OPT3 is a component of the iron-signaling network between leaves and roots and misregulation of OPT3 leads to an over-accumulation of cadmium in seeds

Abstract

Plants and seeds are the main dietary sources of zinc, iron, manganese, and copper, but are also the main entry point for toxic elements such as cadmium into the food chain. We report here that an Arabidopsis oligopeptide transporter mutant, opt3-2, over-accumulates cadmium (Cd) in seeds and roots but, unexpectedly, under-accumulates Cd in leaves. The cadmium distribution in opt3-2 differs from iron, zinc, and manganese, suggesting a metal-specific mechanism for metal partitioning within the plant. The opt3-2 mutant constitutively up-regulates the Fe/Zn/Cd transporter IRT1 and FRO2 in roots, indicative of an iron-deficiency response. No genetic mutants that impair the shoot-to-root signaling of iron status in leaves have been identified. Interestingly, shoot-specific expression of OPT3 rescues the Cd sensitivity and complements the aberrant expression of IRT1 in opt3-2 roots, suggesting that OPT3 is required to relay the iron status from leaves to roots. OPT3 expression was found in the vasculature with preferential expression in the phloem at the plasma membrane. Using radioisotope experiments, we found that mobilization of Fe from leaves is severely affected in opt3-2, suggesting that Fe mobilization out of leaves is required for proper trace-metal homeostasis. When expressed in yeast, OPT3 does not localize to the plasma membrane, precluding the identification of the OPT3 substrate. Our in planta results show that OPT3 is important for leaf phloem-loading of iron and plays a key role regulating Fe, Zn, and Cd distribution within the plant. Furthermore, ferric chelate reductase activity analyses provide evidence that iron is not the sole signal transferred from leaves to roots in leaf iron status signaling.

Keywords: ionomics.; iron deficiency; metal homeostasis; phloem transport; seed loading.

Figure 1

opt3-2 Over-Accumulates Cd in Seeds and Is Cd-Hypersensitive. (A) Ionomic profile of opt3-2 seeds grown on soil supplemented with heavy metals. Metal concentrations were determined by ICP–OES, normalized against Mg, and plotted as standard deviation from the wild-type mean (Z-value) (Lahner et al., 2003). Each line represents seeds from independent plants grown on heavy metal-laden soil. Z-values are considered significant when |z| > 1.96 (p < 0.05). (B) opt3-2 seedlings are hypersensitive to Cd. Wild-type and opt3-2 seeds were grown on ¼ MS media with or without 50 μM CdCl₂ for 2 weeks_.

Figure 2

Cadmium Distribution between Tissues Is Altered in opt3-2 Plants. Cd concentration was measured in roots (n = 5) and rosette leaves (n = 10) of 6-week-old hydroponically grown plants exposed to 20 μM CdCl₂ for 72h and dried seeds of plants (n = 18) grown on soil containing a defined content of heavy metals (Lahner et al., 2003). Data represent mean ± SE (* p < 0.05).

Figure 3

The Distribution of Iron, Zinc, and Manganese Is Different from Cd in opt3-2. Metal concentration in roots, leaves, and seeds was determined as in Figures 1 and 2. (A) Concentration of Fe, Zn, and Mn in opt3-2 seeds was similar to wild-type (n = 18). (B) In leaves, only Zn and Fe were over-accumulated while Mn concentration was unaffected (n = 10). (C) In roots, opt3-2 plants exhibited over-accumulation of Zn, Fe, and Mn compared to wild-type plants (n = 5). Data represent mean ± SE (* p < 0.05).

Figure 4

Ectopic Overexpression of OPT3 in opt3-2 Reduces Cadmium Concentration in Seeds and Rescues the Seedling Hypersensitivity to Cd. (A) Relative OPT3 expression levels of four representative 35S _pro:OPT3 overexpression lines. Wild-type, opt3-2, and four OPT3 overexpression lines were grown on ¼ MS for 2 weeks, and OPT3 expression was determined by qPCR and normalized against wild-type OPT3 expression levels. Data represent mean ± SE (n = 3). (B) OPT3 overexpression reduces the over-accumulation of Cd in seeds and (C) the Cd hypersensitivity of opt3-2 seedlings. Wild-type, opt3-2, and four complemented lines were grown on ¼ MS with or without 50 μM CdCl₂ for 2 weeks. Data represent mean ± SE (n = 6; * p < 0.05).

