A proposed role of zein and glutelin as N sinks in maize

Zea mays grown with high levels of N fertilizer transports more sucrose into kernels than with low N. Sucrose translocation was greatest in genotypes with the highest capacity to deposit nitrogenous compounds as zein and glutelin in the kernel. These two proteins combined contain about 80% of the total N in the kernel and about 60% of the total N in the plant at maturity. They appear to serve as a functional N sink for the deposition of nitrogenous compounds. As the N sink capacity increases with additional available N fertilizer, more sucrose is transported into the kernel, resulting in increased kernel weight and grain yield. Zein functions as a more dynamic N sink than glutelin because the synthesis of zein is readily manipulated by N fertilization and genetic means. Increases in N deposition in the normal endosperm induced by N fertilizer are confined primarily to zein. Early termination of zein accumulation in the opaque-2 mutant results in a reduction of sucrose movement into kernels. By using plants heterozygous for normal and opaque-2 in these studies, interplant variability was eliminated and the hypothesis relating the kernel N sink capacity to productivity was strengthened.

Tissue-specific zein synthesis in maize kernel

Zein may account for as much as 10% of the total protein in the mature embryo of maize inbred W64A. This protein exhibited an electrophoretic pattern on SDS gels similar to that of the endosperm. Like the endosperm system, the synthesis of zein components in the embryo was controlled by the opaque-2 and floury-2-mutations. However, unlike zein synthesis in the endosperm, zein synthesis in the embryo could not be increased by nitrogen fertilizer. Variations in amino acid composition were observed between the zein components of the embryo and those of the endosperm.

Interaction of the opaque-2 gene with starch-forming mutant genes on the synthesis of zein in maize endosperm

The combination of opaque-2 with starch-modified or starch-deficient mutants produced a cumulative and synergistic effect, respectively in regulating zein synthesis. The double mutant, brittle-2 opaque-2, which almost completely prevented the synthesis of Z1 and Z2, had high RNase activity. The possible involvement of RNase in effecting zein synthesis is discussed.

Dissociation of polysome aggregates by protease k

Apparent large size-classes of zein-synthesizing polysomes from developing kernels of Zea mays L. were converted to smaller polysomes after treatment with Protease K. The reduction in polysome size was not a result of ribonuclease activity, inasmuch as the enzyme did not affect the free polysomes or the size of the mRNA from the membrane-bound polysomes. High concentrations of MgCl(2) in polysome buffer inhibited ribonuclease activity and appeared to cause protein interaction between nascent zein polypeptides. Although Protease K inhibited the polysome's capacity for protein synthesis, it was a useful reagent for determining if polysomes were aggregated by protein.

Storage Protein Synthesis in Maize: II. Reduced Synthesis of a Major Zein Component by the Opaque-2 Mutant of Maize

Two zein proteins (Z1 and Z2) represent the majority of the protein synthesized during maize endosperm development. Undegraded membrane-bound polysomes isolated from normal maize synthesized these proteins when incubated in a cell-free protein-synthesizing system from wheat germ. The proteins synthesized in vitro were similar to authentic zein in ethanol solubility and electrophoretic mobility. Zein synthesis was associated with large size classes of membrane bound polysomes in normal maize.Membrane-bound polysomes isolated from developing kernels of opaque-2 mutant synthesized less total zein in vitro, and dramatically reduced incorporation into the Z1 component. The reduction in total zein corresponded to a 50% reduction in the level of membrane-bound polysomes in opaque-2, and the near absence of the large polysome size classes, which synthesized zein in normal maize. We concluded that the opaque-2 mutation results in a decreased "availability" of the zein mRNAs, reflected in a reduced level of membrane-bound polysomes.

Storage Protein Synthesis in Maize: III. Developmental Changes in Membrane-bound Polyribosome Composition and in Vitro Protein Synthesis of Normal and Opaque-2 Maize

Zein synthesis accompanied an increase in large polyribosomes of maize (Zea mays) endosperm cells. The two classes of polyribosomes (free and membrane-bound) had dissimilar size class distributions. Membrane-bound polyribosomes were predominantly large size classes, which were not found in free polyribosomes. The ratio of large membrane-bound polysomes to total membrane-bound polysomes was highest when zein was being synthesized. Appearance of the large polysomes correlated with the onset of zein accumulation in vivo. These large size classes were nearly absent in the opaque-2 mutant at all stages of endosperm development. Similarly, rRNA content was reduced in the mutant from that in normal endosperm development. These differences were associated with reduced in vitro synthesis and in vivo accumulation of zein.

