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Sensitive plants (mail-out 66, April 1999,  by Florianne Koechlin)

Dear friends

This mail-out is not about patents nor about genetic engineering; it's
about most fascinating plant-research-projects and new discoveries of
plants 'senses'.
Because I'm convinced  we should also think about what we want: What are
OUR concepts for a 'sustainable', modern and human future? What kind of
research do we prefer? Here an example.

Soon after the beet army worm (a caterpillar) starts attaking a maize-plant
and eating on the leafs , their natural ennemies arrive too: in this case a
parasitoid little wasp (cotesia margini ventris). They put their eggs into
the caterpillars and destroy them slowly from within.

But how do the wasps find their prey so fast? Ted Turlings (University of
Neuchatel, Switzerland) and a few other research-teams found out: As soon
as the plants get hurt by caterpillars they start to produce volatile
odours (mixtures of indol and terpenoids) which strongly attract the
female wasps. "These odours are actually so strong you can smell them too",
says Turlings. But how do the plants "know" when to call for help? Another
series of experiments revealed: Plants can taste the spit of the
caterpillars. Maize-leafs smeared with caterpillar-spit immediately started
to produce the odours, which attracted female wasps, even without the
caterpillars being present. It took researchers 4 years to find the
component: A chemical they call volicitin in the spit of the caterpillars
is what plants taste. And most amazingly plants seem to have a whole
repertoire of different odours they can produce to specifically attract
certain wasps and not others - and the wasps "learn" in early age what
odour can be associated with what prey."Wasps are good learners", says Ted
Turlings. He and his team also found that different varieties of maize
produce very different quantities of odours, some very much and some
little, that young plants produce much more than old plants, and that
dryness favours the production of odours. This could be a future
perspective: To get maize-plants produce more odours in order to attract
more wasps for a better protection.

Plants have many other strategies to fight off pests. For example, they can
produce different toxins (so called phyto-alexins) against invaders or they
can induce rapid cell-death of invaded cells to hinder the pests of
spreading out. All these strategies depend on quick and reliable
recognition-systems. Research of the last years revealed some amazing
abilities of plants in this area.  Plants not only can taste the spit of
caterpillars, they can also smell, see, "feel" and hear, as Andy Coughlin
reported in the New Scientist (26/9.98).

Plants can smell: Wounded tomatoes are known to produce the volatile odour
methyl-jasmonate as an alarm-signal. Plants in the neighbourhood can then
smell the danger and prepare for the attack by producing chemicals that
defend insects or attract predators. And according to New Scientist:
"Methyl jasmonate is often used in perfumes and this created problems for
the researchers. "We had to warn the women not to use it in the greenhouse
because it would mess up the experiment,"
said  Bud Ryan (Washington State University)."

Plants can see: Many plant-organs contain photo-sensitive compounds, each
reacting very specifically to certain wavelengths of light. These
light-sensors tell the plant  if it's day or night, how long the day is,
how much light is available and from where the light comes. More recently,
researchers discovered that plants also can detect harmful ultraviolet
B-rays and then start  producing pigments which filter out these rays.
"Plants make their own suntan cream in the presence of UVB", says Gareth
Jenkins (University Glasgow).

Plants have a sense of touch. We all know the mimosa plant (Mimosa pudica):
It makes its thin leaves point down at the slightest touch. And carnivorous
plants such as the Venus flytrap snap shut by the touch of insects. But a
sense of touch is something every plant has, as Coughlin describes:
"Ordinary plants need a sense of touch to respond to the buffeting of the
wind, which can cause damage to foliage. They try to resist wind by
strengthening tissues that are being swayed. The extra energy expended
stiffening tissue can cost farmers dear, however. One experiment showed
that when maize plants are shaken for 30 seconds each day, yields drop by
30 to 40% compared with unshaken plants", writes Coughlin (New Scientist).

And can plants hear? Mordecai Jaffe (Wake Forest University) used an
instrument that made a loud "warble" and got a doubling in the growth of
dwarf pea plants. Jaffe suspects that the plant hormone gibberellic acid,
which is instrumental in shoot elongation and seed germination, is involved
in the "hearing" response. When Jaffe added chemicals to the pea plants
inhibiting the biosynthesis of this hormone, he was unable to reproduce the
original effects.

Plants know much more about their surroundings than we tended to believe.
How these new findings can be integrated into  modern ecofarming-systems
still remains to be worked at. It certainly is an exciting perspective.
In the light of all these findings: Is it not so that the genetic dogma
with its mechanistic view of plants appears even more obsolete?

Literature: A.Coughlin 'Sensitive flower', New Scientist, 26.9.98//
T.Turlings and B.Benrey 'Effects of plant metabolites on the behaviour and
development of parasitic wasps',1998,Ecoscience, 5(3),321 //C.M.De Moraes
et al, 'Herbivore-infested plants selectively attract parasitoides',1998,
Nature, 393,570// H.T.Alborn et al, 'An elicitor of plant volatiles from
Beat Armyworm Oral Secretion', 1997, Science, 276,945// W.J.Lewis and
K.Takasu, 'Use of learned odours by a parasitic wasp in accordance with
host and food needs', 1990, Nature, 348,635

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