Developing seeds store reserve levels of phosphorus and minerals, mobilization of which during germination provides the mineral nutrition essential for optimal early seedling growth. This process also functions as a component of the inorganic P and mineral homeostasis mechanisms necessary for nominal cellular function. The storage and resup-ply of phosphorus centers around the synthesis and breakdown of phytic acid (myo-inositol 1,2,3,4,5,6-hexakisphosphate). Phytic acid is often deposited as a mixed, ‘phytin’ salt, primarily of potassium and magnesium. Thus phytin deposition and re-mobilization also represents an important component of mineral cation storage. Exceptions to this may include iron and calcium storage and homeostasis, for which non-phytin mechanisms may play major roles. There has been recent progress in the molecular biology of several components of phosphorus and mineral storage pathways. These include studies of myo-inositol synthesis, some aspects of myo-inositol phosphate metabolism, protein body transport functions, and calcium and iron storage proteins. Recent genetic studies bring into question the paradigm that phytic acid synthesis is essential to phosphorus storage or inorganic P homeostasis. Much additional progress is required, particularly concerning the synthetic pathway to phytic acid, before a thorough or truly detailed understanding of the molecular biology of P and mineral storage processes is obtained.

Accumulation and Storage of Phosphate and Minerals

The amount of nutrients (i.e., N, P, Zn, and Cu) and associated odors emitted from production animals into the environment can be modulated by sev- eral different nutritional strategies, but their practical application is dependent on costs and biological limita- tions. In general, nutrient excretion may be reduced by avoiding the overfeeding of specific nutrients or by us- ing nutritional manipulations to enhance nutrient utili- zation in the animal. Loss to the environment can be avoided by manufacturing and handling the feed in a pelletized form that will minimize waste and improve feed/gain. Other strategies for minimizing nutrient losses include: 1) the development of feeding programs that are specific for sex and strain of the animal; 2) increasing the number of feed phases to better meet the animal's age-related requirements; 3) formulating diets to include the minimal amounts of nutrients re- quired to satisfy production goals; 4) meeting the ani- mal's amino acid requirements; 5) using synthetic amino acid supplements to feed to reduce N emission; 6) using feed ingredients with high digestibility and nutrient bioavailability; and 7) formulating diets based on nutrient availability instead of total nutrient con- tent. Nutrient digestibility of feedstuffs is dependent on processing conditions, genetic characteristics of the grains and oilseeds, and the presence of nutritional antagonists in specific feedstuffs used in the diet. Feed ingredients that lead to odor production can be avoided (e.g., fishmeal and some easily fermentable feed ingre- dients). Feed additives, such as antibiotics, nonstarch polysaccharides, direct-fed microbials, organic acids, microbial enzymes (i.e., phytase, carbohydrases, and proteases) can be used to increase the digestibility and absorption of nutrients or to modulate the microflora. Finally, a cost factor for the control or disposal of nutri- ents or odor should be considered in the feed formula- tion to optimize the various nutritional strategies dis- cussed above. Regardless of biological and economic limitations, significant reductions in nutrient and odor emission from nonruminants can be achieved by appro- priate nutritional strategies, but response may differ for swine and poultry.

Nutritional strategies to reduce environmental emissions from nonruminants

Phytic acid, a phosphorylated derivative of myo-inositol, functions as the major storage form of phosphorus in plant seeds. Myo-inositol phosphates, including phytic acid, play diverse roles in plants as signal transduction molecules, osmoprotectants, and cell wall constituents. D-myo-inositol-3-phosphate synthase (MIPS EC 5.5.1.4) catalyzes the first step in de novo synthesis of myo-inositol. A soybean (Glycine max) MIPS cDNA (GmMIPS1) was isolated by reverse transcriptase-PCR using consensus primers designed from highly conserved regions in other plant MIPS sequences. Southern-blot analysis and database searches indicated the presence of at least four MIPS genes in the soybean genome. Northern-blot and immunoblot analyses indicated higher MIPS expression and accumulation in immature seeds than in other soybean tissues. MIPS was expressed early in the cotyledonary stage of seed development. The GmMIPS1 expression pattern suggested that it encodes a MIPS isoform that functions in seeds to generate D-myo-inositol-3-phosphate as a substrate for phytic acid biosynthesis.

