Inbred Line B73 Research Articles

Thermal Properties of Starch in Corn Variants Isolated After Chemical Mutagenesis of Inbred Line B73

ABSTRACTThe starch from eight ethyl methanesulfonate (EMS) treated M4 families of the corn (Zea mays L.) inbred line B73 was analyzed using differential scanning calorimetry (DSC), a Rapid Visco Analyser (RVA), a texture analyzer (TA), and scanning electron microscopy (SEM) coupled with image analysis. The eight families were chosen from 144 families previously selected for having starch with unusual DSC parameters. Apparent amylose contents of the starch from the eight families generally were lower than that of the control. According to DSC, starches from mutagenized families tended to have lower onset temperature (T0) of gelatinization, enthalpy (ΔH) of gelatinization, and peak height index (PHI), but broader gelatinization range (R) than the B73 control. Their values for ΔH and percentage of retrograzdation (%R) were clustered around that of the control. Pasting properties from the RVA of the starches from the M4 families also were clustered around those of the control B73 starch, except for the setback values which were lower than B73 for M4 starches. Gel firmness values, as measured by TA, of all the M4 starches were generally lower than that of the B73 starch at storage treatments of one day at 25°C or seven days at 4°C. The stickiness of the gels of the M4 starches tended to be greater than that of B73 after seven days of storage at 4°C. These observations were consistent with the lower apparent amylose values for the M4 starches. SEM and image analysis data revealed no differences among the treatments in granule size and shape. Possibly, EMS treatment altered the genes, affecting internal structure of the starch granules. Starch from the mutagenized families likely had lower bonding forces among molecules and fewer long chains in the amylopectin molecules than did B73.

Genetics of resistance to kernel infection by Aspergillus flavus in maize *

AbstractAlthough kernel infection by Aspergillus flavus Link ex Fries and subsequent pre‐harvest aflatoxin contamination of maize (Zea mays L.) grain are major production problems in the south‐eastern United States and elsewhere in the world, limited progress has been made in developing and identifying sources for resistance. Genetics of kernel infection by A. flavus remains poorly understood. A 10‐parent diallel experiment was conducted in 1992 and 1994 to study the genetic nature of percentage kernel infection (PKI) by A. flavus. General combining ability (GCA), specific combining ability (SCA), and reciprocal mean squares for PKI were significant. The GCA and SCA sums of squares were about equal. The GCA, SCA, and reciprocal effects varied across years, which implied that percentage kernel infection was greatly influenced by environments. The percentage kernel infection was always greater when Mo 17 was the male parent in a cross than when it was the female parent. The percentage kernel infection mean for female Mo 17 was 38.5% lower than that for male Mol7. Similarly, percentage kernel infection mean for female L668 was 23.7% lower than that for male L668. The use of inbred lines L729 and B73 as female parents should be avoided, as they showed significant, positive maternal reciprocal effects.

A Diallel Analysis of Ear Moisture Loss Rate in Maize

Maize (Zea mays L.) growers need hybrids with fast ear moisture loss rate (EMLR) to reduce grain drying cost. A 10‐parent diallel experiment with reciprocals (Griffing's Method 3) was conducted for 2 yr to study the genetic nature of EMLR in maize. Field ear moisture was measured with an electronic probe device at 30 and 51 d after midsilk. Analysis of covariance (ANCOVA) was used to increase precision of treatment comparisons and parameter estimation since F1 crosses differed in midsilk dates and in growing degree days (GDD) between the first (Moisture1) and the second probe moisture readings (Moisture2). Adjusted plot means were used to compute combining abilities. General combining ability fixed variance component (GCA, Vg) and reciprocal component were significant, whereas specific combining ability fixed variance component (SCA, Vg) was not significant for EMLR. The 2Vg/(2Vg + Vg) ratio was 0.81, indicating that GCA effects were more important than SCA effects for EMLR. Significant, positive GCA effects for EMLR were detected in L108, L605, and L654 inbred lines and negative GCA effects in Mo17 inbred line. Non‐maternal component was significant for EMLR. Moisture1 and Moisture2 also were genetically controlled. A relatively low Moisture1, high EMLR, and positive GCA and reciprocal effects for EMLR were important in lowering Moisture2. Specific interaction between nuclear genetic component(s) and non‐nuclear genetic factors can be exploited for enhancing EMLR. Inbred lines L668, B73, and H111 had relatively low Moisture2 due to either relatively low Moisture1 or high EMLR or both. In general, Louisiana (L) lines had higher Moisture1, Moisture2, and EMLR than B73, H111, and Mo17.