Isolation and in vitro translation of zein messenger ribonucleic acid

Zein messenger RNA was isolated from membrane-bound polyribosomes of developing maize kernels by oligo(dT)-cellulose chromatography. Translation of the mRNA in vitro yielded protein similar to native zein in amino acid content, ethanol solubility, and mobility on sodium dodecyl sulfate- polyacrylamide gels. The zein mRNA sedimented as a homogeneous peak on sucrose gradients and contained a poly(A)-rich region based upon hybridization to [3H]poly(U). The mRNA had an apparent molecular weight of 540 000 on agarose-acrylamide gels. It synthesized both 21 800 and 19 000 molecular weight zein components in the wheat-germ cell-free protein synthesis system. The possibility of a polycistronic mRNA or two mRNAs of similar molecular weight is considered.

Genetic regulation of storaage protein content in maize endosperm

Sodium dodecylsulfate-polyacrylamide gel electrophoresis reveals that zein prepared from normal maize inbred (Zea mays L.) contains six separable components. Z1 and Z2 are the predominant species, with molecular weights of 21,800 and 19,000 daltons. Amino acid analysis of these two components shows that both are rich in glutamic acid, leucine, and proline, but low in lysine. Of the four minor bands, Z3, Z4, Z5, and Z6, the latter two exist only in trace amounts. A mutation at the opaque-2 locus severely suppresses the synthesis of Z1. The nonallelic mutant, opaque-7, strongly suppresses the synthesis of Z3 and Z4, while slightly reducing Z2. On the other hand, the floury-2 mutant appears to reduce the synthesis of these six proteins in the same relative proportion. In the double mutant combinations, opaque-2 apparently is epistatic to opaque-7 and floury-2 in the synthesis of zein components. The glutelin fraction shows a more complex banding pattern; however, qualitative differences are not apparent among the mutant lines examined.

Storage Protein Synthesis in Maize: Isolation of Zein-synthesizing Polyribosomes

Undegraded free and membrane-bound polysomes were isolated from developing kernels of Zea mays L. frozen in liquid nitrogen. Freezing in liquid nitrogen was a prerequisite for preserving polysome structure in stored kernels. Membrane-bound polysomes from 22-day post-pollination kernels ground in high pH buffers containing 50 mm Mg(2+) contained unique classes of large polysomes. These large polysomes were sensitive to ribonuclease, and electron micrographs verified that they were not formed by aggregation. The membrane-bound polysomes were the principal site of zein synthesis, since the major protein synthesized in vitro was similar to purified zein in its ethanol solubility and mobility on sodium dodecyl sulfate polyacrylamide gels.

Multiple forms of glucosephosphate isomerase in maize

Three apparently different glucosephosphate isomerases are found in the developing seeds of maize (Zea mays L.). Glucosephosphate isomerase I is found in both the endosperm and embryo. It is separable by column chromatography from glucosephosphate isomerase II of the developing endosperm and glucosephosphate isomerase III of the developing embryo and is further distinguished from them by heat stability, temperature activation, and relative insensitivity to the presence of zinc ions in the reaction mixture. Glucosephosphate isomerases II and III elute in the same fractions from diethylaminoethyl cellulose columns but are distinguished by electrophoretic mobility and reaction to the presence of adenosine 5'-triphosphate in the reaction mixture. All three isomerases give multiple banding patterns on electrophoresis. An extensive investigation of the conditions generating additional electrophoretic species and chromatographically separable minor activity peak (Ia) from glucosephosphate isomerase I has shown that these transformations are enhanced by dialysis, column chromatography, ammonium sulfate fractionation, and treatment with urea. The transformations are retarded by the presence of mercaptoethanol during these operations. We concluded that the multiple banding pattern seen on electrophoresis of glucosephosphate isomerase I prepared by certain procedures is artifactual. In germinating seeds of maize, glucosephosphate isomerases I and III are detectable, but II is not. It is possible that glucosephosphate isomerase II specifically catalyzes a step in starch biosynthesis.

