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

In the present study, we report the in vitro interactions between Frankia sp. ACN10a and non-Frankia nodular endophytes (NFNE) isolated from alder. The supernatant of NFNE grown in nitrogen-replete medium had neutral or negative effects on Frankia growth; none had a stimulatory effect. Inhibitory effects were observed for supernatants of some NFNE, notably Mi-cromonospora, Pseudomonas, Serratia and Stenotrophomonas isolates. However, some NFNE-Frankia coculture supernatants could stimulate Frankia growth when used as a culture medium supplement. This was observed for supernatants of Frankia cocultured with Microvirga and Streptomyces isolates. In nitrogen-limited conditions, cocultures of Frankia with some NFNE, including some rhizobia and Cytobacillus, resulted in higher total biomass than Frankia-only cultures, suggesting cooperation, while other NFNE were strongly antagonistic. Microscopic observation of cocultures also revealed compromised Frankia membrane integrity, and some differentiation into stress resistance-associated morphotypes such as sporangia and reproductive torulose hyphae (RTH). Furthermore, the coculture of Frankia with Serratia 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.