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/phytochelati n 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, w ere supplied for two weeks to seedlings growing in a hydroponic system and using Petri dis hes with a gradient of distances between germinating seeds and metal-contaminated agar ized 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 surfac e mainly due to the higher branching and number of lateral roots, accompanied by higher av erage root diameter. The confocal microscopy analysis of auxin accumulation and influx i n 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 a uxin levels and accumulation in the seedling exposed to zinc alone or in combination. Real t ime quantitave PCR analysis of some genes involved in auxin and cytokinin synthesis showed on average a metal up- regulated transcription. The production of thiol-peptides was i nduced by zinc alone or in combination, but the expression of the genes involved in thiol- peptide synthesis was not stimulated by the metals, suggest ing a full post-transcriptiona l control. Results show that the Cd/Cu/Zn-induced changes in root morphology are caused by a hor monal unbalance, mainly governed by the auxin/cytokinin ratio. The remodeling of the ro ot architecture in response to zinc could be a pollution ‘escaping strategy’ aimed at seeki ng metal-free areas. The methods used and the results obtained by this model plant could be transferred to species with bioremediation or agronomic importance. Acknowledgments: This work was p artly supported by a STSM Grant from Zinc-Net COST Action TD1304.