Abstract
Cell-free extracts capable of converting [14C]-labeled gibberellins (GAs) were prepared from spinach (Spinacia oleracea L.) leaves. [14C]-labeled GAs, prepared enzymically from [14C]mevalonic acid, were incubated with these extracts, and products were identified by gas chromatography-mass spectrometry. The following pathway was found to operate in extracts from spinach leaves grown under long day (LD) conditions: GA12 → GA53 → GA44 → GA19 → GA20. The pH optima for the enzymic conversions of [14C]GA53, [14C]GA44 and [14C]GA19 were approximately 7.0, 8.0, and 6.5, respectively. These three enzyme activities required Fe2+, α-ketoglutarate and O2 for activity, and ascorbate stimulated the conversion of [14C]GA53 and [14C]GA19. Extracts from plants given LD or short days (SD) were examined, and enzymic activities were measured as a function of exposure to LD, as well as to darkness following 8 LD. The results indicate that the activities of the enzymes oxidizing GA53 and GA19 are increased in LD and decreased in SD or darkness, but that the enzyme activity oxidizing GA44 remains high irrespective of light or dark treatment. This photoperiodic control of enzyme activity is not due to the presence of an inhibitor in plants grown in SD. These observations offer an explanation for the higher GA20 content of spinach plants in LD than in SD.
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