Endosperm Mutants Research Articles

A mutation causing proline requirement in Zea mays

A conditional seedling lethal, monogenic recessive, endosperm mutant is described. Phenotypic can be accomplished when embryos are cultured in vitro on media supplemented with proline. The efficiency of the repair is proportional to the concentration of proline in the medium. Normal growth is resumed at a dose of 160 mg/l. All the data collected are most easily interpreted by assuming that the mutant, symbolized pro has a genetic block in the biosynthetic route leading to proline.This is probably the first case of a strict genetic requirement for an amino acid to be reported in Zea mays. The possible reasons for the difficulties encountered in isolating obligate auxotrophs in higher plants are briefly reviewed.

Survey of Maize Selections for Endosperm Lysine Content1

Lysine and protein content from 364 white endosperm maize (Zea mays L.) plant introductions showed that 12 accessions had lysine values ≥ 0.22%. The 364 accessions included 195 dent, 28 floury, 9 popcorn, and 132 flint types. Very little difference was noted among the four types for mean lysine content. Two of the 195 dent, 22 of the 28 floury, none of the 9 popcorn, and two of the 132 flint types had opaque endosoerms. Lysine content from the whole kernel was significantly correlated (r = 0.67) with lysine content of the endosperm for 47 accessions. This indicates that about 45% of the variation in lysine content in the whole kernel can be explained by variation in endosperm lysine content. Several of the accessions should be good germplasm sources for cyclic breeding programs for increasing lysine content in normal corn without specific major endosperm mutations or as sources for upgrading the lysine level in the opaque‐2 mutant types.

Influence of Maize Endosperm Mutants on Ground Kernel In Vitro Dry Matter Disappearance1

Ground kernels of the W64A maize (Zea mays L.) inbred and 5 single and 10 double endosperm mutants of maize converted to W64A were evaluated using a twostage in vitro rumen fermentation system with and without urea added to the digestion buffer. In vitro dry matter disappearance (IVDMD) values were not correlated with protein content. Urea addition raised IVDMD values to the same level for most genotypes. Four genotypes were investigated using only first‐stage digestion for 3, 6, 12, 24, and 48 hours. Maximum differences were observed after 12 and 24 hours of digestion. At these times, opaque‐2 was most degraded followed by waxy, normal, and amylose‐extender. The superior rate of opaque‐2 digestion compared to waxy could not be explained by differences in particle sizes nor by known rates of isolated starch granule digestion. We suggest that incubation times used for forage IVDMD studies utilizing the basic procedure of Tilley and Terry be shortened when comparing ground grain samples.

Influence of Opaque-2 and Floury-2 Genes on Formation of Proteins in Particulates of Corn Endosperm

Protein-rich subcellular particulates were isolated by zonal centrifugation from homogenates of endosperms of normal, opaque-2, and floury-2 mutant corn (Zea maize) kernels at different stages of development. In early stages the high lysine mutants vary from normal corn by greater production of a glutelin protein not associated with the matrix. This protein is high in lysine and may become a component of matrix glutelin at later stages of maturity. Differences in size and structure of zein-rich protein bodies were observed in the mutant strains when compared with normal corn. Enhanced production of nonmatrix glutelin as well as the reduction in synthesis of lysine-deficient zein is responsible for the improved lysine content of the mutant endosperms at early stages of development.

PROTEIN QUALITY STUDY WITH THE ENDOSPERM CORN MUTANT GENES OPAQUE-2 AND FLOURY-2

Forty-two single crosses of corn (Zea mays L.) among seven parental lines were used to evaluate the effects of the endosperm mutants opaque-2 and floury-2 on protein quality. Each parental line had the following five versions: (a) original normal, (b) opaque-2, (c) normal derived from the opaque-2 backcross population, (d) floury-2, and (e) normal derived from the floury-2 backcross population. These five versions were used to make five diallel sets of 42 single crosses and these were grown in a split-split plot design with three replications. Pollen contamination between sets was kept at a minimum with sufficient border rows. Differences were found among endosperm types for percentages of lysine, lysine content of protein, and between F1 hybrids within endosperm types for the above traits and for percentage protein. Lysine content of floury-2 crosses was only slightly higher than that of normal and lower than that of opaque-2 crosses. Additive effects for the three traits studied were more important than nonadditive. Phenotypic and genotypic correlations between percentages of lysine and protein in the breeding materials studied were high and indicated that selection for higher protein levels within either opaque-2 or floury-2 types should not reduce the nutritive value of the protein.

