The Medicago truncatula hypermycorrhizal B9 mutant displays an altered response to phosphate and is more susceptible to Aphanomyces euteiches

Inorganic phosphate (Pi) plays a key role in the development of arbuscular mycorrhizal (AM) symbiosis, which is favoured when Pi is limiting in the environment. We have characterized the Medicago truncatula hypermycorrhizal B9 mutant for its response to limiting (P/10) and replete (P2) Pi. On P2, mycorrhization was significantly higher in B9 plants than in wild-type (WT). The B9 mutant displayed hallmarks of Pi-limited plants, including higher levels of anthocyanins and lower concentrations of Pi in shoots than WT plants. Transcriptome analyses of roots of WT and B9 plants cultivated on P2 or on P/10 confirmed the Pi-limited profile of the mutant on P2 and highlighted its altered response to Pi on P/10. Furthermore, the B9 mutant displayed a higher expression of defence/stress-related genes and was more susceptible to infection by the root oomycete pathogen Aphanomyces euteiches than WT plants. We propose that the hypermycorrhizal phenotype of the B9 mutant is linked to its Pi-limited status favouring AM symbiosis in contrast to WT plants in Pi-replete conditions, and discuss the possible links between the altered response of the B9 mutant to Pi, mycorrhization and infection by A. euteiches. We have characterized the response to phosphate of the hypermycorrhizal mutant B9 of Medicago truncatula. We show through physiological approaches, root architecture analyses and transcriptomics that this mutant displays an altered response to phosphate. We propose that the hypermycorrhizal phenotype of the B9 mutant is linked to its Pi-limited status favoring AM symbiosis. In addition this mutant is more susceptible to the oomycete Aphanomyces euteiches, suggesting possible links between symbiosis, pathogenesis and response to phosphate.