Fungal lipochitooligosaccharide symbiotic signals in arbuscular mycorrhiza

Abstract

Arbuscular mycorrhiza (AM) is a root endosymbiosis between plants and glomeromycete fungi. It is the most widespread terrestrial plant symbiosis, improving plant uptake of water and mineral nutrients. Yet, despite its crucial role in land ecosystems, molecular mechanisms leading to its formation are just beginning to be unravelled. Recent evidence suggests that AM fungi produce diffusible symbiotic signals. Here we show that Glomus intraradices secretes symbiotic signals that are a mixture of sulphated and non-sulphated simple lipochitooligosaccharides (LCOs), which stimulate formation of AM in plant species of diverse families (Fabaceae, Asteraceae and Umbelliferae). In the legume Medicago truncatula these signals stimulate root growth and branching by the symbiotic DMI signalling pathway. These findings provide a better understanding of the evolution of signalling mechanisms involved in plant root endosymbioses and will greatly facilitate their molecular dissection. They also open the way to using these natural and very active molecules in agriculture.

Strigolactones fine-tune the root system

“…Those mutants are strongly affected in nodulation and nsp2 and nsp1/nsp2 mutants also show a slight decrease in arbuscular mycorrhization Maillet et al 2011). These data indicate that the low level of strigolactones might contribute to the nodulation phenotype.…”

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.…”

“…Diffusible AM fungal signals were also shown to activate early events, such as changes in cytosolic (Navazio et al 2007;Kosuta et al 2008) and nuclear Ca2+ concentration (Chabaud et al 2011), and delayed processes, such as root branching (Oláh et al 2005) and starch accumulation in the root cortex (Gutjahr et al 2009). Recently, fungal exudates from Glomus intraradices were shown to contain a mixture of sulfated and nonsulfated lipochitooligosaccharides (LCO), which are responsible for the activation of MtENOD11 gene expression and the induction of lateral roots formation (Maillet et al 2011). The accumulation of NO in plants can be rapid (Delledonne 2005) and preliminary experiments where NO could not be visualized in AM fungus-contacted roots suggested the necessity of investigating NO production in the first minutes that follow fungus perception (Calcagno et al, unpublished data).…”

LYR3, a high‐affinity LCO‐binding protein of Medicago truncatula, interacts with LYK3, a key symbiotic receptor

“…Variation in the chemical substitutions on the chitin backbone and the structure of the acyl chain are important for the host-range of different rhizobia [15]. These molecules are structurally related to the MycLCOs [16] and COs [17], recently shown to be produced by AM fungi. The establishment of both rhizobial and AM fungal root endosymbioses requires the same signaling pathway, known as the common symbiosis signaling pathway (CSSP) [18][19][20].…”