Cellular Sequestration of Cadmium in the Hyperaccumulator Plant Species Sedum alfredii      

Abstract

Spatial imaging of cadmium (Cd) in the hyperaccumulator Sedum alfredii was investigated in vivo by laser ablation inductively coupled plasma mass spectrometry and x-ray microfluorescence imaging. Preferential Cd accumulation in the pith and cortex was observed in stems of the Cd hyperaccumulating ecotype (HE), whereas Cd was restricted to the vascular bundles in its contrasting nonhyperaccumulating ecotype. Cd concentrations of up to 15,000 mg g 21 were measured in the pith cells, which was many fold higher than the concentrations in the stem epidermis and vascular bundles in the HE plants. In the leaves of the HE, Cd was mainly localized to the mesophyll and vascular cells rather than the epidermis. The distribution pattern of Cd in both stems and leaves of the HE was very similar to calcium but not zinc, irrespective of Cd exposure levels. Extended x-ray absorption fine structure spectroscopy analysis showed that Cd in the stems and leaves of the HE was mainly associated with oxygen ligands, and a larger proportion (about 70% in leaves and 47% in stems) of Cd was bound with malic acid, which was the major organic acid in the shoots of the plants. These results indicate that a majority of Cd in HE accumulates in the parenchyma cells, especially in stems, and is likely associated with calcium pathways and bound with organic acid (malate), which is indicative of a critical role of vacuolar sequestration of Cd in the HE S. alfredii.

Elemental distribution by cryo-micro-PIXE in the zinc and cadmium hyperaccumulator Sedum plumbizincicola grown naturally

“…mesophyll in leaf, cortex and pith in stem, played more important roles in the storage and detoxification of Cd than Zn in S. plumbizincicola. Different Cd and Zn distributions were also found in the hyperaccumulator S. alfredii supplied with Cd hydroponically (Tian et al 2011). This indicates that different storage and detoxification strategies exist for Cd and Zn in the shoots of S. plumbizincicola.…”

“…However, in the hyperaccumulator N. praecox Zn concentrations in the epidermis were approximately 5 times higher than that in the mesophyll, while Cd concentrations in the epidermis were only twice as high as or even similar to that in the mesophyll (Vogel-Mikuš et al 2008a, b). In the hyperaccumulator Sedum alfredii Zn accumulated preferentially in the epidermis and vascular bundles while Cd was localized mainly in parenchyma cells such as mesophyll in the leaves and cortex and pith in the stems (Tian et al 2009(Tian et al , 2011. On the other hand, most studies on cellular elemental localization use plant materials from artificial hydroponics systems with sufficient nutrients and elevated metal bioavailability.…”

“…For example, in the hyperaccumulators Thlaspi caerulescens (now Noccaea caerulescens) and Noccaea praecox Zn and Cd were mainly concentrated in the epidermal cells of the leaf (Cosio et al 2005;Vogel-Mikuš et al 2008a) and in Arabidopsis halleri they were stored in the trichomes (Küpper et al 2000). Some studies have also indicated a dominant role of mesophyll cells in the compartmental detoxification of abundant heavy metals (Küpper et al 2000;Ma et al 2005;Tian et al 2011). Cd and Zn were found to share similar distribution patterns at tissue and single cell levels in the hyperaccumulators N. caerulescens (Küpper et al 1999;Cosio et al 2005;Ma et al 2005), A. halleri (Küpper et al 2000) and Potentilla griffithii (Hu et al 2009).…”

Cadmium toxicity

“…1 Cadmium is one of the most toxic and non-essential heavy metals. 2 However, the mechanisms underlying the toxic effects are still largely unknown. Due to its chemical similarity to certain essential mineral elements, e.g., Zn, Fe and Ca, cadmium toxicity rises from displacement of these essential elements from a number of essential metalloproteins.…”

Improved cadmium uptake and accumulation in the hyperaccumulator Sedum alfredii: the impact of citric acid and tartaric acid

“…Exudation of organic acids such as tartaric acid (TA) and oxalic acid has been observed from the roots of the hyperaccumulator S. alfredii under Cd exposure (Li et al, 2007). Moreover, Cd in the shoots of hyperaccumulating ecotype (HE) S. alfredii was mainly coordinated with organic acids, mainly malic acid, and also CA (Tian et al, 2011). Preliminary experiments showed that exogenous TA and CA application increased Cd uptake and accumulation in plants of S. alfredii, whereas positive effects were not observed for malic acid or oxalic acid.…”