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The arginine biosynthetic pathway represents an area of plant biochemistry that has been poorly investigated. Recently, the first enzyme of the arginine pathway, encoded by the N-acetyl-L-glutamate synthase gene (SlNAGS1), was isolated and characterized in tomato, and was found to be structurally similar to other predicted NAGS. SlNAGS1 accumulation patterns suggest a possible role of this gene in hypoxia-induced responses. The 35S::SlNAGS1 Arabidopsis plants accumulated ornithine at high levels and exhibited increased tolerance to salt and drought stresses. Ornithine is the intermediate compound in the arginine biosynthesis where the pathway divaricates to the production of compounds, such as proline and polyamines that are known to serve osmoprotective functions. It is therefore likely that the elevated ornithine accumulation in the SlNAGS1-overexpressing plants be coupled with the production of a pool of osmoprotectants that end up to the improved stress tolerance. The possible implications of ornithine accumulation are discussed.
Plant resistance to abiotic stress has been receiving increased attention over the past few decades. Temperature extremes, decreased rainfall and progressive salinization of soils via irrigation may lead to a disruption of plant water status1 ultimately affecting growth, crop quality and yield.2 Osmotic stress can be a result of either reduced water availability or increased salt content.3 One way for plants to cope with osmotic stress is the synthesis and accumulation of compatible solutes. This process of osmotic adjustment develops slowly as a response to tissue dehydration and involves the production of osmolytes like amino acids (proline, citrulline, arginine), polyamines, sugars and sugar alcohols (mannitol, inositol, trehalose) and quaternary ammonium compounds (glycine betaine), which accumulate at high molar concentrations in the cytosol.4
Arginine biosynthesis in plants proceeds through a nine-step process from glutamate involving the production of important pathway intermediates like ornithine and citruline.5 In higher plants, beyond its apparent role in metabolism, arginine is involved in many physiological processes including abiotic stress response.6 Arginine and ornithine can act as precursors of polyamines, ubiquitous molecules that participate in many biological processes including fruit ripening7 and plant protection from osmotic stress.8 Regulation of arginine biosynthesis was originally proposed to be via inhibition of N-acetyl glutamate kinase (NAGK) by the end product, arginine, whereas N-acetyl glutamate restores its activity.9 However, recent studies have demonstrated that this control is mediated by PII, a 2-oxoglutarate and amino acid sensing protein that can alleviate arginine inhibition of NAGK.10,11
The associated work by Kalamaki et al.12 began with the cloning and characterization of a full-length cDNA for N-acetyl glutamate synthase (SlNAGS1) from tomato (Solanum lycopersicum Mill.) which constitutes the first report on the cloning of a plant NAGS. The predicted protein of 604 amino acids appears to be targeted to the plastid with a predicted cleavage site at residue 31 resulting in a mature protein of 66.4 kDa. Targeting predictions of the Arabidopsis and other plant putative NAGS support localization of these enzymes at the plastid.5 Gene expression pattern analysis revealed that SlNAGS1 transcript accumulates mainly in green tissues, which is consistent with the putative chloroplastic localization of the enzyme. During fruit development transcript levels were shown to remain relatively stable with a slight peak at the red fruit stage, whereas hypoxia and anoxia time course experiments revealed that SlNAGS1 mRNA abundance was induced by low oxygen indicating a possible role of this gene in plant responses to early events of hypoxia.
The SlNAGS1 ORF was introduced to Arabidopsis thaliana via Agrobacterium-mediated genetic modification under the transcriptional control of the CaMV 35S promoter and three stable transformed lines (1–7, 3–8, 6–5) possessing a single copy of the transgene were further evaluated for their resistance to osmotic stress. Although a dramatic increase in arginine content was not observed in the overexpressing transgenic lines, a striking increase in ornithine levels of leaves was observed. Several hypotheses may explain the high ornithine levels. For example it might be related to the compartmentation of the ornithine/arginine metabolism where a translocator/carrier malfunction may occur. This leads to an area that has not been characterized in plants.13 The high ornithine levels might also be due to the limited function or inhibition of the enzyme next in the pathway (OCT, ornithine carbamoyltransferase) that fails to convert/translocate the excess of ornithine in the subsequent biosynthetic step. In any case, the anaplerotic role of NAGS5,9 that results into an overloaded pathway is in the limelight.
It is well established that ornithine, apart from being a precursor of arginine biosynthesis serves as a precursor of other metabolic pathways including polyamine biosynthesis via ornithine decarboxylase (ODC) and glutamate and proline via ornithine-δ-aminotransferase (δ-OAT) activities. As discussed earlier, increased polyamine biosynthesis has been associated with increased resistance of plants to salt and drought stresses.8 On the other hand, proline accumulation has been found to protect plant tissues from hyperosmotic stress (reviewed in ref. 14). Furthermore, proline's role as a signaling molecule involved in regulation of gene expression under stress has been suggested.15 The acidic amino acid glutamate plays an essential role in plant metabolism and recently has been proposed to function as a signaling molecule (reviewed in ref. 16). The ability of the overexpressing transgenic lines to withstand osmotic stress was evaluated by different experiments. Seed germination in 250 mM sodium chloride revealed that overexpression of SlNAGS1 resulted in increased germination potential of seeds under salt pressure. Seedlings of two of the three transgenic lines also exhibited a significantly (p < 0.001) higher root tolerance index when grown at 100 mM NaCl than wild type plants whereas no significant difference was observed at 125 mM NaCl (Fig. 1). When mature plants were watered with 300 mM NaCl all three transgenic lines exhibited increased resistance to the imposed stress as evidenced by better chlorophyll retention of transgenic lines compared to wild type. Could this observation be attributed to the overproduction of ornithine that was induced by the overexpression of SlNAGS1? At the moment we can only speculate based on the central role of ornithine as a precursor of many metabolic pathways involved in stress resistance. It is possible that in osmotically stressed plants, feedback inhibition of NAGS by arginine is alleviated and this pathway could serve to increase the N-acetylglutamate pool that is directed to polyamine, proline, glutamate and arginine biosynthesis. For example, proline accumulation by the ornithine pathway was observed in young Arabidopsis plants under salt stress,17 and also in Brassica napus plants under prolonged drought stress.18 Since accumulation of proline under non stress conditions was shown to be toxic to plants,15 overproduction of its precursor, ornithine, may be a way to equip plants with a metabolite that can be readily converted to proline upon osmotic stress induction. In addition, the ornithine pathway for proline synthesis is not feedback inhibited, since the activity of δ-OAT and pyrroline-5-carboxylate reductase (P5CR) is not regulated by proline.19 Engineering plants to constitutively accumulate a non-toxic metabolite that can readily be converted to an osmoprotective molecule upon stress induction might present an alternative way of increasing abiotic stress tolerance.
Addendum to: Kalamaki MS, Alexandrou D, Lazari D, Merkouropoulos G, Fotopoulos V, Pateraki I, et al. Overexpression of a tomato N-acetyl-L-glutamate synthase gene (SlNAGS1) in Arabidopsis thaliana results in high ornithine levels and increased tolerance in salt and drought stressesJ Exp Bot20096018591871 doi: 10.1093/jxb/erp072.
Previously published online: www.landesbioscience.com/journals/psb/article/9873