Abstract
BACKGROUND: Glutamine synthetase (GS) catalyzes the first step of nitrogen assimilation in plant cell. The main GS are classified as cytosolic GS1 and plastidial GS2, of which the functionality is variable according to the nitrogen sources, organs and developmental stages. In maize (Zea mays L.) one gene for GS2 and five genes for GS1 subunits are known, but their roles in root metabolism are not yet well defined. In this work, proteomic and biochemical approaches have been used to study root GS enzymes and nitrogen assimilation in maize plants re-supplied with nitrate, ammonium or both. RESULTS: The plant metabolic status highlighted the relevance of root system in maize nitrogen assimilation during both nitrate and ammonium nutrition. The analysis of root proteomes allowed a study to be made of the accumulation and phosphorylation of six GS proteins. Three forms of GS2 were identified, among which only the phosphorylated one showed an accumulation trend consistent with plastidial GS activity. Nitrogen availabilities enabled increments in root total GS synthetase activity, associated with different GS1 isoforms according to the nitrogen sources. Nitrate nutrition induced the specific accumulation of GS1-5 while ammonium led to up-accumulation of both GS1-1 and GS1-5, highlighting co-participation. Moreover, the changes in thermal sensitivity of root GS transferase activity suggested differential rearrangements of the native enzyme. The amino acid accumulation and composition in roots, xylem sap and leaves deeply changed in response to mineral sources. Glutamine showed the prevalent changes in all nitrogen nutritions. Besides, the ammonium nutrition was associated with an accumulation of asparagine and reducing sugars and a drop in glutamic acid level, significantly alleviated by the co-provision with nitrate. CONCLUSION: This work provides new information about the multifaceted regulation of the GS enzyme in maize roots, indicating the involvement of specific isoenzymes/isoforms, post-translational events and biochemical factors. For the first time, the proteomic approach allowed to discriminate the individual contribution of the GS1 isoforms, highlighting the participation of GS1-5 in nitrate metabolism. Moreover, the results give new insights about the influence of amino acid metabolism in plant C/N balance.
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Claims 9
Claim · model-written summary
A phosphorylated GS2 form alone tracked plastidial GS activity.
Zea maysNot reportedDirection
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Three forms of GS2 were identified, among which only the phosphorylated one showed an accumulation trend consistent with plastidial GS activity”
e1“nitrogen sources differently affect root glutamine synthetase isoforms and amino acid balance among organs in maize”
Claim · model-written summary
Nitrogen availability increased root total GS synthetase activity.
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Nitrogen availabilities enabled increments in root total GS synthetase activity”
e1“nitrogen sources differently affect root glutamine synthetase isoforms and amino acid balance among organs in maize”
Claim · model-written summary
Nitrate nutrition specifically accumulated GS1-5.
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Nitrate nutrition induced the specific accumulation of GS1-5”
e1“nitrogen sources differently affect root glutamine synthetase isoforms and amino acid balance among organs in maize”
Claim · model-written summary
Ammonium increased GS1-1 and GS1-5 accumulation.
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“ammonium led to up-accumulation of both GS1-1 and GS1-5”
e1“nitrogen sources differently affect root glutamine synthetase isoforms and amino acid balance among organs in maize”
Claim · model-written summary
Mineral nitrogen sources changed amino acid accumulation and composition across roots, xylem sap, and leaves.
- Organ / stage
- roots, xylem sap and leavese0
- Treatment
- mineral sourcese0
- Outcome
- amino acid accumulation and compositione0
- Direction
- changede0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“The amino acid accumulation and composition in roots, xylem sap and leaves deeply changed in response to mineral sources”
e1“nitrogen sources differently affect root glutamine synthetase isoforms and amino acid balance among organs in maize”
Claim · model-written summary
Ammonium nutrition was associated with asparagine accumulation.
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“ammonium nutrition was associated with an accumulation of asparagine”
e1“nitrogen sources differently affect root glutamine synthetase isoforms and amino acid balance among organs in maize”
Claim · model-written summary
Ammonium nutrition was associated with reducing sugar accumulation.
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“ammonium nutrition was associated with an accumulation of asparagine and reducing sugars”
e1“nitrogen sources differently affect root glutamine synthetase isoforms and amino acid balance among organs in maize”
Claim · model-written summary
Ammonium nutrition was associated with decreased glutamic acid.
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“ammonium nutrition was associated with an accumulation of asparagine and reducing sugars and a drop in glutamic acid level”
e1“nitrogen sources differently affect root glutamine synthetase isoforms and amino acid balance among organs in maize”
Claim · model-written summary
Nitrate co-provision significantly alleviated the ammonium-associated glutamic acid decrease.
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“a drop in glutamic acid level, significantly alleviated by the co-provision with nitrate”
e1“nitrogen sources differently affect root glutamine synthetase isoforms and amino acid balance among organs in maize”
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