Mechanisms of trichome-specific Mn accumulation and toxicity in the Ni hyperaccumulator Alyssum murale

Background and aims: Mechanisms of Mn accumulation and toxicity in and around trichomes on the Ni hyperaccumulator Alyssum murale were investigated. Methods: Plants were grown aeroponically with variable amounts of Mn and Ni. Total metals were determined and electron microprobe analysis (EMPA) and synchrotron-based micro x-ray fluorescence (μ-SXRF) spectroscopy were used to evaluate metal distribution. Synchrotron techniques (μ-XANES, μ-EXAFS) along with infrared spectroscopy (DRIFT) were used to determine Mn speciation. Results: At lower Mn concentrations or when grown together with Ni, Mn is confined to the trichome basal compartment in the +2 oxidation state in a complex with phosphate. At tissue concentrations >1,150 μg g^-1 Mn-rich lesions develop around some trichomes in which greater amounts of Mn ³⁺ is found. Conclusions: Mn is preferentially stored in trichomes on the plant surface which at higher concentrations enters the cell wall or apoplastic space of neighboring cells resulting in the formation of brown reaction products and oxidized Mn species. We propose a mechanism by which lesion formation and oxidized Mn species around some trichomes is possibly due to induction of the peroxidase system by excess Mn, triggering the accumulation of toxic phenoxy radicals and Mn³⁺.