The role of phosphorus in animal nutrition is well recognized. Eighty percent of the phosphorus in the body of animals is found in the skeleton, providing structure to the animal. Twenty percent of the phosphorus in animals can be found in soft tissues, where it is a constituent compound and therefore involved in a wide series of biochemical reactions. For example, phosphorus is required for the synthesis and activity of DNA, RNA, phospholipids, and some B vitamins.
Though phosphorus is essential for healthy animals, it is also recognized that not all phosphorus in feed is bioavailable. Phytic acid salts (i.e., phytates) are the major storage form of phosphorus in plants. See e.g., "Chemistry and Application of Phytic Acid: an Overview," Phytic Acid: Chemistry and Application; Graf, Ed.; Pilatus Press: Minneapolis, Minn., pp. 1-21; (1986). Phytates are the major form of phosphorus in seeds, typically representing from 50% to 80% of seed total phosphorus.
In corn and soybeans, for example, phytate represents about 60% to 80% of total phosphorus. When seed-based diets are consumed by non-ruminants, the consumed phytic acid forms salts with several nutritionally-important minerals in the intestinal tract. Excretion of these salts reduces the retention and utilization, i.e., bioavailability of the diet's phosphorus and mineral contents. Consequently, this can result in mineral deficiencies in both humans and animals fed the above seed. See e.g., McCance, et al., Biochem. J., 29:4269 (1935); Edman, Cereal Chem., 58:21 (1981).
Phytate, a large source of phosphorus, is not metabolized by monogastric animals. Phytic acid, in fact, is considered to be an anti-nutritional factor because it reduces the bioavailability of proteins and minerals by chelation; see e.g., Cheryan, "Phytic Acid Interactions in Food Systems," CRC Crit. Rev. Food Sci. Nutr., 13:297-335 (1980).
Phytate does not simply cause a reduction in nutrient availability. The phytate-bound phosphorus in animal waste contributes to surface and ground water pollution. See e.g., Jongbloed, et al., Nether. J. Ag. Sci. 38:567 (1990).
Because the phytate content of seed has an impact on diet, phosphorus and mineral retention, and the environment, several approaches have been proposed to reduce this impact. Approaches include removing dietary phytate by post-harvest intervention and reducing seed phytate content genetically.
Post-harvest food processing methods that remove phytic acid either physically or via fermentation, are disclosed for example by Indumadhavi, et al., Int. J. Food Sci. Tech. 27:221 (1992). Hydrolyzing phytic acid is a useful approach to increase the nutritional value of many plant foodstuffs. Phytases, as discussed more fully below, catalyze the conversion of phytic acid to inositol and inorganic phosphate. Phytase-producing microorganisms include bacteria and yeasts. See e.g. Power, et al., J. Bacteriol. 151:1102-1108 (1982); Segueilha, et al., Biotechnol. Lett. 15(4):399-404 (1993) and Nayini, et al., Lebensm. Wiss. Technol. 17:24-26 (1984).
The use of phytases, phytic acid-specific phosphohydrolases, typically of microbial origin, as dietary supplements, is disclosed by Nelson, et al., J. Nutr. 101:1289 (1971). All currently known post-harvest technologies involve added procedures and expense in order to circumvent problems associated with phytate.
The genetic approach involves developing crop germplasm possessing heritable reductions in seed phytic acid. Heritable quantitative variation in seed phytic acid has been observed among lines of several crop species. See Raboy, In: Inositol Metabolism in Plants, Moore D. J., et al., (eds.) Alan R. Liss, New York, pp. 52-73; (1990).
However, this variation has been found to be highly and positively correlated with variation in less desirable characteristics, therefore, breeding for reduced seed phytic acid using traditional breeding methods, could result in germplasm with undesirable correlated characteristics. To date, there have been no reports of commercially acceptable low phytic acid corn germplasm produced by such an approach.
In genetically altering phytate, natural variability for phytate and free phosphorus has been examined. See Raboy, V. and D. B. Dickinson Crop Sci. 33:1300-1305 (1993),and Raboy, V. et al., Maydica 35:383-390(1990). While some variability for phytic acid was observed, there was no corresponding change in non-phytate phosphorus. In addition, varietal variability represented only two percent of the variation observed, whereas ninety-eight percent of the variation in phytate was attributed to environmental factors.
As mentioned above, studies of soybean and other crops have indicated that altering genetic expression of phytate through recurrent selection breeding methods might have correlated undesirable results. See Raboy, V., D. B. Dickinson, and F. E. Below; Crop Sci. 24:431434 (1984); Raboy, V., F. E. Below, and D. B. Dickinson; J. Hered. 80:311-315 (1989); Raboy, V., M. M. Noaman, G. A. Taylor, and S. G. Pickeft; Crop Sci. 31:631-635; (1991).
While it has been proposed that a block in phytic acid accumulation might be valuable in producing low phytic acid germplasm without the introduction of undesirable correlated responses, (See Raboy, et al., Crop Sci. 33:1300 (1993)) employing such a traditional mutant selection approach has, in certain cases, revealed that homozygosity for mutants associated with substantial reductions in phytic acid also proved to be lethal.
Myo-inositol is produced from glucose in three steps involving the enzymes hexokinase (EC 2.7.1.1), L-myo-inositol 1-phosphate synthase (EC 5.5.1.4) and L-myo-inositol 1-phosphate phosphatase (EC 3.1.3.25). The biosynthetic route leading to phytate is complex and not completely understood. Without wishing to be bound by any particular theory of the formation of phytate, it is believed that the synthesis may be mediated by a series of one or more ADP-phosphotransferases, ATP-dependent kinases and isomerases. A number of intermediates have been isolated including for example 2 and 3 monophosphates, 1,3 and 2,6 di-phosphates, 1,3,5 and 2,5,6 triphosphates, 1,3,5,6 and 2,3,5,6 tetra-phosphates, and 1,2,4,5,6 and 1,2,3,4,6 pentaphosphates. Several futile cycles of dephosphorylation and rephosphorylation of the P.sub.5 and P.sub.6 forms have been reported as well as a cycle involving G6P.fwdarw.myo-inositol-1-phosphate.fwdarw.myo-inositol; the last step being completely reversible, indicating that control of metabolic flux through this pathway may be important. This invention differs from the foregoing approaches in that it provides tools and reagents that allows the skilled artisan, by the application of, inter alia, transgenic methodologies to influence the metabolic flux in respect to the phytic acid pathway. This influence may be either anabolic or catabolic, by which is meant the influence may act to decrease the flow resulting from the biosynthesis of phytic acid and/or increase the degradation (i.e., catabolism of phytic acid). A combination of both approaches is also contemplated by this invention.
As mentioned above, once formed phytate may be dephosphorylated by phosphohydrolases, particularly 3-phytases typically found in microorganisms and 6-phytases the dominant form in plants. After the initial event, both enzymes are capable of successive dephosphorylation of phytate to free inositol.
Accordingly, there have also been reports that plants can be transformed with constructs comprising a gene encoding phytase. See Pen, et al., PCT Publication WO 91/14782, incorporated herein in its entirety by reference. Transgenic seed or plant tissues expressing phytases can then be used as dietary supplements. However, this application has not been done to reduce seed phytic acid.
Based on the foregoing, there exists the need to improve the nutritional content of plants, particularly corn and soybean by increasing non-phytate phosphorus and reducing seed phytate with no other obvious or substantial adverse effects.