Response to copper excess in Arabidopsis thaliana: Impact on the root system architecture, hormone distribution, lignin accumulation and mineral profile

Growth, in particular reorganization of the root system architecture, mineral homeostasis and root hormone distribution were studied in Arabidopsis thaliana upon copper excess. Five-week-old Arabidopsis plants growing in hydroponics were exposed to different Cu2+ concentrations (up to 5 μM). Root biomass was more severely inhibited than shoot biomass and Cu was mainly retained in roots. Cu2+ excess also induced important changes in the ionome. In roots, Mg, Ca, Fe and Zn concentrations increased, whereas K and S decreased. Shoot K, Ca, P, and Mn concentrations decreased upon Cu2+ exposure. Further, experiments with seedlings vertically grown on agar were carried out to investigate the root architecture changes. Increasing Cu2+ concentrations (up to 50 μM) reduced the primary root growth and increased the density of short lateral roots. Experiment of split-root system emphasized a local toxicity of Cu2+ on the root system. Observations of GUS reporter lines suggested changes in auxin and cytokinin accumulations and in mitotic activity within the primary and secondary root tips treated with Cu2+. At toxic Cu2+ concentrations (50 μM), these responses were accompanied by higher root apical meristem death. Contrary to previous reports, growth on high Cu2+ did not induce an ethylene production. Finally lignin deposition was detected in Cu2+-treated roots, probably impacting on the translocation of nutrients. The effects on mineral profile, hormonal status, mitotic activity, cell viability and lignin deposition changes on the Cu2+-induced reorganization of the root system architecture are discussed.