Metabolism of deuterium- and tritium-labeled gibberellins in cambial region tissues of Eucalyptus globulus stems

Abstract

Deuterium- and tritium-labeled gibberellins (GAs) were applied to stems of 3-year-old Eucalyptus globulus Labill. saplings and 9-month-old potted seedlings. Cambial region tissues surrounding the application point were collected 6, 24 or 48 h later. Twenty-four hours after application of 5 μg of [2H2]GA20, 7% of the cambial region GA20 pool, 7% of the GA1 pool and 58% of the GA29 pool were labeled with deuterium based on selected ion monitoring of purified extracts subjected to gas chromatography-mass spectrometry. The relatively low percent dilution of endogenous GAs by [2H2]GAs suggests that the exogenous application of [2H2]GA20 did not result in substrate overloading, indicating that these conversions probably occur naturally within cambial region tissues. Extracts from similar cambial region tissues fed tritium-labeled GAs were sequentially fractionated by SiO2 partition chromatography, C18 reversed phase HPLC and N(CH3)2 HPLC. The radioactivity profiles indicated metabolism of GA20 to GA1 and GA29, GA1 conversion to GA8, GA4 to GA34 and GA9 to GA51. Gibberellins GA34, GA51 and GA29 are C-2β-hydroxylated catabolites of low biological activity, whereas GA1 and GA4 are probably effectors of growth in the Eucalyptus stem and shoot. Evidence for C-13 hydroxylation of GA4 to GA1, GA9 to GA4 or GA9 to GA20 in the stem was inconclusive. Thus, although GA4 and GA9 are native to cambial region tissues, GA1 is probably not produced from them in significant quantities. We conclude that the early C-13-hydroxylation pathway; i.e., conversion of GA19 to GA20 to GA1, is the major pathway of GA1 biosynthesis.