Underground communication – the new elements of signalling pathways of arbuscular mycorrhizal symbiosis

Mycorrhiza is a symbiotic relationship between living cells of the roots of higher plants and non-pathogenic fungi which inhabit soil and belong to Glomeromycota (endomycorrhizae) and Basidiomycota, Ascomycota (ectomycorrhizae). Although the phenomenon of mycorrhiza was discovered by a Polish botanist F.D. Kamieński already in 1881, various stages of establishing the symbiotic relationship between the partners are still not fully understood and explained. According to the current knowledge, the roots of host plants release strigolactones, which stimulate germination and branching of spores of arbuscular fungi. As a result, the fungi synthesize molecular signals, i.e. chitooligosaccharides (COs) and lipochitooligosaccharides (LCOS), called MycF factors. Thanks to the development of molecular biology techniques the probable cascade of events during the recognition of fungal MycF factor by the host-plant has been outlined. The enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase 1 (HMGR1) and also its product, mevalonic acid (MVA), play an essential role in the biosynthesis of sterols and isoprenoids in a plant cell. The recent studies indicate that these compounds may also play a very important role during establishing of the symbiotic mycorrhizal relationship. It is believed that MVA detects and transmits MycF factor to a cell nucleus of a host-plant triggering numerous necessary mechanisms in the plant cell to activate next steps of the mycorrhizal symbiosis. The discovery of HMGR1 and MVA sheds new light on symbiotic nature of mycorrhiza. This paper is a review of the current knowledge on the signal exchange during symbiotic interactions between mycorrhizal fungi and host plants.

1. Introduction. 2. Symbiotic nature of arbuscular fungi. 3. Arbuscular mycorrhiza in early stages. 4. Exchange of signaling molecules during arbuscular mycorrhiza formation. 5. Mevalonic acid – secondary signaling molecule messengers in the arbuscular mycorrhiza. 6. Protein kinase CCaMK as a key element in the establishment of arbuscular mycorrhiza. 7. Summary