تنظیم سنتز آمینو اسیدهای ضروری و انباشت در گیاهان The Regulation of Essential Amino Acid Synthesis and Accumulation in Plants

INTRODUCTION Animals, including humans and monogastric livestock that serve as human food, cannot synthesize all of the 20 amino acids that are required for the formation of proteins. Therefore, they must obtain the amino acids that they cannot synthesize (termed essential amino acids) from external sources, which are based on plants. These nine essential amino acids are lysine (Lys), methionine (Met), and threonine (Thr) of the aspartate (Asp) family pathway; phenylalanine (Phe) and tryptophan (Trp) of the aromatic amino acids; valine (Val), isoleucine (Ile), and leucine (Leu) of the branched-chain amino acids (BCAAs); and histidine (His). The levels of four of these amino acids—Lys, Met, Thr, and Trp—are considered to limit the nutritional quality of plants, because their contents in plants are very low compared with the levels required for optimum growth of human and animals. The major factors limiting these essential amino acids in crop plants are (a) regulatory factors that control the synthesis of the essential amino acids by feedback inhibition loops, in which the accumulating amino acids suppress the activity of enzymes in their biosynthesis pathways, and (b) the efficient catabolism of these amino acids. Indeed, amino acids also serve as precursors of a wide variety of plant natural products that play crucial roles in plant growth and development, including responses to biotic and abiotic stresses (205). Amino acids are also efficiently catabolized into the tricarboxylic acid (TCA) cycle to generate the cellular energy required for plant growth, particularly in response to stresses that create energy deprivation (105, 120). METABOLISM OF THE AROMATIC AMINO ACIDS Among the aromatic amino acids, Phe and Trp are considered essential, whereas tyrosine (Tyr) is regarded as nonessential (58). The synthesis of these three amino acids begins with the conversion of phosphoenolpyruvate and erythrose 4-phosphate into chorismate via the shikimate pathway; this chorismate is subsequently converted into Phe and Trp via the aromatic amino acid biosynthetic pathways (18, 65, 190, 199) (Figure 1). Given that the shikimate and aromatic amino acid biosynthesis pathways have been described in considerable detail in recent reviews (125, 188, 189), we merely provide a brief overview of these pathways here. The biosynthesis of the aromatic amino acid Phe from chorismate principally uses two different metabolic routes: one through phenylpyruvate as a metabolic intermediate, and one through arogenate (Figure 1). Chorismate mutase catalyzes the first committed step in Phe biosynthesis. Most plant species have a plastidial and a cytosolic isoform of the enzyme, with the former inhibited by Phe and Tyr and activated by Trp. Prephenate aminotransferase, which was only recently identified at the molecular level (37, 72, 127), catalyzes the reversible transamination between prephenate and arogenate. This route to Phe is completed by the enzyme arogenate dehydratase. Studies of flowers of petunia plants, meanwhile, have revealed that expression of the arogenate dehydratase 1 isozyme was incredibly high in petals and that this high level was positively correlated with the biosynthesis of the endogenous Phe in the flowers (126). It is important to note that arogenate dehydratase isolated from several species additionally possesses prephenate dehydratase activities (126, 201). Aiming to stimulate the production of Phe in plants, the Galili laboratory expressed a recombinant construct encoding a bacterial bifunctional PheA enzyme, containing chorismate mutase and prephenate dehydratase, inArabidopsis(191). The PheA-expressing plants exhibited increased Phe content, indicating that plants, like bacteria, can convert prephenate into Phe, exposing an additional level of complexity in the biosynthesis of the aromatic amino acids in plants. Until very recently, this enzyme had only been speculated to be an aromatic amino acid transferase (190). However, this was subsequently determined at the molecular level when Maeda and coworkers (203) revealed that this conversion is mediated by a cytosollocalized Tyr:phenylpyruvate aminotransferase. This enzyme thereby links Phe production to a coordinated catabolism of Tyr in addition to linking the plastidial biosynthetic