Phytic acid in cereal grains and oilseeds is poorly digested by monogastric animals and negatively affects animal nutrition and the environment. However, breeding programs involving mutants with less phytic acid and more inorganic phosphate (P(i)) have been frustrated by undesirable agronomic characteristics associated with the phytic acid-reducing mutations. We show that maize lpa1 mutants are defective in a multidrug resistance-associated protein (MRP) ATP-binding cassette (ABC) transporter that is expressed most highly in embryos, but also in immature endosperm, germinating seed and vegetative tissues. Silencing expression of this transporter in an embryo-specific manner produced low-phytic-acid, high-Pi transgenic maize seeds that germinate normally and do not show any significant reduction in seed dry weight. This dominant transgenic approach obviates the need for incorporating recessive lpa1 mutations to create maize hybrids with reduced phytic acid. Suppressing the homologous soybean MRP gene also generated low-phytic-acid seed, suggesting that the strategy might be feasible for many crops.

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Mannitol is the most abundant sugar alcohol in nature, occurring in bacteria, fungi, lichens, and many species of vascular plants. Celery (Apium graveolens L.), a plant that forms mannitol photosynthetically, has high photosynthetic rates thought to result from intrinsic differences in the biosynthesis of hexitols vs. sugars. Celery also exhibits high salt tolerance due to the function of mannitol as an osmoprotectant. A mannitol catabolic enzyme that oxidizes mannitol to mannose (mannitol dehydrogenase, MTD) has been identified. In celery plants, MTD Mannitol is a six-carbon noncyclic sugar alcohol found in diverse organisms ranging from bacteria to higher plants. Mannitol is present in more than 100 species of higher plants, where it can be a significant portion of the soluble carbohydrate (1-3). For instance, celery (Apium graveolens) translocates up to 50% of its photoassimilate as mannitol, with the remainder being sucrose (4). Both translocated carbohydrates are assimilated during growth of nonphotosynthetic heterotrophic (i.e., sink) tissues. Other postulated roles for mannitol include carbon storage, free radical scavenging, and osmoprotection (4-7).The use of mannitol as a photoassimilate and translocated carbohydrate is reported to be advantageous to the plant in several ways. Celery, a C3 plant, has carbon fixation rates equivalent to those of many C4 plants (8). This may result from both increased NADP/NADPH turnover compared to plants that exclusively form sugars and from the additional cytosolic sink for photosynthetically fixed CO2 provided by mannitol synthesis (7,9,10). In addition to the increased carbon fixation that accompanies mannitol biosynthesis, the initial step of mannitol utilization generates NADH, thus giving a higher net ATP yield than the catabolism of an equal amount of sucrose (7). Finally, mannitol-producing plants also exhibit a high degree of salt tolerance due to the function of mannitol as an osmoregulator and compatible solute (6,11,12). Celery plants grown in hydroponic culture with a salinity equivalent to 30% that of sea water show dry weight gains equal to plants grown at normal nutrient levels (12). In addition, tobacco that was genetically engineered to synthesize mannitol through the introduction of the Escherichia coli NAD-dependent mannitol-1-phosphate dehydrogenase acquired significant salt tolerance (6).Metabolite pool sizes in plants are usually determined by relative rates of synthesis and utilization. The isolation and characterization of a plant NAD-dependent mannitol dehydrogenase (MTD), the enzyme responsible for the oxidation of mannitol to mannose in celery, was reported by our laboratory (13). MTD is a monomeric mannitol:mannose 1-oxidoreductase with a molecular mass of "40 kDa (13,14). In celery plants, the expression of MTD is highly regulated. MTD activity is highest in young actively growing root tips, is also high in young rapidly growing (sink) leaves, but is not detected in mature photosynthetic (source) leaves. Extractable MTD activity ...

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The effect of excess macronutrients in the root environment on mannitol and sucrose metabolism was investigated in celery (Apium graveolens 1. var dulce [Mill.] Pers.). Plant growth was inhibited progressively as macronutrient concentration in the media, as measured by eledrical condudivity (E.C.), increased from 1.0 to 11.9 decisiemens m-'. Plants grown for 35 d at higher E.C. had a lower water content but similar dry weight in their roots, leaves, and petioles compared to plants grown at lower E.C. Macronutrient concentrations of leaves, roots, and petioles were not affeded by the imposed stress, indicating that the macronutrient stress resulted in a water-deficit stress response rather than a salt-specific response. Mannitol accumulated in sink tissues and was accompanied by a drastic decrease in adivity of mannitol-1-oxidoredudase. Sucrose concentration and adivities of sucrose-metabolizing enzymes in sink tissues were not affeded by the macronutrient stress. Mature leaves exhibited increased concentrations of both mannitol and sucrose, together with increased activity of mannose-6-phosphate redudase and sucrose phosphate synthase, in response to macronutrient stress. Thus, mannitol accumulation in osmotically stressed celery is regulated by diminished catabolism in sink tissues and increased capacity for mannitol biosynthesis in source leaves.