© 2010 American Society of Plant Biologists
First published on May 17, 2010: e0132. 10.1199/tab.0132
The Biosynthetic Pathways for Shikimate and Aromatic Amino Acids in Arabidopsis thaliana
Vered Tzin a and Gad Galili a,1
Department of Plant Sciences, The Weizmann Institute of Science, Rehovot 76100 Israel. Address correspondence to firstname.lastname@example.orgThe aromatic amino acids phenylalanine, tyrosine and tryptophan in plants are not only essential components of protein synthesis, but also serve as precursors for a wide range of secondary metabolites that are important for plant growth as well as for human nutrition and health. The aromatic amino acids are synthesized via the shikimate pathway followed by the branched aromatic amino acidmetabolic pathway, with chorismate serving as a major branch point intermediate metabolite. Yet, the regulation of their synthesis is still far from being understood. So far, only three enzymes in this pathway, namely, chorismate mutase of phenylalanine and tyrosine synthesis, tryptophan synthase of tryptophan biosynthesis and arogenate dehydratase of phenylalanine biosynthesis, proved experimentallyto be allosterically regulated. The major biosynthesis route of phenylalanine in plants occurs via arogenate. Yet, recent studies suggest that an alternative route of phynylalanine biosynthesis via phenylpyruvate may also exist in plants, similarly to many microorganisms. Several transcription factors regulating the expression of genes encoding enzymes of both the shikimate pathway and aromaticamino acid metabolism have also been recently identiﬁed in Arabidopsis and other plant species.
INTRODUCTION The aromatic amino acids (AAA), phenylalanine (Phe), tyrosine (Tyr) and tryptophan (Trp) (Fig. 1), are central molecules in plant metabolism. Besides their function as building blocks of proteins, the three AAA serve as precursors for a variety of plant hormones, such as auxin andsalicylate, as well as for a very wide range of aromatic secondary metabolites with multiple biological functions and biotechnological value in the health promoting, medical and food industries (Bartel, 1997; Vogt, 2010). The AAA of plants are also essential nutritive compounds in the diets of humans and monogastric livestock, which are unable to synthesize them (Li and Last, 1996; Galili et al., 2002).Additionally, the shikimate pathway enzyme 5-enolpyruvylshikimate-3-phospate synthase (EPSP synthase) is the target of the glyphosate herbicide, and non-plant EPSP synthase provides the herbicide-resistance trait in a number of commercial transgenic crops (Duke and Powles, 2008). These important properties account for the major motivation to elucidate the regulation of the shikimate and AAAbiosynthesis pathways in plants. The biosynthesis of AAA from core primary metabolism initiates via the shikimate pathway, leading to the synthesis of chorismate (Fig. 2). Chorismate is the initial branch point metabolite in the synthesis of all three AAA (Fig. 2) and the wide range of aromatic secondary metabolites derived from it (Gilchrist and Kosuge, 1980; Herrmann, 1995). Hence, the shikimate andAAA
Figure 1. Structures of chorismate and the three aromatic amino acids.
biosynthesis pathways also represent a major regulatory link of primary and secondary metabolism in plants. Despite the extreme signiﬁcance of the AAA to the life cycles of plants, the regulation their biosynthesis via the shikimate and AAA biosynthesis pathways has been largely ignored and even not reviewed in the lastdecade. Yet, these biosynthesis pathways have been re-visited in recent years by a number of studies. The present review focuses on new insights into the regulation of AAA biosynthesis, which are based on: (i) recent studies, focusing mainly on Phe and to a smaller extent also on Tyr and Trp biosyn-
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Figure 2. The shikimate pathway. Enzymes involved in the...