Throughout the world, over 400 species of plants are known to accumulate large quantities of metals in their shoots (`hyperaccumulators'), but of these, relatively few accumulate manganese (Mn). We have identified for the first time an Australian native hyperaccumulator of Mn, Austromyrtus bidwillii (Benth.) Burrett (Myrtaceae). Concentrations of Mn up to 19 200 µg g-1 were measured in dried leaves of this rainforest tree, and young bark was found to contain up to 26 500 µg g-1 Mn. Approximately 40% of the Mn in the leaves is readily extracted with water, suggesting that some of the Mn is associated with water-soluble compounds such as organic acids. Organic acids present in appreciable amounts in leaf extracts of A. bidwillii were identified and quantified by HPLC and gas chromatography-mass spectrometry. The following organic acids (in order of concentration) were present: succinic > malic ≥ malonic > oxalic >> citric acid. The concentration of total organic acids was on average 123 000 µg g-1 dry tissue, which amounted to approximately three times the molar equivalent of Mn and two times the molar equivalent of total cations (Mn, Mg and Ca), demonstrating that organic anions were in excess. The Mn remaining after water extraction ((61 ± 3.9%) could be extracted with 0.2M HCl, suggesting that a significant portion of the Mn is associated with the cell wall (perhaps replacing Ca) or is present as other insoluble compounds.

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A basic, galactose-rich style glycoprotein (GaRSGP) encoded by a previously characterized style-specific cDNA (NaPRP4) has been isolated from the styles of Nicotiana alata and structurally characterized. The glycoprotein is associated with cell walls in the transmitting tract and is composed of approximately 25% (w/w) protein and 75% (w/w) carbohydrate. The purified glycoprotein appears as a smear of between 45-120 kDa on SDS-PAGE; the deglycosylated protein backbone has an apparent molecular weight of approximately 30 kDa. The glycoprotein is rich in the amino acids lysine, proline, and hydroxyproline and in the monosaccharides galactose and arabinose. It is one of only a few proline/hydroxyproline-rich glycoproteins (P/HRGPs) to be characterized both as a cDNA-clone and protein. Glycans are attached to the protein backbone through both O- and N-glycosidic linkages with the majority of the carbohydrate being O-linked and consisting of short, highly branched chains terminating primarily in galactose residues. A carbohydrate epitope(s) is found on both GaRSGP and another style-specific glycoprotein but not on glycoproteins from other tissues. This finding provides further evidence for the existence of a style-specific carbohydrate epitope(s) which may play a role in style function.

The cellular and subcellular distribution of Ni within leaves of Hybanthus floribundus (Lindley) F. Muell, a hyperaccumulator of Ni, was investigated at relatively high spatial resolution using energy-dispersive X-ray microanalysis (EDAX). Elemental distribution maps showed that Ni was predominantly localized in the vacuoles of epidermal cells in the leaves. Quantification of Ni revealed concentrations up to 275 mmol kg ----1 (embedded tissue) in some epidermal vacuoles. The accumulation of Ni in these cells was associated with a decrease in the concentration of Na and K. There was no indication that Ni was associated with P, S or Cl in the vacuoles. Ni was also concentrated on the outside of cell walls throughout the leaves, indicating that apoplastic compartmentation is also involved in Ni tolerance and accumulation in this plant.

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