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

Abstract

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 Micromonospora, 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 (IAA) 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.

Root nodules of red alder (Alnus rubra) and Sitka alder (Alnus viridis ssp. sinuata) are inhabited by taxonomically diverse cultivable microbial endophytes

“…We found Pseudomonas with Burkholderia and Pseudomonas with Bacillus to be frequent intranodular bacterial pairings. Except for Garneau et al (2023a, 2023b), almost no other studies have reported on cultivable non‐ Frankia bacterial endophytes of alder root nodules. In contrast, many studies conducted on legumes have revealed diverse intranodular bacteriomes that include genera such as Pseudomonas , Burkholderia , and Bacillus (Martínez‐Hidalgo & Hirsch, 2017).…”

“…Bacillus to be frequent intranodular bacterial pairings. Except for Garneau et al (2023aGarneau et al ( , 2023b, almost no other studies have reported on cultivable non-Frankia bacterial endophytes of alder root nodules.…”

“…While information on the taxonomic diversity of non-Frankia root nodule endophytes of actinorhizal plants is limited, we know even less about their functional roles in plant physiology and ecosystem processes (but see Garneau et al, 2023aGarneau et al, , 2023b. The factors that drive the curation of the root nodule microbiome and determine the diversity and composition of root nodule endophytes also remain elusive, but some studies have shown that the nodule microbiomes of both legumes and actinorhizal plants can be influenced by host genetics, and abiotic factors such as water status (Lipus & Kennedy, 2011;Pozzi et al, 2018;Sharaf et al, 2019).…”