Genotype-specific heat shock proteins in two maize inbreds

Leaf blade tissue of maize inbred lines B73 and Mo17 was analyzed for intraspecific genetic variability in the heat shock response. The maize inbreds were characterized for acquired thermal tolerance and patterns of heat shock protein synthesis. The leakage conductivity assay of membrane stability during stress indicated that Mol7 possesses greater potential than B73 to acquire thermal tolerance. Poly(A)(+) RNA, extracted from leaf blades, was translated in vitro in the presence of (35)S-methionine and the translation products separated by twodimensional gel electrophoresis. Major genotypic differences were observed in the translation products. Mo 17 synthesized twelve unique heat shock proteins in the 15-18 kD range, but B73 synthesized only three unique heat shock proteins in the same range. DNA polymorphisms were observed between the maize lines using (32)P labeled heat shock protein gene probes.

Improved tissue culture response of an elite maize inbred through backcross breeding, and identification of chromosomal regions important for regeneration by RFLP analysis

The frequency of initiation of friable, embryogenic callus from immature embryos of the elite maize inbred line B73 was increased dramatically by introgression of chromosomal segments from the inbred line A188 through classical backcross breeding. Less than 0.2% of the immature B73 embryos tested (5 of 3,710) formed embryogenic callus. The breeding scheme consisted of six generations of backcrossing to B73 with selection at each generation for high frequency initiation of embryogenic cultures. BC6 individuals were selfed for four generations to select homozygous lines. The average embryogenic culture initiation frequency increased to 46% (256/561). Nearly all (91%) of the embryos from one BC6 S4 plant formed embryogenic cultures. RFLP analysis was used to determine the locations and effects of the introgressed A188 chromosomal segments. Five segments were retained through at least the fifth backcross generation. The hypothesis that one or more of these five regions contains genes controlling somatic embryogenesis in maize was tested using an F2 population of the cross A188 X Mo17. A set of five DNA markers (three of them linked) explained 82% of the observed phenotypic variance for percentage of immature embryos forming embryognic callus. Four of the five markers were located in or near introgressed A188 chromosome segments.The region marked by probe c595 on the long arm of chromosome 9 was highly associated with several measures of in vitro culture response (percent embryogenic embryos, plants per embryo, and plants per embryogenic embryo). We propose that there is a major gene (or genes) in this region in A188 that promotes embryogenic callus initiation and plant regeneration in B73, Mo17, and probably many other recalcitrant inbred lines of maize.

Mutations in corn (Zea mays L.) conferring resistance to imidazolinone herbicides

Three corn (Zea mays L.) lines resistant to imidazolinone herbicides were developed by in vitro selection and plant regeneration. For all three lines, resistance is inherited as a single semidominant allele. The resistance alleles from resistant lines XA17, XI12, and QJ22 have been crossed into the inbred line B73, and in each case homozygotes are tolerant of commercial use rates of imidazolinone herbicides. All resistant selections have herbicide-resistant forms of acetohydroxyacid synthase (AHAS), the known site of action of imidazolinone herbicides. The herbicide-resistant phenotypes displayed at the whole plant level correlate directly with herbicide insensitivity of the AHAS activities of the selections. The AHAS activities from all three selections have normal feedback regulation by valine and leucine, and plants containing the mutations display a normal phenotype.