Inheritance of Test‐Weight Components in Normal, Opaque‐2, and Floury‐2 Corn (Zea mays L.)1

Diallel mating systems were used to study kernel‐endosperm properties in normal corn (Zea mays L.) and its opaque‐2 and floury‐2 endosperm mutants. Seven parental lines were involved. Each line had five endosperm versions: original normal, opaque‐2, normal derived from opaque‐2 backcross population, floury‐2, and normal derived from a floury‐2 backcross population. All 21 possible single crosses and reciprocals were made among the seven lines within each of the five groups. The opaque‐2 and floury‐2 groups gave similar responses, each decreasing test weight, 500‐kernel weight, and kernel density; and increasing number of kernels per 100 g and percent kernel breakage. Addition of opaque‐2 and floury‐2 genes changed the means of the kernel traits but did not change appreciably the combining ability of the lines. Additive gene effects accounted for a larger part of the variation among crosses than nonadditive effects in the inheritance of the kernel traits. Phenotypic and genetic correlations among test weight, kernel density, and kernel breakage were high; chemical components were correlated poorly with physical kernel traits.

Inbred and Hybrid Effects on Establishment of in Vitro Cultures of Zea Mays L. Endosperm1

Zea mays L. endosperm explants from 23 starchy inbred and hybrid lines and several endosperm mutants were tested for their ability to grow in vitro. Explants from two starchy inbreds, two hybrids, and the mutants amylose‐extender, sugary‐1, and shrunken‐2 grew actively and have been maintained in culture for over 2 years. Other lines produced limited growth or no growth following the first transfer. These data demonstrate the importance of testing several different inbreds or hybrids when attempting to establish in vitro cultures of corn endosperm.

Effect of Various Endosperm Mutants on Oil Content and Fatty Acid Composition of Whole Kernel Corn (Zea mays L.)1

Abstract Oil content as extracted with chloroform - methanol ranged from 5.2 to 18.4% among the 12 mutant genotypes examined. GLC analysis of the methyl esters of the corn oil fatty acids showed widely significant differences in composition among the genotypes. Action of the opaque-2 gene on the inbred caused the greatest decrease in oil content and greatest increase in the proportion of polyunsaturates. The brittle-2 gene caused the greatest increase in oil content and greatest decrease in the proportion of polyunsaturates. A positive relationship was evident between saturated and oleic acids and oil content. An inverse relationship was evident between polyunsaturated acids and oil content.

Nucleoside Diphosphate Glucose Pyrophosphorylase Isoenzymes of Developing Normal, Brittle‐2, and Shrunken‐2 Endosperms of Zea mays L.1

Maize (Zea mays L.) endosperms from normal, brittle‐2, and shrunken‐2 near‐isogenic sublines of the Oh43 inbred were assayed for nucleoside diphosphate glucose pyrophosphorylase activity at 10, 20, and 30 days post‐pollination. Polyethaleneamine‐cellulose thin layer chromatography and polyacrylamide gel electrophoresis were used for product assay and identification of isoenzyme patterns from crude enzyme preparations, respectively.Evidence is presented for the existence in corn endosperm of pyrophosphorylose activity with guanosine triphosphate and thymidine triphosphate in addition to adenosine triphosphate, cytidine triphosphate, and uridine triphosphate. Two adenosine diphosphate glucosepyrophosphorylase isoenzyme bands from normal and shrunken‐2 endosperms were detected with the same pattern, suggesting that the shrunken‐2 gene may be regulatory in nature. Brittle‐2 showed an altered mobility of the adenosine diphosphate glucose pyrophosphorylase banding pattern, which indicates that brittle‐2 may be a structural gene. Three isoenzymes for uridine diphosphate glucose pyrophosphorylase were found in normal and mutant endosperms. Evidence indicated that guanosine triphosphate may be utilized by adenosine diphosphate glucose pyrophosphorylase. Thymidine triphosphate and possibly cytidine triphosphate may be utilized by uridine diphosphate glucose pyrophosphorylase.There were no differential appearances of the pyrophosphorylase isoenzymes during corn endosperm development. However, quantitative differences in the isoenzyme bands were observed.

