Selective Transport of Zinc, Manganese, Nickel, Cobalt and Cadmium in the Root System and Transfer to the Leaves in Young Wheat Plants

• Background and Aims The uptake, translocation and redistribution of the heavy metals zinc, manganese, nickel, cobalt and cadmium are relevant for plant nutrition as well as for the quality of harvested plant products. The long-distance transport of these heavy metals within the root system and the release to the shoot in young wheat (Triticum aestivum ‘Arina’) plants were investigated. • Methods After the application of 65Zn, 54Mn, 63Ni, 57Co and 109Cd for 24 h to one seminal root (the other seminal roots being excised) of 54-h-old wheat seedlings, the labelled plants were incubated for several days in hydroponic culture on a medium without radionuclides. • Key Results The content of 65Zn decreased quickly in the labelled part of the root. After the transfer of 65Zn from the roots to the shoot, a further redistribution in the phloem from older to younger leaves was observed. In contrast to 65Zn, 109Cd was released more slowly from the roots to the leaves and was subsequently redistributed in the phloem to the youngest leaves only at trace levels. The content of 63Ni decreased quickly in the labelled part of the root, moving to the newly formed parts of the root system and also accumulating transiently in the expanding leaves. The 54Mn content decreased quickly in the labelled part of the root and increased simultaneously in leaf 1. A strong retention in the labelled part of the root was observed after supplying 57Co. • Conclusions The dynamics of redistribution of 65Zn, 54Mn, 63Ni, 57Co and 109Cd differed considerably. The rapid redistribution of 63Ni from older to younger leaves throughout the experiment indicated a high mobility in the phloem, while 54Mn was mobile only in the xylem and 57Co was retained in the labelled root without being loaded into the xylem.

Redistribution of Nickel, Cobalt, Manganese, Zinc, and Cadmium via the Phloem in Young and Maturing Wheat

The phloem mobility of heavy metals is relevant to the redistribution of micronutrients and pollutants and, ultimately, to the quality of harvested plant parts. The relative mobility in wheat may vary considerably between different cations. In the study reported here, radio-labeled nickel (Ni), cobalt (Co), manganese (Mn), zinc (Zn) and cadmium (Cd) were introduced into either intact young winter wheat (Triticum aestivum L. cv. Arina) via a leaf flap, or detached maturing shoots via the cut stem. Elements fed into the lamina of the second leaf of 21-day-old plants were translocated to the younger (expanding) leaves and to the roots but not or only in trace amounts to the first (already fully expanded) leaf. The 63Ni and 65Zn were exported more rapidly compared with the other heavy metals. Most of 54Mn was retained in the originally labeled leaf. The peduncle of some maturing shoots was steam-girdled below the ear to distinguish between xylem and phloem transport. This phloem interruption reduced the content of 63Ni in the ear to about 25%. Intermediate effects were observed for 65Zn, 57Co, and 109Cd. Total 54Mn accumulation in the ear was hardly affected by steam-girdling, indicating a transport of this element within the xylem to the ear. These results suggest that the relative phloem mobility of Ni and Zn in young wheat plants and in maturing wheat shoots is higher than the mobility of Co and Cd, whereas the mobility of Mn is very low.

Wounding of Arabidopsis halleri leaves enhances cadmium accumulation that acts as a defense against herbivory

Approximately 0.2 % of all angiosperms are classified as metal hyperaccumulators based on their extraordinarily high leaf metal contents, for example >1 % zinc, >0.1 % nickel or >0.01 % cadmium (Cd) in dry biomass. So far, metal hyperaccumulation has been considered to be a taxon-wide, constitutively expressed trait, the extent of which depends solely on available metal concentrations in the soil. Here we show that in the facultative metallophyte Arabidopsis halleri, both insect herbivory and mechanical wounding of leaves trigger an increase specifically in leaf Cd accumulation. Moreover, the Cd concentrations accumulated in leaves can serve as an elemental defense against herbivory by larvae of the Brassicaceae specialist small white (Pieris rapae), thus allowing the plant to take advantage of this non-essential trace element and toxin. Metal homeostasis genes are overrepresented in the systemic transcriptional response of roots to the wounding of leaves in A. halleri, supporting that leaf Cd accumulation is preceded by systemic signaling events. A similar, but quantitatively less pronounced transcriptional response was observed in A. thaliana, suggesting that the systemically regulated modulation of metal homeostasis in response to leaf wounding also occurs in non-hyperaccumulator plants. This is the first report of an environmental stimulus influencing metal hyperaccumulation.