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Legume-Nodulating Rhizobia are Widespread in Soils and Plants Across the Island of O’ahu, Hawai’i

Summary

This research examined beneficial soil bacteria called rhizobia that help legume plants grow by providing them with nitrogen. The study mapped these bacteria across the Hawaiian island of O’ahu and tested their ability to help grow native Hawaiian plants. The findings show these helpful bacteria are widespread but vary in their distribution and effectiveness with different plant species. This research has several practical implications: • Helps nurseries grow endangered native Hawaiian trees more effectively • Provides guidance for agricultural practices involving legume crops • Contributes to conservation efforts of native Hawaiian plants • Improves understanding of how beneficial soil microbes are distributed across islands • Demonstrates the importance of selecting specific bacterial strains for optimal plant growth

Background

The Hawaiian Islands, due to their isolation, have developed a biogeographic landscape prone to invasion by introduced organisms, including soil microbes. While most microbial introductions were not recorded, the Nitrogen Fixation for Tropical Agricultural Legumes (NifTAL) Project introduced numerous non-native rhizobia strains starting in 1975. Hawaii has 16 endemic legume species, 7 indigenous species, and at least 174 recorded introduced species, but the current occurrence and distribution of rhizobia and their host associations remains largely unknown.

Objective

To characterize rhizobia genera from soils and nodules across O’ahu, Hawaii and determine their distribution patterns and host associations. Additionally, to test selected rhizobia strains as potential nursery inoculants for the endemic and threatened tree Erythrina sandwicensis.

Results

18 genera of rhizobia were identified from soils and nodules. Cupriavidus was found at all sites but at lower abundance than other common genera like Rhizobium, Bradyrhizobium, and Devosia. Agricultural sites had higher rhizobia diversity than undisturbed sites. Bradyrhizobium was the most common nodulator, with strain JA1 capable of nodulating nine different host species including two native species. Both tested Bradyrhizobium strains successfully nodulated E. sandwicensis and significantly increased seedling biomass in nursery trials.

Conclusion

The soils across O’ahu contain diverse rhizobia genera, with some strains showing broad host ranges while others appear more specific. The presence of rhizobia across sites suggests a constant source of natural inoculants in these soils, though specific strains may be needed for certain host plants. For nursery applications with E. sandwicensis, Bradyrhizobium strain JA9 is recommended as it produced the greatest growth benefits.
  • Published in:PLoS One,
  • Study Type:Field Survey and Experimental Study,
  • Source: 10.1371/journal.pone.0291250
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