Thursday, October 8, 2015
Zinc and nickel tissue distribution and translocation in accumulating and non-accumulating species: histidine-mediated differences
Ilya V. Seregin & Anna D. Kozhevnikova
Laboratory of Root Physiology, Timiryazev Institute of Plant Physiology, Russian Academy of Sciences, Moscow
The aim of this work was to compare Ni and Zn toxic effects, accumulation and tissue distribution in three non-accumulators Thlaspi arvense, Lepidium ruderale and Capsella bursa-pastoris and in four accessions of hyperaccumulator Noccaea caerulescens. All the N. caerulescens accessions were much more tolerant to Ni and Zn than non-accumulating species and differed remarkably in metal tolerance. The calamine accessions (SF, LC) were more tolerant to Ni than the non-metallicolous accession LE, and all of them were less tolerant and accumulated less Ni than the serpentine accession, MP. Non-metallicolous accession accumulated more Zn, but was less tolerant than the accessions originating from calamine soils. Thus, Zn tolerance may at least in part be connected with lower accumulation capacity. High tolerance of N. caerulescens is defined by high efficiency of detoxification mechanisms, including metal sequestration in the compartments with low metabolic activity such as the cell wall (Zn) and the vacuoles of large water-storage cells in leaf epidermis (Zn, Ni). In non-accumulators, Zn and Ni were detoxified by sequestration mainly in the cell vacuoles (Ni, Zn) or also in the apoplast (Zn) of root tissues (cortex, endodermis). The mechanisms of enhanced root to shoot metal transport in metal hyperaccumulators are incompletely understood. We investigated the role of free histidine in Ni and Zn xylem loading and transport across the tonoplast of root cells in hyperaccumulator Noccaea caerulescens and non-accumulator T. arvense. Compared to T. arvense, the concentration of free histidine in N. caerulescens was 10-fold enhanced in roots, but was only slightly higher in leaves. Exogenous histidine supply significantly enhanced Ni and Zn xylem loading in N. caerulescens but not in T. arvense. Ni and Zn uptake in energized root-derived tonoplast vesicles was significantly inhibited in N. caerulescens, but not significantly affected in T. arvense, when Ni and Zn were supplied as complexes with histidine. Thus, the high rate of root to shoot translocation of Ni and Zn in N. caerulescens as compared to T. arvense seems to depend on the combination of two distinctive characters, i.e. a greatly enhanced root histidine concentration and a strongly decreased ability to accumulate histidine-bound Ni and Zn in root cell vacuoles.
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