Genetic analysis of pathway regulation for enhancing branched-chain amino acid biosynthesis in plants

• Published in: The Plant journal : for cell and molecular biology Vol. 63; no. 4; pp. 573 - 583
• Main Authors: Chen, Hao, Saksa, Kristen, Zhao, Feiyi, Qiu, Joyce, Xiong, Liming
• Format: Journal Article
• Language: English
• Published: Oxford, UK Oxford, UK : Blackwell Publishing Ltd 01.08.2010; Blackwell Publishing Ltd; Blackwell
• Subjects: Acetolactate Synthase - genetics; Acetolactate Synthase - metabolism; ACT domain; AHAS; Amino Acid Sequence; Amino acids; Amino Acids, Branched-Chain - biosynthesis; Amino Acids, Branched-Chain - pharmacology; Arabidopsis - enzymology; Arabidopsis - genetics; Arabidopsis - metabolism; Arabidopsis Proteins - genetics; Arabidopsis Proteins - metabolism; BCAA; Biological and medical sciences; Biosynthesis; Biosynthetic Pathways - genetics; branched-chain amino acids; Cloning, Molecular; Drug Resistance - genetics; essential amino acids; Feedback, Physiological; Fundamental and applied biological sciences. Psychology; Gene expression; Gene Expression Profiling; Gene Expression Regulation, Enzymologic; Gene Expression Regulation, Plant; Leucine - biosynthesis; Leucine - pharmacology; Molecular Sequence Data; Mutation; Plant biology; Plant physiology and development; Plants, Genetically Modified; Protein Subunits - genetics; Protein Subunits - metabolism; Reverse Transcriptase Polymerase Chain Reaction; Sequence Homology, Amino Acid; valine resistance; Vegetals; AHAS; Biosynthesis; branched-chain amino acids; Genetic determinism; Resistance; valine resistance; Branched chain; Plant; Regulation(control); essential amino acids; Valine; Aminoacid; ACT domain; BCAA
• ISSN: 0960-7412; 1365-313X
• DOI: 10.1111/j.1365-313X.2010.04261.x

The branched-chain amino acids (BCAAs) valine, leucine and isoleucine are essential amino acids that play critical roles in animal growth and development. Animals cannot synthesize these amino acids and must obtain them from their diet. Plants are the ultimate source of these essential nutrients, and they synthesize BCAAs through a conserved pathway that is inhibited by its end products. This feedback inhibition has prevented scientists from engineering plants that accumulate high levels of BCAAs by simply over-expressing the respective biosynthetic genes. To identify components critical for this feedback regulation, we performed a genetic screen for Arabidopsis mutants that exhibit enhanced resistance to BCAAs. Multiple dominant allelic mutations in the VALINE-TOLERANT 1 (VAT1) gene were identified that conferred plant resistance to valine inhibition. Map-based cloning revealed that VAT1 encodes a regulatory subunit of acetohydroxy acid synthase (AHAS), the first committed enzyme in the BCAA biosynthesis pathway. The VAT1 gene is highly expressed in young, rapidly growing tissues. When reconstituted with the catalytic subunit in vitro, the vat1 mutant-containing AHAS holoenzyme exhibits increased resistance to valine. Importantly, transgenic plants expressing the mutated vat1 gene exhibit valine tolerance and accumulate higher levels of BCAAs. Our studies not only uncovered regulatory characteristics of plant AHAS, but also identified a method to enhance BCAA accumulation in crop plants that will significantly enhance the nutritional value of food and feed.