In vitro and in vivo analyses of plant-growth-promoting potential of bacteria naturally associated with spruce trees growing on nutrient-poor soils

Abstract

In natural ecosystems, plants associate with numerous microbes that play a crucial role in sustaining their growth and health on disturbed and nutrient-poor soils. The West Chilcotin in British Columbia, Canada is a secluded region having poorly-developed and coarse-textured soils containing extremely limited amounts of plant-available nutrients. However, natural forest stands of hybrid white spruce are thriving in this region unaffected by such nutrient-limited edaphic conditions. Looking for plant-growth-promoting bacteria that may be involved in supporting the growth of spruce trees in this region, several potential endophytic diazotrophic bacteria were previously isolated from spruce tissues. We analyzed the plant-growth-promoting potential of six selected bacterial strains in vitro using enzyme assays (qualitative and quantitative tests) and in vivo using 18-month long greenhouse growth trials. In lab-based enzyme assays, these bacterial strains showed significant ability to solubilize inorganic and organic phosphate and produce siderophores, indole-3-acetic acid, 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase enzyme and cell wall degrading enzymes – cellulase, protease, chitinase and β-1,3-glucanase. These bacteria also showed considerable in situ ACC deaminase activity by enhancing the root growth (84–479%) of canola and tomato (ethylene-sensitive plants). In addition, each bacterial strain was able to significantly enhance the length (>40%) and biomass (>90%) of not only its original host (hybrid white spruce) but also a foreign host (lodgepole pine) in the greenhouse growth trials. This plant growth promotion could be attributed to the rhizospheric and endophytic colonization observed for these bacterial strains in both tree species. Notably, one bacterial strain, Caballeronia sordidicola LS-S2r, showed the highest colonization ability (105–108 colony forming units/g tissue) in spruce and pine seedlings along with enhancing their biomass by 7-fold and 4.25-fold, respectively, which could be related to the highly significant in vitro plant growth promoting ability observed for this strain in all enzyme assays. In summary, the potential of these bacteria to form key ecological relationships with multiple tree hosts to promote their growth may explain the ability of trees to naturally regenerate on nutrient-poor soils. Moreover, these bacteria, particularly C. sordidicola LS-S2r, could potentially be used as comprehensive biofertilizers for boreal forest trees under highly disturbed and nutrient-poor edaphic conditions.