Amino acid metabolism in maize earshoots. Implications for assimilate preconditioning and nitrogen signaling

Nitrogen stress and the expression of asparagine synthetase in roots and nodules of soybean (Glycine max)

The difficulty of assaying asparagine synthetase (AS) (EC 6.3.5.4) activity in roots of soybean has been circumvented by measuring expression of the AS genes. Expression of three soybean asparagine synthetase (SAS) genes (SAS1, SAS2 and SAS3) was observed in roots of non-nodulated soybean plants cultivated on nitrate. Expression of these genes was reduced to very low levels within days after submitting the plants to a N-free medium. The subsequent return to a complete medium (containing nitrate) restored expression of all three AS genes. Roots of nodulated plants, where symbiotic nitrogen fixation was the exclusive source of N (no nitrate present), showed very weak expression of all three AS genes, but on transfer to a nitrate-containing medium, strong expression of these genes was observed within 24 h. In nodules, all three genes were expressed in the absence of nitrate. Under conditions that impair nitrogen fixation (nodules submerged in aerated hydroponics), only SAS1 expression was reduced. However, in the presence of nitrate, an inhibitor of N(2) fixation, SAS1 expression was maintained. High and low expressions of AS genes in the roots were associated with high and low ratios of Asn/Asp transported to the shoot through xylem. It is concluded that nitrate (or one of its assimilatory products) leads to the induction of AS in roots of soybean and that this underlies the variations found in xylem sap Asn/Asp ratios. Regulation of nodule AS expression is quite different from that of the root, but nodule SAS1, at least, appears to involve a product of N assimilation rather than nitrate itself.

Wheat nitrogen metabolism during grain filling: comparative role of glumes and the flag leaf

The mobilization of nitrogen (N) compounds and the roles played by glumes and the flag leaf during grain filling were studied in bread wheat (Triticum aestivum L. cv. Florida) grown under field conditions. Glumes lost twice as much of their total N content as that lost by the flag leaf between the milk and early dough stages. In the flag leaf, glumes and grains, Glu, Asp, Ser and Ala accounted for 85% of all the reductions in the free amino acid pool. Principal component analysis of free amino acid pools separated grains from the glumes and the flag leaf, suggesting grain specific regulations in the use of free amino acids in protein synthesis. In all three organs, no decrease in Gln was detected, probably due to steady glutamine synthetase (GS; EC 6.3.1.2) activities per soluble protein in both the flag leaf and glumes. Compared with the flag leaf, glumes presented relatively smaller amounts of the chloroplast GS associated isoform. This we show is due to a lower relative number of mesophyll cells in glumes as supported by the different anatomy and the cellular pattern of the GS immunolocalization. We argue that cellular distribution plays a key role in supporting metabolism to enable the various functions undertaken by glume tissue.

Nuclear magnetic resonance spectroscopy-based metabolite profiling of transgenic tomato fruit engineered to accumulate spermidine and spermine reveals enhanced anabolic and nitrogen-carbon interactions

Polyamines are ubiquitous aliphatic amines that have been implicated in myriad processes, but their precise biochemical roles are not fully understood. We have carried out metabolite profiling analyses of transgenic tomato (Solanum lycopersicum) fruit engineered to accumulate the higher polyamines spermidine (Spd) and spermine (Spm) to bring an insight into the metabolic processes that Spd/Spm regulate in plants. NMR spectroscopic analysis revealed distinct metabolite trends in the transgenic and wild-type/azygous fruits ripened off the vine. Distinct metabolites (glutamine, asparagine, choline, citrate, fumarate, malate, and an unidentified compound A) accumulated in the red transgenic fruit, while the levels of valine, aspartic acid, sucrose, and glucose were significantly lower as compared to the control (wild-type and azygous) red fruit. The levels of isoleucine, glucose, gamma-aminobutyrate, phenylalanine, and fructose remained similar in the nontransgenic and transgenic fruits. Statistical treatment of the metabolite variables distinguished the control fruits from the transgenic fruit and provided credence to the pronounced, differential metabolite profiles seen during ripening of the transgenic fruits. The pathways involved in the nitrogen sensing/signaling and carbon metabolism seem preferentially activated in the high Spd/Spm transgenics. The metabolite profiling analysis suggests that Spd and Spm are perceived as nitrogenous metabolites by the fruit cells, which in turn results in the stimulation of carbon sequestration. This is seen manifested in higher respiratory activity and up-regulation of phosphoenolpyruvate carboxylase and NADP-dependent isocitrate dehydrogenase transcripts in the transgenic fruit compared to controls, indicating high metabolic status of the transgenics even late in fruit ripening.

Two cytosolic glutamine synthetase isoforms of maize are specifically involved in the control of grain production

The roles of two cytosolic maize glutamine synthetase isoenzymes (GS1), products of the Gln1-3 and Gln1-4 genes, were investigated by examining the impact of knockout mutations on kernel yield. In the gln1-3 and gln1-4 single mutants and the gln1-3 gln1-4 double mutant, GS mRNA expression was impaired, resulting in reduced GS1 protein and activity. The gln1-4 phenotype displayed reduced kernel size and gln1-3 reduced kernel number, with both phenotypes displayed in gln1-3 gln1-4. However, at maturity, shoot biomass production was not modified in either the single mutants or double mutants, suggesting a specific impact on grain production in both mutants. Asn increased in the leaves of the mutants during grain filling, indicating that it probably accumulates to circumvent ammonium buildup resulting from lower GS1 activity. Phloem sap analysis revealed that unlike Gln, Asn is not efficiently transported to developing kernels, apparently causing reduced kernel production. When Gln1-3 was overexpressed constitutively in leaves, kernel number increased by 30%, providing further evidence that GS1-3 plays a major role in kernel yield. Cytoimmunochemistry and in situ hybridization revealed that GS1-3 is present in mesophyll cells, whereas GS1-4 is specifically localized in the bundle sheath cells. The two GS1 isoenzymes play nonredundant roles with respect to their tissue-specific localization.

Correlation between AS1 gene expression and seed protein contents in different soybean (Glycine max [L.] Merr.) cultivars

In higher plants, asparagine synthetase (AS) plays an important role in regulating the nitrogen sink-source relationship. We studied the expression of AS genes in five Chinese soybean cultivars exhibiting contrasting seed protein contents. We found that only the AS2 but not the AS1 gene was induced by dark treatment. On the other hand, the expression of AS1 in leaves (especially in trifoliate leaves of young seedlings) showed a positive correlation with seed protein contents in the soybean cultivars tested. Therefore, in spite of the fact that the principle transporting compounds in soybean plants for nitrogen acquired via symbiotic fixation are ureides, AS may still play an important role in the process of nitrogen assimilation.