Figure 5

OPT3 Is a Plasma Membrane Transporter Expressed in the Phloem. (A, B) GUS staining was performed in fixed and sectioned stems of OPT3 _pro:GUS plants under (A) iron-deficient conditions and (B) Fe-sufficient conditions. (C) OPT3 localizes to the plasma membrane. N. benthamiana epidermal cells were infiltrated with Agrobacterium carrying 35S _pro:YFP–OPT3 and imaged using confocal microscopy 3 d later. Fluorescence in the cell perimeter is indicative of plasma membrane localization. ER fluorescence is also present (arrows) surrounding the nucleus and as strands traversing the cytoplasm. (D) Leaves of N. benthamiana epidermal cells transiently expressing YFP–OPT3 as in the panel were plasmolyzed with 4% NaCl. Note the Hechtian strands (arrows) connecting the cell wall to the plasmolyzed protoplast (double arrowheads), indicative of plasma membrane localization.

Figure 6

Shoot-Specific Expression of OPT3 Is Sufficient to Complement the Fe-Deficiency Response in opt3-2 Roots. (A) CAB2 _pro is preferentially active in shoots and is not active in roots. GUS staining in a whole seedling expressing CAB2 _pro:GUS is evident only in shoots. (B) RT–PCR confirmed the shoot specificity of CAB2 _pro:OPT3. Wild-type, opt3-2, and three independent CAB2 _pro:OPT3 lines were grown vertically on ¼ MS plates for 2 weeks, and cDNA was prepared separately from root and leaf RNA. OPT3 expression was determined in roots and shoots of wild-type, opt3-2, and three independent CAB2 _pro:OPT3 lines. ACT2 was used a loading control, and the number of PCR cycles is shown to the right of each gel image. Note that complete knockout of OPT3 causes embryo lethality (Stacey et al., 2002), and opt3-2 shows reduced expression of OPT3 transcript. (C) CAB2 _pro:OPT3 successfully restores regulation of IRT1 in opt3-2. IRT1 expression in roots of wild-type, opt3-2, and CAB2 _pro:OPT3 was determined by RT–PCR as in panel (A). RT–PCR was performed for 22 cycles, and ACT2 was used as a loading control. (D) opt3-2 plants expressing CAB2 _pro:OPT3 accumulate wild-type levels of Cd in seeds. Wild-type, opt3-2, and three CAB2 _pro:OPT3 lines were grown on heavy metal-laden soil, and their seed metal concentration was determined by ICP–OES as in Figure 1. Data represent mean ± SE (n = 6; * p < 0.05). (E) CAB2 _pro:OPT3 complements seedling sensitivity to Cd in opt3-2. Wild-type, opt3-2, and three CAB2 _pro:OPT3 lines were grown on ¼ MS with or without 50 μM CdCl₂ for 2 weeks.

Figure 7

Mobilization of Iron between Leaves Is Impaired in opt3-2. (A) ⁵⁹Fe was applied to a mature wild-type or opt3-2 leaf and the distribution of ⁵⁹Fe was monitored after a 12-h incubation period. Lower panel: Signal coming from ⁵⁹Fe was detected in wild-type leaves adjacent to the leaf where the ⁵⁹Fe was applied while only a fraction of the signal was detected in opt3-2 leaves. (B) The specific activity measured in the four adjacent leaves to which the ⁵⁹Fe was originally applied show that the movement of ⁵⁹Fe in opt3-2 was marginal. Data represent mean ± SE (n = 4, * p < 0.05). (C–I) Visualization of Fe using Perls’ stain shows that, compared to a mature wild-type leaf (C), opt3-2 contains substantial amounts of stainable Fe (D). (E) Over-accumulation of Fe in opt3-2 leaves is less evident in younger leaves. Accumulation of Fe in opt3-2 leaves is more evident close to (F) secondary veins, (G) the base of trhichomes, and (H, I) surrounding the vasculature.