Nodulation of Shepherdia ×utahensis ‘Torrey’ and the Diversity of Symbiotic Frankia Strains

Shepherdia × utahensis ‘Torrey’ (hybrid buffaloberry) is an actinorhizal plant that can form symbiotic nodules with the actinobacterial genus Frankia . However, little research has been conducted to investigate the presence of Frankia in their nodules and the effects on plant growth. In this study, plants were grown in a Metro-Mix ® 820 substrate and inoculated with soils collected from Mohave County, AZ, or in a low organic-matter substrate inoculated with soils from North Logan, UT. The presence of Frankia was quantified using PolF/PolR primers to amplify their nitrogenase ( nif H) gene sequences. In the Metro-Mix 820 substrate, plants irrigated with nitrogen (N)-free Hoagland’s solution at pH 6.5 formed nodules at week 12 after experiment initiation, whereas those receiving the same solution with 2 m m ammonium nitrate (NH 4 NO 3 ) appeared healthy, but no nodules formed. In the low organic-matter substrate, nodules formed in 5 weeks when plants were irrigated with N-free Hoagland’s solution at pH 7.5. Four 300-bp fragments of query sequences (SU1, SU2, SU3, and SU4) were obtained from nodules. When compared with nif H gene sequences reported in the literature using the Basic Local Alignment Search Tool (BLAST), more than 90% similarity to the nif H of Frankia spp. was obtained. The Frankia strains in the nodules shared nif H sequences similar to those of the same host-specific group of Shepherdia . Furthermore, Frankia strains with similar nif H genes have been reported in nodules of Shepherdia argentea (silver buffaloberry). Additionally, Frankia strains belonging to cluster 3 infective strains consisting of Elaeagnaceae and Rhamnaceae infective Frankia showed high similarity to the query sequences. This research demonstrates that nodulation of S. ×utahensis is inhibited at 2 m m NH 4 NO 3 . Apart from N, nodule formation may be associated with the substrate type and pH of the nutrient solution. Based on nif H gene sequence amplification, Frankia strains in the root nodules may have the potential to fix atmospheric nitrogen (N 2 ). These Frankia strains have signature gene sequence characteristics of Elaeagnaceae-infective Frankia , suggesting that S. × utahensis shares Frankia strains similar to its parents.