Nitrogen Metabolism in the Stalk Tissue of Maize

Abstract

During ear development in maize (Zea mays L.), nitrogenous compounds are translocated from vegetative organs to the kernels. At anthesis, the stalk contains approximately 40% of the total plant N, and contributes 45% of the N remobilized to the ear. Therefore, the stalk has an important function as a temporary reservoir for N. Little is known of the metabolism of maize stalks, and this paper describes initial studies of enzymes of N metabolism. High in vitro activity of glutamine synthetase (GS) in maize stalk samples throughout ear development contrasted with a peak in activity of glutamate synthase soon after anthesis and negligible nitrate reductase. With fresh sections of stalk tissue collected at anthesis, '5N-feeding experiments confirmed high GS and low nitrate reductase activities. Two isoforms of GS were separated from extracts from stalk tissue: GS1, the cytoplasmic form, increased to maximum levels at 2 weeks postanthesis and remained fairly high thereafter; whereas the plastidic form, GS2, declined progressively during kemel development. Westem blot analysis confirmed the presence of constantly high levels of GS protein after anthesis. The levels of GS proteins decreased after transfer of N-starved, hydroponically grown plants to N-rich conditions in order to restrict remobilization of N. In contrast, transfer of plants grown under abundant N conditions to N-free medium, which encourages N remobilization, resulted in a relative increase in GS protein. Because glutamine is the major form of N transported in maize, the results indicate that GS, specifically the GS1 isoform, has a central role in the remobilization on nitrogenous compounds from the stalk to the ear.

Root- and shoot-specific responses of individual glutamine synthetase genes of maize to nitrate and ammonium

“…These results are consistent with the idea that the multiple GS genes of plants facilitate the differential regulation of GS synthesis in various tissues in response to different requirements for assimilation of ammonium. They also provide indirect support for the hypothesis [30,47,57] that at least some of the multiple GS isoenzymes are involved with different aspects of nitrogen metabolism, given that the amide nitrogen of glutamine is utilized in many distinct metabolic pathways [reviewed in 43].…”

Seasonal Changes of Shoot Nitrogen Concentrations and15N/14N Ratios in Common Reed in a Constructed Wetland

“…Redistribution of stored N in plants requires the transformation of N into mobile N compounds, and this process is initiated by various enzymes, including nitrate reductase, protease, and glutamine synthetase (Ta, 1991). Because of the discrimination against 15 N-bearing compounds (faster reaction rate of 14 N than 15 N) during the enzymatic reactions (Yoneyama et al, 1998), 15 N enrichment of the remaining N in the N source (substrate) has often been reported (Gebauer and Sculze, 1991;Gebauer et al, 1994;Chambers et al, 2004).…”

Differential expression of six glutamine synthetase genes in Zea mays

“…Yamaya and Oaks [27], Muhitch [14], and Ta [22] have proposed that the multiple GS isoenzymes may be involved in different aspects of nitrogen metabolism because the amide nitrogen of glutamine is used in a large number of metabolic pathways [20]. For example, glutamine is the major form of transported nitrogen in maize, and thus GS isoenzymes play a key role in the mobilization of nitrogenous compounds throughout the plant.…”