UNIVERSITY PARK (Centre
Co.) A gene long suspected of
controlling the self-incompatibili
ty mating system in plants has
finally been caught in the act by a
team of Penn State biologists.
Led by Teh-hui Kao, associate
professor of biochemistry and
molecular biology, the team is the
first to show directly that this gene
determines whether a plant is able
to fertilize itself.
For more than 130 years, since
Darwin observed that some plants
can fertilize themselves with their
own pollen while others cannot,
scientists have been trying to
understand exactly what controls
this aspect of plant mating. Now,
in a paper published in the Feb. 10
issue of the journal Nature, the
Penn State biologists have pro
vided the first direct evidence con
firming a theory of genetic self
incompatibility that is the founda
tion for years of research in plant
genetics.
The cornerstone of this theory
is the self-incompatibility gene, or
“S gene.” According to the theory,
a plant that cannot fertilize itself
has an S gene that is “turned on,”
enabling it to produce in its pistil a
protein that recognizes and rejects
its own pollen. If a plant’s S gene
is “turned off,” it does not produce
the S protein, so it is able to fertil
ize itself.
“A protein identified in the ear
ly 1980 s seemed to be the predict
ed S protein,” said Kao, “but our
strongest clues until now were on
ly from indirect evidence.”
By harnessing standard genetic
engineering techniques, Kao’s
team was able to neutralize the
gene in a group of petunia plants,
reversing their inherited inability
to fertilize themselves and en
abling them to produce seeds. The
biologists also inserted the gene
into another group of plants, giv
ing them the ability to reject pol
Win Top
Awards In Contest
Farmers
DEKALB, 111. Richard
Schmaltz of Doylestown, Pa. won
the state first-place award in the
National Grain Sorghum Produc
ers (NGSP) yield and manage
ment contest, non-irrigated divi
sion. This is his third consecutive
win.
R. Gregory Manners of Rin
goes, NJ. won the state first-place
award in the National Grain Sor
ghum Producers (NGSP) yield
and management contest, non-irri
gated division. This is his third
consecutive win.
Also, Schmaltz won the award
with DeKalb DK37 sorghum
which produced 90.32 bushels per
acre. Tbe yield ranked him among
DeKalb’s 14 national and state
first-place winners.
He will be honored along with
other state and national winners at
the NGSP's annual convention in
Nashville, Tenn. DeKalb Plant
Genetics also will present him
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Plant Mating Mystery Solved
len with a specific genetic identi
ty-
The biologists performed two
experiments to show that a plant’s
ability to produce seeds when self
pollinated depends on the pre
sence or absence of an active S
gene. In the first experiment, they
disabled an S gene in aline of self
incompatible plants, then attempt
ed to fertilize them with their own
pollen.
“We reasoned that if an S pro
tein is required for self-incompati
bility interactions between pistil
and pollen then inhibition of its
synthesis should lead to the break
down of self incompatibility,”
Kao said.
Each plant has two varieties of
the S gene, called S alleles, which
it inherits from the parent plants.
Kao used petunia plants that had
alleles called S 2 and S 3. He used a
genetic engineering technique to
produce an “antisense” S 3 allele
whose DNA sequence order is the
reverse of a normal S 3 allele’s.
Normal alleles produce RNA in a
normal sequence order that makes
genetic “sense.”
“Antisense RNA is able to
block the synthesis of protein
from sense RNA in a mysterious
way that we do not yet under
stand,” Kao said.
Next, the team, including post
doctoral fellow Hyun-Sook Lee
and graduate student Shihshieh
Huang, incorporated the antisense
S 3 allele into a bacterium that they
then used to infect the petunia
leaves. From these leaves, they
grew transgenic plants containing
the three alleles, S 2, S 3 and anti
sense S 3.
“Although this is a standard
procedure, it turned out to be the
most critical step in this experi
ment,” Kao says. “We struggled
for about a year before we were
able to successfully grow trans
genic petunias.”
with an award during a special re
ception.
