Plasma-treated water applied as a foliar spray promotes root growth in barley

Plasma-treated water (PTW) contains reactive oxygen and nitrogen species and can therefore induce mild oxidative stress in plants. In a previous study, PTW treatment of leaves resulted in both short- and long-term systemic signalling effects. In this study, we analysed the adaptive response of the barley leaf and root transcriptome triggered by PTW treatment under short- and long-term conditions to further understand the regulatory mechanisms and the mode of action of PTW. The application of PTW to the leaves resulted in stronger long-term transcriptional changes in the roots. PTW treatment significantly increased root biomass, while it had no effect on shoot growth. In roots, the altered gene expression indicated that the signalling of auxin, ethylene, brassinosteroids and gibberellin was stimulated, while cytokinin signalling was impeded. Many genes encoding cell wall modifying enzymes, structural proteins and receptor kinases were upregulated in the roots in long-term response to PTW. In addition, foliar treatment with PTW led to changes in root architecture: root length, surface area, diameter and number of forks increased in the long term, as did the content of soluble phenolic compounds. All these changes indicate that PTW treatment affected root growth possibly by promoting cell elongation. Thus, PTW could be a suitable stimulant to improve the root growth of plants.