The Requirement for Organic Nitrogen in Zea mays Embryos

Abstract

Previous investigations have shown that, during germination, there is a transfer of organic nitrogen from storage tissues to the developing embryo (16). Indeed in both barley (10) and pea (5) specific storage proteins are the first components to disappear. (7), and which have been grown to maturity (1), present an apparent exception to the general picture. However, Nason (18) has shown that excised maize embryos have abnormally low levels of trypotophan, and when our own preliminary investigation showed that the amount of protein in such embryos was drastically reduced, an investigation was made of the importance of organic nitrogen normally supplied by the endosperm. A progress report of this work has appeared previously (20). MethodsMaize grain (hybrid variety Wf 9 X 38-11) was sterilized briefly with chlorox, rinsed, and allowed to germinate on a thin layer of agar (1.5%) in the dark at 260. When the root was 2 cm long (about 40 hr) the embryos (including scutella) were removed by sterile excision. Three or four normal seedlings (intact) or excised embryos were transferred to 10 ml of a mineral-salts solution (14,18) for soluble nitrogen and a-amino nitrogen determinations. The amino acids were washed free of other components in the alcohol extract by passage through a 6 X 1-cm column of Dowex-50 X 8 (H+) cation exchange resin and eluted with iN NH4OH. They were further fractionated into acidic, neutral, and basic amino acids by the procedures of Hirs, Moore and Stein (13,17). In this procedure glutamic and aspartic acids were adsorbed on a 20 X 1-cm Dowex-1 X 10 (acetate) column. Glutamine and asparagine in the water effluent from this column were hydrolyzed in iN HCl for 4 hours at 1000. The resulting glutamic and aspartic acids were adsorbed on a second Dowex 1-X 10 (acetate) column. The water effluent from this column was passed through a 15 X 1 cm Dowex-50 X 98 (Na +) column to adsorb the neutral and basic amino acids. The neutral amino acids were eluted with 15 nml of 0.1 mr Na citrate (pH 5.0) and the basic amino acids, including y-amino butyric acid, with 0.5 m Na citrate (pH 7.5). Glutanlic and aspartic acids were eluted serially from the Dowex-1 X 10 (acetate) columns with 0.2 N acetic acid. After each step in the procedure the eluates were dried and the a-amino nitrogen of each fraction was determined with ninhydrin (24). ResultsTransfer of Nitrogen front the Endosperm to tihe Embryo. Results with intact maize seedlings (fig 1) showed that during germination the alcohol-insoluble nitrogen fraction increased in the embryo while it declined in the endosperm. The level of alcoholsoluble nitrogen increased temporarily in the endosperm and had not fallen below the initial level at 96 hours. There was a progressive increase in this fraction in the embryo after 20 hours. Usually about three-quarters of the loss front the endosperm was recovered as embryo nitrogen. This picture is sufficiently similar to well documented results obtained with barley (10, 11) to assume that organic nitrogen is transferre...

Synthesis and Turnover of Nitrate Reductase in Corn Roots

“…During early growth in maize seedlings, enzymes involved in the reduction of nitrate to ammonia may be considered luxury enzymes, since there is a rich source of amino acids supplied by the hydrolysis of endosperm proteins (13,14). In addition, amino acids supplied by the endosperm are potential regulators of the embryo nitrate reductase (4,7).…”

The soluble leucine pool in maize root tips.

“…It was therefore the aim of this investigation to define the nature of the soluble pool for a model amino acid in the maize root tip. Since leucine is one of the amino acids potentially supplied in large amounts by the maize en(losperm (21) it was chosen for this purpose.…”

FIFTY YEARS OF PLANT SCIENCE: Was There Really No Place for A Woman?

“…With this work we had established that amides were significant regulators in maize seedling roots. In view of the fact that sucrose inhibits the production of asparagine in seedling tissues (8) and enhances the induction of NR, it may be that the sucrose released from the endosperm is responsible for an inhibition of asparagine production and thus, indirectly leads to the induction of NR by five days post germination. However, there was still another problem.…”

“…The factor that caused this turned out to be amino acids, but not just any amino acid mix. I made a synthetic mixture of amino acids similar in concentration to the amino acids released by the endosperm and found that this mixture, but not a casein hydrolysate, enhanced the accumulation of alcohol-insoluble nitrogen in the excised embryos (8). I knew from those early experiments that small peptides were also released and I thought that they were probably important in the overall utilization of endosperm nitrogen, protecting leucine from degradation in the scutellum, for example (see Reference 4 for details).…”