SummaryHomoserine kinase (HSK) produces O-phospho-L-homoserine (HserP) used by cystathionine c-synthase (CGS) for Met synthesis and threonine synthase (TS) for Thr synthesis. The effects of overexpressing Arabidopsis thaliana HSK, CGS, and Escherichia coli TS (eTS), each controlled by the 35S promoter, were compared. The results indicate that in Arabidopsis Hser supply is the major factor limiting the synthesis of HserP, Met and Thr. HSK is not limiting and CGS or TS control the partitioning of HserP. HSK overexpression had no effect on the level of soluble HserP, Met or Thr, however, when treated with Hser these plants produced far more HserP than wild type. Met and Thr also accumulated markedly after Hser treatment but the increase was similar in HSK overexpressing and wild-type plants. CGS overexpression was previously shown to increase Met content, but had no effect on Thr. After Hser treatment Met accumulation increased in CGS-overexpressing plants compared with wild type, whereas HserP declined and Thr was unaffected. Arabidopsis responded differentially to eTS expression depending on the level of the enzyme. At the highest eTS level the Thr content was not increased, but the phenotype was negatively affected and the T1 plants died before reproducing. Comparatively low eTS did not affect phenotype or Thr/Met level, however after Hser treatment HserP and Met accumulation were reduced compared with wild type and Thr was increased slightly. At intermediate eTS activity seedling growth was retarded unless Met was supplied and CGS expression was induced, indicating that eTS limited HserP availability for Met synthesis.

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To examine the effects of N nutrition upon endosperm development, maize (Zea mays) kernels were grown in vitro with either 0, 3.6, 7.1, 14.3, or 35.7 millimolar N. Kernels were harvested at 20 days after pollination for determination of enzyme activities and again at maturity for quantification of storage products and electrophoretic separation of zeins. Endosperm dry weight, starch, zein-N, and nonzein-N all increased in mature kemels as N supply increased from zero to 14.3 millimolar. The activities of sucrose synthase, aldolase, phosphoglucomutase, glutamatepyruvate transaminase, glutamate-oxaloacetate transaminase, and acetolactate synthase increased from 1-to 2.5-fold with increasing N supply. Adenosine diphosphate-glucose pyrophosphorylase and both ATP-and PPi-dependent phosphofructokinases increased to lesser extents, while no significant response was detected for hexose kinases and glutamine synthetase. Nitrogen-induced changes in enzyme activities were often highly correlated with changes in final starch and/or zein-N contents. Separation of zeins indicated that these peptides were proportionately enhanced by N supply, with the exception of C-zein, which was relatively insensitive to N. These data indicate that at least a portion of the yield increase in maize produced by N fertilization is induced by a modification of kernel metabolism in response to N supply.

Maize (Zea mays L.) kernel pedicels, including vascular tissues, pedicel parenchyma, placento-chalazal tissue, and the surrounding pericarp, contained two forms of glutamine synthetase (EC 6.3.1.2), separable by anion exchange chromatography under mildly acidic conditions. The earlier-eluting activity (GSp,), but not the later-eluting activity (GSp2), was chromatographically distinct from the maize leaf and root glutamine synthetases. The level of GSp1 activity changed in a developmentally dependent manner while GSp2 activity was constitutive. GSpi and GSp2 exhibited distinct ratios of transferase to hydroxylamine-dependent synthetase activities (5 and 23, respectively), which did not change with kernel age. Purified pedicel glutamine synthetases had native relative molecular masses of 340,000, while the subunit relative molecular masses differed slightly at 38,900 and 40,500 for GSp1 and GSp2, respectively. Both GS forms required free Mg2+ with apparent Kms = 2.0 and 0.19 millimolar for GSp1 and GSp2, respectively. GSp1 had an apparent Km for glutamate of 35 millimolar and exhibited substrate inhibition at glutamate concentrations greater than 90 millimolar. In contrast, GSp2 exhibited simple MichaelisMenten kinetics for glutamate with a Km value of 3.4 millimolar. Both isozymes exhibited positive cooperativity for ammonia, with S0.5 values of 100 and 45 micromolar, respectively. GSp, appears to be a unique, kernel-specific form of plant glutamine synthetase. Possible functions for the pedicel GS isozymes in kemel nitrogen metabolism are discussed.During seed development, nitrogen and carbon compounds are translocated from the vegetative portions of the plant to the seed where they are converted into water-insoluble storage polymers. In maize (Zea mays L.), the vascular system terminates in the pedicel tissue of the kernel, and assimilates are unloaded and pass symplastically through this basal maternal tissue region before entering the endosperm via the basal endosperm transfer cells (1 1). The form in which nitrogen is transported to the developing seed is species specific (28) and the results of amino acid pool analysis (3, 15, 16) and radiolabeled precursor studies (26) suggest that in maize nitrogen is transported predominantly as aspartate, seine, glutamate, glutamine, and alanine.In both legumes (22) and cereals (7, 15, 16), considerable metabolism of the incoming nitrogenous transport compounds appears to take place in the seed-associated maternal tissues prior to their uptake into the developing endosperm or cotyledons. In the pedicel region of the maize kernel, the free amino acid pool is altered such that the relative glutamine and glutamate levels are increased and aspartate, glycine, and serine levels are decreased when compared to the cob vascular sap (15). Incubation of whole kernels in ['4C]aspartic acidcontaining media showed that the carbon from aspartate is rapidly metabolized into organic acids within the pedicel (16). The most striking change in the transport free amino acid pool, h...