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 D. 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 D. caryophyllus.MethodsipdC dependent IAA production of SGM 81 was quantified using LC-MS/MS and localised proximal to D. caryophyllus 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 gfp 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 Klebsiella SGM 81 (accession number PRJEB21197) was determined to validate PGP traits and phylogenic relationships.ResultsInoculation of D. caryophyllus roots with Klebsiella 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 Klebsiella SGM 81 genome, identified the presence of a canonical ipdC gene in Klebsiella 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 Klebsiella SGM 81 to be a rhizoplane and endophytic coloniser of D. caryophyllus 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.ConclusionKlebsiella SGM 81 effectively improves root traits of D. caryophyllus 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 Klebsiella SGM 81 and carnation that potentially extends to the wider rhizosphere microbiota.

中文翻译：

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克雷伯菌 SGM 81 和石竹在调节根可塑性和根际细菌密度方面的互利作用


目的石竹是一种具有重要商业价值的观赏花卉。促进植物生长的根际细菌越来越多地用作生物肥料和生物强化剂。我们研究了根际分离株克雷伯氏菌 SGM 81 菌株在无菌和非无菌条件下促进石竹生长、内生定植其根系的作用及其对可培养微生物群落的影响。我们将克雷伯菌 SGM 81 的生长素吲哚-3-乙酸 (IAA) 产量确定为最有可能促进丁香石竹生长的主要细菌性状。方法使用 LC-MS/MS 和局部近端对 SGM 81 的 SipdC 依赖性 IAA 产量进行定量D. caryophyllus 根并与根生长促进和特征形态变化相关。使用表达 SGM 81 的 gfp 的 3D 渲染共聚焦显微镜将 SGM 81 细胞定位在植物根部之上和之内。使用 Salkowski 试剂对 IAA 产生进行定量并原位定位于根部附近。不同细菌滴度对根际细菌种群的影响以营养琼脂上的CFU计数。确定了克雷伯氏菌 SGM 81（登录号 PRJEB21197）的基因组序列，以验证 PGP 性状和系统发育关系。结果用克雷伯氏菌 SGM 81 接种石竹根，在琼脂和土壤中生长时，可显着促进植物生长，同时根毛爆发。形成，表明根部生长素活性增加。我们对克雷伯氏菌 SGM 81 基因组进行了测序，鉴定了克雷伯氏菌 SGM 81 中是否存在典型的 ipdC 基因，使用 LC-MS/MS 确认了分批培养物中 IAA 的细菌产生和分泌，以及 SGM 81 靠近根部的局部植物依赖性 IAA 产生。 我们发现克雷伯氏菌 SGM 81 是石竹根的根平面和内生定植菌，且呈剂量依赖性。我们发现 SGM 81 对整个根际微生物种群没有不利影响，除非以非常高的滴度供应到土壤中。结论 克雷伯氏菌 SGM 81 以剂量依赖的方式有效改善了石竹的根性状，可能是通过根平面中色氨酸依赖性 IAA 的产生和可能位于根组织的细胞间隙内。在非无菌土壤中的最佳植物生长促进条件下，根际的高微生物总滴度支持克雷伯氏菌 SGM 81 和康乃馨之间的互利关系，这种关系有可能扩展到更广泛的根际微生物群。