As one of the important micronutrient, iron participates in many important physiological and biochemical processes. Although iron is abundant in the environment, plants often suffer from iron deficiency because of the alkaline and calcareous soils (Guerinot and Yi, 1994). In the long-term evolution process, plants have evolved into tight mechanisms such as reduction-based strategy I and chelation-based strategy II for iron uptake, transport, utilization, or storage (Marschner et al., 1986).
In the process of Fe absorption, intracellular or intercellular and long-distance transport, many genes are involved including the root epidermal cell membrane transporters: iron-related transporter 1 (IRT1) and natural resistance-associated macrophage protein 1 (Nramp1) (Curie et al., 2000; Korshunova et al., 1999), vacuolar membrane transporter (Nramp3, Nramp4) and vacuolar iron transporter 1 (Kim et al., 2006; Lanquar et al., 2005), ferric reductase defective 3 (FRD3), citrate synthase 1 (CS1), nicotianamine synthase 1 (NAS1), and yellow stripe1-like (YSL) (Kobayashi and Nishizawa, 2012). Among these genes, the ZIP (ZRT, IRT-like proteins) metal transporter family and NRAMP family catch our attention all the time.
The absorption of metal elements in plants is a complicated process; they often use the same transport system for absorption, transportation, or storage (Rogers et al., 2000). Research suggests that there is a competition between Zn2+ and Fe2+ absorption in Arabidopsis thaliana (Fukao et al., 2011). Mn2+ deficiency can increase the transport of Fe2+ efficiently (Yang et al., 2008). Ni2+ accumulation may act as an iron-deficiency signal and induce the Fe-deficient response to upregulate Fe2+ absorption genes expression (Nishida et al., 2012). In addition, all of the previous studies were performed with the interaction between Fe2+ and other divalent metals. The mechanism of divalent metal ions absorption under iron deficiency in woody plants is little known.
Malus xiaojinensis is a native apple rootstock in China and has been characterized by its high efficiency for iron uptake (Han et al., 1998; Wu et al., 2012; Zha et al., 2014). In this study, we detected the spatial distribution of selected divalent metals in the root of Malus xiaojinensis, which belongs to the strategy I plant by using SR-μXRF as well as the expression of iron absorption- and transport-related genes in roots under iron-deficiency treatment, which will help to investigate the tissue-specific distribution of divalent metals when plants are subjected to iron deficiency.
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