Crop Science 42:355-359 (2002)
© 2002 Crop Science Society of America
CROP BREEDING, GENETICS & CYTOLOGY
Starch Particle Volume in Single- and Double-Mutant Maize Endosperm Genotypes Involving the Soft Starch (h) Gene
O. A. Gutiérreza,
M. R. Campbellb and
D. V. Glover*,c
a USDA-ARS, Crop Science Res. Lab., Genetics and Precision Agriculture Research Unit, Mississippi State, MS 39762
b Truman State University, Division of Science, 162 Barnett Hall, Kirksville, MO 63501
c Dep. of Agronomy, 1150 Lilly Hall of Life Sciences, Purdue University, West Lafayette, IN 47907-1150
* Corresponding author (dglover{at}purdue.edu)
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ABSTRACT
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Larger starch particle volume of maize (Zea mays L.) endosperms is beneficial for many aspects of starch processing, and may improve the performance of some starch attributes. The recessive soft starch (h) locus increases starch granule size compared with normal (+/+) genotypes. The objective of this study was to compare starch particle volume of normal maize inbreds, and their near-isoline conversions to single- and corresponding double-mutant endosperm starch genotypes, including the h locus. Eighteen public inbreds and their near-isogenic conversions to amylose-extender (ae/ae), dull (du/du), waxy (wx/wx), sugary-2 (su2/su2), h/h, ae/ae h/h, du/du h/h, wx/wx h/h, and su2/su2 h/h, were planted in replicated trials in two environments in 1995 and one in 1996. Starch was extracted from mature kernels collected from each plot and examined for starch particle volume with a laser-based, particle size analyzer. Starch particle volume was greater in all near-isogenic line conversions containing the h locus than in their respective normal inbred. Starch particle volume for all double-mutant h/h combination genotypes was greater across all inbreds than in their respective single-mutant starch counterparts. These results demonstrate that the h allele is epistatic to the ae, du, wx and su2 alleles for increasing starch particle volume. Starch particle volume for single-mutant genotypes averaged across inbreds and environments ranked from the smallest to the largest as follows: du/du, ae/ae, su2/su2, normal (+/+), wx/wx, and h/h. The ranking among the double-mutant genotypes from smallest to largest starch particle volume was: du/du h/h, su2/su2 h/h, ae/ae h/h, and wx/wx h/h. The epistatic nature of the h allele further enhances its usefulness for increasing starch particle volume in combination with other endosperm starch mutants utilized to develop specialty starches with value-added traits. This study also demonstrated that the genetic variation present among maize inbreds could be useful in breeding for increased starch particle volume.
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INTRODUCTION
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STARCH PARTICLE VOLUME in maize is an important property for food and industrial applications of starches, because it may influence the performance characteristics during modification of granular starch products and, consequently, the functional properties of the starch in addition to wet-milling properties of the grain. It is believed that by increasing starch particle size, improvements can be made in starch yield during processing. According to the Iowa Grain Quality Initiative Traits Task Team, improving the wet-milling efficiency of maize leading to increased starch yield is a high priority due to the relatively high gross added value which was estimated to be $280 million per year or 0.01568 Mg ha-1 (Johnson et al., 1999). Examples of applications where starch granule size is important include the manufacturing of degradable plastic films (Lim et al., 1992), carbonless copy paper (Nachtergade and Nuffer, 1989), dusting powder, baking powder, laundry-stiffening agents, and utilization of small granules as a fat substitute (Daniel and Whistler, 1990; Jane et al., 1992).
In maize, many endosperm starch mutants have been identified and their effects on the enzymes that regulate carbohydrate metabolism and starch synthesis have been described (Shannon and Garwood, 1984). Geneticists have developed double- and triple-mutant combinations of these endosperm mutants with starch properties that are similar to chemically modified starches. There are starch use patents held on several of these starch mutant genotypes (Wurzburg and Fergason, 1984; Friedman et al., 1988a–g).
