Effect of Auxin on Ascorbic Oxidase Activity in Tobacco Pith Cells.

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Abstract
Auxin (3-indoleacetic acid, IAA) at a concn. of 3.5 mg./l. in the medium causes a marked increase in the ascorbic oxidase (AO) activity of tobacco pith sections cultured in vitro on modified White''s medium containing 2% sucrose. The increase in oxidase activity is evident by the 2d day and is rapid thereafter for several days. By the 6th day the activity had risen 1900% on a dry wt. basis, compared with a 37% rise for control sections grown without IAA. The rise in oxidase activity is accompanied by an increase in respiratory rate, detectable by the 3d day, by pronounced cell enlargement, and by gains in fresh and dry wt. No cell divisions occur. Control sections show only relatively slight gains in respiratory rate and fresh and dry wt. during this period. The AO activity of homogenates prepared with the Potter homogenizer is localized in the cell wall fraction obtained by differential centrifugation, and it is suggested that the enzyme occurs in cytoplasm intimately associated with the wall. Intact cells oxidize ascorbate at the same rate as homogenates of such cells. IAA at a concn. of 17.5 mg./l. inhibits AO activity and cell enlargement without decreasing the respiratory rate. IAA does not stimulate AO activity when added directly to homogenates. Vigorous AO activity is characteristic of callus tissues and a variety of other actively growing plant tissues examined. It is suggested that AO may be causally related to cell growth.

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