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Biochemistry of plant pathogens -

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Biochemistry of plant pathogens

Some plants make potent insecticidal molecules. Flowers are rich in antibiotics. After Marilyn
Anderson isolates the chemicals, Kim Plummer tries to determine how these chemicals work.
Applications of this work is in engineering crops such as corn to be resistant to attack from


Robyn Williams: Plenty of research being done in that field in Australia as well, not least in
Melbourne at La Trobe University. Marilyn Anderson and Kim Plummer work on fungi but on other
plants as well, with all kinds of properties, like hurrying up childbirth. Marilyn Anderson is a
professor of biochemistry at La Trobe.

Marilyn Anderson: Accelerated childbirth, this is an area we got into when we were studying a
natural tribal medicine from the Congo region in Africa. The story started when the Red Cross were
there when the wars were on in the Congo, and a doctor discovered that the families were coming to
visit women in labour and were smuggling in this tea. The women had this tea and then the
childbirth would accelerate and be out of control. So this Norwegian doctor took this tea home or
found the plant, it was called Kalata kalata, and extracted the molecule and found it was a very
interesting and new molecule. David Craik subsequently in 1995 found it was a cyclic peptide, a
tiny little peptide, and that it was able to irritate the uterus enough to accelerate childbirth.

Robyn Williams: I see, so has that been exploited to use it in a clinical sense?

Marilyn Anderson: David is trying to exploit it to use it in a clinical sense but not to accelerate
childbirth because it turns out that a lot of molecules stimulate childbirth because the uterus is
a twitchy organ. But actually the thing that was exciting about this molecule was that that they
could boil the plant, prepare a tea and that the women could take this orally and that it would
work, and this suggests that it was absorbed. So David Craik's group in Queensland is interested in
having this as an oral pharmaceutical and he wants to use this molecule to graft other activities
on it. So his dream is to have an oral inulin or some other molecule that doesn't have to be

Robyn Williams: Have you been involved directly yourself?

Marilyn Anderson: Our role has actually been to see why does the plant make this molecule, because
obviously plants aren't interested in women having more rapid childbirth, and so we extracted the
molecule and actually found out how it's made because this is the first cyclic molecule ever found
that's gene encoded, we discovered that. And then we went aside and said, well, why are the plants
making it? And we found that they're really potent insecticidal molecules.

When we first did this we thought we'd feed some insects this molecule, and we had a little
artificial diet, and we put some caterpillars in with the artificial diet and they did everything
they could to escape. They ate their way out of the container, and the first three experiments
failed until we found a way of keeping them in there, and then we found that they would take a
taste and then they would just not eat anymore and they failed to grow. So it's a very efficient
insecticidal molecule.

Robyn Williams: How wonderful, of course a tremendous source of these chemicals. Before you
introduce me to Kim, how do you two combine your work together?

Marilyn Anderson: My interest with Kim is with another set of naturally occurring plant molecules
but these are molecules that protect plants against fungal disease, and specifically the molecules
I'm working with here are molecules from flowers because it's very important that flowers are not
infected because it's really important that they produce seeds and go on to make the next
generation. So we're finding that flowers are very rich in natural antibiotics, and so we went
ahead to try and isolate these molecules and we found one that was really very effective. Now my
lab is trying to understand why it is that it kills a fungus. So we're trying to understand exactly
how it works. But we needed to go and talk to Kim because she's an expert on fungi and how a plant
interacts with a fungus, and we really needed a highly trained fungal plant pathologist.

Robyn Williams: Hello Kim.

Kim Plummer: That's where I step in. So yes, we're looking specifically at how the molecules that
Marilyn has isolated from those flowers go about their business in attacking the fungus and killing
the fungus off and indeed killing a range of different fungi as well.

Robyn Williams: Any successes so far?

Kim Plummer: Absolutely. The majority of the fungi that have been looked at so far have actually
been found to be killed by this molecule, and we're looking at various different mechanisms to
dissect the interaction. One of Marilyn's students, Nicole van der Weerden, has done some really
nice work in looking at biochemically treating fungi to work out what perhaps the molecules from
the plant bind to on the fungal surface. And we've had some other students who are working away at
being able to biochemically treat and look at different layers of where the protein might bind to,
and we're also looking at modifying the genetics of the particular fungus to see if we can try and
get the genes involved with the structures that the plant proteins attack to then try and dissect
how that's getting in and how that's working.

Robyn Williams: Of course plenty of agricultural products are vulnerable to such things. I was
going to say Phytophthora infestans on potatoes, but you're going to tell me it's not a fungus.

Kim Plummer: That's true, that's right, it's not a true fungus as was previously believed. The
classic fungal pathogen is no longer accepted as being a fungus. But yes, a lot of these plant
pathogens, including fungi and oomycetes, which...Phytophthora infestans is an oomycete...a lot of
these pathogens have been and are fundamentally controlled by chemicals. And various different
chemicals have been used and are the primary weapon against plant diseases to this date. Fungi and
other plant pathogens are really good, however, at overcoming different chemicals. They have very
plastic genomes, there are vast populations of pathogens with the ability to mutate to overcome the
mode of action of those different chemicals.

