St. Louis, Missouri
October 1, 2007
Botanical 'cloak and dagger'
That clover necklace you make for
your child could well be a ring of poison. That’s because some
clovers have evolved genes that help the plant produce cyanide –
to protect itself against little herbivores, such as snails,
slugs and voles, that eat clover. Other clover plants that do
not make cyanide are found in climates with colder temperatures.
So, in picking your poison, er, clover, ecology and geography
play important roles.
A plant evolutionary biologist at
Washington University in St. Louis is trying to get to the
bottom of this botanical cloak and dagger tale. Kenneth Olsen,
Ph.D., Washington University assistant professor of biology in
Arts & Sciences, is looking at the genetics of a wide variety of
white clover plants to determine why some plants do and some
plants don’t make cyanide – what biologists call polymorphism,
or two types.
“We are documenting the effect of natural selection at the DNA
sequence level to understand the molecular evolution of this
polymorphism,” said Olsen. “Usually, researchers study model
plants such as Arabidopsis or tobacco to understand genetics.
But with clover we have a system where we can look in detail at
DNA sequence variation and at the same time have a thorough
understanding of the plant’s ecology.”
In a study published the week of Sept. 24 in the journal
Molecular Ecology, Olsen and his colleagues report findings on
the molecular basis of the cyanide polymorphism.
Cyanide “bomb” in cell
White clover is native to Europe and Asia and was introduced
some 300 years ago in North America. The cold factor-acyanogenic
relationship has been known a long time in Europe and Asia and
it re-evolved in North America when the plant was introduced,
indicating that natural selection was a powerful force in
shaping the geographical distributions of the two plant types.
The genetic basis behind cyanide production in clover plants
boils down to just two genes.
“A cyanogenic plant sets up a little cyanide bomb in the cell,”
Olsen explained. “You have a cyanogenic glucoside – basically a
sugar with a cyanide group stuck onto it, in the cell vacuole,
and then in the cell wall there is an enzyme required to
hydrolyze the cyanide. If something damages the cell, these two
compounds come into contact with each other and free cyanide is
released.” One gene, Li, encodes the enzyme, which is called
linamarase; another gene, Ac, is responsible for the presence or
absence of cyanogenic glucosides.
Olsen’s recent findings have revealed that plants that do not
synthesize linamarase are lacking the Li gene altogether: the
gene’s DNA is absent from genomes of these plants.
Olsen and colleagues are also testing hypotheses on why
acyanogenic plants occur in cold climates. One poses that there
are fewer herbivores in colder climates.
“If a plant can get by without investing in all the resources it
takes to be cyanogenic, it can concentrate those resources in
other forms of growth and reproduction, then it would be out
competing the plants that are cyanogenic,” Olsen said.
Suicide not ruled out
The second hypothesis explores the lurid possibility of plant
suicide. “In hypothesis two, we question the role of frequent
frosts. The frosts could cause cell rupture and the release of
cyanide leading to autotoxicity. If cyanogenic plants are
poisoning themselves in cold climates, then those plants will be
at a disadvantage.”
To examine both the weather factor and suicide possibility,
Olsen and his colleagues are testing different types of clovers
in freeze chambers at controlled temperatures to see if survival
is higher for either acyanogenic or cyanogenic plants.
“The advantage of the clover cyanogenesis system is there’s
already so much known about its ecology,” Olsen said. “What
we’re able to do now is get to the molecular level and look at
the molecular basis of ecologically important variation.” |
|
