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Published in Crop Sci. 44:2027-2031 (2004).
© 2004 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

CROP BREEDING, GENETICS & CYTOLOGY

Agronomic and Seed Characteristics of Soybean with Reduced Phytate and Palmitate

Brent S. Hulke, Walter R. Fehr* and Grace A. Welke

Dep. of Agronomy, Iowa State Univ., Ames, IA 50011-1010

* Corresponding author (wfehr{at}iastate.edu)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Nonruminant animals fed soybean [Glycine max (L.) Merr.] meal cannot metabolize the P that is in the form of phytate. A soybean line CX1834-1-6 was developed that had a major reduction in phytate P and a concomitant increase in the inorganic P that is available to nonruminants. The objective of this study was to determine the impact of low phytate (LP) on agronomic and seed traits of lines with reduced palmitate in the seed oil. A population of soybean was developed by crossing CX1834-1-6 to a reduced-palmitate line B01769B019 and backcrossing the F1 plants to B01769B019. Twenty BC1F2–derived lines with LP and reduced palmitate and 20 lines with normal phytate (NP) and reduced palmitate from the population were evaluated at three Iowa environments in 2003. The LP lines had a mean seedling emergence that was 22.3 percentage units less than the NP lines. Although the plant density of the LP lines was less than the NP lines, the mean yield of the two types was not significantly different. The palmitate and stearate content of the LP lines was significantly greater than that of the reduced-palmitate parent. The reduced emergence and greater saturate content of the LP lines may make it difficult to develop acceptable cultivars for production of LP soybean meal and low-saturate oil.

Abbreviations: LP, low phytate • NP, normal phytate


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
SOYBEAN SEED OF CONVENTIONAL CULTIVARS has {approx}75% of its total P as phytate [myo-inositol 1,2,3,4,5,6-hexakisphosphate] P and {approx}25% as inorganic P (Raboy et al., 1984). Phytate may be involved in the storage of energy and initiation of dormancy in seeds (Reddy et al., 1982). A soybean mutant M153-1-4-6-14 with {approx}25% phytate P and {approx}75% inorganic P was developed by chemical mutagenesis (Wilcox et al., 2000). The mutant line was crossed to ‘Athow’ and the low-phytate (LP) line CX1834-1-6 was selected from the population (J.R. Wilcox, 2001, personal communication). The LP trait in the line was controlled by pha1 and pha2, which are recessive alleles at two independent loci that exhibit duplicate dominant epistasis (Oltmans et al., 2004a).

Soybean meal is a common feed source for nonruminant animals that lack the phytase enzyme necessary to break down phytate. Producers of nonruminant livestock add phytase to rations to improve digestion of phytate P and supplement the rations with inorganic P to provide an adequate amount of P for animal health (Cromwell et al., 1993). Undigested phytate P is excreted in the feces, which can cause water contamination (Cromwell et al., 1993; Leske and Coon, 1999; Parry, 1998). Swine and chickens fed soybean meal derived from LP soybeans with the mips allele for reduced phytate and raffinose saccharides absorbed significantly more P than those fed conventional soybean meal (Cromwell et al., 2000; Spencer et al., 2000). Decreasing the amount of phytate P in the feed would reduce dependence on phytase and inorganic P supplements, while limiting the amount of P that could become a pollutant.