The soft starch (h) gene, originally reported by Mumm (1929), causes a loose packing of starch granules in the endosperm, but it has not been associated with any major changes in storage proteins, starch composition, or starch granule structure (Glover, 1988; Shannon and Garwood, 1984). Penetrance and expressivity of the h locus is variable and can create difficulty in phenotypic classification of segregating material (Wilson et al., 2000b). Previous studies have shown that the h gene produces larger starch granules than the normal genotype. Brown et al. (1971) evaluated 24 different mutant endosperm genotypes, as well as non-mutant genotypes, including normal, at 24 d postpollination in hybrid backgrounds related to W64A x W23 and found that normal starch granule size ranged from 7.99 to 8.53 µm in diameter. Starch granules from endosperms homozygous for the h gene had the largest diameter (8.96 to 9.45 µm), whereas the double mutant sugary-1 sugary 2 (su1/su1 su2/su2) had the smallest diameter (2.56 to 2.98 µm). Wilson et al. (2000b) detected no dosage effect at the h locus for starch particle volume in mature kernels. The starch particle volume of the recessive endosperm genotype hh/h (16.073 µm3) was significantly different from the normal genotypes ++/+, ++/h, and hh/+ (14.452 µm3). Wang et al. (1993a) studied the native starch granule structure and morphology of starches from 17 endosperm mutant genotypes of the inbred Oh43 with scanning electron microscopy (SEM). They observed that the h mutant had the largest average starch granule diameter (13.8 µm) compared with the normal (11.6 µm), amylose–extender (ae) [7.0 µm], dull (du) [7.8 µm], and waxy (wx) [10.3 µm] genotypes. Katz et al. (1993) described the granules of starch from maize containing the h gene as significantly larger than normal starch (mode 19.2 µm) with similar amylose: amylopectin ratios, gelatinization peak temperature (Tp) and enthalpy (
H) values to normal genotypes. Wang et al. (1993b) observed that the h gene possesses similar starch physicochemical properties including amylose content, pasting patterns, swelling, gel firmness and stickiness values when compared with normal genotypes. Campbell et al. (1996) reported the genetic variation for starch particle volume among 35 tropical and semitropical maize populations. Starch particle volume means ranged from 16.2 to 18.2 µm3. They also observed several positive correlations between starch particle volume values and starch thermal properties (To, Tp, and Tc). Other studies have shown that starch thermal properties may reflect differences in starch granule size (Knutson et al., 1982; Okechukwu and Rao, 1995). In a diallel and generation mean analysis study, Wilson et al. (2000a) reported that the h locus had a pronounced effect in increasing the starch particle volume of granules, and the predominance of additive genetic effects and some dominance effects for other loci were responsible for the increase in starch particle volume in both normal starch and h starch backgrounds. Their results indicated the usefulness of the soft starch gene as well as the additional genetic variation among inbreds in the improvement of starch particle volume for increased starch recovery in wet milling.
The purpose of this study was to compare starch particle volume of normal maize inbreds, their near-isoline conversions to single-mutant starch genotypes and corresponding double-mutant combination genotypes including the h locus. In addition, variation among inbreds within specific endosperm mutant genotypes was examined.
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MATERIALS AND METHODS
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Genetic Material and Locations
Eighteen public maize inbreds and their near-isogenic conversions to ae/ae, du/du, wx/wx, su2/su2, h/h, ae/ae h/h, du/du h/h, wx/wx h/h, and su2/su2 h/h were selected for this study. The inbreds (Table 1) represent a sample of heterotic groups that are presently used in the U.S. seed corn industry (Gerdes et al., 1993; Troyer, 1999). The experiment was replicated at three environments: Purdue University Agronomy Research Center, West Lafayette, IN, during the summers of 1995 and 1996 on a fine-silty, mixed, mesic Typic Haplaquoll soil, and in 1995–1996 in Nuevo Vallarta, Nayarit, Mexico, under short day conditions (winter nursery) on sandy clay loam soil. The experimental design was a split-plot design with four replicates. The 18 inbreds were the main plots and the 10 genotypes [+/+, ae/ae, du/du, wx/wx, su2/su2, h/h, ae/ae h/h, du/du h/h, wx/wx h/h, and su2/su2 h/h] were the subplots. Plots were one row 5.2 m long in Indiana and 4.9 m long in Mexico with 0.76 m row spacing at both locations. In all locations, recommended production practices were used. All plants in each row were self-pollinated and ears were individually harvested at maturity, dried at 37°C, and stored at room temperature. Composite samples of eight kernels from four selected ears, two kernels per ear, were drawn from each plot for starch extraction and particle volume analysis.
Starch Isolation and Purification
Starch was extracted from composite kernel samples according to the method described by Schoch (1957) and Boyer et al. (1976) with some modifications. Whole kernel samples were placed in a test tube containing a 0.45% Na2S2O5 solution to a level of 6 cm over the kernels and steeped in a 50°C water bath for 24 h. After this period, germ and pericarp were removed. The endosperms were macerated with a mortar and pestle and placed in a Warning blender. Ten mL of 0.05 M NaCl solution were added and the sample was homogenized for 1 min with a brief pause after 30 s to assure a fully ground sample. The resulting slurry was filtered through a 45-µm sieve to remove large debris and 15 mL more of 0.05 M NaCl solution was used to finish the washing process. The collected solid was placed in a 250 mL beaker and allowed to stand for at least 1 h, after which the supernatant was poured off and the precipitated starch was transferred in a 15-mL polypropylene screw cap centrifuge tube. The extracted samples were purified five times with a 5:1 (v/v) 0.05 M NaCl-toluene ration, mixed with a vortex mixer, 15-min shaking time, and 5-min centrifugation at 1900 x g. Purified samples were then washed with 10 mL distilled water, and once with 10 mL acetone. With the last two washes, the samples were once again mixed, shaken for 15 min, and centrifuged for 5 min. The resulting starch pellets were covered and left to air-dry under a laboratory hood for 24 h at room temperature. The pellets were then powdered, placed in labeled vials, and left to air-dry an additional 72 h before the vials were covered.
