The Pre-Symbiotic Growth of Arbuscular Mycorrhizal Fungi Is Induced by a Branching Factor Partially Purified from Plant Root Exudates

Abstract

Arbuscular mycorrhizal (AM) symbiosis is an association between obligate biotrophic fungi and more than 80% of land plants. During the pre-symbiotic phase, the host plant releases critical metabolites necessary to trigger fungal growth and root colonization. We describe the isolation of a semipurified fraction from exudates of carrot hairy roots, highly active on germinating spores of Gigaspora gigantea, G. rosea, and G. margarita. This fraction, isolated on the basis of its activity on hyphal branching, contains a root factor (one or several molecules) that stimulates, directly or indirectly, G. gigantea nuclear division. We demonstrate the presence of this active factor in root exudates of all mycotrophic plant species tested (eight species) but not in those of nonhost plant species (four species). We negatively tested the hypothesis that it was a flavonoid or a compound synthesized via the flavonoid pathway. We propose that this root factor, yet to be chemically characterized, is a key plant signal for the development of AM fungi.

The exudate from an arbuscular mycorrhizal fungus induces nitric oxide accumulation in Medicago truncatula roots

“…Most studies on early events in the AM interaction have used simplified experimental systems where the plant was physically separated from the fungus (Weidmann et al 2004;Kosuta et al 2008;Gutjahr et al 2009) or plant or fungal exudate was applied to the reciprocal symbiotic partner as a means of simulating the sudden appearance of diffusible signaling compounds (Buée et al 2000;Navazio et al 2007;Besserer et al 2006;Tamasloukht et al 2003;Chabaud et al 2011;Mukherjee and Ané 2011;Maillet et al 2011). In the present work, we followed the latter approach: a fungal exudate was obtained following the protocol that allowed Navazio et al (2007) to detect a transient rise of cytosolic Ca2+ in cultured soybean cells and Chabaud et al (2011) to highlight a nuclear Ca2+spiking in M. truncatula ROC.…”

Root Factors Induce Mitochondrial-Related Gene Expression and Fungal Respiration during the Developmental Switch from Asymbiosis to Presymbiosis in the Arbuscular Mycorrhizal FungusGigaspora rosea

“…Hyphal proliferation (branching) was the main fungal response studied. Morphological and growth responses could be scored within 24 h in a very reproducible and sensitive manner (Buée et al, 2000;Nagahashi and Douds, 2000). A lipophilic fraction (branching factor) with a strong stimulatory activity on Glomus intraradices and several Gigaspora spp.…”

“…was isolated from carrot (Daucus carota) root organ cultures and partially purified. The branching factor is believed to be present in all mycotrophic plants because it was found in root exudates of host plants representing several families and not in root exudates of non-hosts (Buée et al, 2000). The chemical nature of the factor is not known.…”

“…However, assays with various compounds such as abietic acid, brassinolide, jasmonic acid, salicylic acid, and quercetin were all negative. Flavonoids in general were also excluded as branching factor candidates because root exudates from maize mutants, deficient in chalcone synthase (necessary for flavonoid synthesis), contained the same branching activity as the wild type (Buée et al, 2000). The branching factor is believed to be a signal rather than a nutrient because it is active irrespective of the culture medium used, including when germinating fungal spores are growing in pure water.…”

Kultur akar rambut in vitro serta pemanfaatan kultur ganda untuk pertumbuhan dan perkembangan endomikoriza (Gigaspora sp. dan Acaulospora sp.) Hairy root culture in vitro and the application of dual culture for growth and development of endomycorrhiza ( Gigaspora sp. and Acaulospora sp.)

“…dan Acaulospora sp. yang sudah disterilkan menurut Buce et al (2000), diinokulasikan dengan cara memipet masingmasing sebanyak 10 µL yang berisi 10-15 butir spora, dan menyemprotkannya secara perlahan-lahan dekat ujung akar baik pada kecambah wortel dan tomat (umur empat minggu) serta akar rambut wortel (umur satu minggu). Kultur tanaman wortel dan tomat diinkubasi dalam ruang kultur bercahaya 12 jam/hari dan Rh 80-90%, sedangkan kultur akar rambut wortel diinkubasikan dalam ruang gelap.…”