How arbuscular mycorrhizal fungi talk to plants before colonisation

Plant roots do not sit alone in the soil. In the earth you will also find various microbes. Lurking in the dirt is Rhizophagus irregularis, an arbuscular mycorrhizal fungus that colonises wheat. In a new study Hui Tian and colleagues investigated if what happened before a fungus makes contact with the wheat roots.

There are over 200 species of arbuscular mycorrhizal fungi in the soil, but Rhizophagus irregularis is a particularly useful fungus to examine, as Dr Tian explained. “We chose the fungi Rhizophagus irregularis because this is the only arbuscular mycorrhizal fungal species whose genome data is available up to date.” The team needed the genome data to check the fungus hadn’t found its way into the roots. “In our RNA-seq experiment, we needed to map mRNA sequences against the arbuscular mycorrhizal fungal genome to verify roots were not colonized by the arbuscular mycorrhizal fungus.”

The reason Rhizophagus irregularis is after the wheat roots is that it cannot make its own food. Dr Tian said that arbuscular mycorrhizal fungi have a variety of relationships with plants. “Beneficial or parasitic relationship between arbuscular mycorrhizal fungi and host plants may be determined by the trade balance between them. The relationship could be significantly influenced by arbuscular mycorrhizal fungal species and plant species, or varieties. A parasitic relationship between wheat and the fungus Rhizophagus irregularis has been observed in a number of studies. Even where there is a parasitic relationship, arbuscular mycorrhizal fungi may still supply mineral nutrients to host plants, but they may consume too much carbon from host plants.”

To see if the fungi could be affecting plants without physically touching them, Tian and colleagues buried filter bags near the plant roots. The filters had to be big enough to allow chemicals released by the fungi to pass, but small enough to prevent them from escaping. That had to be small as the fungi reach out with hyphae, filaments under a tenth of the width of a human hair. Dr Tian said these filters were critical to the success of the experiment. “We used a 0.25 µm filter membrane to prevent the fungi from physically interacting with the plants. When sampling, membrane bags were checked carefully, and the pots whose bags broken were abandoned.”

What the team found was evidence of a ‘conversation’ going on between the fungi and plants. Dr Tain said, “After receptor proteins located on root epidermal cells received signals produced by arbuscular mycorrhizal fungi, expression of a large number of genes in roots were changed. This means roots are preparing for colonization. On the other side, roots also produce chemical signals that stimulate the growth of arbuscular mycorrhizal hyphae and guide the colonization process. Arbuscular mycorrhizal spores will germinate in soil if there is enough water and moisture; however, if there are no chemical signals produced by plants, spores would not colonize roots and be dead.”

As there were no fungi in the roots, Tian and colleagues knew that any changes in the plants were the result of chemical interaction. They didn’t find just a few changes, as Dr Tian said. “Many genes involved in key processes during arbuscular mycorrhizal symbiosis development are regulated at the pre-physical contact stages, this is our most exciting finding in this work. This finding may change our traditional thinking about the arbuscular mycorrhizal symbiosis formation processes.”

“Our research explains why the expression of some genes is regulated by arbuscular mycorrhizal colonization. And also, it is interesting to investigate roles of the transcription factor genes, activated by symbiosis molecular signals, in regulating gene expression of plant roots in the future.”

The authors conclude there would be a benefit in “a combination of transcriptomic and GWAS studies to more accurately determine the genes that determine AM colonization degree of wheat roots in future studies.”