The Distribution and Biosynthesis of Niacin in Germinating Corn

Experiments with excised maize root tips have shown that an exogenous supply of amino acids can effectively inhibit the synthesis of threonine, valine, leucine, lysine, arginine, and proline from acetate-2-C14 (18). In addition it has been suggested that the metabolites in the root tip region are normally maintained by the supply from other regions of the embryo (3,18,26). If this is true, it is possible that the ready supply of amino acids from older regions of the embryo could control the endogenous synthesis within the tip region as the exogenous amino acids (lo. This prediction would require a reduction of the pool level followed by an increased rate of synthesis of the amino acids after the removal of the transport system. 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. MethodsMaize seed (hybrid var. Wf 9 X 38-11) wvas soaked overnight in water, and allowed to germinate on damp filter paper for an additional 48 hours at 30°. At this time the roots were 4 to 5 cm long. were located on X-ray film, and the material contained was eluted for the determination of radioactivity and a-amnino nitrogen content. When leucine-C'4 was fed, only 1 radioactive area corresponding to leucine was detected on the chromiatogramn. When the residue was exposed to trypsin, or wvas hydrolyzed with 1 N HCI for 48 hours at 250, several radioactive areas were prominent, while the region corresponding to leucine was scarcely visible on the X-ray film. Thus adsorption of C'4-leucine to the residue was not an important source of error.A Kjehldahl digestion followed by nesslerization was used for the nitrogen determinations (11). The a-amino nitrogen was (leterminedl with ninh-drin (28).A balance sheet for the utilization of leucine-U-C'' by excised root tips is shown in

Section: Methodsmentioning

confidence: 99%

The soluble leucine pool in maize root tips.

Oaks¹

1965

“…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.…”

Section: Methodsmentioning

confidence: 99%

“…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) supplenmented with 2 % glucose. After a period of 50 to 70 hours in liquid culture in the dark the embryos were washed and placed in an oven at 700 for 12 hours.…”

Section: Methodsmentioning

confidence: 99%

“…Excised maize embryos, which show normal increases in dry weight and shoot length when supplied with glucose (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).…”

mentioning

confidence: 99%

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The Requirement for Organic Nitrogen in Zea mays Embryos

Oaks

Beevers

1964

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...

“…The high percentage of bound auxin residual in the resting endosperm of maize (3,4,7,13,14), and the increasing discoveries of gibberellin-like substances in the seeds of higher plants (22,24) also have made the contribution by the endosperm to the growth of the axis seemingly more complex than merely that of a sugar and inorganic ion source. Furthermore, Nason (19) found that the rate and extent of niacin accumulation in intact maize embryos were three times as great as that of excised embryos after 10 days of germination. His data suggest that this is related to the absorption of tryptophan from the endosperm rather than to the absorption of niacin itself.…”

mentioning

confidence: 99%

Gross Nutritional Contributions of Maize Endosperm and Scutellum to Germination Growth of Maize Axis

Dure¹

1960

In 1890 Brown and Morris (8) showed that starch paste, when applied to the absorbing surface of the scutellum, could replace the endosperm during the germination of barley embryos. Andronescu (2) first demonstrated that the maize embryo (axis & scutellum) could develop into a normal plant when separated from its endosperm and supplied with sucrose. Most investigators, however, have emphasized the endosperm as a source of growth factors, either essential or stimulatory to the germination of the axis, rather than as a tissue serving singularly as a source of sugars and inorganic ions. Schander (26) considered the aleurone layer of the maize endosperm to be the source of an accelerator which is absorbed by the scutellum during germination. DeRopp (i l) found that embryos which were allowed to remain in the intact grain during the early stages of germination exhibited more vigorous growth after excision than did embryos removed from the dry rye seed. He also interpreted this as due to the absorption by the scutellum early in germination of a growth factor from the endosperm. The "Z" factor of Robbins (25) and the "blastinin" hormone of Cholodny (10) are other compounds said to originate in the endosperm. The high percentage of bound auxin residual in the resting endosperm of maize (3,4,7,13,14), and the increasing discoveries of gibberellin-like substances in the seeds of higher plants (22,24) also have made the contribution by the endosperm to the growth of the axis seemingly more complex than merely that of a sugar and inorganic ion source. Furthermore, Nason (19) found that the rate and extent of niacin accumulation in intact maize embryos were three times as great as that of excised embryos after 10 days of germination. His data suggest that this is related to the absorption of tryptophan from the endosperm rather than to the absorption of niacin itself.The extent to which the scutellum can provide nutrition to the axis has received little attention, although it has been shown that lipids are rapidly lost from the maize scutellum during germination (18). 'Received May 9, 1960. 2 Research supported by National Science Foundation under NSF grant G6076.3 Present address: Department of Chemistry, University of Georgia, Athens.The object of this research was to determine the relative nutritional contributions of the scutellum and the endosperm of the maize kernel to the growth of the axis during germination. METHODS AND MATERIALSThe material used was a non-waxy, inbred maize (Zea mays L.) line T854, secured from the Agricultural and Mechanical College of Texas. The endosperm of this line contain 85 % carbohydrate and 10 % protein on a dry weight basis (12). Oil is present in trace amounts in the aleurone layer, demonstratable by staining with Sudan IV. The predominate carbohydrates found in non-waxy maize endosperm during germination are starch, maltose, dextrins of various chain lengths, and a trace of glucose (29,6). Amylose comprises about 25 % of the starch of non-waxy maize endosperm and the remainder is...