The Biosynthesis of Indoleacetic Acid in the Styles and Ovaries of Tobacco Preliminary to the Setting of Fruit.

The hormone responsible for the swelling of the ovraries of tobacco has been identified as indoleacetic acid (5). Indoleacetic acid (IAA) is probably the active principle responsible for the growth of strawberry receptacles (14). The biosynthesis of this compound has been studied and demonstrated in many plants and plant parts (3). Wildman, Ferri, and Bonner (21) were the first to suggest tryptophan as a precursor of IAA in higher plants. Prior to this, Thimann (18) had ascertained tryptophan to be an active precursor of IAA in Rhizopus suinus. The biochemical mechanism of this oxidation is not clear for all plants, but it is fairly well established thalt tryptophan is a precursor of IAA. Free tryptophan has been studied in pollen of tobacco, acorn squash, and Lilium regale (13). Its occurrence has been reported in the ovaries of the same plants (13) and in the achenes of strawberries (14). The increase in tryptophan correlates nicely with previously found changes in IAX production (11). There is a strong correlation between hormone synthesis in the styles of tobacco and the rate of pollen tube growth (5, 6). Mluir (7) was the first to study hormone changes of pollen tubes grown in vitro on pure agar and sugar. Nitsch (12) found substantial increases in hormone after germination of pollen 13.5 and 20.5 hours on such a medium containing tryptophan. These experiments suoggest that pollen tubes possess an enzyme system capable of converting, tryptophan to IAX. These results were in question, however, because of possible bacterial contamination interfering with the oxidative process (13). The possibility that the factor supplied by the pollen which brings about hormone production is an enzyme has been suggested (7, 19). Homogenates of resting pollen and unpollinated styles of Lilium regale contain a tryptophan-IAA enzyme (13). This enzyme activity increases with time after pollination in the ovaries of Lilium regale (13) and corn (17). However, doubt was cast on the possible participation of the enzyme in fruit development (8), because of the observation that both unfertilized and fertilized ova-