It may be desirable commercially to grow a cultivar that can be processed into LP soybean meal and modified soybean oil of greater value than that obtained from conventional cultivars. A premium is paid for soybean oil with reduced palmitate that meets the labeling requirements in the USA for a low-saturate oil. For commercial production of a soybean low in phytate and saturates, cultivars with acceptable agronomic and seed traits would be required. The objective of this study was to determine the influence of LP on agronomic and seed traits of lines with reduced palmitate in the seed oil.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
The cross CX1834-1-6 x B01769B019 was made during July 2001 at the Agronomy and Agricultural Engineering Research Center near Ames, IA. CX1834-1-6 was a LP line of Maturity Group II with normal palmitate content that was selected by the USDA-ARS as a F3–derived line from the cross Athow x M153-1-4-6-14 (J.R. Wilcox, 2001, personal communication). B01769B019 was a NP line of Maturity Group II developed by Pioneer Hi-Bred International, Inc., Johnston, IA, that had reduced palmitate. The reduced-palmitate trait of B01769B019 was derived from A18, which was developed by Iowa State University. A18 had the fap1 and fap3 alleles for reduced palmitate (Fehr et al., 1991; Schnebly et al., 1994).

The F1 seeds and seeds of B01769B019 were planted at the Iowa State University–University of Puerto Rico soybean breeding nursery at Isabela, Puerto Rico, during October 2001. Artificial lighting was used to extend the daylength to obtain flowers suitable for crossing. Marker analysis of DNA was used to confirm that the four F1 plants were hybrids. Each F1 plant was used as the male for the backcross to B01769B019, and 36 BC1F1 seeds were obtained.

The BC1F1 seeds were planted in Puerto Rico during February 2002 under artificial lights to promote seed production. The BC1F1 plants had one of four genotypes for phytate, Pha1 Pha1 Pha2 Pha2, Pha1 Pha1 Pha2 pha2, Pha1 pha1 Pha2 Pha2, or Pha1 pha1 Pha2 pha2. The 35 BC1F1 plants that reached maturity were threshed individually and kept separate as families.

The BC1F2 seeds of each BC1F1 family were planted during June 2002 at Ames in four-row plots with 0.69 m between rows within a plot and 0.86 m between adjacent plots. The seeding rate was 10 seeds m–1. The number of seeds planted for each family ranged from 47 to 419.

The genotype of the BC1F1 plants had to be determined by testing BC1F3 seeds for phytate content because the amount of BC1F2 seed was limited. The expected frequency of BC1F3 seeds with the genotype pha1 pha1 pha2 pha2 from BC1F1 plants with the genotype Pha1 pha1 Pha2 pha2 was 9/64. A total of 23 individual BC1F3 seeds from each family were analyzed to find at least one LP seed with 95% probability (Sedcole, 1977). To obtain seed for analysis, one pod from each of 23 mature plants from each family was harvested and one seed from each pod was analyzed for phytate content. There were five BC1F1 families with at least one LP seed in the test. Each BC1F2 plant from the five families was harvested and threshed individually.

Phytate content was evaluated by a modification of the technique described by Wilcox et al. (2000). Each seed was placed into an envelope and crushed by striking it with a small steel weight. The pieces were placed into a 12-by-75-mm glass tube. Into each tube was added 1 mL aqueous 12.5% (v/w) trichloroacetic acid and 25 mM MgCl2, followed by the addition of 1 mL of 1 volume aqueous 3 M H2SO4, 1 volume aqueous 0.02 M ammonium molybdate, 1 volume aqueous 10% (v/v) ascorbic acid, and 2 volumes double-distilled H2O at room temperature. The solution was a dark-blue color after 15 to 20 min if the seed had LP content and a colorless to light-blue color if the seed had NP content.

To identify BC1F2 plants that would produce homogeneous LP or NP progeny, one BC1F3 seed from each plant was tested for phytate content. If the seed had reduced phytate, three more individual seeds were tested for phytate content. If the three seeds had reduced phytate, there was a 99% probability that the plant had the genotype pha1 pha1 pha2 pha2 (Sedcole, 1977). BC1F2 plants that would produce homogeneous NP progenies had one of five genotypes: Pha1 Pha1 Pha2 Pha2, Pha1 Pha1 Pha2 pha2, Pha1 Pha1 pha2 pha2, Pha1 pha1 Pha2 Pha2, or pha1 pha1 Pha2 Pha2. To eliminate plants heterozygous at both loci that would segregate for phytate content, 47 BC1F3 seeds had to be analyzed to be 95% certain of finding an LP seed (Sedcole, 1977). It was not possible to analyze 47 seeds from each plant. Instead, plants heterozygous at one locus were eliminated with a 95% probability by analyzing 11 individual seeds and discarding those with at least one LP seed (Sedcole, 1977).

