Starch-dependent sodium accumulation in the leaves of Vigna riukiuensis

This research provides insight into a unique salt tolerance mechanism of Vigna riukiuensis . V. riukiuensis is one of the salt-tolerant species identified from the genus Vigna . 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 species V. angularis . 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 ( 22 Na) 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. Our results could provide the second evidence of the Na-trapping system by starch granules, following the case of common reed that accumulates starch granule at the shoot base for binding Na.