Regulation of sucrose phosphate synthase by gibberellins in soybean and spinach plants

Exogenous applications of gibberellins (GAs) increased the extractable activity of leaf sucrose phosphate synthase (SPS) in soybean (Glycine max [L.]) and spinach (Spinacia oleracea [L.]). The response to GA applications was detectable within 2 h postapplication and was still observed 6 h, 24 h, and 7 d after treatment. When paclobutrazol, a GA biosynthesis inhibitor, was applied to intact soybean and spinach plants, decreased extractable SPS activity resulted within 24 h following the treatment. Different methods of GA application (spray, injection, capillary wick, and excised leaf systems) produced similar effects on SPS activity of soybean leaves. Protein synthesis in soybean leaves appeared to be necessary for GA-promoted SPS activity because gibberellic acid only partially reversed the inhibitory effect of pretreatment with cycloheximide. Levels of SPS protein from crude extracts of spinach plants were measured by a dot blot technique using monoclonal antibodies against SPS. Application of gibberellic acid to spinach leaves increased levels of SPS protein 2 h, 24 h, and 7 d after treatment. The results suggest that, in both soybean and spinach, GA is one of the endogenous hormonal factors that regulate the steady-state level of SPS protein and, hence, its activity.