The LP and NP plants that were selected based on the phytate assay were evaluated for fatty ester content. A five-seed bulk sample was analyzed by gas chromatography using the method described by Hammond (1991). Plants with ≤45 g kg–1 palmitate were selected for increase.

The BC1F2:3 seeds of 26 LP lines and 60 NP lines, the parents, and eight check cultivars and lines were planted during December 2002 at the Illinois Crop Improvement nursery near Ponce, PR. Each plot was a single row 7.62 m in length. There were two plots spaced 0.51 m apart on an irrigation bed and 1.32 m between rows of adjacent beds. The seeding rate ranged from 3 to 8 seeds m–1, depending on the amount of seed available for each line. Each plot was harvested in bulk with a stationary thresher.

After harvest, 23 individual BC1F2:4 seeds of the NP lines were tested to be 95% certain that all lines heterogeneous for phytate content were identified and discarded (Sedcole, 1977). The homogeneous LP and NP lines were analyzed for fatty ester content with five individual seeds. Lines that had a mean palmitate content < 50 g kg–1 and no noticeable segregation for palmitate were identified. On the basis of the phytate and fatty ester analyses, 20 LP lines with reduced palmitate and 20 NP lines with reduced palmitate were chosen for planting in 2003.

The 20 LP lines, 20 NP lines, two parent lines, and eight check cultivars and lines were grown during 2003 in two replications of a randomized complete-block design at the Ames, Carlisle, and Rippey environments in Iowa. The soil at Ames and Rippey is a Nicollet loam (fine-loamy, mixed, superactive, mesic Aquic Hapludoll), and at Carlisle is a Tama silty clay loam soil (fine-silty, mixed, superactive, mesic Typic Agriudoll). The plots were two rows spaced 0.68 m apart within a plot and 0.91 m between adjacent plots. The seeding rate was 30 seeds m–1. The Ames environment was planted on 19 May, Carlisle on 23 May, and Rippey on 27 May.

Each plot was evaluated for seedling emergence, plant density, yield, maturity, lodging, height, seed weight, protein content, oil content, and fatty ester content. Emergence percentage and plant density were determined by counting the number of plants in a plot at the V2 to V4 stage (Fehr and Caviness, 1977). Emergence percentage was calculated by dividing the number of plants by the 160 seeds planted and multiplying by 100. Plant density in plants per square meter was calculated by dividing the number of plants in a plot by the plot area (5.187 m2). Maturity was measured as days after 31 August when at least 95% of pods had reached their mature color. Lodging was visually scored on a scale of 1 (all plants erect) to 5 (all plants prostrate). Plant height was measured in centimeters from the soil surface to the highest node on the main stem at maturity. Plots were harvested with a self-propelled combine, and the seed moisture and weight were measured to determine seed yield on a 13%-moisture basis. Seed weight in miligrams per seed was measured by weighing 400 random seeds and dividing by 400. Protein and oil content were determined with an Infratech 1221 near-infrared whole grain analyzer (Tecator AB, Hooganas, Sweden) and adjusted to a 13%-moisture basis. Fatty ester content was determined by gas chromatography.

The data were analyzed with the general linear model procedure (PROC GLM) of the SAS statistical software package release 8.02 (SAS Institute, 2001). The main effects of environments and replications were considered random, and phytate types and lines within phytate types were considered fixed. Sums of squares for lines within phytate types were partitioned into lines within the LP type and lines within the NP type. F tests were used to determine the significance of each main effect or interaction effect. Correlation coefficients between the agronomic and seed traits were calculated based on line means across environments with the correlation procedure (PROC CORR) of the SAS statistical software package release 8.02 (SAS Institute, 2001).