Genetic studies of corn (Zea mays L.) anther culture response

Anthers of two maize (Zea mays L.) inbred lines, DBTS (P1) and B73 (P2), their F1, F2 and first backcross generations - F1 x DBTS (B1), and F1 x B73 (B2) - were float cultured in YP medium to study the inheritance of corn anther culturability using generation mean analysis. Significant effects of generation were observed for the three traits measured: anther response (%), frequency of embryos (%) and anther productivity. Variation among the generations was similar for anther response and frequency of embryos: no significant differences were found among the P1, F1, F2 and B1 means, but the means of P2 and B2 were significantly lower than those of the other generations. For anther productivity, the F2 generation tended to have a slightly higher tendency for multiple embryo formation. A simple additive-dominance model was adequate in explaining the inheritance of anther response and frequency of embryos, but digenic epistasis (additive x dominance) was involved in the inheritance of anther productivity. Additive genetic variance was higher than non-additive genetic variance for all the traits; however, only environmental variance was significant. Narrow-sense heritability estimates were 65% and 75% for anther response and frequency of embryos, respectively. Significant inter-plant variation was observed within generations, even for the inbred line DBTS, but isozymic analysis involving five enzyme loci did not reveal any genotypic variability within the inbred lines DBTS and B73.

Relation of Corn Leaf Aphid (Homoptera: Aphididae) Colonization to DIMBOA Content in Maize Inbred Lines

Five inbred maize (Zea mays L.) lines (B37, B73, C103, Mo17, and 41:2504B) were evaluated from emergence to the eight-leaf stage for corn leaf aphid (Rhopalosiphum maidis (Fitch)) colonization. Concentration of 2,4-dihydroxy-7-methoxyl-l,4-benzoxazin-3- one (DIMBOA) in these inbred lines was analyzed from emergence to anthesis to determine its effect on corn leaf aphid colony development. Inbred line 41:2504B, which was colonized by corn leaf aphids as it emerged from the soil, had the highest DIMBOA concentration. Inbred lines B73 and B37 were low in DIMBOA and were colonized in the three- and six-leaf stages, respectively. Inbred lines C103 and Mo17 also were low in DIMBOA and supported small colonies after the six- and seven-leaf stages, respectively. Contrary to findings of previous studies, DIMBOA does not seem to be the primary factor conditioning resistance to the corn leaf aphid in these inbreds.

Hybrid reconstruction in maize

The production of seed of the maize single cross hybrid F68*NE2 is uneconomic because of the low grain yield of the maternal line. Therefore the aim was to produce it from newly developed inbred lines obtained by reshuffling the genes in the hybrid, accompanied by selection. Thus in open pollinating generations derived from this hybrid, i.e. in C0, C1, C2, C3 and C4, honeycomb selection for grain yield improvement was applied. Selfing of one ear and open pollination of another ear of selected prolific C4 plants yielded 20 pairs of S1/half sib progenies. Plants grown from remnant S1 seed corresponding to superior progeny pairs were selfed. In each S2-line a single plant was selected and selfed. The S3-lines were evaluated for yield. Two S3-lines, i.e. 6D and 2B, attracted attention because they yielded two and a half times as much as the best commercial inbred line B73.

PRECONDITIONING REDUCES THE SUSCEPTIBILITY TO DRYING INJURY IN CORN SEED

The characteristics of drying injury and drying tolerance of corn (Zea mays L.) seed were investigated on the inbred lines A632, B73, and Mo17. Initial drying at low temperature (preconditioning) to 30–39% seed moisture rendered high-moisture seed in most cases tolerant to subsequent drying at 50 °C. Drying at 20 or 35 °C resulted in more rapid and complete preconditioning of ear samples than storage at slow- or non-drying conditions. Drying of shelled samples at 35 °C did not accelerate the preconditioning effect compared with slower drying ear samples. Physiological factors other than moisture content are important criteria of high-temperature tolerance. These results offer an opportunity to more clearly define corn seed drying and begin to explain drying tolerance.

