By Sara LaJeunesse

A team of scientists, including Hong Ma, Penn State distinguished professor
of biology, 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 Sept. 28 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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