Endosperm Protein Synthesis in Maize Mutants with Increased Lysine Content

The endosperm proteins of the maize mutants, opaque-2, opaque-7, floury-2, brittle-2, and the double mutant of opaque-2 and brittle-2, were separated into five soluble fractions by the Landry-Moureaux method. As compared to their isogenic normal counterparts, the mutant endosperms had higher concentrations of albumins, globulins, and glutelin-3, and lower concentrations of prolamines. The combination of the opaque-2 and brittle-2 genes enhanced this difference. Although the four mutant genes are located on three different chromosomes, they exert a similar effect on endosperm protein composition. Five other starchmodifying mutants with high lysine content resemble the brittle-2 mutant in endosperm protein composition, when the gene is present either singly or combined with opaque-2. Therefore, the pattern of protein synthesis in all maize mutants with high lysine concentrations may be either identical or very similar. Because no synergistic effect on lysine concentration is obtained when floury-2 is combined with opaque-2, different pathways leading to reduced zein synthesis may exist in the floury and starch-modifying mutants with high lysine concentrations.

Genetic Modifications of Sweet Corn Quality1

Abstract Carbohydrate analyses were made at harvest and after 7 days of postharvest storage at 65°F on sweet corn hybrid cultivars with endosperm mutant genotypes ae wx, ae du wx, and sh2 and of standard sweet corn (su1). Cultivars with the mutant genotypes contained from 1.5 to 2.0 times as much total sugar at harvest as the standard. They also lost more total sugar during the post-harvest period. Still, the hybrids with ae du wx and sh2 genotypes contained significantly more sugars than the standard after the 7-day storage treatment. The mutant cultivars were low in water soluble polysaccharides but relatively high in starch. All cultivars had approximately the same amount of total carbohydrate. Moisture loss from the kernels during the storage period was significantly less in the mutant cultivars. Therefore, they maintained their fresh appearance longer, and they were slower than standard sweet corn to show kernel denting. Taste tests indicated a preference for the sweetness of the ae wx and ae du wx genotypes over the standard cultivars Golden Security and ‘Iobelle’ (Florida 104), but they were rated below the standards for pericarp toughness. These studies indicated that, with further refinement of certain horticultural and quality characteristics, the ae wx, ae du wx, and sh2 endosperm mutant genotypes have a definite potential for improving the initial quality and the ability for maintaining good quality during normal post-harvest handling of fresh sweet corn.

An Invertase Inactivator in Maize Endosperm and Factors Affecting Inactivation

A protein present in the developing endosperm of maize (Zea mays L.) causes a loss of invertase activity under certain conditions of incubation. This protein, designated an inactivator, inactivates invertase I of maize even in the presence of other proteins. No inactivation of invertase II of maize or yeast invertase has been observed. The inactivator and invertase I are found only in the endosperm. The quantity of inactivator increases in the normal endosperm during development while invertase I activity decreases. However, the altered levels of invertase I activity in several endosperm mutant lines do not result from different quantities of inactivator. The inactivator can decrease invertase I activity during a preincubation period before addition of sucrose; inactivation is noncompetitive. Invertase I activity decreases curvilinearly with an increase in inactivator concentration. At high buffer concentrations or low inactivator concentrations in the reaction mixture, a latent period is observed when invertase I is not inactivated. Inactivation increases with an increase in temperature and a decrease in pH.

Invertase Activity in Normal and Mutant Maize Endosperms during Development

A bound invertase and two soluble invertases are found in the developing endosperm of maize (Zea mays L.). The two soluble invertases can be separated on diethylaminoethyl-cellulose and Sephadex columns and distinguished by their kinetic constants. One soluble invertase, invertase I, is present from the 10- to 28-day stages of endosperm development with maximal activity per normal endosperm at the 12-day stage. In two endosperm mutant lines, shrunken-1 and shrunken-2, there is a second increase in invertase I activity later in development which could be a secondary effect caused by the abnormal metabolism in these lines. Another soluble invertase, invertase II, is present in the embryo upon germination and is also found in the very young developing endosperm (6-day stage). The third form of invertase, bound invertase, is present in the endosperm by the 6-day stage, and its activity remains approximately constant during development.

Influence of Single Recessive Endosperm Mutants on Yield and Dry Matter Distribution in Maize (Zea mays L.)1

SummarySeed stocks of recessive endosperm mutants su1, fl2 and o2 were each crossed with several inbred lines of maize. F1 plants were self‐pollinated to produce ears segregating for normal and mutant kernels. Careful examination of ears from F2 plants allowed harvest of samples from homozygous normal, heterozygous, and homozygous mutant plants. The heterozygotes were superior in grain yield to either homozygote, and the homozygous normal yielded more than the mutant in su1 and o2 crosses. Husk dry weights were lowest in normal homozygotes of su1 and o2, and in homozygous recessive of fl1. Leaf and stem dry weights showed inconsistent patterns. Plants from normal kernels had a lower ear, husk, and leaf moisture than those of the homozygous recessive. The harvest index (ear wt./total plant wt.) was greater in homozygous normal and heterozygous plants of su1 and o2 maize. The heterozygotes studied appeared to have a more totally efficient system for the conversion of light into dry matter.