Enzymes of carbohydrate metabolism in the developing endosperm of maize

A number of enzymes presumably implicated in starch synthesis were assayed at various stages of endosperm development ranging from 8 days to 28 days after pollination. Activity for invertase, hexokinase, the glucose phosphate isomerases, the phosphoglucomutases, phosphorylase I, uridine diphosphate glucose pyrophosphorylase, and the starch granule-bound nucleoside diphosphate glucose-starch glucosyltransferase was present at the earliest stage of development (8 days) studied. Activity was detectable for phosphorylase III, the soluble adenosine diphosphate glucose-starch glucosyltransferase, adenosine diphosphate glucose pyrophosphorylase, and sucrose-uridine diphosphate glucosyltransferase at 12 days. For phosphorylase II and cytidine diphosphate glucose pyrophosphorylase, activity was first detectable at the 14- and 16-day stages, respectively. Rapid increases in starch content are observed prior to detectable activity for adenosine diphosphate glucose pyrophosphorylase, the soluble adenosine diphosphate glucose-starch glucosyltransferase and phosphorylases II and III. For all enzymes, except invertase, activity per endosperm rises to a peak at 22 or 28 days. Greatest activity for invertase is found at 12 days with a steady decline thereafter. The pattern of invertase activity in comparison with that of sucrose-uridine diphosphate glucosyltransferase supports previous suggestions, that the latter plays a key role in the conversion of sucrose to starch. In addition to phosphorylases I, II, and III, multiple forms of glucosephosphate isomerase and phosphoglucomutase were detected.

Two additional phosphorylases in developing maize seeds

Two additional phosphorylases (III and IV) have been detected in developing seeds of maize. Phosphorylase IV is found only in the embryo (with scutellum). It is also present in the embryo of the germinating seed where its activity is 90-fold greater than the activity in the developing embryo 22 days after pollination. Phosphorylase IV is eluted from a DEAE-cellulose column in the same fraction as phosphorylase I of the endosperm, and the 2 enzymes are similar in many respects. Phosphorylase IV is distinguished from phosphorylase I by electrophoretic mobility, by pH optimum, and because its properties are not affected by the shrunken-4 mutation. Phosphorylase III is found both in the endosperms and embryos of developing seeds. Activity for this enzyme is not detected in crude homogenates nor eluates from a DEAE-cellulose column apparently because it complexes with a non-dialyzable, heat-labile inhibitor. High activity is found after protamine sulfate fractionation. Phosphorylase III is bound to protamine sulfate and is then removed by washing with 0.3 m phosphate buffer. Phosphorylase III activity in the endosperm is not detectable 8 days after pollination but is present 12 days after pollination. Phosphorylase III differs from phosphorylases I, II, and IV in several respects-pH optimum, pH-independent ATP inhibition, time of appearance in the endosperm, and because purine and pyrimidine nucleotides are equally inhibitory. In common with phosphorylase II, phosphorylase III apparently does not require a primer to initiate the synthesis of an amylose-like polymer.

Phosphorylases I and II of Maize Endosperm

Two phosphorylases have been found in the endosperm of Zea mays. Phosphorylase I is found through all stages of endosperm development and seed germination investigated. The other enzyme, phosphorylase II appears only at the stage of rapid starch biosynthesis and is not found during germination. At 22 days after pollination, the activity of phosphorylase II is 10 times that of phosphorylase I. These 2 phosphorylases are separable by column chromatography and behave differently in several respects.Phosphorylase I cannot utilize maltose as a primer while phosphorylase II does so readily. Furthermore, phosphorylase II can synthesize an amylose-like polymer from a "primer free" system after a lag phase.Phosphorylase II is inhibited severely at pH 5.8 by ATP, GTP, ADP, and GDP, and less drastically by UTP, CTP, UDP and CDP. Phosphorylase I is somewhat inhibited by purine nucleotides but not by pyrimidine nucleotides. In all cases, the inhibition is pH-dependent. Phosphorylase I is inhibited competitively by ATP while phosphorylase II is inhibited non-competitively.Phosphorylase II is markedly stimulated by 10 mm Mg(2+) and by 2 mm ethylenediamine tetraacetic acid while phosphorylase I is relatively little affected.

Starch-deficient maize mutant lacking adenosine dephosphate glucose pyrophosphorylase activity

The maize mutant shrunken-2 synthesizes only 25 to 30 percent as much starch as normal maize; it completely lacks adenosine diphosphate glucose pyrophosphorylase activity in both endosperm and embryo tissue. Identification of the mutant block indicates that the greater portion of starch in the endosperm of normal maize is synthesized by way of enzyme systems that utilize adenosine diphosphate glucose as a substrate, and that the latter is formed chiefly by adenosine diphosphate glucose pyrophosphorylase.