Lack of fructose-1,6-bisphosphatase in a range of higher plants that store starch

Abstract

The aim of this work was to discover whether fructose-1,6-bisphosphatase (FBPase) is present in higher-plant cells that synthesize storage starch. The following were examined: suspension cultures of soybean (Glycine max), tubers of potato (Solanum tuberosum), florets of cauliflower (Brassica oleracea), developing endosperm of maize and of sweet corn (Zea mays), roots of pea (Pisum sativum), and the developing embryos of round and wrinkled varieties of pea. Unfractionated extracts of each tissue readily converted fructose 1,6-bisphosphate to fructose 6-phosphate in assays for both plastidic and cytosolic FBPase. These conversions were not inhibited by 20 microM-fructose 2,6-bisphosphate. Except in extracts of pea embryos and sweet-corn endosperm, treatment with affinity-purified antibodies to pyrophosphate: fructose-6-phosphate 1-phosphotransferase reduced the above fructose 6-phosphate production to the rate found with boiled extracts. The antibody-resistant activity from sweet corn was slight. In immunoblot analyses, antibody to plastidic FBPase did not react positively with any protein in extracts of soybean cells, potato tuber, cauliflower florets, maize endosperm and pea roots. Positive reactions were found for extracts of embryos of both round and wrinkled varieties of peas and endosperm of sweet corn. For pea embryos, but not for sweet-corn endosperm, the Mr of the recognized protein corresponded to that of plastidic FBPase. It is argued that soybean cells, potato tuber, cauliflower florets, maize (var. White Horse Tooth) endosperm and pea roots lack significant activity of plastidic FBPase, but that this enzyme is present in developing embryos of pea. The data for sweet corn (var. Golden Bantam) are not decisive. It is also argued that, where FBPase is absent, carbon for starch synthesis does not enter the amyloplast as triose phosphate.