Indigenous soil bacteria as bioinoculants for promoting seedling growth of native plants in arid land rehabilitation

The depletion and degradation of native plant communities is a major threat to the long-term health and functionality of many ecosystems worldwide. Some of the current challenges in mine rehabilitation programs are poor recruitment and establishment of native plants. Indigenous soil bacteria, including cyanobacteria from soil biocrusts, have shown promise as bio-fertilizers as they may promote germination and enhance seedling growth of native plants in reconstructed soil profiles. In this research, we assessed the potential of bioinoculants composed by locally sourced soil bacteria from the rhizosphere and cyanobacteria from biocrusts, to promote germination and growth of native arid plants from Western Australia and New South Wales (Australia). Individual cyanobacteria species (e.g. Leptolyngbya sp, Nostoc sp. and Microcoleus sp), a cyanobacteria mix of these three species, and enrichments of soil bacteria from the rhizosphere were considered as inoculum for seed bio-priming. Overall, our results showed that lower concentrations of cyanobacteria inoculants (1 g l^-1) are more effective for promoting seedling growth than highly concentrated inoculum (5 g l^-1). The effects of the cyanobacterial/bacterial inoculants were specific to each plant species. However, biopriming seeds with soil bacteria and the cyanobacteria mix resulted in three times larger roots in hummock grasses (e.g. Triodia epactia) compared to the control treatment. We also identified the bio-active components or metabolites produced by targeted cyanobacteria species through GC/MS analyses. Our results showed that some of the cyanobacterial inoculants produced substances chemically like plant hormones such as auxins, i.e. indole-3-acetic acid. The positive effects of the native soil bacteria and cyanobacteria inoculants on native plants could be related to their ability of promoting nutrient bioavailability, improving stress resistance, protection against other microbes, and production of substances that may act as hormones. The findings of this research can allow selecting the most effective bio-active inoculants for application in seed-based land rehabilitation programs.