A gravitropic stimulation-induced growth inhibitor, beta-(isoxazolin-5-on-2yl)-alanine, is a possible mediator of negative gravitropic bending of epicotyls in etiolated Pisum sativum seedlings

摘要

Negative gravitropic bending and its possible mediator in etiolated Alaska pea seedlings were intensively studied in comparison with seedlings of an agravitropic mutant, ageotropum. When 3.5-day-old etiolated Alaska seedlings were horizontally placed, the growth suppression at the upper side of the epicotyls began 10 min after the onset of the gravitropic stimulation, whereas the growth acceleration at the lower side began at 30 min, resulting in negative gravitropic bending. In contrast, no gravitropic bending was observed in the etiolated ageotropum seedlings, for which the epicotyls show an automorphogenesis-like growth. Strenuous efforts to identify a possible mediator that induces the gravitropic bending resulted in successfully identifying beta-(isoxazolin-5-on-2yl)-alanine (beta IA). The unilateral application of beta IA to the etiolated Alaska epicotyls substantially induced epicotyl bending toward the application site, indicating that beta IA could act as a growth inhibitor. Analyses of the distribution of beta IA in the upper and lower flanks of the etiolated Alaska epicotyls revealed that its content rapidly increased twice in the upper flanks compared with that in the lower ones in response to gravitropic stimulation, whereas its content in the lower flanks was almost equal to that in the vertical control. In etiolated ageotropum epicotyls, an almost equal amount of beta IA was distributed in the upper and lower flanks of epicotyls. These results suggest that a gravitropic stimulation increases beta IA in the upper flank, resulting in the negative gravitropic bending of epicotyls via the suppression of the growth rate at the upper side of epicotyls in the etiolated Alaska pea seedlings.