Growth parameters, hormonal balance and thiol-peptide compound metabolism in Arabidopsis thaliana seedlings growing under excess zinc

To date, almost no information is available in roots and shoots of the model plant Arabidopsis thaliana (L.) Heynh. about the hierarchic relationship between metal accumulation, phytohormone levels, and glutathione/phytochelatin content, and how this relation affects root and shoot development. For this purpose, specific concentrations of zinc, alone or in triple combination with cadmium and copper, were supplied for two weeks to seedlings growing in a hydroponic system and using Petri dishes with a gradient of distances between germinating seeds and metal-contaminated agarized medium. Zinc accumulation was determined by anodic stripping voltammetry in plant tissues and digested agar samples, and a significant competition in metal uptake was observed. Microscopic and high-resolution scanning analyses revealed that root morphology was affected by metal exposure, with increases in root system total length and surface mainly due to the higher branching and number of lateral roots, accompanied by higher average root diameter. The confocal microscopy analysis of auxin accumulation and influx in the cells by the use of transgenic Arabidopsis lines (DR5:GUS, LAX3:GUS and AUX1:GUS) and the mass spectrometry of plant tissues revealed significant changes in auxin levels and accumulation in the seedling exposed to zinc alone or in combination. Real time quantitave PCR analysis of some genes involved in auxin and cytokinin synthesis showed on average a metal upregulated transcription. The production of thiol-peptides was induced by zinc alone or in combination, but the expression of the genes involved in thiol-peptide synthesis was not stimulated by the metals, suggesting a full post-transcriptional control. Results show that the Cd/Cu/Zn-induced changes in root morphology are caused by a hormonal unbalance, mainly governed by the auxin/cytokinin ratio. The remodeling of the root architecture in response to zinc could be a pollution ‘escaping strategy’ aimed at seeking metal-free areas. The methods used and the results obtained by this model plant could be transferred to species with bioremediation or agronomic importance.