Biologists identify
genes controlling rhythmic plant growth
Source:
September 16, 2008
CheckBiotech
The researchers
report in this week's issue of the journal PLoS
Biology that these genes control the complex interplay of plant growth
hormones, plant light sensors and circadian rhythms that permit plants to
undergo rhythmic growth spurts at specific times of the day or year in response
to varying levels of light and other environmental conditions.
Their discovery of the genetic underpinnings of the rhythmic
plant movements that enthralled Charles Darwin more than a century ago could
eventually allow scientists to design crops that can grow substantially faster
and produce more food than the most productive varieties today.
"This paper builds on our previous findings that almost
all plant genes are expressed only at a particular time of the day," said
Howard Hughes Medical Institute investigator Joanne Chory,
a professor in the Salk Institute's Plant Biology Laboratory.
"What we found is a whole raft of genes that could be
the actual molecular switches that define plant growth at the molecular
level," said Steve Kay, Dean of the Division of Biological Sciences at UC
San Diego and one of the leaders of the research team. "The more we understand
about these genetic mechanisms and how they switch on and off plant growth, the
better we will be at designing tailor-made crops to increase our production of
food and fuel for the world's rapidly growing population."
"It was known that the circadian clock confers an
adaptive advantage to plants in nature, and these findings provide a direct
mechanism by which plants optimize their growth by synchronizing hormone
signaling with the environment," said first-author Todd Michael, a former
postdoctoral fellow at Salk who is now an assistant professor of genomics and
bioinformatics at the Waksman Institute and Rutgers University. Other coauthors
included UCSD postdoctoral fellows Ghislain Breton
and Samuel Hazen, and assistant professor of genome biology, Todd Mockler, and graduate student Henry Priest of
How plants grow to maximize their survival in different
environments has long fascinated biologists. In 1880, Charles Darwin published
The Power of Movement in Plants, one of his lesser-known books in which he
describes his studies on the manner by which different types of plants grow and
move in response to various stimuli.
While most people might assume that plants grow at a slow
and steady rate throughout the day and night,
"Plants actually grow rhythmically," said Kay.
"Some plants, like sorghum, have the ability to elongate a centimeter or
more each night."
Why plants have evolved mechanisms to grow rhythmically at
night or in the hours just before dawn is a mystery. But a similar interplay of
light sensing, plant hormones and circadian rhythms that leads to a pronounced
rhythmic growth by plants during certain seasons and when shaded by other
plants has a clear survival value.
"Any plant that is growing is in a situation in which
it has to compete with the plants growing around it, so it has to develop ways
in which it can measure its environment to enable it to compete," said
Kay. "Plant cells have phytochromes, which are
essentially shade detectors that measure the ratio of different colors of light
that can tell a plant whether it's a cloudy day or whether it's being shaded by
another plant. And being shaded by another plant is bad news, because that
plant is devouring all of the right color of light for photosynthesis. If
plants detect they're shaded, they elicit growth hormones to elongate. You can
see that in the extreme when you leave something on your lawn and the grass
around it has grown taller."
To determine which genes control these rhythmic patterns of
growth, the research team turned to Arabidopsis thaliana, a tiny mustard plant
used as a laboratory model by plant geneticists. Because Arabidopsis, like many
other plants, grows fastest in the hours before dawn when exposed to day and
night cycles of light, the scientists sought to determine which of its genes
were being turned on during that period. Using DNA microarray chips, they were
able to test thousands of genes at a time to determine which ones were active
during that period.
"We did many hundreds of thousands of
measurements," said Kay, "and then asked what genes are rhythmically
being turned on and are correlated with this rhythmic growth pattern just prior
to dawn? What we found was that a whole bunch of genes all scattered around the
Arabidopsis genome that deal with hormone biosynthesis, hormone signaling and
hormone metabolism are all tightly correlated with rhythmic plant growth. This
told us that this set of genes could be the actual molecular signature that
defines plant growth at the molecular level."
The scientists said these disparate genes act together to
regulate rhythmic plant growth much like a gate with its hinges controlled by
photoreceptors and the biological clock—opening in the predawn hours to allow a
wave of multiple plant growth hormones to act within the cells, then closing
the gate to put the brakes on plant growth until the next 24-hour cycle.
"This temporal integration of hormone pathways allows
plants to fine tune phytohormone responses for
seasonal and shade-appropriate growth regulation," they write in their
paper. "Many different plant hormone genes, including genes for hormones
that promote and antagonize growth, are co-expressed at the time of day that
plants grow," said Chory. "That such an
extensive gene regulatory module exists was quite a surprise."
To illustrate their model, the scientists attached a glowing
enzyme, luciferase, to the genes they identified as
responsible for rhythmic growth in Arabidopsis plants. As the
plants go through their rhythmic growth phase, the Arabidopsis plants glow on
and off as genes that regulate the opening and closing of the gate to plant
hormones are activated, then deactivated. See video with narration at: www.biology.ucsd.edu/scicomm/video/sprouts.mov
The scientists also discovered that most of the genes
involved in this rhythmic predawn growth have a DNA sequence in common, a
master controller that they dubbed the HUD element—for "Hormone Up at
Dawn." This HUD element, they noted, must have a protein that attaches to
it that regulates its function.
"We don't know what that is, because we haven't found
it yet," said Kay. "Identifying that protein regulator is going to be
a key goal for the future because that protein is going to be very, very
important for controlling plant growth and yield."
"It's a very exciting time for biologists," Chory added, "because the tools now exist to answer
questions about complex processes, such as how plants grow or how human
metabolism goes awry."
CONTACT:
Kim McDonald
kimmcdonald@ucsd.edu
858-534-7572
Source:
greenbio.checkbiotech.org