Isolation and characterization of Rhizobium from non-leguminous potato plants: New frontiers in Rhizobium research

Rhizobium is well-documented for its symbiotic relationship with legume plants, where it plays a crucial role in biological nitrogen-(N)-fixation within their root nodules. However, the isolation, identification, and association of Rhizobium as a free-living diazotroph with potato plants remain relatively less explored. The present study reports the isolation and characterization of free-living Rhizobium strain from the rhizosphere of potato plants and its potential for promoting growth and N-fixation. Diazotrophic strain (TN04) was isolated from rhizosphere of potato plants on nitrogen-free media and identified on the basis of 16S rRNA gene sequence (Accession number: LN833444). TN04 strain also contained nifH gene and showed N-fixation potential (151.70 nmolmg/protein/h) through ARA activity, indicating its ability to fix atmospheric nitrogen. TN04 exhibited potential for phosphate solubilization (272.5 µg/mL) and produced indole acetic acid at concentration of 3.50 µg/mL. To assess the N-fixing ability of TN04 diazotroph, a ¹⁵N dilution experiment was conducted in pots using sterilized sand and sterilized soil under various fertilizer doses. The results of pot experiments demonstrated significant improvement in N content and growth parameters of inoculated potato plants compared to un-inoculated controls, suggesting that diazotrophic strain effectively fixed atmospheric N through isotopic dilution. Moreover, Rhizobium sp. TN04 remarkably improved plant growth and agronomic parameters under field conditions. Significant improvements were observed in N uptake, N utilization, and N use efficiency in field trails. In addition, microscopic analysis using transmission electron and confocal laser scanning microscopy provided insights into the colonization patterns of TN04 strain at the junctions between the secondary and primary roots, forming strong association with potato roots. Our study presents novel insights into the presence and interaction of Rhizobium with non-host plants, shedding light on its N-fixing capabilities in non-leguminous crops. These findings pave the way for developing strategies to explore microbiome of non-leguminous crops and exploit the N-fixation of Rhizobium in non-host crops.