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. Com- pounds that induce the same morphogenetic or bio- chemical response as light are referred to as photo- mimetic compounds (PCs). This is the first report of a synergistic response by Gigaspora gigantea, an ar- buscular mycorrhizal fungus, to blue light and natu- rally occurring photomimetic compounds isolated from the exudate of host roots. The blue light treat- ment and exposure to photomimetic compounds were effective whether applied sequentially or simul- taneously. 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, trig- gered 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.