Janet Braam and Ronald Davis, two molecular biologists at Stanford University,
have studied the common wall, or thale, cress (Arabidopsis thaliana). Initially,
they set out to study genes that are turned on in plants when they are exposed
to certain hormones. But this work gave them insight into ‘touch-induced’
genes and how they are switched on (Cell, vol 60, p 357).

The biologists chose the wall cress for several reasons. First, it has
the smallest known genome, or complement of genetic material, of any of
the higher plants: less than 1 per cent of the genetic material that wheat
has, and only five times as much as yeast. It is easier to clone genes from
wall cress than from any other plant. A further advantage of wall cress
is that it is small and grows quickly.

Braam and Davis sprayed wall cress plants with a solution of hormone
called gibberellin. They then used standard techniques of molecular cloning
to isolate nine genes that appeared to be switched on by the hormone. But,
to their surprise, they found that five of the genes were also turned on
when the plants were sprayed with water alone. The genes began to be active
within an hour of spraying.

In further experiments, Braam and Davis found that the same five genes
were turned on in many other circumstances. For instance, they became active
when the researchers rubbed or touched the plants’ leaves, or subjected
them to cool air from a hairdryer. The biologists concluded that it was
not the hormone that stimulated the genes to turn on. Instead, the disturbance
when the plants were later taken from a growth room to the laboratory for
analysis was sufficient.

Braam and Davis had another surprise when they analysed the touch-induced
genes by sequencing their DNA. One of the genes turned out to be the gene
responsible for making a small protein called calmodulin in wall cress.

The protein is found in all fungi, plants and animals – although not
in bacteria. It is known to have a very important role in processes within
cells that are controlled by the concentration of calcium ions – for instance,
muscle contraction and the release of neurotransmitters, or chemical messengers,
at the synapses between nerves. Each calmodulin molecule binds to four calcium
ions. Once it has bound them, it binds in turn to important enzymes, triggering
several biochemical important events.

The link between the gene and the proteins suggests that a calcium signal
is somehow involved in the touch response – an idea which is strengthened
by the finding that two of the other touch-induced genes code for new proteins
that are rather similar to calmodulin.

It will take many more experiments before we understand how and why
the wall cress responds to touch in the way that it does. What is already
clear, however, is that this may be a breakthrough in understanding an earlier
observation that plants exposed to wind tend to be less elongated than protected
plants, and that this effect can be reproduced in the laboratory by touching.
Scientists call the touching response: thigmomorpho-genesis.

Being Californian, the biologists did try playing music to their plants.
For a minute, they played the music of the rock group ‘Talking Heads’ at
60 decibels, a sound level a little below ordinary conversation on a standard
scale of loudness. The touch-induced genes, however, were unaffected. Maybe
plants like their rock music loud, or perhaps they prefer classical music.
Or it may be that they react to music by turning on a different set of genes,
the biologists speculate.

Willem Broekaert and his colleagues at the Catholic University, Leuven,
have studied lectins, proteins that recognise and bind to specific sugar
molecules. Some lectins are toxic and are abundant in seeds, helping to
deter animals from eating them. But why should the adult plant make lectins?
Scientists know that a lectin found in wheatgerm binds to the polysaccharide,
chitin. Chitin is strong, chemically resistant and makes up the shells of
crustaceans and the exoskeletons of insects. Chitin is also a component
of the thick cell wall of many types of fungi. For this reason, scientists
have suspected that the chitin-bonding lectin in wheatgerm might protect
wheat seedlings that are germinating from attack by fungi.

Unfortunately, the earlier experiments designed to test this idea used
a lectin preparation contaminated with traces of an enzyme, chitinase. This
substance attacks the chitin molecule and breaks it down into its component
sugars.

Now Broekaert and his colleagues have managed to extract from the common
stinging nettle another lectin that bonds to chitin. They have purified
it to remove all traces of chitinase enzyme. In the laboratory, this pure
lectin did indeed block the growth of several types of fungus. Encouraged
by this result, they measured the lectin in the nettle plant. They found
that in the rhizome, or the underground stem of the plant, the protein is
present at about 10 times the level needed to block the growth of fungi
in the laboratory.

Broekaert and his colleagues found that the lectin is concentrated in
the outer layers of the rhizomes and roots, not in stems or leaves. This
suggests that it helps to defend parts of the nettle that are exposed to
fungi in the soil.

The scientists suggest that the nettle lectin may be able to penetrate
deep into the cell wall of invading fungi and interfere with the continuous
remodelling of the fungal wall that happens as the fungus grows. The small
size of the lectin molecule makes it particularly attractive to genetic
engineers working to devise new strains of crops. Biotechnologists may be
able to create crops resistant to some fungi.