Starch Particle Volume Analysis
Determination of particle volume or size analysis was done on each starch sample with a Galai WCIS-100 particle size analyzer (Galai WCIS 100, Galai Production Ltd., Israel). This instrument has the capability of measuring particle size in three ways: diameter, area, or volume. We chose volume to represent particle size. The distribution of starch granule diameter was represented on a volume density basis, i.e., the projected volume of starch particles found at a given diameter. Starch granule diameter mean was based on volume moment (Allen, 1981). Starch samples were prepared for particle volume analysis according to Wilson (2000b). Approximately 30–40 mg of starch were suspended in 75 mL of deionized water. In addition, 10 drops of an iodine stock solution (20.7 g of KI/L and 4.14 of I2/L) diluted to 4% were added to define the surfaces of the individual starch granules. The samples were stirred for 10 min by a magnetic stir bar set to a high speed to disrupt starch agglomerates and avoid settling of the granules. Following stirring, samples were allowed to settle for 10 s before an aliquant of the suspension was drawn and placed into a clear plastic cuvette with a small magnetic stir bar. The cuvette was then immediately placed into the particle size analyzer and allowed to stir for 1 min before data acquisition was initiated. All samples underwent a 5 min data acquisition period by a particle acquisition range of 0.5 to 30 µm. A minimum granule count of 500000 was obtained for the majority of the samples. An aqueous suspension containing polymer microspheres of known sizes (20.49 ± 0.20 µm) was used as a standard (Duke Scientific, Palo Alto, CA) and run following every 10 starch samples to monitor the performance of the particle size analyzer.
Statistical Analysis
A combined analysis of variance across environments was performed with the MIXED procedure of SAS (SAS Institute, Cary, NC) to test for significant differences (P < 0.05) among inbreds, genotypes, and inbred x genotype interaction. Inbreds, genotypes, and inbred x genotype were considered as fixed effects and all others factors [environments, replication (environment), environment x inbred, replication x inbred (environment), environment x genotype, environment x inbred x genotype, and residual] as random. Least Significant Differences (LSD) were calculated for pair-wise comparisons among inbreds, genotypes, and genotype by inbred interactions for starch particle volume by using the restricted maximum likelihood (REML) estimator of the corresponding standard error of the mean differences (Hoi et al., 1999). For estimating the significance of random effects, likelihood ratio statistic tests and best linear unbiased predictors (BLUPS) were calculated (Littell et al., 1996).
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RESULTS AND DISCUSSION
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Differences among inbreds, genotypes, and inbreds x genotypes for starch particle volume were highly significant (P < 0.0001) across environments. Among inbreds, B37 had the smallest mean (13.33 µm3) and B73 the largest mean (15.01 µm3) when averaged over genotypes across environments (Table 2). There were differences among the inbreds, when averaged over genotypes across environments, indicating the presence of genetic variation and diversity for starch particle volume. However, there were no significant differences among A634, B73, B84, C103, CM105, Mo17, and W64A for starch particle volume. Starch particle volume means among the normal inbred versions averaged across environments ranged from a high of 14.95 µm3 in B84 +/+ to a low of 12.89 µm3 in B37 +/+. Among the 10 evaluated genotypes averaged over inbreds across environments, du/du and ae/ae had the smallest mean starch particle volume (12.28 and 12.60 µm3, respectively) followed by su2/su2 (13.76 µm3), normal (13.97 µm3), and wx/wx (13.97 µm3) which had identical values. Genotypes du/du h/h (14.85 µm3), su2/su2 h/h (15.00 µm3), and ae/ae h/h (15.45 µm3) were not significantly different whereas wx/wx h/h (16.22 µm3) and h/h (16.28 µm3) had the largest mean starch particle volume. These results corroborate those of Brown et al. (1971), Wang et al. (1993a), and Wilson et al. (2000a)( 2000b) where it was reported that the h gene increased the starch particle size. All double mutant genotypes containing the h gene, as well as the h genotype, showed an increase in starch particle volume. The increase in starch particle volume among the double mutant genotypes du/du h/h, su2/su2 h/h, and ae/ae h/h was less than observed in the wx/wx h/h genotype whereas wx/wx h/h was not significantly different from the h genotype. Differences in the inbred x genotype interaction across environments indicated that the starch particle volume values among maize inbreds within genotypes were not the same. Normal versions of inbreds B84 (14.95 µm3), B73 (14.78 µm3), A634 (14.55 µm3), and Mo17 (14.49 µm3) exhibited the largest values, whereas B37 (12.89 µm3), B90 (13.38 µm3), B89 (13.44 µm3), H111 (13.44 µm3), and H95 (13.50 µm3) showed the smallest values. These results indicate the presence of genetic variation among normal genotypes and support the observations by Wilson