Different magnesium uptake and transport activity along the rice root axis revealed by ^<28>Mg tracer experiments(Plant nutrition)

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We characterized magnesium (Mg) uptake and transport along the primary root of six-day-old rice seedlings (Oryza sativa L. cv. Nipponbare) using ^<28>Mg as a radioactive tracer. We used a multi-compartment transport box, which separated the root region every 1 cm in order to apply ^<28>Mg to a particular root region. Then, the root region was defined as R-A, R-B, and R-C in order from the root tip region to the lateral root-developing area. The amount of ^<28>Mg taken up in the R-C region in 15 min, 1 h, and 3 h was nearly three times as much as that of R-A and R-B. Thirty percent to 50% of ^<28>Mg absorbed from R-C was transported toward the lower part of the root. Furthermore, when ^<28>Mg was absorbed from R-C, it was detected only in the root part lower than R-C after 5 min. In contrast, less than 5% of ^<28>Mg absorbed from R-B was transported downward. This characteristic uptake and transport behavior was not observed with ^<32>P(phosphorus)-phosphate or ^<45>Ca (calcium). Our results demonstrate that Mg uptake and transport activity differ between root regions and further suggest that phloem loading of Mg occurs at R-C within minutes after uptake from the culture solution. Then, Mg is transported toward the root tip directly without going up the shoot.
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