Short‐chain chitin oligomers from arbuscular mycorrhizal fungi trigger nuclear Ca²⁺ spiking in Medicago truncatula roots and their production is enhanced by strigolactone

Abstract

SummaryThe primary objective of this study was to identify the molecular signals present in arbuscular mycorrhizal (AM) germinated spore exudates (GSEs) responsible for activating nuclear Ca 2+ spiking in the Medicago truncatula root epidermis.Medicago truncatula root organ cultures (ROCs) expressing a nuclear-localized cameleon reporter were used as a bioassay to detect AM-associated Ca 2+ spiking responses and LC-MS to characterize targeted molecules in GSEs. This approach has revealed that short-chain chitin oligomers (COs) can mimic AM GSE-elicited Ca 2+ spiking, with maximum activity observed for CO4 and CO5. This spiking response is dependent on genes of the common SYM signalling pathway (DMI1/DMI2) but not on NFP, the putative Sinorhizobium meliloti Nod factor receptor. A major increase in the CO4/5 concentration in fungal exudates is observed when Rhizophagus irregularis spores are germinated in the presence of the synthetic strigolactone analogue GR24. By comparison with COs, both sulphated and nonsulphated Myc lipochito-oligosaccharides (LCOs) are less efficient elicitors of Ca 2+ spiking in M. truncatula ROCs. We propose that short-chain COs secreted by AM fungi are part of a molecular exchange with the host plant and that their perception in the epidermis leads to the activation of a SYM-dependent signalling pathway involved in the initial stages of fungal root colonization.

Symbiosis with an endobacterium increases the fitness of a mycorrhizal fungus, raising its bioenergetic potential

“…The presence of the endobacterium affects the basal intracellular calcium concentration One of the most investigated aspects of the AMF-plant interaction is the calcium-mediated signalling pathway, which is elicited in the plant host by molecules from the AMF, by the symbiotic pathway (Genre et al, 2013;Oldroyd, 2013). To examine intracellular (Oldroyd, 2013).…”

Phosphate Treatment Strongly Inhibits New Arbuscule Development But Not the Maintenance of Arbuscule in Mycorrhizal Rice Roots

“…Biosynthesis, transport, and signaling events of strigolactone-related molecules are required for normal colonization (Floss et al, 2008;Vogel et al, 2010;Kretzschmar et al, 2012;Yoshida et al, 2012;Foo et al, 2013;Gutjahr et al, 2015). Strigolactones also induce hyphal branching (Akiyama et al, 2005) and increase the production of symbiotic fungal signals (Genre et al, 2013). However, normal arbuscules are formed in plant mutants that are defective in strigolactone biosynthesis genes (Gutjahr et al, 2012), suggesting that reduced strigolactone levels are not involved in the early stages of P inhibition.…”

Ecological relevance of strigolactones in nutrient uptake and other abiotic stresses, and in plant-microbe interactions below-ground

“…In turn, it helps the plant in the acquisition of water and mineral nutrients, mainly P and nitrogen. AM fungi are obligate biotrophs that colonize the root cortex of the host plant, forming specialized and highly branched tree-like structures called arbuscules in the cells of the host, where the nutrient exchange between the two partners takes place (Genre et al 2013;Gutjahr and Parniske 2013). The hyphae of the fungus grow into the soil far beyond the root rhizosphere and develop an extensive hyphal network that takes up P via fungal high-affinity transporters (Harrison 2005;Smith and Smith 2011), thus acting as 'helper roots' that can search for P beyond the P depletion zone.…”

“…In addition to SL signalling by the plant, and also before symbiosis establishment, AM fungi produce and release diffusible compounds -Myc factors and short chitin oligomers -into the rhizosphere that act as molecular cues indicating the presence of the fungus in the vicinity of the host root and inducing the plant responses required for a successful colonisation (Bucher et al 2014;Genre et al 2013;Maillet et al 2011). Myc factors consist of a mixture of sulphated and nonsulphated simple lipochito-oligosaccharides that have structural similarities with the rhizobial Nod factors (Maillet et al 2011).…”