University Park, Pennsylvania
September 29, 2008
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The
researchers created transgenic lines of rice (G-2 and
G-8) in which the GIF1 gene was overexpressed. Compared
to the normal strain (WT), they found that the
transgenic rice had larger and heavier grains. In this
figure, the grains on the top are encased in their
shells and the grains on the bottom have had their
shells removed.
Credit: Zuhua He, Chinese Academy of Sciences |
A team of scientists, including
Penn State University
Distinguished Professor of Biology Hong Ma, has identified a
gene in rice that controls the size and weight of rice grains.
The gene may prove to be useful for breeding high-yield rice
and, thus, may benefit the vast number of people who rely on
this staple food for survival. "Our work shows that it is
possible to increase rice's yield by enhancing the expression of
a particular gene," said Ma. The team's results will be
published on 28 September 2008 in an early online edition of the
journal Nature Genetics, and in the November print issue of the
journal.
The researchers first searched for and identified mutant strains
of rice that exhibited underweight grains. "We found a
particular mutant that is defective in its ability to produce
normal-sized grains," said Zuhua He, a biology professor at the
Chinese Academy of Sciences
and the leader of the team. The group then examined the mutant
and found that it carried a mutation within the GIF1 gene. "The
GIF1 gene is responsible for controlling the activity of the
enzyme invertase, which is located in the cell wall and converts
sucrose to substances that then are used to create starch," said
He. "Invertase is important in the formation of starch within
developing grains of rice. If invertase is not active, the rice
plant cannot produce edible grains."
Next, to test the ability of the
GIF1 gene to control the production of invertase, the team
measured the activity of invertase within a normal strain of
rice, in which the GIF1 gene lacked any mutations, and within a
mutant strain of rice, in which the GIF1 gene contained a
mutation that caused a defect in the invertase activity. The
scientists found that invertase activity in the mutant strain
was only 17 percent of the activity that was observed in the
normal strain, suggesting that the GIF1 gene does, indeed,
control invertase activity. The team then created transgenic
lines of rice in which the GIF1 gene is overexpressed and found
that, compared with normal strains, the transgenic rice had
larger and heavier grains.
According to Ma, the team was surprised to find that the GIF1
gene was so specialized in controlling invertase activity in a
particular part of the grain -- the vascular tissue, which
transports nutrients, including sugars generated by invertase,
to the developing grain. "The expression pattern was not
expected, in part, because invertase is a general enzyme that is
used by many cell types. In fact, the corresponding gene in wild
rice is not expressed specifically."
The team also found that the GIF1 gene is one of the genes that
were selected during the domestication of rice. "By selectively
growing only those strains of rice with heavier grains, humans
for thousands of years unknowingly have been increasing the
frequency of rice populations that had modifications in the GIF1
gene," said Ma. "This process has caused GIF1 to be expressed
specifically in the vascular tissue and, thus, to produce larger
rice grains," said Ma.
The scientists hope that their findings will help others to
create hybrid varieties of rice that produce even larger grains.
In the meantime, they plan to perform additional analyses that
will help them to understand how other genes might be involved
in the process of improving rice yield. "The goal is to
understand what controls grain weight and other factors, and to
look for ways to increase yield," said Ma.
This research was supported by grants from the Ministry of
Science and Technology of China, the National Science Foundation
of China, and the Shanghai Institutes for Biological Sciences. |
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