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Route and Regulation of Zinc, Cadmium, and Iron Transport in Rice Plants (Oryza sativa L.) during Vegetative Growth and Grain Filling: Metal Transporters, Metal Speciation, Grain Cd Reduction and Zn and Fe Biofortification.

Yoneyama T, Ishikawa S, Fujimaki S - Int J Mol Sci (2015)

Bottom Line: Zn and Fe concentrations in rice grains harvested under different levels of soil/hydroponic metals are known to change only within a small range, while Cd concentrations show greater changes.Transgenic techniques to increase concentrations of the metal chelators (nicotianamine, 2'-deoxymugineic acid) are useful in increasing grain Zn and Fe concentrations.The elimination of OsNRAMP5 Cd-uptake transporter and the enhancement of root cell vacuolar Cd sequestration reduce uptake and root-to-shoot transport, respectively, resulting in a reduction of grain Cd accumulation.

View Article: PubMed Central - PubMed

Affiliation: Department of Applied Biological Chemistry, The University of Tokyo, Tokyo 113-8657, Japan. tadakatsu_yoneyama@opal.ocn.ne.jp.

ABSTRACT
Zinc (Zn) and iron (Fe) are essential but are sometimes deficient in humans, while cadmium (Cd) is toxic if it accumulates in the liver and kidneys at high levels. All three are contained in the grains of rice, a staple cereal. Zn and Fe concentrations in rice grains harvested under different levels of soil/hydroponic metals are known to change only within a small range, while Cd concentrations show greater changes. To clarify the mechanisms underlying such different metal contents, we synthesized information on the routes of metal transport and accumulation in rice plants by examining metal speciation, metal transporters, and the xylem-to-phloem transport system. At grain-filling, Zn and Cd ascending in xylem sap are transferred to the phloem by the xylem-to-phloem transport system operating at stem nodes. Grain Fe is largely derived from the leaves by remobilization. Zn and Fe concentrations in phloem-sap and grains are regulated within a small range, while Cd concentrations vary depending on xylem supply. Transgenic techniques to increase concentrations of the metal chelators (nicotianamine, 2'-deoxymugineic acid) are useful in increasing grain Zn and Fe concentrations. The elimination of OsNRAMP5 Cd-uptake transporter and the enhancement of root cell vacuolar Cd sequestration reduce uptake and root-to-shoot transport, respectively, resulting in a reduction of grain Cd accumulation.

No MeSH data available.


Changes in Zn and Fe amounts in the blade of the 10th leaf along with dry weight growth. (Adapted from Obata and Kitagishi [49]). The dry weight of the leaf was drawn arbitrarily as the dry weight at 23/7 had reached the maximum. ○: change in the content of Zn; □: change in the content of Fe. (Adapted from Obata and Kitagishi [49]).
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ijms-16-19111-f005: Changes in Zn and Fe amounts in the blade of the 10th leaf along with dry weight growth. (Adapted from Obata and Kitagishi [49]). The dry weight of the leaf was drawn arbitrarily as the dry weight at 23/7 had reached the maximum. ○: change in the content of Zn; □: change in the content of Fe. (Adapted from Obata and Kitagishi [49]).

Mentions: With respect to the transport of Zn in xylem sap to rapidly growing leaves, Obata and Kitagishi [47] indicate that some Zn ascending via the xylem (transpiration stream) is transferred to the phloem at the vegetative nodes in addition to the Zn which is mobilized from mature leaves (Figure 4). Together this Zn with different origins is terminated to the intercalary meristem to synthesize ribosomes [48]. Such active transport through both routes supports the high Zn accumulation at the meristem, which has been reported to reach 300 mg Zn·kg−1 dry weight (DW) in the rapidly growing leaf part as compared to 30 mg Zn·kg−1 DW in the mature leaves [49]. Figure 5 shows this unique and quick accumulation of Zn in the early stage of leaves compared to the slow accumulation of Fe and also the early initiation of the decrease after the peak of Zn content, which was one week before reaching the DW peak [49]. The amount of Zn transferred to the phloem via the xylem-to-phloem transfer system seems to be regulated and constant: the transfer rate of Zn from xylem sap is relatively high when the xylem sap Zn concentration is low, and low when the xylem sap Zn concentration is high [50]. The ratio of the Zn deprived by the xylem-to-phloem transfer to that remobilized from the leaves is estimated to be around 3:7 [49]. Partitioning of Cd to the top may be similar to that of Zn and the vegetative nodes may also be the site of the xylem-to-phloem transfer of Cd [51]. 115mCd absorbed by roots is partitioned to the dwarf stem and leaves by the pattern of accumulation in the nodal part of the stem, and transport is then predominant to the rapidly growing leaves with little to the mature leaves [52], similar to that of 65Zn [50]. Recently, preferential movement of 109Cd and 45Ca, which were administered to a root–bathing solution as 109CdCl2 and 45CaCl2, to the newest leaf by transfer to the phloem at the stem, was confirmed by radioisotope imaging [53]. The rate of Fe involved in xylem-to-phloem transfer could be smaller than that of Zn, since the demand-time of Fe is late compared to that of Zn (Figure 5, [49]). It is interesting to note that Fe in rice xylem sap has been found to contain small amounts of Fe–DMA and large amounts of Fe–citrate while the Fe in the xylem sap of barley plants includes a considerable fraction of Fe–mugineic acid (MA) [36]. Barley plants can transfer such xylem sap Fe–MA to the young leaves through the xylem-to-phloem system at nodes as revealed by 52Fe tracing [54].


