EXCISED TASSEL-SEED TUNICATE (TS-5 TU) KERNELS AS A MODEL IN-VITRO SYSTEM FOR STUDYING AMINO-ACID-METABOLISM IN DEVELOPING MAIZE SEEDS

Previous studies suggest that nitrogenous transport compounds may be m etabolized in the pedicel (basal maternal tissue) of the developing ma ize (Zea mays L.) kernel. In this report, excised tassel-seed tunicate (Ts-5 Tu) kernels were tested as a model in vitro kernel system to st udy the metabolic fate of [C-14]aspartate within the pedicel and the e ndosperm. Glume-covered kernels of Ts-5 Tu maize are born on individua l elongated stalks and can be removed from the parent plant without da mage to the basal kernel tissues. Radiolabeled aspartate supplied to e xcised Ts-5 Tu kernels in a 30-min pulse was rapidly metabolized withi n the pedicel, with 60% of the C-14 recovered in the acidic (organic-a cid-containing) fraction at the beginning of the chase period. By 1 h into the chase period, both glutamine and glutamate were more heavily labeled than aspartate. In the endosperm, 60% of the ethanol-soluble, aspartate-derived C-14 was recovered in the basic (amino acid-containi ng) fraction, 35 % in the acidic fraction, and the remainder in the ne utral (sugar-containing) fraction. Radioactivity in glutamate plus glu tamine accounted for 70% of the C-14 contained in the basic fraction o f the endosperm. Inclusion of 2 mM methionine sulfoximine, an irrevers ible inhibitor of glutamine synthetase (GS), had little effect on pedi cel aspartate catabolism but did inhibit incorporation of aspartate-de rived C-14 into glutamine in the pedicel, resulting in more C-14 being taken up by the endosperm as organic acids and less being incorporate d into the prolamin fraction. It was concluded that the results obtain ed using Ts-5 Tu kernels incubated in vitro reflect more closely what occurs in the intact plant than those obtained with cob-borne kernels. The role of pedicel-amino-acid metabolism in maize-kernel nitrogen as similation is also discussed.