A fungus which causes blackleg, a devastating disease afflicting some
members of the cabbage family, makes a previously unknown type of toxin,
say researchers in Canada. The toxin enables the fungus to penetrate a plant
so that it can use its more general chemical weapons.

The fungus, Phoma lingam, was known to produce a wide range of toxins,
such as sirodesmin PL, which affect several different plants. But in addition
to these toxins, which affect all the plants attacked by the fungus, Soledade
Pedras and Janet Taylor from the Plant Biotechnology Institute in Saskatoon
have isolated a toxin specific to rapeseed crops (Brassica napus and Brassica
campestris). They have named the toxin phomalide (Journal of Organic Chemistry,
vol 58, p 4778).

Phomalide has a very unusual chemical structure, quite unrelated to
the structures of the non-specific molecules made by the fungus. It is
a type of molecule called a cyclic depsipeptide, which consists of a large
ring based on amino acids. Phomalide has five amino acid derivatives in
the ring. Depsipeptides usually contain either four amino acids or more
than six amino acids, but very rarely five.

When Pedras and Taylor used phomalide alone, they found that it caused
lesions on a rape plant that were very similar to those found on diseased
plants, where the lower stems blacken and decay. But the mustard plant (B.
juncea), which is resistant to blackleg, was only slightly sensitive to
phomalide.

Curiously, phomalide is produced only in the very early stages of the
plant infection by the germinated fungus spores. After about 60 hours, the
fungus makes only the non-specific toxins.

Once the spores have secreted phomalide, the plant cells are damaged
irreversibly. This allows the fungus to penetrate deeper into the plant,
so the rape is rapidly taken over by the disease.

The researchers also discovered that the the non-specific toxin sirodesmin
PL inhibits the production of phomalide. And the compounds produced by the
rape to fight the fungus invasion, called phytoalexins, inhibit the production
of sirodesmin PL. Yet by producing phytoalexins the plants accelerate their
own destruction, as less sirodesmin PL means more phomalide and so more
damage to the plant.

Dye houses badly need better methods for removing dye from their effluent.
In Britain, for instance, the 1989 Water Act affected the textile industry
by formalising the principle that the ‘polluter pays’.

Wolfgang Saus, Dierk Knittel and Eckhard Schollmeyer of the German Textile
Research Centre in Krefeld applied a little lateral thinking to the
problem. Rather than cleaning up waste water, they hit on the idea of using
no water in the first place (Textile Research Journal, vol 63, p 135).

The medium they chose instead was ‘supercritical’ carbon dioxide. Substances
which are either a gas or liquid at room temperature and pressure can be
made into supercritical fluids by greatly raising the temperature and
pressure. There comes a point when it is impossible to differentiate
between the gas and the liquid, as the substance is in a state somewhere in
between. Carbon dioxide becomes supercritical at a temperature of about 31
°C and at a pressure of 73 atmospheres. If the pressure is raised
still further, the fluid’s ability to dissolve a substance is sharply
increased.

Saus and his colleagues exploited this improved solubility to dye a number
of artificial fibres, notably the polyester polyethylene terephthalate or
PET (more commonly known in the guise of such brand names as Terylene and
Trevira). The dyes normally used for such fibres are often insoluble in
water; their solubility can be improved only by adding large amounts of
other chemicals, such as surfactants and dispersants. But when a
supercritical fluid such as carbon dioxide is used, these chemicals become
unnecessary because the dye dissolves in the supercritical carbon dioxide
alone. The effluent problem is reduced because there are fewer chemicals to
be removed.

The German researchers carry out their dyeing in a vessel called an
autoclave. They wrap the sample around a perforated stainless steel tube and
place the dye powder at the bottom of the vessel. They then fill the vessel
with carbon dioxide gas, heating the gas to the required temperature and
increasing the pressure by constantly stirring it. It is then left for an
hour.

After the pressure has been returned to atmospheric, the carbon dioxide will
have evaporated, leaving the dyed sample completely dry. It is sometimes
necessary to rinse the sample quickly with acetone in order to remove the
residual dye powder.

The procedure proved successful not only for PET but also for nylon and
other artificial fabrics, notably bullet-proof Kevlar which cannot be dyed
by conventional aqueous methods.

A pilot plant for dyeing spools of PET yarn by the new method is currently
under construction in Germany.