Route and Regulation of Zinc, Cadmium, and Iron Transport in Rice Plants (Oryza sativa L.) during Vegetative Growth and Grain Filling: Metal Transporters, Metal Speciation, Grain Cd Reduction and Zn and Fe Biofortification.

Yoneyama T, Ishikawa S, Fujimaki S - Int J Mol Sci (2015)

Changes in Zn and Fe amounts in the blade of the 10th leaf along with dry weight growth. (Adapted from Obata and Kitagishi [49]). The dry weight of the leaf was drawn arbitrarily as the dry weight at 23/7 had reached the maximum. ○: change in the content of Zn; □: change in the content of Fe. (Adapted from Obata and Kitagishi [49]).
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4581289&req=5

ijms-16-19111-f005: Changes in Zn and Fe amounts in the blade of the 10th leaf along with dry weight growth. (Adapted from Obata and Kitagishi [49]). The dry weight of the leaf was drawn arbitrarily as the dry weight at 23/7 had reached the maximum. ○: change in the content of Zn; □: change in the content of Fe. (Adapted from Obata and Kitagishi [49]).
Mentions: With respect to the transport of Zn in xylem sap to rapidly growing leaves, Obata and Kitagishi [47] indicate that some Zn ascending via the xylem (transpiration stream) is transferred to the phloem at the vegetative nodes in addition to the Zn which is mobilized from mature leaves (Figure 4). Together this Zn with different origins is terminated to the intercalary meristem to synthesize ribosomes [48]. Such active transport through both routes supports the high Zn accumulation at the meristem, which has been reported to reach 300 mg Zn·kg−1 dry weight (DW) in the rapidly growing leaf part as compared to 30 mg Zn·kg−1 DW in the mature leaves [49]. Figure 5 shows this unique and quick accumulation of Zn in the early stage of leaves compared to the slow accumulation of Fe and also the early initiation of the decrease after the peak of Zn content, which was one week before reaching the DW peak [49]. The amount of Zn transferred to the phloem via the xylem-to-phloem transfer system seems to be regulated and constant: the transfer rate of Zn from xylem sap is relatively high when the xylem sap Zn concentration is low, and low when the xylem sap Zn concentration is high [50]. The ratio of the Zn deprived by the xylem-to-phloem transfer to that remobilized from the leaves is estimated to be around 3:7 [49]. Partitioning of Cd to the top may be similar to that of Zn and the vegetative nodes may also be the site of the xylem-to-phloem transfer of Cd [51]. 115mCd absorbed by roots is partitioned to the dwarf stem and leaves by the pattern of accumulation in the nodal part of the stem, and transport is then predominant to the rapidly growing leaves with little to the mature leaves [52], similar to that of 65Zn [50]. Recently, preferential movement of 109Cd and 45Ca, which were administered to a root–bathing solution as 109CdCl2 and 45CaCl2, to the newest leaf by transfer to the phloem at the stem, was confirmed by radioisotope imaging [53]. The rate of Fe involved in xylem-to-phloem transfer could be smaller than that of Zn, since the demand-time of Fe is late compared to that of Zn (Figure 5, [49]). It is interesting to note that Fe in rice xylem sap has been found to contain small amounts of Fe–DMA and large amounts of Fe–citrate while the Fe in the xylem sap of barley plants includes a considerable fraction of Fe–mugineic acid (MA) [36]. Barley plants can transfer such xylem sap Fe–MA to the young leaves through the xylem-to-phloem system at nodes as revealed by 52Fe tracing [54].

Bottom Line: Zn and Fe concentrations in rice grains harvested under different levels of soil/hydroponic metals are known to change only within a small range, while Cd concentrations show greater changes.Transgenic techniques to increase concentrations of the metal chelators (nicotianamine, 2'-deoxymugineic acid) are useful in increasing grain Zn and Fe concentrations.The elimination of OsNRAMP5 Cd-uptake transporter and the enhancement of root cell vacuolar Cd sequestration reduce uptake and root-to-shoot transport, respectively, resulting in a reduction of grain Cd accumulation.

View Article: PubMed Central - PubMed

Affiliation: Department of Applied Biological Chemistry, The University of Tokyo, Tokyo 113-8657, Japan. tadakatsu_yoneyama@opal.ocn.ne.jp.

ABSTRACT
Zinc (Zn) and iron (Fe) are essential but are sometimes deficient in humans, while cadmium (Cd) is toxic if it accumulates in the liver and kidneys at high levels. All three are contained in the grains of rice, a staple cereal. Zn and Fe concentrations in rice grains harvested under different levels of soil/hydroponic metals are known to change only within a small range, while Cd concentrations show greater changes. To clarify the mechanisms underlying such different metal contents, we synthesized information on the routes of metal transport and accumulation in rice plants by examining metal speciation, metal transporters, and the xylem-to-phloem transport system. At grain-filling, Zn and Cd ascending in xylem sap are transferred to the phloem by the xylem-to-phloem transport system operating at stem nodes. Grain Fe is largely derived from the leaves by remobilization. Zn and Fe concentrations in phloem-sap and grains are regulated within a small range, while Cd concentrations vary depending on xylem supply. Transgenic techniques to increase concentrations of the metal chelators (nicotianamine, 2'-deoxymugineic acid) are useful in increasing grain Zn and Fe concentrations. The elimination of OsNRAMP5 Cd-uptake transporter and the enhancement of root cell vacuolar Cd sequestration reduce uptake and root-to-shoot transport, respectively, resulting in a reduction of grain Cd accumulation.

No MeSH data available.