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Characterization of zinc uptake and translocation visualized with positron-emitting 65Zn tracer and analysis of transport-related gene expression in two Lotus japonicus accessions.

BACKGROUND AND AIMS
Zinc (Zn) is an essential element for humans and plants. However, Zn deficiency is widespread and 25% of the world's population is at risk of Zn deficiency. To overcome the deficiency of Zn intake, crops with high Zn content are required. However, most crop-producing areas have Zn-deficient soils, therefore crops with excellent Zn uptake/transport characteristics (i.e., high Zn efficiency) are needed. Our objective was to identify the crucial factors responsible for high Zn efficiency in the legume Lotus japonicus.


METHODS
We evaluated Zn efficiency by static and real-time visualization of radioactive Zn ( 65Zn) uptake/transport in two L. japonicus accessions, MG-20 and B-129, that differ in Zn efficiency. The combination of visualization methods verified the dynamics of Zn accumulation and transport within the plant. We compared gene expression under a normal Zn concentration (control) and Zn deficiency to evaluate genetic factors that may determine the differential Zn efficiency of the accessions.


KEY RESULTS
The accession B-129 accumulated almost twice the amount of Zn as MG-20. In the static 65Zn images, 65Zn accumulated in meristematic tissues, such as root tips and the shoot apex, in both accessions. The positron-emitting tracer imaging system (PETIS), which follows the transport process in real time, revealed that 65Zn transport to the shoot was more rapid in B-129 than in MG-20. Many genes associated with Zn uptake and transport were more highly expressed in B-129 than in MG-20 under the control condition. These gene expression patterns by Zn deficiency differed from that under the control Zn condition.


CONCLUSIONS
PETIS confirmed that the real-time transport of 65Zn to the shoot was faster in B-129 than in MG-20. The high Zn efficiency of B-129 may be due to the elevated expression of a suite of Zn uptake- and transport-related genes.