About 80 percent of all terrestrial plants enter into a symbiotic relationship with fungi living in the soil. The fungi provide the plant with water, important nutrients like phosphate and nitrate, and certain trace elements like zinc; the plant, on the other hand, supplies the fungus with carbohydrates. It is assumed that plants were only able to migrate onto land 400 million years ago thanks to this symbiosis.
The formation of this symbiosis is a strictly regulated process that the plant activates in low nutrient levels. The roots release the hormone strigolactone, which is detected by the fungi. The fungal hyphae grow towards the roots, penetrate the epidermis and isolated passage cells, and enter the root cortex. There, the fungal hyphae form tiny branch-like networks, which resemble little trees (arbusculum) and gave the symbiotic relationship its name: vesicular-arbuscular mycorrhizal symbiosis.
Until about five years ago, the hormone strigolactone was known to induce and entice parasitic plant seeds in the soil to germinate. At that stage, no-one understood why plants produced this substance, which is harmful to them. Only when the new role of strigolactone in mycorrhiza formation was discovered did it become clear that the attraction of the parasites was a harmful side effect of the symbiosis.
How do strigolactones get into the soil?
Exactly how strigolactones are released into the soil from the roots and how the fungi find the specialized entry points in the roots was not known until now. The research group headed by Professor Enrico Martinoia from the University of Zurich has now found the answers to these questions in collaboration with Professor Harro Bouwmeester’s team from Wageningen in the Netherlands. “Based on the model plant the petunia, we were able to demonstrate that the protein PhPDR1 transports strigolactones,” explains Professor Martinoia. The protein belongs to the ABC-transporter family found in simple organisms like bacteria, but also in humans.
The researchers observed that PhPDR1 is expressed more highly in a low nutrient content in order to attract more symbiotic fungi, which then supply more nutrients. But there are also plants like the model plant Arabidopsis (mouse-ear cress) that do not form any mycorrhiza. If the researchers added PhPDR1, however, the Arabidopsis roots transported strigolactones again.
Improvements in yield and weed control
“Our results will help to improve the mycorrhization of plants in soils where mycorrhization is delayed,” Professor Martinoia is convinced. “Mycorrhization can thus be triggered where it is inhibited due to dryness or flooding of the soils.” This would enable the plants to be nourished more effectively and achieve a greater harvest. Moreover, thanks to the discovery of the strigolactone transporter the secretion of strigolactone into the soil can be halted, which prevents parasitic plants that use up the host plants’ resources from being attracted. “This is especially important for regions in Africa, where the parasitic weed Striga and other parasitic plants regularly destroy over 60 percent of harvests,” says Martinoia.
Tobias Kretzschmar, Wouter Kohlen, Joelle Sasse, Lorenzo Borghi, Markus Schlegel, Julien B. Bachelier, Didier Reinhardt, Ralph Bours, Harro J. Bouwmeester and Enrico Martinoia. A petunia ABC protein controls strigolactone-dependent symbiotic signalling and branching. Nature, doi:10.1038/nature10873.