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 signals. RNA-seq was applied to analyse the adaptive response triggered by PTW treatment and to further determine the mode of action of PTW in barley leaves and roots under short- and long-term conditions. 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 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. This indicates that PTW treatment appears to affect root growth by promoting cell elongation. 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 could be a suitable stimulant for improving the root growth of plants. Overall design: To investigate the priming effect of plasma-treated water (PTW) as a leaf-spray on barley, plants were harvested 1 day or 4 weeks after PTW or control (sprayed with deionized water) treatment. RNA was extracted from leaf and root samples (each pooled from 5 plants) for RNA-seq. Gene expression profiling was performed for 2 treatments (control & PTW) at 2 timepoints in both organs.