Effects of Endosperm Genes on Dry Matter Accumulation and Moisture Loss in Corn (Zea mays L.)

The effects of nine endosperm mutant genes on dry matter accumulation and percent moisture decrease were studied in three near‐isogenic hybrid backgrounds of corn (Zea mays L.) during the period 35 to 77 days after pollination. Eight of the endosperm mutants were inferior to normal in dry matter accumulation while floury‐l was slightly superior. The shrunken‐1 and sugary‐1 mutants showed the greatest reduction in dry matter, accumulating only 75% and 64% of the total dry matter of normal. The mutants which accumulate greater quantities of sugar and water soluble polysaccharides produced less total dry matter.It appeared that lower dry matter accumulation was accompanied by retention of more moisture. The floury‐l mutant had a slightly lower percent moisture than normal. All other mutants had a greater percent moisture, sugary‐1 being the highest.

Effect of Endosperm Mutants on Germination and Early Seedling Growth Rate in Maize (Zea mays L.)1

Endosperm mutants in three near‐isogenic backgrounds of maize (Zea mays L.) were used to determine their effect on germination and early seedling growth. Two environments were utilized: average greenhouse temperature regimes (20 to 25 C) and 15 C in a controlled environment chamber. Floury‐1 had a significantly greater early seedling growth rate than normal in both environments. while sugary‐1 had a significantly slower seedling growth rate in both environments, all mutants showed significantly slower seedling growth rates than normal at 15 C. Shrunken‐1 showed a significantly lower maximum number of plants germinated than normal under greenhouse conditions. Sugary‐1 was significantly different from normal in both environments.

Effect of Endosperm Mutants on Drying Rate in Corn (Zea mays L.)1

Nine endosperm mutants in three hybrid backgrounds were used to study the effect of the endosperm of the coru (Zea mays L.) kernel on drying rate. Drying rate was determined by the percent moisture loss from husked ears in a forced‐air dryer at 38 C for 24 hours.Dull, ae, sh1, and su1 had a significantly greater drying rate than normal at each of the seven harvests from 35 to 77 days after pollination. Opaque‐2 showed a significantly greater drying rate at the first five harvests. Sugary‐2 had a significantly lower drying rate than normal at the first five harvests, while h was significantly lower than normal at 35 and 70 days after pollination. Floury‐1 and wx did not differ significantly in drying rate from normal at any of the harvests.A relationship was found between percent moisture and drying rate per ear within harvest dates. Osmotic pressure increased as days to harvest increased but was not correlated with drying rate.It is proposed that drying rate is regulated in part by hydrophilic compounds in the endosperm of the coru kernel.

Effect of Endosperm Mutants on Pericarp Thickness in Corn1

Pericarp thickness was measured for mutant and normal kernels obtained from ears segregating for various endosperm mutants in the corn (Zea mays L.) inbred lines Oh43 and B37. The pericarp, being maternal tissue, was not greatly influenced by the genotype of the endosperm when averaged over all mutants. In the B37 series, only the pericarp thickness for the sh2 mutant significantly exceeded its normal sibbed kernels. while in the Oh43 series, ae and su2 mutant kernels had significantly thicker pericarp tissue and su1 mutant kernels had thinner pericarp. While significant these differences were small in magnitude. There is a lack of evidence for a metaxenia effect of endosperm phenotype on pericarp thickness when averaged over all nine mutants.

Ribonuclease activity in normal and opaque-2 mutant endosperm of maize.

Three strains of maize heterozygous for the opaque-2 mutant gene were self-pollinated to produce ears bearing both normal and opaque kernels. The ribonuclease activity of the opaque-2 endosperm was two to more than four times as high as the activity in the normal endosperm.

Photoreactivation of endosperm mutations induced by ultraviolet light in maize

Changes in relative mutation frequencies in maize endosperm were studied after visible light treatment of u.v. or γ-irradiated pollen grains. For the u.v. irradiated groups, mutation frequencies of whole endosperm mutants were considerably decreased by post treatment with visible light, while the frequencies of partial mutations (chimeras) were slightly increased by the treatment. By a similar post treatment of γ-irradiated pollen the mutation frequencies were not modified.