52Mn translocation in barley monitored using a positron-emitting tracer imaging system

Until now, the real-time uptake and movement of manganese (Mn), an essential plant nutrient, has not been documented in plants. In this study, the real-time translocation of Mn in barley (Hordeum vulgare L. cv. Ehimehadaka no. 1) was visualized using the positron-emitting tracer Mn-52 and a positron-emitting tracer imaging system (PETIS). PETIS allowed the non-destructive monitoring of Mn translocation in barley under various conditions. In all cases, Mn-52 first accumulated in the discrimination center (DC) at the basal portion of the shoot, suggesting that this region may play an important role in Mn distribution in graminaceous plants. Manganese-deficient barley showed greater translocation of Mn-52 from roots to shoots than did Mn-sufficient barley, demonstrating that Mn deficiency causes enhanced Mn uptake and loading into vascular bundles. In contrast, the translocation of Mn-52 from roots to shoots was suppressed in Mn-excess barley. In these plants, the uptake of Mn may be suppressed or Mn may accumulate in the intercellular organelles of root cells, resulting in low rates of Mn translocation to shoots. In Mn-sufficient barley, the dark treatment did not suppress the translocation of Mn-52 to the youngest leaf, suggesting that the translocation of Mn to the youngest leaf is independent of the transpiration stream. When Mn-52 was supplied to the cut end of an expanded leaf, 52 Mn was transported to the DC within 27 min and then retranslocated to roots and other leaves. Our results show that the translocation of Mn from the roots to the DC depends passively on water flow, but actively on the Mn transporter(s).