Neighbours in nodules: the interactions between sp. ACN10a and non- nodular endophytes of alder.

In the present study, we report the in vitro interactions between sp. ACN10a and non- nodular endophytes (NFNE) isolated from alder. The supernatant of NFNE grown in nitrogen-replete medium had neutral or negative effects on growth; none had a stimulatory effect. Inhibitory effects were observed for supernatants of some NFNE, notably and isolates. However, some NFNE- coculture supernatants could stimulate growth when used as a culture medium supplement. This was observed for supernatants of cocultured with and S isolates. In nitrogen-limited conditions, cocultures of with some NFNE, including some rhizobia and resulted in higher total biomass than -only cultures, suggesting cooperation, while other NFNE were strongly antagonistic. Microscopic observation of cocultures also revealed compromised membrane integrity, and some differentiation into stress resistance-associated morphotypes such as sporangia and reproductive torulose hyphae (RTH). Furthermore, the coculture of with sp. isolates resulted in higher concentrations of the auxinic plant hormone indole-3-acetic acid and related indolic compounds in the culture supernatant. This study sheds new light on the breadth of microbial interactions that occur amongst bacteria that inhabit the understudied ecological niche of the alder nodule.