Efficient xylem transport and phloem remobilization of Zn in the hyperaccumulator plant species Sedum alfredii

Abstract

SummarySedum alfredii is one of a few species known to hyperaccumulate zinc (Zn) and cadmium (Cd). Xylem transport and phloem remobilization of Zn in hyperaccumulating (HP) and nonhyperaccumulating (NHP) populations of S. alfredii were compared.Micro-X-ray fluorescence (l-XRF) images of Zn in the roots of the two S. alfredii populations suggested an efficient xylem loading of Zn in HP S. alfredii, confirmed by the seven-fold higher Zn concentrations detected in the xylem sap collected from HP, when compared with NHP, populations. Zn was predominantly transported as aqueous Zn (> 55.9%), with the remaining proportion (36.7-42.3%) associated with the predominant organic acid, citric acid, in the xylem sap of HP S. alfredii.The stable isotope 68 Zn was used to trace Zn remobilization from mature leaves to new growing leaves for both populations. Remobilization of 68 Zn was seven-fold higher in HP than in NHP S. alfredii. Subsequent analysis by l-XRF, combined with LA-ICPMS (laser ablationinductively coupled plasma mass spectrometry), confirmed the enhanced ability of HP S. alfredii to remobilize Zn and to preferentially distribute the metal to mesophyll cells surrounding phloem in the new leaves. The results suggest that Zn hyperaccumulation by HP S. alfredii is largely associated with enhanced xylem transport and phloem remobilization of the metal. To our knowledge, this report is the first to reveal enhanced remobilization of metal by phloem transport in hyperaccumulators.

Phytoremediation of Chromium: Distribution and Speciation of Chromium in Typha angustifolia

“…13 This pattern was also observed in Sedum alfredii, in which Zn was distributed mainly along the xylem. 14 The accumulation of Cr in Gynura pseudochina treated with Cr(III) was also located in the stem cortex and vascular bundle. Distribution, translocation and accumulation of Cr in G. pseudochina primarily depended on the oxidation state of Cr and on the plant tissue.…”

“…Researchers have found that increased xylem loading is the main factor affecting absorption and translocation of heavy metal from root to shoot; as shown in increasing uptake of Zn and Cd by S. alfredii, Solanum melongena, and Solanum torvum. 14,20 The xylem loading capacity of heavy metal hyperaccumulator plants is higher than in non-hyperaccumulator plants. 14,20 Hyperaccumulator plants also have special mechanisms to tolerate high concentration of heavy metals in tissue by using low-molecular-weight ligands, small metal-binding proteins such as histidine, organic acids, phytochelatine, metalothionine in the sequestration, transport, and accumulation of heavy metals.…”

“…14,20 The xylem loading capacity of heavy metal hyperaccumulator plants is higher than in non-hyperaccumulator plants. 14,20 Hyperaccumulator plants also have special mechanisms to tolerate high concentration of heavy metals in tissue by using low-molecular-weight ligands, small metal-binding proteins such as histidine, organic acids, phytochelatine, metalothionine in the sequestration, transport, and accumulation of heavy metals. 21 EtOAcCr(III) was the Cr species found to be accumulated in root and shoot of T. angustifolia in this study.…”

“…In Trifolium brachycalycium, Atriplex canescence G. pseudochina and S. alfredii, Cr(VI) is reduced to Cr(V) then further reduced to Cr(III), or Cr(VI) is directly reduced to Cr(III). 14,15,26 EtOAcHyperaccumulator plants may develop different mechanisms applicable in the phytoremediation process. In this study, T. angustifolia tolerated Cr(VI) by reducing it to Cr(III) in the rhizosphere.…”

Evolutionary aspects of elemental hyperaccumulation

“…It has been shown that citrate and malate bind to Zn and Ni to allow for loading and unloading into and out of xylem, phloem and across the tonoplast. In Sedum alfredii, Lu et al (2013) showed increased levels of citrate in the xylem sap of plants from a hyperaccumulating population relative to a non-hyperaccumulating population and that the levels of citrate increased with increasing Zn concentration. Nickel hyperaccumulator Alyssum murale has increased citric acid and malic acid synthesis in the root mitochondria relative to non-hyperaccumulator A. montanum (Agrawal et al 2013).…”