et al. (2000a). The normal version of maize inbreds with larger mean starch particle volumes, in general, did not maintain the same propensity for increased values in the respective ae/ae, du/du, wx/wx, su2/su2, and h/h genotype conversions. Inbreds B37 and B73 seemed to be more consistent in manifesting either the smaller or larger mean values, respectively, across all single gene conversions. These results showed the influence of inbred background on starch particle volume and indicated the presence of modifying genes on this trait. For example, the CM105 normal counterpart had an intermediate value (13.98 µm3) but the CM105 ae/ae converted isoline had the smallest value (11.73 µm3) and the CM105 h/h near-isogenic version had the largest starch particle volume (17.78 µm3) in this study. Inbred B84 also had the largest mean starch particle volume (14.95 µm3) in the normal counterpart, but the B84 du/du mean (11.96 µm3) was among the smallest for this genotype among inbred conversions. Furthermore, the B84 h/h conversion was among the largest (17.19 µm3) for this genotype among inbreds.
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Table 2. Mean starch particle volume of 18 maize inbreds, 10 genotypes per inbred, and inbred by genotype interaction averaged across three environments in 1995 and 1996.
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Among the double-mutant combinations with the h/h starch genotypes, inbred B37 had the smallest starch particle volume across inbreds, except for the case of B90 ae/ae h/h. Starch particle volume in all double-mutant endosperm h/h starch genotypes was increased across all inbreds compared with their respective single-mutant starch genotype counterpart. It is apparent that the h allele is epistatic to the ae, du, wx, and su2 endosperm starch alleles for increasing starch particle volume.
Variance component estimates (Table 3) due to environments were large (P < 0.001) and indicated differences across environments. BLUPS for environments were 13.945, 14.379, and 14.993 µm3 indicating that there was an increase in starch particle volume from environment one (West Lafayette, 1995) to environment three (West Lafayette, 1996). Environment x inbred variance components also indicated differences among inbreds averaged across genotypes (P < 0.001) for the three environments. BLUPS for inbred by environment interaction (Table 4) also showed an increase in particle volume from environment one to environment three. Estimates of variance components for the environment x genotype interaction were substantial and showed statistically significant evidence of differences among genotypes averaged across inbreds (P < 0.001) at each environment. In this case, BLUPS for genotype x environment interaction showed an increase in magnitude from environment one to environment three (Table 5). Finally, the variance component for environment by inbred by genotype interaction was significant (P < 0.001), indicating that the starch particle volume among maize inbreds within genotypes was different in each of the environments.
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Table 3. Variance component estimates and their standard errors for starch particle volume from 18 maize inbreds, 10 genotypes per inbred, and inbred by genotype interaction averaged across three environments in 1995 and 1996.
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Table 4. Best linear unbiased predictors (BLUPS) for starch particle volume from 18 maize inbreds across genotypes at three environments.
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Table 5. Best linear unbiased predictors (BLUPS) for starch particle volume from 10 maize endosperm mutant genotypes across inbreds at three environments.
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The results of this study further corroborate the research findings of Wilson (2000a) in regard to the apparent variability in starch particle volume among normal (+/+) maize inbreds as well as among near-isogenic h/h inbred line conversions. Furthermore, we demonstrated that the h allele is epistatic to the ae, du, wx, and su2 endosperm starch alleles for increasing starch particle volume. This finding further enhances the usefulness of the h locus for increasing starch particle volume in combination with other endosperm starch mutants utilized to develop specialty starches with value-added traits.
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ACKNOWLEDGMENTS
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The research was partly supported by Cerestar USA, Inc., Hammond, IN. The senior author would like to thank Julie A. Wilson and Wayne Witlow for their technical assistance as well as Drs. Linda Pollak and Arnel R. Hallauer for their helpful comments. We also thank Dr. J.B. Holland for his valuable assistance with the statistical analysis.
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NOTES
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The research was partially supported by Cerestar U.S.A. Inc., Hammond, IN. Contribution of the Purdue Agricultural Research Programs Journal paper No. 16363.
Received for publication March 12, 2001.
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ABSTRACT
INTRODUCTION