Expression of D-myo-inositol-3-phosphate synthase in soybean. Implications for phytic acid biosynthesis.

The time-course of phosphorus (P) accumulation in the phytic acid fraction of developing soybean ( Glycine max [L.] Merr. cv `Williams 799) seeds as well as the relation of phytic acid P to total P content were determined. Phytic acid was detected early in embryogenesis in field-grown soybeans and accumulated in a linear fashion throughout most of seed development. Although the observed rates of accumulation ranged from 18.7 micrograms phytic acid P per seed per day in pods positioned low on the plant to 33.6 micrograms in pods positioned high on the plant, the final concentrations were the same in all cases. Nearly all of the P translocated to developing seeds was incorporated into phytic acid from the third week after flowering until physiological maturity, with the sum of nonphytic acid P compounds remaining constant. Phytic acid accumulation was also linear throughout development when soybean plants were grown in solutions having nutrient P levels that ranged from severely limiting (2.0 milligrams P per liter) to excess (50 milligrams P per liter). However, there was a pronounced effect on rate of accumulation, which ranged from 7.2 micrograms phytic acid per seed per day with limiting nutrient P to 44.7 micrograms with excess P. The change in level of phytic acid accounted for most of the alteration in total seed P that was caused by altering the P status of the plants. These results support the view that phytic acid synthesis is involved in P homeostasis of the developing soybean seed.

The Timing and Rate of Phytic Acid Accumulation in Developing Soybean Seeds

The time-course of phosphorus (P) accumulation in the phytic acid fraction of developing soybean (Glycine max [L.1 Merr. cv 'Williams 79') seeds as well as the relation of phytic acid P to total P content were determined. Phytic acid was detected early in embryogenesis in fieldgrown soybeans and accumulated in a linear fashion throughout most of seed development. Although the observed rates of accumulation ranged from 18.7 micrograms phytic acid P per seed per day in pods positioned low on the plant to 33.6 micrograms in pods positioned high on the plant, the final concentrations were the same in all cases. Nearly all of the P translocated to developing seeds was incorporated into phytic acid from the third week after flowering until physiological maturity, with the sum of nonphytic acid P compounds remaining constant. Phytic acid accumulation was also linear throughout development when soybean plants were grown in solutions having nutrient P levels that ranged from severely limiting (2.0 milligrams P per liter) to excess (50 milligrams P per liter). However, there was a pronounced effect on rate of accumulation, which ranged from 7.2 micrograms phytic acid per seed per day with limiting nutrient P to 44.7 micrograms with excess P. The change in level of phytic acid accounted for most of the alteration in total seed P that was caused by altering the P status of the plants. These results support the view that phytic acid synthesis is involved in P homeostasis of the developing soybean seed.

The Timing and Rate of Phytic Acid Accumulation in Developing

Altering the level of phytic acid phosphorus by nutritional means had no effect on the ability of soybean ( Glycine max L. [Merr.], cv `Williams 799) seeds to germinate under laboratory or greenhouse conditions. Dry matter moved out of the cotyledons at similar rates whether the germinating seeds initially contained low (0.19), medium (0.59), or high (1.00 milligram per seed) phytic acid phosphorus. Growth of roots and shoots from 3 to 9 days after planting was similar for seeds containing low and medium levels of phytic acid phosphorus. The medium level of phytic acid resembles that found in field-grown seed, so it is clear that soybean seeds normally contain a phosphorus reserve far above that needed for germination and early seedling growth.

Stephen J. Hudson

Papers

Reduced phytic Acid content does not have an adverse effect on germination of soybean seeds.