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

Abstract

During spore germination, arbuscular mycorrhizal (AM) fungi show limited hyphal development in the absence of a host plant (asymbiotic). In the presence of root exudates, they switch to a new developmental stage (presymbiotic) characterized by extensive hyphal branching. Presymbiotic branching of the AM fungus Gigaspora rosea was induced in liquid medium by a semipurified exudate fraction from carrot (Daucus carota) root organ cultures. Changes in RNA accumulation patterns were monitored by differential display analysis. Differentially appearing cDNA fragments were cloned and further analyzed. Five cDNA fragments could be identified that show induced RNA accumulation 1 h after the addition of root exudate. Sequence similarities of two fragments to mammalian Nco4 and mitochondrial rRNA genes suggested that root exudates could influence fungal respiratory activity. To support this hypothesis, additional putative mitochondrial related-genes were shown to be induced by root exudates. These genes were identified after subtractive hybridization and putatively encode a pyruvate carboxylase and a mitochondrial ADP/ATP translocase. The gene GrosPyc1 for the pyruvate carboxylase was studied in more detail by cloning a cDNA and by quantifying its RNA accumulation. The hypothesis that respiratory activity of AM fungi is stimulated by root exudates was confirmed by physiological and cytological analyses in G. rosea and Glomus intraradices. Oxygen consumption and reducing activity of both fungi was induced after 3 and 2 h of exposition with the root factor, respectively, and the first respiration activation was detected in G. intraradices after approximately 90 min. In addition, changes in mitochondrial morphology, orientation, and overall biomass were detected in G. rosea after 4 h. In summary, the root-exuded factor rapidly induces the expression of certain fungal genes and, in turn, fungal respiratory activity before intense branching. This defines the developmental switch from asymbiosis to presymbiosis, first by gene activation (0.5-1 h), subsequently on the physiological level (1.5-3 h), and finally as a morphological response (after 5 h).Arbuscular mycorrhizal (AM) fungi are obligate biotrophic root symbionts that cannot be propagated in pure culture. Therefore, they are difficult to study, and analyses concerning the structure and the function of their genes are rare and mainly based on PCR techniques (Franken and Requena, 2001). Establishment of the symbiosis after spore germination includes hyphal branching, appressorium development after contacting the root, symbiotic colonization of the cortex, formation of the intracellular arbuscules, and, concomitantly, production of a sporulative extraradical mycelium (Bianciotto and Bonfante, 1998; Smith and Read, 1997). These developmental stages presumably require molecular communication between the fungus and the plant. Signals should be exchanged between the partners, leading to stagespecific patterns of gene expression. The corresponding gene products in turn would be re...

GR24, a Synthetic Analog of Strigolactones, Stimulates the Mitosis and Growth of the Arbuscular Mycorrhizal FungusGigaspora roseaby Boosting Its Energy Metabolism

“…It has been shown previously that the semipurified root exudate fraction (the so-called branching factor; Nagahashi and Douds, 1999Buée et al, 2000) affects the expression of genes with a mitochondrial function (Tamasloukht et al, 2003(Tamasloukht et al, , 2007. The rapid (within minutes) mitochondrial activation of G. rosea by GR24 that we observed suggests that it does not rely on enhanced transcription.…”

“…Although our gene-targeted approach does not allow us to exclude specific and early gene induction at the nuclear or mitochondrial level, our results suggest that during the first hours of GR24 treatment, regulations that will lead to greater hyphal branching are mostly posttranscriptional. Although, a previously isolated root exudate fraction, and here the synthetic strigolactone (GR24), have been shown to induce similar effects on mitochondrial changes (Tamasloukht et al, 2003;Besserer et al, 2006), the results on gene expression are different. The rapid up-regulation of several genes observed in response to root exudates (Tamasloukht et al, 2003(Tamasloukht et al, , 2007 suggests that other, yet unknown, molecules act in synergy or in parallel with strigolactones (Nagahashi and Douds, 2007).…”

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.…”

An STE12 gene identified in the mycorrhizal fungus Glomus intraradices restores infectivity of a hemibiotrophic plant pathogen

“…GintSTE is up-regulated in germinating spores and upon root penetration in G. intraradices Host root exudates are known to enhance fungal transcription during spore germination and especially for genes associated with mitochondrial activity (Tamasloukht et al, 2003), mobilization and transport of glycogen and lipid reserves (Park et al, 2002), protein synthesis or stress responses (Seddas et al, 2008). GintSTE expression levels increased significantly upon spore germination but effects did not differ between water and root exudates, suggesting that the up-regulation process for this gene is not dependent on a diffusable signal from the plant partner.…”