Mineral nitrogen sources differently affect root glutamine synthetase isoforms and amino acid balance among organs in maize

Bhakti Prinsi,Luca Espen

doi:10.1186/s12870-015-0482-9

Abstract

BackgroundGlutamine 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.ResultsThe 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.ConclusionThis 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.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-015-0482-9) contains supplementary material, which is available to authorized users.

Highlights

Glutamine synthetase (GS) catalyzes the first step of nitrogen assimilation in plant cell
The GS catalyzes the fixation of NH4+ on glutamic acid (Glu) to form glutamine (Gln), and in the assimilation process it is generally coupled with plastidial glutamate synthase (GOGAT, EC1.4.1.13/14) that incorporates C skeletons
Since in field conditions maize N fertilization consists of a single application at sowing [29], the thirdleaf stage corresponds to a vegetative phase in which plants are exposed to a high level of inorganic N and that is indicated as one of the more susceptible to NH4+ toxicity [14]

Summary

Introduction

Glutamine synthetase (GS) catalyzes the first step of nitrogen assimilation in plant cell. Nitrogen (N) represents one of the main minerals required throughout plant development In agronomic terms, this results in a worldwide ever-increasing use of fertilizers and its consequent environmental and socioeconomic costs [1]. This results in a worldwide ever-increasing use of fertilizers and its consequent environmental and socioeconomic costs [1] This N requirement is emphasized with regard to cereal crops [2], for which maize (Zea mays L.) is a model species [4,5], and it is deeply influenced by C metabolism [6]. The GS catalyzes the fixation of NH4+ on glutamic acid (Glu) to form glutamine (Gln), and in the assimilation process it is generally coupled with plastidial glutamate synthase (GOGAT, EC1.4.1.13/14) that incorporates C skeletons. The inter-conversion with other amino acids greatly influences N plant economy, especially regarding asparagine (Asn) and alanine (Ala) [10]

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