Roots and microbiome jointly drive the distributions of 17 phytohormones in the plant soil continuum in a phytohormone‐specific manner

Phytohormones are a group of signal compounds that regulate plant growth, development, health and plant-soil interactions. We aimed to investigate the distribution and controlling factors of a diverse set of phytohormones found in the plant-soil continuum. Twenty phytohormones belonging to seven categories were studied in maize roots, rhizosphere and bulk soil with ultra-high-performance liquid chromatography–tandem mass spectrometry. In addition, the microbiomes shaped by gamma-sterilization and reinoculation were analyzed using high throughput sequencing. We found three major distribution patterns of the 17 phytohormones detected along the root-soil continuum. In the first of these, the phytohormones were predominant in roots, followed by rhizosphere and then by bulk soil. In the second pattern, concentrations of the phytohormones were smallest in the rhizosphere. While, in the third pattern, concentrations of the phytohormones were greatest in the rhizosphere. The concentrations of the phytohormones were closely related to those of root phytohormone or roots biomass as well as relative abundances of wide range of microbial taxa. The results indicated that the distribution patterns of six categories of phytohormones in the root-soil continuum are driven by a balance between the production and uptake by the roots and the production, utilization and degradation by the soil microbiome in a phytohormone-specific manner.