INFLUENCE OF NITROGEN-SOURCE ON THE GROWTH, PROLAMIN CONTENT, AND GLUTAMINE-SYNTHETASE ISOZYME PROFILES OF ENDOSPERM-DERIVED SUSPENSION-CULTURES OF MAIZE

In order to assess the suitability of endosperm-derived suspension cultures (EDSC) as a model system for nitrogen metabolism in intact maize kernel endosperm, the isozyme complements of glutamine synthetase (GS), a key enzyme of nitrogen metabolism in higher plants, were compared from both tissue sources. Anion exchange profiles revealed five GS isozymes from intact endosperm. These same isozymes were also found in EDSC, although in very different relative proportions, along with a sixth isozyme with elution characteristics corresponding to the chloroplast GS from maize leaves. Isoelectric focusing under denaturing conditions followed by immunoblotting revealed the presence of five GS subunits from both intact endosperm and EDSC. The relative abundance of the GS isozymes and subunits from intact endosperm changed strikingly with development. Developmental changes in the isozyme patterns were less dramatic in the EDSC. Culturing the EDSC on alternative nitrogen sources changed the relative proportions of isozymes and affected growth and zein accumulation. It was concluded that while the EDSC do resemble intact endosperm in that they express the same GS subunits, the differences in the relative proportions of those subunits and the corresponding isozymes and differences in developmental changes make extrapolations of results obtained from studies of nitrogen metabolism using the endosperm suspension cultures to the intact endosperm inappropriate.