Long-lasting impact of chito-oligosaccharide application on strigolactone biosynthesis and fungal accommodation promotes arbuscular mycorrhiza inMedicago truncatula

Summary The establishment of arbuscular mycorrhiza (AM) between plants and Glomeromycotina fungi is preceded by the exchange of chemical signals: fungal released Myc-factors, including chitoligosaccharides (CO) and lipo-chitooligosaccharides (LCO), activate plant symbiotic responses, while root exuded strigolactones stimulate hyphal branching and boost CO release. Furthermore, fungal signaling reinforcement through CO application was shown to promote AM development in Medicago truncatula , but the cellular and molecular bases of this effect remained unclear. Here we focused on long-term M. truncatula responses to CO treatment, demonstrating its impact on the transcriptome of both mycorrhizal and non-mycorrhizal roots over several weeks and providing a novel insight into the mechanistic bases of the CO-dependent promotion of AM colonization. CO treatment caused the long-lasting regulation of strigolactone biosynthesis and fungal accommodation related genes. This was mirrored by an increase in root didehydro-orobanchol content, and the promotion of accommodation responses to AM fungi in root epidermal cells. Lastly, an advanced down-regulation of AM symbiosis marker genes was observed at the latest time point in CO-treated plants, in line with an increased number of senescent arbuscules. Overall, CO treatment triggered molecular, metabolic and cellular responses underpinning a protracted acceleration of AM development.