Growth and gibberellin A1 metabolism in excised lettuce hypocotyls

• Published in: Plant physiology (Bethesda) 1977-02, Vol.59 (2), p.211-216
• Main Authors: Silk, Wendy Kuhn, Jones, Russell L., Stoddart, John L.
• Format: Article
• Language: English
• Subjects: Gibberellins; Hormone metabolism; Hormones; Hypocotyls; Lettuce; Metabolism; Peas; Plants; Radioactive decay; Seedlings
• ISSN: 0032-0889; 1532-2548
• DOI: 10.1104/pp.59.2.211

Excised lettuce (Lactuca sativa L. cv. Arctic) hypocotyls retain the ability to elongate in response to exogenously supplied gibberellic acid and gibberellin A1 ($\text{GA}_{1}$). We have studied the relationship between metabolism of $\text{GA}_{1}$ and elongation in this tissue. In 24 hours at 28 C, hypocotyls treated with 3 μM $\text{GA}_{1}$ double in length while controls elongate less than 45%. After an exogenous hormone supply is removed, hypocotyls continue to grow faster than untreated controls, although as the hormone application time is decreased, the $\text{GA}_{1}$ concentration required to effect a given length change increases. $[{}^{3}\text{H}]\text{GA}_{1}$ was used to determine rates of hormone uptake, efflux, and metabolism. In the presence of $[{}^{3}\text{H}]\text{GA}_{1}$, hypocotyls accumulate and metabolize label for at least 24 hours. When the exogenous label is removed, the amount of acidic GA in the hypocotyl declines rapidly to a constant level while ethyl acetate-insoluble metabolites increase rapidly to a constant level. Lable accumulation and metabolism at any time are proportional to the external $\text{GA}_{1}$ concentration below 50 μM $\text{GA}_{1}$. Chromatographic analysis of radioactive compounds present in tissue extracts suggests that unaltered $\text{GA}_{1}$ is the major component of the acidic ethyl acetate-soluble fraction, and gibberellin A8 is a minor component. The ethyl acetate-insoluble fraction appears to contain an unidentified $\text{GA}_{1}$ metabolite with chromatographic properties similar to those of $\text{GA}_{1}$. The strong retention of accumulated $\text{GA}_{1}$ confirms the possibility of a continuing requirement for $\text{GA}_{1}$ during the sustained response to a $\text{GA}_{1}$ "pulse" but raises the question of accessibility of the stored hormone for growth promotion.