ISOLATION AND CHARACTERISATION OF PLANT GROWTH PROMOTING BACTERIA FROM AERIAL ROOTS OF HONG KONG PLANTS

In Hong Kong (as in much of SE Asia), rainfall surpasses 2200 millimetres per year. In addition, local granite-derived soils are poorly retentive of nutrients, creating oligotrophic conditions with < 0.47% nitrogen in surface soil levels and negligible concentrations below. Despite such challenges, numerous non-leguminous (and often lithophytic) plants, such as Ficus microcarpa and Glochidion hongkongense, are not only abundant but also grow rapidly to great size. These very high levels of carbon sequestration imply correspondingly large rates of nitrogen uptake that can only be explained by microbial nitrogen fixation. In this project, samples of aerial roots were recovered from a number of common Hong Kong plant species. nitrogen free (NF) media was used to identify potentially nitrogen fixing bacteria, which were then streaked to purity before DNA extraction and sequencing. Complete genomes of three isolates, 9ba2, Kosakonia radicincitans JS2a2 and Gluconobacter thailandicus ISBL3, were generated by hybrid assembly, using both Illumina MiSeq and Oxford Nanopore MinION platforms. Genomic analysis revealed numerous highly-conserved nitrogenase related (nif) genes across the different genera. Mineral transport systems, particularly for iron and molybdate, were also strongly represented. For example, in K. variicola 9bα-2, a 166 kbp plasmid not only encodes the fec(RABCDE) operon for ferric citrate uptake, but also the molybdate-responsive ModE transcription factor. The probiotic use of nitrogen fixing bacteria such as these has been proposed as a more sustainable measure for plant growth promotion that could reduce requirements for chemical fertilisers. Significantly, the extent of microbial nitrogen fixation that we estimate in Hong Kong suggests that the contribution to the nitrogen cycle of non-legume-associated microbes may be greatly underestimated.