CHANGES IN MOISTURE, TEMPERATURE, AND QUALITY OF CORN SEED DURING HIGH-TEMPERATURE DRYING

Mechanical drying has frequently caused injury in corn seed. Changes in seed moisture, temperature, and quality were determined for inbred lines A632, B73 and Mo17 to define the relationship between these variables. Ears harvested at ca. 48 and 38% seed moisture could be dried at 50 °C for 4–15 h and 18–24 h, respectively, before germination started to decline linearly with prolonged 50 °C drying. Drying time at 50 °C, seed moisture, or embryo moisture after 50 °C drying could be used equally well for prediction of seed quality. Seedling dry weights often declined even when seed was dried for only a few hours at 50 °C. Temperature measurements within seeds indicated that evaporation cooled the seed no more than 5 °C. Drying susceptibility of seed parents varied greatly between years.Key words: Moisture, temperature changes, seed corn, drying

Heterosis for Embryo Size and Source and Sink Components of Maize

Assimilate source and sink components controlled by genotype and environment determine grain yield of maize (Zea mays L.). Field experiments were conducted in 1986 and 1987 at Gainesville, FL (29° 38′ N) on a Lakeland sand (hyperthermicoated Thermic Quartzipsamment) to assess heterosis for vegetative and yield components. In order to equalize leaf area index and light interception, parent inbred lines B73, MO17, MS71, and VA22 were planted at normal plant populations density (PPD) of 55 000 and also at 110 000 plants ha‐1, while three F1 hybrids from these inbreds were planted at 55000 and 27 500 PPD. In hybrid seeds, embryo size was associated with seed size. At common seed size (8 ram), lengths embryo and embryo axis of hybrids were greater than those of midparent and inbred line means, manifesting heterosis for embryo size. At common PPD, all biomass and grain yield source and sink components were significantly greater for hybrids than for inbreds. At common LAI and light interception achieved by doubling inbred PPD, hybrids produced significantly more biomass and kernel yield, kernel growth rate and weight, and grain‐filling duration, but kernel number for hybrids and inbreds was similar at common LAI. We conclude that hybrids as compared with their parent inbreds express heterosis in source components, including embryo size, as well as sink components, all contributing to higher biomass and grain yields of maize.

Effect of X‐ray Irradiation on Maize Inbred Line B73 Tissue Cultures and Regenerated Plants

In order to enhance variation induced by the tissue culture process and to obtain agronomically desirable mutants, friable embryogenic tissue cultures of maize (Zea mays L.) inbred line B73 were x‐ray irradiated with 11 doses [0–8.4 kilorads (kR)]. Reductions in callus growth rate and embryogenic callus formation occurred with increasing x‐ray doses 20 d and 3 months (90 d) after irradiation. Callus irradiated with 0.8 kR showed a significant increase in growth rate and a 20% increase in embryogenic callus 9 months (270 d) after irradiation. A total of 230 R0 plants were regenerated for evaluation. Pollen fertility and seed set of R0 plants decreased with increasing x‐ray dosage. Days to anthesis and plant height of R0 plants varied among x‐ray treatments but were generally reduced with higher dosages. The number of chromosomal aberrations increased with x‐ray dosage. The R1 seeds taken from R0 plants were also grown and tested for mutant segregation. Plants regenerated from irradiated calli had a two‐ to 10‐fold increase in mutations over plants regenerated from unirradiated control callus. Germination frequency of seeds from R0 plants decreased with increasing x‐ray dosage. Although chlorophyll mutants were most frequently observed, a number of vigorous plants with earlier anthesis date were also recovered.

Incidence of Bipolaris and Exserohilum Species in Corn Leaves in North Carolina

Samples of 42–60 corn leaves were collected without regard to disease symptoms at 2-m intervals in each of eight cornfields in western and two in eastern North Carolina in 1985. Sampling dates for the 10 fields ranged from 16 July to 8 August. The leaves were surface-sterilized and incubated in moist chambers for isolation of large-spored species of Bipolaris and Exserohilum. Incidence of B. zeicola infection was high (41–81%) in leaves from nine of the 10 fields. B. maydis and E. turcicum, which are more aggressive pathogens of hybrid corn than B. zeicola, occurred at lower incidences of 0–48% and 0–50%, respectively, in leaves from these fields at the time of sampling. This indicated that initial population densities of these aggressive pathogens were much lower than that of B. zeicola. Forty-five isolates of E. turcicum collected before 28 August were all race 1. Three isolates of race 2 were obtained later from fields of hybrids with the Ht 1 gene for resistance to race 1. All 75 isolates of B. maydis obtained from the 10 extensively sampled fields were race O; they were equally virulent on inbred line B73 with normal cytoplasm or B73 with C, S, or T male sterile cytoplasm. Frequencies of mating types of B. maydis and B. zeicola varied from field to field, with no correlation between the frequencies in the two species. Races 2 and 3 of B. zeicola, however, had similar mating type frequencies in these fields. B. sorokiniana was isolated from leaves from six of the 10 fields; the greatest incidence was 37%. E. rostratum was found in leaves from five fields at incidences up to 18%, and E. holmii occurred in leaves in one field at 11% incidence. These data indicate that a variety of species of Bipolaris and Exserohilum are able to infect green leaves of mature corn plants in the field. Thus, corn may contribute to the survival of species that are primarily pathogenic on other gramineous hosts.

