Gibberellins in seeds of Arabidopsis thaliana: biological activities, identification and effects of light and chilling on endogenous levels

Abstract

The activities of several gibberellins in stimulating germination of wild-type and GA-deficient gal seeds of Arabidopsis thaliana were compared. Of the six compounds tested GA4 and GA7-isolactone had the highest activity and GA7 and GA9 the lowest; activities of GA1 and GA3 were intermediate. Combined application of pure GAs presented no indications that more than one GA receptor is involved. Four GAs were identified in extracts from wild-type and GA-insensitive gai seeds by combined gas chromatography mass spectrometry: GA1, GA3, GA4 and GA9. Effects of light and chilling on levels of GA1, GA4 and GA9 were studied using deuterated standards. Light increased both GA levels and germination in unchilled wild-type and gai seeds. As a result of irradiation GA levels in gai seeds were 7–10 times as high as in wild-type seeds. In the dark germination was 0%, in the light 14% of gai seeds and 95% of wild-type seeds germinated. A chilling pre-treatment of 7 days at 2°C was required to enhance further the germination of gai seeds in the light. Light did not increase GA levels of chilled seeds of either genotype; levels of GA4 and GA9 of chilled gai seeds, in the light were respectively 7 and 12 times lower than in non-chilled seeds, whereas the latter seeds germinated better. Slightly elevated levels of GA4 were detected in darkness after chilling, but germination capacity was still 0%. These results strengthened the conclusion that GAs are required for germination of A. thaliana seeds, whereby GA4 has intrinsic biological activity. However, it is unlikely that light and chilling stimulate germination primarily by increasing levels of GA. Instead GA sensitivity is a possible alternative.