Mycorrhizas – a successful symbiosis

One of the two genetically modified barley lines used in the trial contained a gene for a chitinase that had been inserted from the soil fungus Trichoderma harzianum, a fungus that has been used in biological plant protection for many years. The chitinase breaks down the hyphal walls of parasitic fungi, making the plants more resistant to fungal infections. The second transgenic line contains a gene for a glucanase taken from a soil bacterium. It is designed to improve the fodder quality of barley used as poultry feed.

In order to find out whether the chitinase and glucanase in the transgenic barley also attack beneficial fungi, the relevant conventional parent varieties were planted on the trial field as a control. On one half of the trial field a commercially available mycorrhizal fungus preparation was worked into the soil. If the beneficial mycorrhizal fungi were damaged, this could become apparent in reduced plant growth because mycorrhiza-free plants cannot take up as many nutrients from the soil.

The mutual benefits for plant and fungus

Mycorrhizal fungi live in symbiosis with plant roots, i.e. the two partners – plant and fungus – form a biocoenosis for mutual benefit. The fungus supplies the plant with nutrients such as phosphate, while the plant offers the fungus a protected habitat and supplies it with carbohydrates. Scientists estimate that mycorrhizal fungi have been in existence stabilising ecosystems for nearly 500 million years. It is suspected, for instance, that they played an important role in the transfer of plants from sea to land.

The plant benefits from its partner in a variety of ways. It grows better, is protected against diseases, and can tolerate certain conditions like heat or high salt concentrations better. Today it is known that mycorrhizal fungi increase the plant’s antioxidant potential, which is probably one explanation for the plant’s increased stress tolerance.

Mycorrhizal fungi that live in symbiosis with barley develop endophytically, i.e. within the host plant. They grow in the root cells, where they form hyphal structures – so-called arbuscles – with which they take up nutrients from the plants. Arbuscular mycorrhizal fungi, as they are known, are very widespread and are found in almost all soil ecosystems. More than eighty per cent of land plants live in symbiosis with such root endophytes.

What is being studied?

Following the harvest of the released plants, the roots of transgenic and non-transgenic barley will now be investigated for infestation with mycorrhizal fungi and parasitic fungi. Firstly, this will indicate the plants’ resistance to phytotoxic fungi commonly occurring in the soil; secondly, it will give a clear picture of the effect of the chitinase and glucanase on mycorrhizal fungi. These studies will initially involve using the microscope – put simply, the scientists will be counting fungal hyphae and spores in the plant roots. This will be followed by a molecular biological method (quantitative PCR), to gain a more detailed picture of the quantities of the different fungi in the roots. If these investigations provide indications of damage to (or promotion of) the mycorrhizal fungi in the transgenic plants, more detailed studies of fungal organs, e.g. the structure of the arbuscles, will be carried out with the help of a modern cellular biological procedure – confocal laser microscopy.