Mutualism Between SGM 81 and in Modulating Root Plasticity and Rhizospheric Bacterial Density

Aims: is a commercially important ornamental flower. Plant growth promoting rhizobacteria are increasingly applied as bio-fertilisers and bio-fortifiers. We studied the effect of a rhizospheric isolate SGM 81 strain to promote growth under sterile and non-sterile conditions, to colonise its root system endophytically and its impact on the cultivatable microbial community. We identified the auxin indole-3-acetic acid (IAA) production of SGM 81 as major bacterial trait most likely to enhance growth of .

Methods: dependent IAA production of SGM 81 was quantified using LC-MS/MS and localised proximal to roots and correlated to root growth promotion and characteristic morphological changes. SGM 81 cells were localised on and within the plant root using 3D rendering confocal microscopy of expressing SGM 81. Using Salkowski reagent IAA production was quantified and localised proximal to roots in situ. The effect of different bacterial titres on rhizosphere bacterial population was CFU enumerated on nutrient agar. The genome sequence of SGM 81 (accession number PRJEB21197) was determined to validate PGP traits and phylogenic relationships.

Results: Inoculation of roots with SGM 81 drastically promoted plant growth when grown in agar and soil, concomitant with a burst in root hair formation, suggesting an increase in root auxin activity. We sequenced the SGM 81 genome, identified the presence of a canonical gene in SGM 81, confirmed bacterial production and secretion of IAA in batch culture using LC-MS/MS and localised plant dependent IAA production by SGM 81 proximal to roots. We found SGM 81 to be a rhizoplane and endophytic coloniser of roots in a dose dependent manner. We found no adverse effects of SGM 81 on the overall rhizospheric microbial population unless supplied to soil in very high titres.

Conclusion: SGM 81 effectively improves root traits of in a dose dependent manner, likely through tryptophan dependent IAA production in the rhizoplane and potentially within the intercellular spaces of root tissue. Under optimal plant growth promoting conditions in non-sterile soil, the high total microbial titre in the rhizosphere supports a mutualistic relationship between SGM 81 and carnation that potentially extends to the wider rhizosphere microbiota.