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

Treatment with S-3307, a newly developed growth retardant, causedswelling of leaf sheaths in 2- to 3-leafed young onion seedlings(Allium ccpa L. cv. Senshu-Chuko), which otherwise showed noswelling even under long-day conditions. Before swelling becameevident, changes occurred in the arrangement of cortical microtubulesin leaf sheath cells of S-3307-treated seedlings. Cortical microtubulesin leaf sheath cells, which are normally oriented transverselyto the cell axis in untreated seedlings, were oriented longitudinallyor obliquely in treated seedlings. S-3307 exerted these effectsonly in seedlings grown under long-day conditions, but not undershort-day conditions. Simultaneously applied gibberellin reversed the effect of S-3307on swelling, indicating that S-3307 acts as an inhibitor ofgibberellin biosynthesis in onion seedlings. Endogenous gibberellinseems to play an important role in arranging cortical microtubulestransversely to the cell axis. (Received July 14, 1984; Accepted September 26, 1984)