LOCALIZATION OF SUCROSE SYNTHASE ACTIVITY IN DEVELOPING MAIZE KERNELSBY IN-SITU ENZYME-HISTOCHEMISTRY

Sucrose synthase is usually localized by immunocytochemistry, but this method does not show the actual activity of the localized enzyme, A h istochemical assay is presented here showing the activity of sucrose s ynthase by tetrazolium salt precipitation on sections of developing ma ize kernels. The advantages of the assay are a high sensitivity for lo w amounts of active sucrose synthase and the independence of specific antibodies. In this study the activity of endosperm sucrose synthase i s shown to move gradually from the apical part of the endosperm to the basal endosperm during kernel development. This shift in sucrose synt hase activity correlates well with the localization of starch synthesi s during kernel development. The assay also shows the early loss of ac tivity in the aleurone layer bordering the embryo, and a loss of activ ity in the apical aleurone during the final stage of kernel developmen t while the enzyme was still found by immunocytochemistry. This is in contrast to a high sucrose synthase activity in the epithelium of the scutellum, where hardly any labelling was found with antibodies agains t maize sucrose synthase. Low sucrose synthase activities were found i n the pericarp and pedicel parenchyma. Possible functions of the high and low activity patterns in the developing maize kernels and differen ces between the enzyme assay and immunocytochemistry are discussed.