Manners won the award with
DeKalb DK37 sorghum which
produced 109.93 bushels per acre.
The yield ranked him among De-
Kalb’s 14 national and state first
place winners.
He also will be honored at the
NGSP’s annual convention in
Nashville, Tenn. DeKalb Plant
Genetics also will present him
with an award during a special re
ception.
Schmaltz, who has been farm
ing 40 years and has been a De-
Kalb dealer more than 30 years,
grows com, sorghum, soybeans
and wheat on 573 acres. He plants
193 acres of that to sorghum.
Manners, who has been farming
21 years, plants 80 of his 700
acres tp sorghum. On the other
acreage he grows com, soybeans,
oats, rye, hay, sorghum/Sudan
grass. He also raises about 30 head
of cattle.
The team tested these transgen
ic plants, found they were not pro
ducing any S 3 protein and at
tempted to fertilize them with S 3
pollen.
“A normal plant with S 2 and S 3
alleles, when pollinated with S 3
pollen, will reject the pollen be
cause the S allele types match.
The flower’s pistil recognizes the
pollen as 'self pollen,’ fertilization
fails, and the plant does not pro
duce seeds,” said the Penn State
biologists. “But our transgenic pe
tunias produced the same large
number of seeds as you would get
from compatible pollination,
showing that they had lost the
ability to reject self pollen.”
Kao says this is the first suc
cessful attempt to use the anti
sense approach in any self-incom
patible plant species.
Growers of self-incompatible
crops such as apples could benefit
from this part of Kao’s research,
according to George Greene, asso
ciate professor of pomology at
Penn State.
Because apples arc self-incom
patible, commercial apple growers
typically mix, in a single orchard
block, three varieties that they
carefully select to provide sources
of compatible pollen. Cultivation
of a single self-compatible variety
would increase efficiency. Greene
said by reducing several cultural
and harvesting problems.
In their second experiment,
Kao’s team put an S 3 gene into
petunias that contained SI and S 2
alleles. A normal plant with SI
and S 2 alleles will accept S 3 pol
len because the S 3 allele carried
by the pollen is different from the
SI and S 2 alleles carried by the
flower’s pistil. However, Kao’s
team found that some of the trans
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genic plants produced no seeds at
all when pollinated with S 3 pol
len.
'The transgenic plants that fail
ed to produce any seeds at all had
normal levels of S 3 protein for a
plant containing an S 3 gene,
which enabled them to acquire the
ability to completely reject S 3 pol
len,” Kai said. “The transgenic
plants that produced a few seeds
when pollinated with S 3 pollen
had levels of S 3 protein that were
much lower than normal, and
those transgenic plants producing
the most seeds did not have any
detectable amount of S 3 protein.”
He said that this experiment
shows that S-protein levels alone
control a plant’s ability to reject its
own pollen or pollen whose S
allele type is identical to one of
those contained in the flower’s
pistil.
“The ability to prevent plants
from fertilizing themselves could
double the yield and reduce by
one-third to two-thirds the labor
costs involved in hybrid seed pro
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Styer Professor of Horticultural
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Virtually all commercially im
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FI hybrid seeds, the result of
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in order to assure the uniformity
of hybrid seeds, growers typically
must remove by hand the pollen
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self-incompatible, you could har
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using much less costly and more
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Craig said. “In addition, Dr. Kao’s
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“Confirmation that the S gene
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scientists who have published
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Craig said, “Many generations
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Keo said his team’s next re
search goals are to determine
whether the S protein, a ribonucle
ase, digests the pollen’s RNA or
blocks its growth in some other
way and to identify exactly which
of the protein’s amino acids re
cognize self-pollen.
“We have not yet captured the
holy grail of this field, which is to
determine the precise bio-chemi
cal mechanism of self-incompati
bility,” Kao ;aid, “but this goal is
the focus of our work, which looks
like it could turn out to be a life
time project. ’
This research was supported by
grants from the National Science
Foundation and the United States
Department of Agriculture.
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