PHYTOCHROME, GIBBERELLINS, AND HYPOCOTYL GROWTH - A STUDY USING THE CUCUMBER (CUCUMIS-SATIVUS L) LONG HYPOCOTYL MUTANT

The possible involvement of gibberellins (GAs) in the regulation of hypocotyl elongation by phytochrome was examined. Under white light the tall long hypocotyl (Ih) cucumber (Cucumis sativus L.) mutant, deficient in a type B-like phytochrome, shows an increased ''responsiveness'' (defined as response capability) to applied GA(4) (the main endogenous active GA) compared to the wild type. Supplementing far-red irradiation results in a similar increase in responsiveness in the wild type. Experiments involving application of the precursor GA(9) and of an inhibitor of GA(4) inactivation suggest that both the GA(4) activation and inactivation steps are phytochrome independent. Endogenous GA levels of whole seedlings were analyzed by combined gas chromatography-mass spectrometry using deuterated internal standards. The levels of GA(4) (and those of GA(34), the inactivated GA(4)) were lower in the Ih mutant under low-irradiance fluorescent light compared with the wild type, similar to wild type under higher irradiance light during the initial hypocotyl extension phase, and higher during the phase of sustained growth, in which extension involved an increase in the number of cells in the upper region. In all cases, growth of the Ih mutant was more rapid than that of the wild type. It is proposed that GA(4) and phytochrome control cell elongation primarily through separate mechanisms that interact at a step close to the terminal response.