Mutualism between Klebsiella SGM 81 and Dianthus caryophyllus in modulating root plasticity and rhizospheric bacterial density. Klebsiella and Dianthus caryophyllus mutualism

AimsDianthus caryophyllus 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 Klebsiella SGM 81 strain to promote Dianthus caryophyllus 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 Klebsiella SGM 81 as major bacterial trait most likely to enhance growth of Dianthus caryophyllus. MethodsPlant growth promotion assays through bacterial root inoculation, bacterial growth on roots and root colonisation microscopy, PCR, LC-MS/MS. ResultsInoculation of Dianthus caryophyllus roots with Klebsiella SGM 81 drastically promoted plant growth when grown in agar and soil, concommittant with a burst in root hair formation, and suggesting an important role of IAA production secretion to promote Dianthus growth. We identified the presence of a canonical ipdC gene in Klebsiella SGM 81 and confirmed bacterial production and secretion of IAA using LC-MS/MS. We found Klebsiella SGM 81 to be an endophytic coloniser of Dianthus caryophyllus roots in a dose dependent manner and found no adverse effects on the overall rhizospheric microbial population unless supplied to soil in very high titres. ConclusionKlebsiella SGM 81 effectively improves root traits of Dianthus caryophyllus in a dose dependent manner, likely through tryptophan dependent IAA production, and colonising the intercellular spaces of root tissue. Under best plant growth promoting conditions in non-sterile soil, the resulting apparent high overall microbial titres in the rhizosphere support a mutualistic relationship between Klebsiella SGM 81 and carnation that potentially extends to the wider rhizosphere microbiota.