Branching Of Arbuscular Mycorrhizal Fungi Research Articles

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

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. 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. 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.

Sharing a Signal for Good and Bad

The majority of terrestrial plants rely on symbiotic relationships to obtain necessary nutrients. One of these relationships is between the roots of plants and arbuscular mycorrhizal fungi (see Parniske). Akiyama et al. isolated and identified the so-far elusive chemical that is the signal, called branching factor (BF), released by plants to attract the fungi. The compound was determined to be a strigolactone, which belongs to a class of sesquiterpene lactones that also serve as seed germination stimulants for parasitic weeds. Natural and synthetic strigolactones stimulated the formation of hyphal branches in an in vitro bioassay. Strigolactones are unstable compounds that are released by plants under conditions of low phosphate availability, which are the same conditions that allow parasitic weeds to prevail. The sharing of the signal for symbiotic root colonization by fungi with seed germination of parasitic weeds may lead to new strategies for managing beneficial symbiotic relationships and preventing the invasion by parasitic plants. M. Parniske, Plant-fungal association: Cue for the branching connection. Nature 435 , 750-751 (2005). [PubMed] K. Akiyama, K.-i. Matsuzaki, H. Hayashi, Plant sesquiterpenes induce hyphal branching in arbuscular mycorrhizal fungi. Nature 435 , 824-827 (2005). [PubMed]

Synergism between Blue Light and Root Exudate Compounds and Evidence for a Second Messenger in the Hyphal Branching Response of Gigaspora gigantea

Light and chemical components of the host root exudate can induce hyphal growth and branching of arbuscular mycorrhizal fungi. Compounds that induce the same morphogenetic or biochemical response as light are referred to as photo-mimetic compounds (PCs). This is the first report of a synergistic response by Gigaspora gigantea, an arbuscular mycorrhizal fungus, to blue light and naturally occurring photomimetic compounds isolated from the exudate of host roots. The blue light treatment and exposure to photomimetic compounds were effective whether applied sequentially or simultaneously. The number of hyphal branches induced by blue light and photomimetic compounds together was greater than the sum of the branches generated by each separate treatment, and the synergism was greatest at the higher levels or orders of branches. The fact that blue light and PCs, individually, triggered the same hyphal branching response and when given together, they produced a synergistic response, indicated the activation of a second messenger in the induced-branching process. Delaying the application of PCs, after the initial light exposure, showed the second messenger was stable up to 3 h.

Synergism between blue light and root exudate compounds and evidence for a second messenger in the hyphal branching response of Gigaspora gigantea

Light and chemical components of the host root exudate can induce hyphal growth and branching of arbuscular mycorrhizal fungi. Compounds that induce the same morphogenetic or biochemical response as light are referred to as photo-mimetic compounds (PCs). This is the first report of a synergistic response by Gigaspora gigantea, an arbuscular mycorrhizal fungus, to blue light and naturally occurring photomimetic compounds isolated from the exudate of host roots. The blue light treatment and exposure to photomimetic compounds were effective whether applied sequentially or simultaneously. The number of hyphal branches induced by blue light and photomimetic compounds together was greater than the sum of the branches generated by each separate treatment, and the synergism was greatest at the higher levels or orders of branches. The fact that blue light and PCs, individually, triggered the same hyphal branching response and when given together, they produced a synergistic response, indicated the activation of a second messenger in the induced-branching process. Delaying the application of PCs, after the initial light exposure, showed the second messenger was stable up to 3 h.