pathways to the downstream metabolic pathways of the aromatic amino acids. Phenylpyruvate also serves as precursor for several secondary metabolites, including phenylacetaldehyde, 2-phenylethanol, and 2-phenylethyl β-D-glucopyranoside (99, 186, 197). In addition, Phe itself is the precursor for a wide range of intermediary and secondary metabolites of considerable importance for both plant structure and defense (44). The synthesis of Trp from chorismate requires the enzymes (a) anthranilate synthase, (b) phosphoribosylanthranilate transferase, (c) phosphoribosylanthranilate isomerase, (d ) indole3-glycerol phosphate synthase, and (e) α and β Trp synthase. In plants, anthranilate synthase is a heterotetramer consisting of two α and two β subunits and is feedback inhibited via the binding of Trp to the β subunit (146). Because anthranalite fluoresces a distinctive blue under UV light, it can be used as a phenotypic marker for identifying mutants in this step (88, 122); these mutants were additionally characterized as being feedback insensitive and thus accumulated Trp to three times the wild-type levels. The second enzyme in Trp biosynthesis converts anthranilate and phosphoribosylpyrophosphate into phosphoribosylanthranilate and inorganic pyrophosphate, and the expression of the gene encoding this enzyme is controlled by regulatory elements located inside the first two introns (165). The third enzyme is responsible for the conversion of phosphoribosylanthranilate into 1-(O-carboxyphenylamino)-1-deoxyribulose 5-phosphate (Figure 1). Arabidopsis has three genes that are differentially regulated in response to UV irradiation and the elicitor silver nitrate in a tissue- and cell-specific manner (83). The subsequent enzyme, indole-3-glycerol phosphate synthase, catalyzes the formation of indole-3- glycerol phosphate from 1-(O-carboxyphenylamino)-1-deoxyribulose 5-phosphate and is the only enzyme known that catalyzes the formation of the indole ring (195). This reaction is therefore important in the production of indolic secondary metabolites, including auxin (indole-3-acetic acid), camalexin, indole glucosinolates, and indole alkaloids. The final step in Trp biosynthesis is carried out in two parts by the α and β subunits of Trp synthase: First, indole-3-glycerol phosphate is cleaved to indole and glycerol-3-phosphate by the α subunit, and then the indole is transferred to the β subunit, which catalyzes its condensation with serine (Ser) to form Trp (137). Recent studies have shown that Phe and Trp levels are commonly upregulated during such diverse environmental conditions as light, water, and cold stress as well as during dark-induced senescence. Furthermore, it appears that plants are able to convert Phe and Trp into 2-oxoglutarate in a pathway that includes either isovaleryl coenzyme A (CoA) dehydrogenase or D-2-hydroxyglutarate dehydrogenase (9) in an as yet uncharacterized manner. The pattern of accumulation of Phe and Trp levels across the broad range of stress conditions mentioned above hints to potential in planta roles for either the free amino acids themselves or the metabolites thereof when plants are under stress. However, unequivocal elucidation of the functional importance of the aromatic amino acids under such conditions is presently lacking. Because of the essential nature of aromatic amino acids, boosting their levels in plants was attempted by transforming Arabidopsis with a recombinant bacterial AroG gene encoding 3-deoxyD-arabinoheptulosonate 7-phosphate (DAHP), the product of which was insensitive to feedback inhibition by the aromatic amino acids (192). Expression of this gene yielded a major effect on the levels of intermediate primary metabolites, such as shikimate, Phe, and Trp, and a broad range of secondary metabolites derived from these amino acids, including phenylpropanoids, glucosinolates, and various hormone conjugates (192). Expression of this gene in tomato fruit and also in petunia resulted in enhanced levels of the aromatic amino acids as well as enhanced levels of volatile and nonvolatile phenylpropanoids (151, 193). Interestingly, the Phe levels did not correlate with those of phenylpropanoids in a tomato introgression line population (2); thus, these studies suggest that modifying aromatic amino acid metabolism can be an effective metabolic engineering strategy only when the feedback regulation mechanisms are circumvented.