Hormone concentrations in tobacco roots change during arbuscular mycorrhizal colonization with Glomus intraradices

Phytohormones are known to play a pivotal role in various developmental processes in plants and in arbuscular-mycorrhizal (AM) fungal-host symbiosis. This study focuses on characterizing the changes in the concentrations of auxins, cytokinins and gibberellins in tobacco (Nicotiana tabacum) during the early stages of colonization by Glomus intraradices, using advanced analytical detection techniques. High-pressure liquid chromatography analysis followed by radioimmunoassay detection revealed that AM colonization induced the accumulation of specific zeatin riboside-like and isopentenyl adenosine-like compounds in both roots and shoots. Use of the gas chromatography-mass spectrometry technique on the same developmental stage revealed that gibberellins (GA) of the earl-13-hydroxylation biosynthetic pathway (GA₁, GA₈, GA₁₉ and GA₂₀) were significantly more abundant in roots, but not shoots, of AM inoculated plants than in those of nonmycorrhizal plants. Indoleacetic acid concentrations (total and free) remained unaltered by AM colonization. This study demonstrates that hormonal changes do occur during AM symbiosis with tobacco, before the fungal benefits manifest.