roots became stunted and strangely angled.
Light versus
chemicals
To check whether light was directly
transmitted through the plant rather than it activating signalling chemicals
that travelled to the roots, the researchers attached a light source to the
stem of plants via an optical fibre. An underground detector at the end of the
roots confirmed that light was transmitted through.
Moreover, when they treated Arabidopsis
thaliana specimens in the dark with common plant signalling chemicals such as
sucrose, no significant increase in root growth was observed – suggesting that
such chemicals were not driving growth.
Red light was found to move most
efficiently through the plants. The long wavelengths of such light may be
favourable because they can travel further than shorter blue and green
wavelengths, says Lee.
However, the light’s intensity
would be too low for creatures in the soil to see it illuminating the roots, or
for bacteria to use it for photosynthesis, he says.
Common
function?
Most plants have phytochromes,
suggesting that directly piping sunlight down the stem is a common mechanism
used to optimise root growth, says Lee.
This makes sense, because light
signalling would be faster than chemical signalling, says Mike Haydon at the
University of Melbourne, Australia.
The study does not, however,
provide definitive proof, he says. The researchers found that it took 2 hours
from initial illumination for plants to activate their root phytochromes –
longer than might be expected if the light is directly conducted.
In addition, the study only
investigated a handful of signalling chemicals that could act as mobile
intermediaries between light, the stem and the roots, says Haydon. “This
doesn’t really exclude the possibility of a mobile intermediate signal.”
These
are partial studies, still unfinished, which reveal daily to us new capacities
of the plant world, probably one of the greatest sources of scientific
discoveries in the years to come.
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