Robyn Williams: Like bacteria in hospitals and us.

Kim Plummer: Precisely, exactly like bacteria and their ability to overcome or to mutate and
change, and thereby being not able to be targeted by an antibiotic. So the process of mutation,
it's quite a natural process that happens, and if the mutation ends up resulting in the fungus
being able to avoid that interaction with that chemical, or indeed avoid recognition by a naturally
occurring resistant plant, then you get a fungus that's more effective in causing disease.

Robyn Williams: Just checking, as you've knocked down one of my iconic funguses, what about the
ones that attack vines in vineyards, which you treat in the old-fashioned way with copper sulphate?
Are they still fungi?

Kim Plummer: They are. Actually the two different pathogens that attack grapes, the classic
botrytis which gives us the lovely botrytised, sticky, sweet, yummy wines, a bit of a friend and a
foe, that's a true fungus. But the downy mildew that attacks grapes also is not a true fungus, that
also is an oomycete. So it's been kicked out of the true fungi.

Robyn Williams: Back to yourself with the biochemistry. What's your next stage in this quest to
keep one step ahead of these amazing creatures?

Marilyn Anderson: First of all, as Kim said, we're trying to actually understand exactly how this
molecule works so that if a fungus does become resistant we understand exactly how that happens.
But we're not restricting ourselves to this molecule, we're looking at other naturally occurring
plant molecules that are toxic to fungi, and then we're trying to find ones that work by different
mechanisms of action. So the ultimate aim is that we will use combinations of molecules so that if
a fungus becomes resistant to one, it will still be affected by the other one.

Robyn Williams: Is there a scale of financial impact that occurs to you in Australia, say, how many
crops are knocked off in the classic sense...even though it's not a fungus, the one that used to be
a fungus changed the course of history in Ireland, as we all know.

Marilyn Anderson: Actually Kim and I were talking about this before we came to talk to you and we
said; just what are the losses? The losses vary from year to year. Sometimes they're just happy to
get a crop at all. So Kim was saying that quite often in the best seasons...the best season for the
best crop is also the best season for fungal disease, so you may lose 20% of the crop but you still
get enough to go on with. Other years you might lose the entire crop, and then that's especially
devastating. That's especially true for small crops where there's not the same amount of research
gone into developing the ideal chemical, or organic crops where they're only dependent on copper
and that doesn't always hold the fungus.

Robyn Williams: And there they are in the soil standing by almost forever because soil is such a
complex thing, and I suppose they can last for thousands of years if they want to.

Kim Plummer: Well, it depends on how they survive. Some fungi require a host and so they need a
living host to be available pretty much all the time. A lot of them have developed survival
structures so that they can survive in the soil for 10 or 15 or more years. Often those pathogens
are the ones that are the most difficult to control because they've got those nice survival
structures that hang out in the soil. Another major issue with a lot of the pathogens that we work
with is how specific they are in terms of the plants that they will infect.

One of the pathogens that I work with, a sclerotinium, it produces one of these little resting
structures that survive in the soil for many years, and it can infect just about every broad-leafed
crop species that we grow. Many of the broad-leafed fruit and vegetables and the broad-leaf crops
such as canola are susceptible to sclerotinia, so because this one species of fungus can attack so
many different hosts and devastate many of those different crop species it's quite hard to
understand how it's actually working, and therefore any kind of control that would be useful for
one crop could have many different applications for other crops as well. Another pathogen I work on
just infects apple. It infects apple and very closely related species of apple, crab-apple.

Robyn Williams: It comes from New Zealand perhaps?

Kim Plummer: It's all over the place.

Robyn Williams: We mustn't always blame the New Zealanders.

Kim Plummer: No, indeed not, in fact I worked for ten years in New Zealand so we should be very
careful about our buddies overseas, and I still collaborate with them on that particular apple
pathogen. So there are lots of different types of pathogens, and the way they survive and the way
they grow and how plants can defend themselves against those pathogens is very much a part of what
I'm doing and the work that Marilyn is involved with.

Robyn Williams: And the question I put before, the scale of the reward should you have
success...what was the figure that occurred to you?

Kim Plummer: With something like the protein that Marilyn's laboratory have developed, it could
have a very broad-spectrum range of pathogen that will be able to be controlled by this, and
therefore...I wouldn't like to even attempt to estimate it, but it could be extremely valuable for
such an impact.

Robyn Williams: How long will it take to get it into the field?

Marilyn Anderson: It's actually been in the field already. So as a spin-off of our work we have a
biotechnology company called Hexima and we've had three years of field trials with transgenic
cotton, and I shouldn't say that we were surprised but we were surprised, we could see an effect.
We could see an effect from the air. It's the first time I've ever done an experiment where we
could look at results from the air, and we saw protection. We're still working on it of course but
we're very encouraged. And we started a new project in collaboration with Pioneer DuPont to try to
make corn more fungal resistant, and of course that's a very valuable crop worldwide.

Robyn Williams: Marilyn Anderson, professor of biochemistry at La Trobe University in Melbourne,
with botanist Kim Plummer.