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
The mean seedling emergence of the LP lines was significantly lower than the NP lines in the three individual environments (Table 1). The LP lines had 36.1 percentage units lower emergence than the NP lines at Ames, 14.4 percentage units lower emergence at Carlisle, and 16.4 percentage units lower emergence at Rippey. The variation in the differences between the means of the two types among environments resulted in a significant (P < 0.01) environment x phytate type interaction. Combined across environments, the mean seedling emergence of the LP lines was 22.3 percentage units less than the NP lines, which was only significant at the 0.10 probability level due to the large environment x phytate type interaction. Oltmans et al. (2004b) found that the mean seedling emergence for LP lines from single crosses of CX1834-1-6 to three NP lines with normal palmitate ranged from 19 to 27 percentage units less than NP lines from the same populations. The similar results of the two studies indicated that the reduced emergence of our LP lines was not due to a negative interaction of the reduced-phytate and reduced-palmitate traits. Combining the two traits in a cultivar should not be more detrimental to emergence than the integration of LP into a cultivar with a conventional fatty ester profile in the oil.


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  		Table 1. Mean seedling emergence of low- and normal-phytate soybean lines grown at three Iowa environments in 2003.

 
Reduced emergence associated with the LP trait in soybean was first reported by Meis et al. (2003), who evaluated lines with the mips allele that reduced both phytate and raffinose saccharide content. They found that seed source significantly affected the emergence of the mips lines. Emergence of mips lines was 58 to 85 percentage units less than Mips lines for the subtropical seed sources and 8 to 24 percentage units less for temperate seed sources in that study. The reduction in emergence of our LP lines, the LP lines of Oltmans et al. (2004b), and the mips lines indicated that the reduction in phytate has an adverse effect on seed viability. Unlike the mips lines, the emergence of LP lines with the pha1 and pha2 alleles did not seem to be adversely influenced by production of seed in a subtropical environment. The difference in emergence between our LP and NP lines grown from seed produced in Puerto Rico was similar to the difference between the LP and NP lines grown from seed produced in Iowa by Oltmans et al. (2004b). The lines of both studies were grown in the same environments in 2003.

The average plant density of the LP lines was 6.9 plants m–2 lower than the NP lines, which was significant at the 0.10 probability level (Table 2). The correlation coefficient between plant density and yield of 0.53 was significant. Despite the lower plant density of the LP lines, there was no significant difference in the mean yield of the two types, a LP line had the highest mean yield, and nine of the 20 LP lines were not significantly different (P < 0.05) in yield than the recurrent parent, B01769B019. Soybean plants have the ability to compensate for reduced plant density by branching. Egli (1988) evaluated the yield of a soybean cultivar at plant densities ranging from 0.6 to 24 plants m–2. He found that yield was maximized at 17.5 plants m–2 in the first year of the study and 7.3 plants m–2 in the second year. Any increase in plants m–2 beyond those levels did not result in an increase in yield in his study. In our study, the mean plant density of 20.0 plants m–2 for the LP lines apparently was sufficient to maximize yield. The results indicated that the LP trait per se did not have a significant negative impact on seed yield.


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  		Table 2. Agronomic and seed traits of low- and normal-phytate soybean lines averaged across three Iowa environments in 2003.

 
The LP and NP lines were not significantly different for mean maturity, lodging, height, protein, or oil (Table 2). Seed weight was significantly higher in the LP lines than the NP lines, but the difference was only 6 mg seed–1. It should be possible to develop LP lines that are similar to NP cultivars for the six traits.

