Strigolactones: chemical signals for fungal symbionts and parasitic weeds in plant roots

Abstract

Aims: Arbuscular mycorrhizae are formed between >80 % of land plants and arbuscular mycorrhizal (AM) fungi. This Botanical Briefing highlights the chemical identification of strigolactones as a host-recognition signal for AM fungi, and their role in the establishment of arbuscular mycorrhizae as well as in the seed germination of parasitic weeds.

Scope: Hyphal branching has long been described as the first morphological event in host recognition by AM fungi during the pre-infection stages. Host roots release signalling molecules called 'branching factors' that induce extensive hyphal branching in AM fungi. Strigolactones exuded from host roots have recently been identified as an inducer of hyphal branching in AM fungi. Strigolactones are a group of sesquiterpenes, previously isolated as seed germination stimulants for the parasitic weeds Striga and Orobanche. Parasitic weeds might find their potential hosts by detecting strigolactones, which are released from plant roots upon phosphate deficiency in communication with AM fungi. In addition to acting as a signalling molecule, strigolactones might stimulate the production of fungal symbiotic signals called 'Myc factors' in AM fungi.

Conclusions: Isolation and identification of plant symbiotic signals open up new ways for studying the molecular basis of plant-AM-fungus interactions. This discovery provides a clear answer to a long-standing question in parasitic plant biology: what is the natural role for germination stimulants? It could also provide a new strategy for the management and control of beneficial fungal symbionts and of devastating parasitic weeds in agriculture and natural ecosystems.

Fig. 1.

Symbiotic interactions between the model legume Lotus japonicus and the AM fungus Gigaspora margarita. (A) Arbuscular mycorrhizae formed between L. japonicus and G. margarita. Fungal hyphae are stained with trypan blue after the root has been cleared by heating in KOH. Arrow, appressorium formed on root surface; asterisk, root's stele. Scale bar = 60 μm. (B) Hyphal branching of G. margarita in the vicinity of roots of L. japonicus. Secondary, tertiary, fourth and fifth hyphae emerge from primary hypha. Arrow, direction of growth of primary hypha (germ tube); double arrow, auxiliary cells of fungus. Scale bar = 300 μm.

Fig. 2.

Chemical structures of natural strigolactones and synthetic analogues: (A) 5-deoxy-strigol; (B) four natural strigolactones; (C) alectrol (tentative structure); (D) synthetic analogue GR24; (E) synthetic analogue GR7. The absolute configuration of the natural 5-deoxy-strigol (A), (+)-strigol and (+)-sorgolactone (B) is 3a(R), 8b(S), 2'(R).

Fig. 3.

Hyphal branching of Gigaspora margarita induced by 5-deoxy-strigol using the paper disc diffusion method: (A) control (70 % ethanol in water); (B) natural 5-deoxy-strigol (30 pg per disc). Extensive formation of hyphal branches from secondary hyphae is induced by treatment with 5-deoxy-strigol. Control hyphae, which treated with 70 % ethanol in water, form no hyphal branches from secondary hyphae. Arrows indicate direction of growth of primary hyphae. Scale bars = 1 mm.