Starch-dependent sodium accumulation in the leaves of Vigna riukiuensis.

Abstract This research provides insight into a unique salt tolerance mechanism of Vigna riukiuensis. V. riukiuensis is one of the great genetic resources of salt tolerance. We have previously reported that V. riukiuensis accumulates a higher amount of sodium in the leaves, whereas V. nakashimae, a close relative of V. riukiuensis, suppresses sodium allocation to the leaves. We first suspected that V. riukiuensis would have developed vacuoles for sodium sequestration, but there were no differences compared to a salt-sensitive accession. However, many starch granules were observed in the chloroplasts of V. riukiuensis. In addition, forced degradation of leaf starch by shading treatment resulted in no radio-Na (22Na) accumulation in the leaves. We performed SEM-EDX to locate Na in leaf sections and detected Na in chloroplasts of V. riukiuensis, especially around the starch granules but not in the middle of those. Our results could provide the second evidence of Na-trapping system by starch granules, following the case of common reed that accumulates starch granule at the shoot base for binding Na.