Science: Turning over a new leaf

A bacterium without cell walls could prevent disease in cabbages and other crops

It's no longer fashionable, or even permissible, for scientists to spend time simply thinking or studying natural phenomena for any other reason than to develop a saleable product or process. More's the pity - as a paper published this month in Letters in Applied Microbiology shows. Written by five University of Aberdeen microbiologists, it establishes that bacteria deprived of their cell wall can live in close association with Chinese cabbage cells and thereby protect the plant against disease. The work has considerable practical possibilities. Yet it was initiated by the researchers' late colleague Alan Paton, as a result of observations made while peering for hours down a simple, old-fashioned microscope.

The Aberdeen research is based on varieties of bacteria known as L-forms. Bacteria of this sort have lost, either permanently or temporarily, the ability to synthesise the materials that constitute their cell walls. They are bounded only by the delicate membrane that normally lies under the rigid wall.

Most bacteria can be turned into unstable L-forms by treating them with penicillin or lysozyme (a natural antiseptic in tears and other secretions). Some of them can be stabilised in that state. Many L-forms also change their appearance as they go through a complex life cycle, which includes tiny granular forms that pass like viruses through very fine filters.

Studying potato, carrot and turnip cells from plants infected by bacterial soft-rot diseases, Paton noticed some of the cells were densely packed with bacteria - not only the bacteria that had caused the disease, but other harmless ones too. Further studies suggested that L-form granules could have entered through apertures denied to much larger, "normal" bacteria. By injecting ordinary bacteria together with penicillin or lysozyme into plant tissues, the Aberdeen microbiologists have since created associations between a wide variety of different bacteria and plants.

The fact that the bacteria in such partnerships continue to show the biochemical activities they conducted when living alone suggests that one potential practical use of these associations might be to enhance the nutritional value of crops. Other new characteristics, such as drought and cold resistance, might be introduced into crops too.

The other putative range of applications is resistance to the fungi, viruses and bacteria that cause diseases. In theory there are several ways in which an L-form, living in close association with a plant, might protect it against infection. One would be simply by being there, barring the way to invasion by a harmful microbe. More actively, an L-form could produce an antibiotic that destroyed the invader.

This month's report by Anne Glover and her colleagues provides evidence that L-forms can indeed protect plants against disease-causing bacteria. Their experiments were in two parts. First, they prepared L-forms of the bacterium Pseudomonas syringiae and added them to germinating Chinese cabbage seeds. They grew the seedlings, removed any surface bacteria and confirmed the presence of the bacterium inside the plant.

To determine whether the Chinese cabbage could now resist a potentially serious infection, they used Xanthomonas campestris, a bacterium responsible for spot disease of peppers and other infections. To facilitate infection, they damaged the leaves of the seedlings and then applied X. campestris.

The result was striking. Six days later, plants carrying the L-forms were healthy and showed no sign of disease, while the plants that had not been augmented by the L-forms, were in a bad way. The leaves infected by X. campestris were dead, and the others were wilting. Experiments with the original Ps. syringae showed it failed to protect Chinese cabbage in the same way: only the L-form worked.

It's not yet clear how the L-forms safeguard the plant cells against invasion. However, the effect seems not to be one of simple physical exclusion, because Ps. syringae with its cell wall intact does not work. What seems to happen is that the plant responds to the L-form, with which it can co-exist, in such a way as to repel more threatening invaders.