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Gene boosts yield, sweetness in tomato hybrids


April 5, 2010
By Fruit & Vegetable

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NEWS HIGHLIGHT

Gene boosts yield, sweetness in tomato hybrids

Giving
tomato breeders and ketchup fans something to cheer about, a Cold Spring Harbor
Laboratory (CSHL) scientist and his colleagues at the Hebrew University in
Israel have identified a gene that pushes hybrid tomato plants to spectacularly
increase yield.



March 30, 2010 — Giving
tomato breeders and ketchup fans something to cheer about, a Cold Spring Harbor
Laboratory (CSHL)
scientist and his colleagues at the Hebrew University in
Israel have identified a gene that pushes hybrid tomato plants to spectacularly
increase yield.

The yield-boosting power
of this gene, which controls when plants make flowers, works in different
varieties of tomato, and crucially, across a range of environmental conditions.

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“This discovery has
potential to have a significant impact on both the billion-dollar tomato
industry, as well as agricultural practices designed to get the most yield from
other flowering crops,” says CSHL’s Dr. Zach Lippman, one of the three authors
on the study, which appears in the journal Nature Genetics online on March
28th. The study is co-authored by Israeli scientists Uri Krieger and Professor
Dani Zamir.

The team made the
discovery while hunting for genes that boost hybrid vigour, a revolutionary
breeding principle that spurred the production of blockbuster hybrid crops like
corn and rice a century ago. Hybrid vigour, also known as heterosis, is the
miraculous phenomenon by which intercrossing two varieties of plants produces
more vigorous hybrid offspring with higher yields. First observed by Charles
Darwin in 1876, heterosis was rediscovered by CSHL corn geneticist George Shull
30 years later, but how heterosis works has remained a mystery.

Shull’s studies suggested
that harmful, vigour-killing gene mutations that accumulate naturally in every
generation are exposed by inbreeding, but hidden by crossbreeding. “But there
is still no consensus as to what causes heterosis,” says Lippman. “Another
theory for heterosis, supported by our discovery, postulates that improved
vigour stems from only a single gene – an effect called ‘superdominance’ or
‘overdominance.’”

To find overdominant
genes, the team developed a novel approach by turning to a vast tomato “mutant
library” – a collection of 5,000 plants, each of which has a single mutation in
a single gene that causes defects in various aspects of tomato growth, such as
fruit size, leaf shape, etc. Selecting a diverse set of mutant plants, most of
which produced low yield, the team crossed each mutant with its normal
counterpart and searched for hybrids with improved yield.

Among several cases, the
most dramatic example increased yield by 60 per cent. This hybrid, the team
found, produced greater yields because there was one normal copy and one
mutated copy of a single gene that produces a protein called florigen. This
protein, touted as the breakthrough discovery of the year in 2005 in Science
magazine, instructs plants when to stop making leaves and start making flowers,
which in turn produce fruit.

In plants such as
tomatoes, flowering (and therefore yield) is controlled by a delicate balance
between the florigen protein, which promotes flowering, and another related
protein, that delays flowering. A mutation in only one copy of the florigen
gene causes the hybrid to produce more flowers in less time – the key to
improved yield.

“It’s the Goldilocks
concept,” explains Lippman. “What we find is that to maximize yield, you can’t
have too much or too little florigen. A mutation in one copy of the gene
results in the exact dose of florigen required to cause heterosis.”

The scientists have
observed the gene’s heterosis effect in different varieties of tomatoes and in
plants grown in different climate and soil conditions, both in Israel and
locally in New York at CSHL and the Cornell Horticultural Experiment Station at
Riverhead, NY.

In addition to superior
yield, the hybrids also display another, perhaps equally important quality –
taste. Tomato plants only produce a finite amount of sugar, which they
distribute equally among their fruits. So higher yields usually result in each
fruit having less sugar. But, remarkably, the florigen gene also boosted the
sugar and sweetness of individual fruits.

The researchers are
already planning to explore if genes related to florigen in other crops can
cause heterosis and improve yield. The concept that a mutation in only one copy
of a single gene can improve yield has broad implications for plant breeding.
“Mutant plants are usually thrown away because of the notion that mutations
would have negative effects on growth,” says Lippman. “Our results indicate
that breeding with hybrid mutations could prove to be a powerful new way to
increase yields, not only in tomato, but all crops.”

The research was funded by
grants from the National Science Foundation, the Israel Science Foundation and
EU-SOL.