Combined effects of fungal inoculants and the cytokinin-like growth regulator thidiazuron on growth, phytohormone contents and endophytic root fungi in Miscanthus × giganteus

Aim of this study was to investigate main effects and interactions between symbiotic fungi and the cytokinin-like growth regulator thidiazuron (TDZ) in Miscanthus × giganteus. The arbuscular mycorrhiza fungus Rhizophagus intraradices (AMF) and the endophyte Piriformospora indica (PI) were chosen as model symbionts. The fungal inoculants and TDZ had no significant effect on plant growth but modulated phytohormone levels in the leaves. TDZ induced accumulation of salicylic acid in controls, but not in plants inoculated with fungi. Leaf concentrations of abscisic acid (ABA) derivatives, auxin (indole-3-acetic acid) precursors and catabolites and numerous cytokinins were increased by R. intraradices but lowered by P. indica. TDZ raised concentrations of ABA compounds, the non-indole auxin phenylacetic acid, jasmonate and some cytokinins, but decreased cis-zeatin and N6-(Δ2-isopentenyl)adenine levels. Inoculation with AMF reduced abundance of endogenous clampless endophytes. TDZ application strongly reduced formation of arbuscular mycorrhiza and increased occurrence of clamped mycelia (i.e. basidiomycetous endophytes). Our study provides a thorough outline of the phytohormone homeostasis under the combined influence of beneficial inoculants and a growth regulator, highlighting the necessity to study their interaction in the whole plant-microbial context.