Aspartate-Derived Amino Acid Biosynthesis inArabidopsis thaliana

Abstract

The aspartate-derived amino acid pathway in plants leads to the biosynthesis of lysine, methionine, threonine, and isoleucine. These four amino acids are essential in the diets of humans and other animals, but are present in growthlimiting quantities in some of the world's major food crops. Genetic and biochemical approaches have been used for the functional analysis of almost all Arabidopsis thaliana enzymes involved in aspartate-derived amino acid biosynthesis. The branch-point enzymes aspartate kinase, dihydrodipicolinate synthase, homoserine dehydrogenase, cystathionine gamma synthase, threonine synthase, and threonine deaminase contain well-studied sites for allosteric regulation by pathway products and other plant metabolites. In contrast, relatively little is known about the transcriptional regulation of amino acid biosynthesis and the mechanisms that are used to balance aspartatederived amino acid biosynthesis with other plant metabolic needs. The aspartate-derived amino acid pathway provides excellent examples of basic research conducted with A. thaliana that has been used to improve the nutritional quality of crop plants, in particular to increase the accumulation of lysine in maize and methionine in potatoes.

Deciphering the Role of Aspartate and Prephenate Aminotransferase Activities in Plastid Nitrogen Metabolism

“…Cell wall preparation and lignin quantification were performed following the method described by Lange et al (1995). Briefly, tubes containing about 200 mg of frozen samples were treated with 1 mL of methanol, shaken for 1 h, and then centrifuged at 7,500g for 5 min.…”

“…ALS, Acetolactate synthase; AS, Asn synthetase; ASN, asparaginase; BCAT, branched-chain aminotransferase; CBL, cystathionine b-lyase; CGS, cystathionine g-synthase; DAP-D, diaminopimelate decarboxylase; DAP-F, diaminopimelate epimerase; DHAD, dihydroxy acid dehydratase; DHDPR, dihydrodipicolinate reductase; DHDPS, dihydrodipicolinate synthase; HSDH, homoserine dehydrogenase; HSK, homoserine kinase; KARI, ketol-acid reductoisomerase; MS, Met synthase; TD, Thr deaminase; TS, Thr synthase. main pathways: (1) the biosynthesis of Asn in the cytosol (Lea and Azevedo, 2003;Lea et al, 2007); and (2) the biosynthesis of Lys, Met, Thr, and Ile through the socalled Asp metabolic pathway in the plastids (Azevedo et al, 2006;Jander and Joshi, 2009;Galili, 2011;Fig. 1).…”

Analysis of Loss-of-Function Mutants in Aspartate Kinase and Homoserine Dehydrogenase Genes Points to Complexity in the Regulation of Aspartate-Derived Amino Acid Contents

“…In major field crops, such as corn (Zea mays), rice (Oryza sativa), soybean (Glycine max), and potato (Solanum tuberosum), Asp-derived amino acids Lys, Met, Thr, and Ile are present at levels that limit the nutritional values (Müntz et al, 1998;Debabov, 2003;Pfefferle et al, 2003;Stiller et al, 2007). Increasing the contents of these amino acids in crop plants has long been a major goal of traditional breeding and genetic engineering Ufaz and Galili, 2008;Jander and Joshi, 2009). Genetic improvement requires a better understanding of the pathways and regulation of Asp-derived amino acids.…”

“…For example, the Arabidopsis (Arabidopsis thaliana) nuclear genome contains three AKencoding genes (AK1 [At5g13280], AK2 [A5g14060], and AK3 [At3g02020]) and two AK-HSDH-encoding genes (AK-HSDH1 [At1g31230] and AK-HSDH2 [At4g19710]; Jander and Joshi, 2009). Public microarray data indicated that the five enzymes are expressed in all plant tissues (Schmid et al, 2005;Winter et al, 2007).…”

“…The first step of the biosynthetic pathway for Asp-derived amino acids is the activation of Asp to Asp-4-P by monofunctional Asp kinases (AKs) and dual-functional Asp kinase-homoSer dehydrogenases (AK-HSDHs; Fig. 1; Jander and Joshi, 2009). Asp-4-P is then converted to Asp semialdehyde, the branch-point intermediate for Lys or Met, Thr, and Ile biosynthesis.…”