Genetic Mapping of Loci Associated with Seed Phytic Acid Content in CX1834-1-2 Soybean

Soybean [Glycine max (L.) Merr.] seed phosphorus is stored primarily as phytic acid, a form in which it is unavailable to monogastric mammals and birds. Because of the nutritional and environmental problems caused by phytic acid, development of cultivars with low phytic acid (lpa) mutations has become an important objective in many soybean breeding programs. Information about the inheritance of the low phytate trait would facilitate these efforts. The objectives of the current research were (i) to map low phytate loci in populations derived from the lpa mutant line CX1834-1-2, (ii) to identify closely linked molecular markers, and (iii) to characterize inheritance of the trait. We identified two loci associated with the low phytate phenotype of CX1834-1-2 and discovered an epistatic interaction between the loci. A locus on linkage group (LG) N near Satt237 accounted for 41% of the observed variation in seed inorganic phosphorous (Pi) levels, which are inversely correlated with phytate levels in plants carrying the lpamutation. Another locus near Satt527 on LG L explained 11% of the variation in seed Pi levels, and an interaction between the LG L and N loci accounted for an additional 8 to 11%. The loci on LG N and LG L are probably the previously designated pha1 and pha2 loci. D low phytate cultivars has become an important objective in soybean breeding programs. Phytate, a mixed cation salt of phytic acid (myo-inositol 1,2,3,4,5,6-hexakisphosphate), is the form in which 67 to 77% of the phosphorus (P) in soybean seed is stored (Raboy et al., 1984). Phytate P is nutritionally unavailable to monogastric animals such as poultry, swine, and humans (Erdman, 1979; Larson, 1998). In addition, phytic acid chelates cations such as potassium, magnesium, iron, zinc, and calcium, thereby reducing their nutritional availability (Erdman, 1981; McCance and Widdowson, 1935). Diet rations fed to poultry and swine are typically supplemented either with phytase, which catalyzes the stepwise removal of phosphate from phytate (Lei and Porres, 2003), or with inorganic phosphate (Pi) to increase P availability (Adeola et al., 1995). Eutrification of surface water can result from runoff and leaching of excessive P in soils to which livestock or poultry manure has been applied (Ertl et al., 1998). Excretion of phytate P by nonruminant animals is therefore potentially detrimental to the environment, and supplementation of soy meal with Pi can exacerbate this problem. Low phytic acid (lpa) mutations induced by treatment of seeds with ethyl methanesulfonate (EMS) have been used to lower phytate levels in soybean (Wilcox et al., 2000; Hitz et al., 2002), barley (Hordeum vulgare L.; Larson et al., 1998; Rasmussen and Hatzack, 1998), maize (Zea mays L.; Raboy and Gerbasi, 1996; Raboy et al., 2000), rice (Oryza sativa L.; Larson et al., 2000), and wheat (Triticum aestivum L.; Guttieri et al., 2004). Wilcox et al. (2000) developed lpa mutants of the soybean breeding line CX1515-4 by treating seeds with EMS and then testing M3 seeds for elevated levels of Pi. Two M2 plants, M153 and M766, were identified as having lpa mutations resulting in phenotypes in which the increase in Pi was associated with an equivalent decrease in phytic acid, as had been observed in the lpa1-1mutants of maize and barley (Larson et al., 1998). In the maize lpa1-1 mutant, a 60% reduction in phytic acid was accompanied by a molar-equivalent increase in Pi (Ertl et al., 1998; Raboy and Gerbasi, 1996). These single-gene mutations are recessive, and translocation of gene products or metabolites in heterozygous (Lpa/lpa) barley plants does not complement the loss of gene function in their homozygous mutant (lpa/lpa) seeds (Larson et al., 1998). In the seeds of M6 progenies descended from the mutant soybean line M153-1-4, Pi accounted for 60 to 70% of the sum total of phytate P and inorganic P, as compared with only 15% in the cultivar Athow (Wilcox et al., 2000). Overall, approximately 75% of seed total P in M153-1-4 should be available to monogastric animals, whereas only about 25% of the total P in seeds from soybean with normal phytate levels would be available. At the Univ. of Georgia, we began in 2001 to investigate the inheritance of the low phytate trait in soybean with the assumption, based on data from Wilcox et al. (2000), that a single locus with a mutated allele was responsible for the low phytate phenotype. The lpa mutations in maize, barley, and rice had each been mapped to a single locus (Larson et al., 1998, 2000; Raboy et al., 2000). These results and the lowmutation rates expected from EMS mutagenesis suggested that the low phytate phenotypes in the mutant soybean lines of Wilcox et al. (2000) were also the result of a mutation in a single gene. We therefore expected to be able to map this locus using small F2:3 populations from Athow 3 M153-1-4-6-15-3 (37 F2 individuals) and Savoy 3 M153-1-4-6-29-2 (40 F2 individuals), and a BC1F2 population of 94 individuals D.R. Walker and H.R. Boerma, Dep. of Crop and Soil Sciences/ Center for Applied Genetic Technologies, 111 Riverbend Road, Univ. of Georgia, Athens, GA 30602-6810; A. Scaboo and V.R. Pantalone, Dep. of Plant Sciences, 2431 Joe Johnson Drive, Univ. of Tennessee, Knoxville, TN 37996-4561; J.R. Wilcox, USDA-ARS, Crop Production and Pathology Research and Dep. of Agronomy, Purdue Univ., West Lafayette, IN. Received 21 Mar. 2005. *Corresponding author