The Association of Ascorbate and Ascorbate Oxidase in the Apoplast with IAA-Enhanced Elongation of Epicotyls from <italic>Vigna angularis</italic>

The level of (ascorbic acid (AA) plus dehydroascorbic acid (DHA)) and the ratio of the level of AA to that of AA plus DHA in intercellular washing fluid (IWF) of epicotyl segments from Vigna angularis decreased from 2.8±0.3 to 1.2±0.5nmol (g fr wt)−1 and from 0.23±0.03 to 0.13±0.01, respectively, after incubation of the segments without IAA for 20 h at 27°C. However, these values changed to 5.3±1.7 nmol (g fr wt)−1 and 0.07±0.05 after incubation with 0.1 mM IAA. The activity of cell wall-bound ascorbate oxidase increased by about 20% and 70% after incubation without IAA and with IAA, respectively. However, the activity of cell wall-bound peroxidase was not affected by incubation with or without IAA. The activities of ascorbate oxidase and peroxidase in IWF decreased by about 85 and 75% after incubation without IAA. IAA did not affect these decreases to any great extent. A lignin-like compound was formed during the incubation of epicotyl segments in the absence of IAA. Formation of this compound was inhibited by IAA. The results suggest that one of the causes of the enhancement of elongation growth by IAA is the inhibition of peroxidase-dependent lignification as a result of increases in levels of AA and DHA and in ascorbate oxidase activity.