Effects of S-3307, an Inhibitor of Gibberellin Biosynthesis, on Swelling of Leaf Sheath Cells and on the Arrangement of Cortical Microtubules in Onion Seedlings

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Abstract
Treatment with S-3307, a newly developed growth retardant, caused swelling of leaf sheaths in 2- to 3-leafed young onion seedlings (Allium ccpa L. cv. Senshu-Chuko), which otherwise showed no swelling even under long-day conditions. Before swelling became evident, changes occurred in the arrangement of cortical microtubules in leaf sheath cells of S-3307-treated seedlings. Cortical microtubules in leaf sheath cells, which are normally oriented transversely to the cell axis in untreated seedlings, were oriented longitudinally or obliquely in treated seedlings. S-3307 exerted these effects only in seedlings grown under long-day conditions, but not under short-day conditions. Simultaneously applied gibberellin reversed the effect of S-3307 on swelling, indicating that S-3307 acts as an inhibitor of gibberellin biosynthesis in onion seedlings. Endogenous gibberellin seems to play an important role in arranging cortical microtubules transversely to the cell axis.