Callus induction and plant regeneration from maize mature embryos

Calli were induced from mature embryos of maize (Zea mays L.) inbred lines A632, B73 and Mol7 on MS medium supplemented with 1-2 mg/1 2,4-dichlorophenoxyacetic acid. Callus induction frequency ranged from 23-100%, with Mol7 having the highest frequency. Plants were regenerated from 4-5% of the B73 and Mol7 explants. Embryogenic and organogenic calli of B73 were maintained for more than two and one half years without losing regenerability. Of 95 regenerated plants, only one R0 plant with abnormal pollen was detected, and no morphological variants were observed in the R1 progeny.

Effect of Ear Removal on CO2 Exchange and Activities of Ribulose Bisphosphate Carboxylase/Oxygenase and Phosphoenolpyruvate Carboxylase of Maize Hybrids and Inbred Lines

The effects of ear removal on gas exchange traits, chlorophyll, and leaf N profiles, and activities of ribulose 1,5-bisphosphate carboxylase/oxygenase and phosphoenolpyruvate carboxylase were examined using four maize hybrids (B73 x Mo17, B73 x LH38, FS854, and CB59G x LH38) and four inbred lines (B73, Mo17, LH38, and CB59G) as experimental material. A diverse genotypic response to ear removal was observed which was generally typified by (a) greatly accelerated loss of chlorophyll, leaf N, enzyme activities, and CO(2) exchange relative to controls for B73, B73 x Mo17, and B73 x LH38, (b) intermediate rate of decline for leaf constituents for FS854, LH38, and Mo17, or (c) loss of leaf constituents at similar or slower rates than for control plants for CB59G and CB59G x LH38. For all genotypes which had accelerated senescence relative to controls, loss of CO(2) exchange activity was correlated with increased internal CO(2) concentrations. Thus, it was concluded that metabolic factors and not stomatal effects were responsible for loss of CO(2) exchange activity. Loss of chlorophyll, leaf N, and enzyme activities correlated well with loss of CO(2) exchange activity only for some of the genotypes. Accelerated leaf senescence in response to ear removal for the inbred line B73 and the hybrids B73 x Mo17 and B73 x LH38, as well as the apparent delayed leaf senescence for the inbred line CB59G and the hybrid CB59G x LH38 show that the contrasting responses to ear removal, rapid versus delayed senescence, can be transmitted as dominant traits to F(1) hybrids. The intermediate response by some genotypes, and the dominance of contrasting senescence traits, suggested a relatively complex inheritance for expression of the ear removal response.

Plant regeneration via organogenesis and embryogenesis in the maize inbred line B73

Immature embryos from Zea mays L. inbred line B73 were cultured on Murashige and Skoog (MS) medium containing 0.5 mg/1 2,4-dicholorophenoxyacetic acid (2,4-D) and 12% (w/v) sucrose. A compact white callus proliferated from the scutella. When this tissue was transfered to equivalent medium containing 2% (w/v) sucrose, a green organogenic tissue was selected from the compact callus. After over 2 years in culture, this tissue was still capable of regenerating plants. A sector of this tissue spontaneously became embryogenic. Embryogenic callus can be distinguished by its friability, rapid growth rate and its mucilaginous texture. Embryogenic callus and suspensions have been maintained for approx. 2 years. Plants from organogenic tissue and embryogenic callus have been grown to maturity.