Garden Soaps - intimacy, deceit and betrayal in the plant world.
Garden Soaps.
Many plants produce natural soaps (called saponins, or technically, plant glycosides) that have anti-fungal and anti-bacterial activity and so help defend the plant against disease attach. Saponins can be present in high concentrations (up to 10% of dry weight) in healthy plants. Microbes that can successfully invade saponin-containing plants produce enzymes that break down the saponins into less toxic chemicals, thus allowing them to breach the plant's first line of defence.
Oats are the only cereal known to produce saponins and the avenacin type of saponin (found in oat roots) is of particular interest because it has anti-fungal properties. It has been suggested that the resistance of oats to the fungal root disease ‘take-all’ is due to the presence of avenacin in their roots. Fungi that can invade oats have been found to produce enzymes that breakdown the avenacin into less toxic compounds. Oat plants that do not produce avenacin in their roots, due to a genetic mutation, are susceptible to the ‘take-all’ fungus. Other cereals do not produce avenacin, but introducing this ability could have major benefits in reducing ‘take-all’ disease, and could reduce the need for chemical sprays or fumigants.
A similar story has been found in the plant family Solanaceae (tomato,
potato and their relatives). The main saponin in tomato is -tomatine, which
has very strong anti-fungal activity. Scientists have discovered that the
tomato leaf spot fungus produces an enzyme (called tomatinase) that breaks
down -tomatine, so allowing the fungus to invade the tomato plant. Mutants
of the leaf spot fungus that cannot produce tomatinase are unable to cause
disease. They also found that the breakdown products of -tomatine interfere
with essential communication processes in the plant that normally activate
plant defence responses. So in overcoming one line of defence the fungus also
disrupts the plant’s ability to trigger its other defence systems. Hence,
although these garden soaps are an important first line of defence it seems
that the pathogens have evolved methods to overcome saponin-based defences,
and can even turn them against the plant.
Soaps, applied as sprays or drenches, can be used to control a large range of plant pests and certain plant pathogens (notably Powdery Mildew). Not all soaps have insecticidal properties, but soaps developed for horticulture and agriculture use are specially formulated to have insect-killing properties, while being safe for most plant species. Typically these soaps disrupt the structure and permeability of insects' cell membranes, causing cell contents to leak out leading to the insect’s death. They can also interfere with the feeding of pests and be used to help deliver small particles into the spiracles (respiratory system) of insects, causing suffocation. The ‘wetting’ effect of soaps can assist in simply washing insects from plant surfaces, if applied in a sufficient volume of water. As soaps work by contact and only when wet, once the soap spray has dried there is no residual activity on insects, however repeated applications may have damaging effects on some types of plants.