Location: Soybean and Nitrogen Fixation Research
Title: Inositol metabolism and phytase activity in normal and low phytic acid soybean seed Authors
Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 23, 2010
Publication Date: January 1, 2011
Citation: Israel, D., Taliercio, E.W., Kwanyuen, P., Burton, J.W., Dean, L.L. 2011. Inositol metabolism and phytase activity in normal and low phytic acid soybean seed. Crop Science. 51:282-289. Interpretive Summary: Phytic acid is a phosphorous compound found in soybean seeds. It is a seed storage form of phosphorous which the plant uses for nutrition during germination and early seedling growth. But phytic acid cannot be readily digested by those who consume soybeans for food, both animals and humans. The nondigested phytic acid is excreted and becomes a source of phosphorous pollution in the environment. Several low phytic acid soybeans have been developed. These mutants affect the biosynthesis of phytic acid in different ways. This research was aimed at describing the metabolism of phytic acid in developing seeds of normal and low phytic acid soybeans to help in understanding how the low phytic acid mutants actually work. The study revealed the low phytic acid trait was not a result of increased phytase, an enzyme that breaks down phytic acid. The pentose sugars, also thought to be involved, developed the same in both normal and mutant soybeans throughout the seed filling process. Thus, other biosynthesis factors were causing the low phytic acid mutant.
Technical Abstract: The genetic basis for the low seed phytic acid trait in soybean lines derived from the low phytic acid line (CX1834) of Wilcox et al (2000) is under investigation in several laboratories. Our objective was to measure metabolite levels associated with the phytic acid and raffinosaccharide biosynthetic pathways in seed of normal lines and low seed phytic acid lines derived from CX 1834. Concentrations of inositol, phytic acid, phosphate, partially phosphorylated intermediates of inositol (IP3 IP4, P5), sucrose, raffinose and stachyose were measured in seed of normal and low seed phytic acid lines throughout development. Plants were cultured in growth chambers with 650 to 700 µM.m-2.s-1 of photosynthetically active radiation, and a 26:22 C day:night temperature regime and supplied complete nutrient solution daily. Inositol concentrations were high and similar at 20 days after flowering (DAF) in seed of normal and low phytic acid lines and decreased by more the 95% at maturity. Inositol P-3 concentrations were low in seed of all genotypes and decreased to barely detectable levels at maturity and IP4 and IP5 intermediates were not detected. Stachyose and raffinose concentrations in seed of all genotypes increased rapidly after 30 DAF and reached similar levels at maturity. A 40% decline in sucrose concentration was associated with the increase in raffinosaccharide concentrations. Phytase activity in seeds of normal and low phytic acid lines was similar throughout development. These results rule out defects in genes coding myoinositol-1-P synthase (MIPS) and inositol kinases and differences in expression of phytase activity as the basis for the low seed phytic acid trait in CX1834 derived lines. The metabolite measurements are consistent with genetic evidence that defects in genes coding for a multiple drug resistant-associated protein (MRP) ATP-binding cassette (ABC) transporter may be responsible for the low seed phytic acid trait in CX1834 derived lines.