The mean palmitate, stearate, and palmitate + stearate contents of the LP lines were not significantly different than the NP lines, and there was some overlap in the distributions of the two types of lines (Table 3). Although the similarity between the two types suggested that LP did not influence the content of the saturated fatty esters, all of the LP lines were significantly greater than the recurrent parent B01769B019 for their palmitate + stearate content. In contrast, six of the 20 NP lines were not significantly different from the recurrent parent for palmitate + stearate content. None of the LP lines likely would have been acceptable for commercial use as low-saturate cultivars. The U.S. Food and Drug Administration specifies that an oil labeled as low in saturated fat must have 1 g or less of total saturated fatty acids in a 14-g serving, which is equivalent to 71.4 g kg–1 of saturated fat (U.S. Food and Drug Administration, 1999). When saturated fatty acids total 1.25 g or less per 14-g serving (89 g kg–1 saturated fat), the number of grams can be rounded down to 1 g and the oil can be labeled as low in saturated fat. Although the two primary saturated fatty acids in soybean oil are palmitic and stearic, the content of other minor saturated fatty acids must be included in the total saturated fat content, including myristic (14:0), arachidic (20:0), behenic (22:0), and lignoceric (24:0) (Hammond et al., 2004; White, 2000). The LP lines with the lowest palmitate + stearate content would have total saturates close to the limit of 89 g kg–1 when averaged across environments. Oil from a cultivar with such a high content of total saturates could not be labeled as low in saturated fat if further increases in the saturated fatty acids occurred due to adverse environmental conditions or accidental mixing with conventional soybeans during commercial production.


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  		Table 3. Fatty ester content of low- and normal-phytate soybean lines averaged across three Iowa environments in 2003.

 
The elevated palmitate + stearate content of the LP lines was associated with greater-than-normal content of the two fatty esters in the donor parent CX1834-1-6 (Table 3). Both the palmitate and stearate content of the donor were significantly greater than that of IA2052, the conventional soybean cultivar with the greatest palmitate + stearate content of any of the six conventional lines and cultivars included in the experiment as checks. The elevated palmitate + stearate content of the LP lines indicated that there may be one or more modifiers for elevated saturated fat content in the donor that may be linked to one or both of the pha alleles or that one or both of the pha alleles may have a pleiotropic effect on the content of the two fatty esters. Additional research will be needed to determine if the association between LP and elevated palmitate + stearate can be broken.

The lack of significant differences between of the LP and NP lines for palmitate and stearate could occur, even though there was linkage or pleiotropic effects associated with the pha alleles. The homogeneous NP lines could contain one or two pha alleles, except for those derived from the genotype Pha1 Pha1 Pha2 Pha2. An average of 5/6 of the NP lines would be expected to have at least one of the pha alleles. The NP lines with a pha allele could be influenced by linked genes for elevated palmitate and stearate or by a pleiotropic effect of pha1 or pha2.

The mean oleate content of the LP lines was significantly greater and the linoleate and linolenate content were significantly lower than that of the NP lines (Table 3). The distributions for the two types of lines overlapped and the content of the recurrent parent for the three unsaturated fatty esters was within the range of the LP lines. It should be possible to develop low-saturate LP cultivars with oleate, linoleate, and linolenate contents similar to that of existing low-saturate NP cultivars.

The decreased emergence and increased saturate content of the LP lines could make it difficult to develop acceptable low-saturate LP cultivars. Additional breeding will be needed to determine if the negative associations of LP with emergence and saturate content can be overcome.


   ACKNOWLEDGMENTS
 
The authors gratefully acknowledge the assistance of Dan N. Duvick for the phytate and fatty ester analyses; Silvia R. Cianzio for crosses made in Puerto Rico; and Susan L. Johnson and Kevin O. Scholbrock for the collection of field data.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
This journal paper of the Iowa Agric. and Home Economics Exp. Stn., Ames, IA, Project No. 3732 was supported by the Hatch Act, State of Iowa, Iowa Soybean Promotion Board, Raymond F. Baker Center for Plant Breeding, and the United Soybean Board.

Received for publication January 7, 2004.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 




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