Tryptophan Mutants in Arabidopsis: The Consequences of Duplicated Tryptophan Synthase b Genes

Abstract

The cruciferous plant Arabidopsis thaliana has two closely related, nonallelic tryptophan synthase beta genes (TSB1 and TSB2), each containing four introns and a chloroplast leader sequence. Both genes are transcribed, although TSB1 produces greater than 90% of tryptophan synthase beta mRNA in leaf tissue. A tryptophan-requiring mutant, trp2-1, has been identified that has about 10% of the wild-type tryptophan synthase beta activity. The trp2-1 mutation is complemented by the TSB1 transgene and is linked genetically to a polymorphism in the TSB1 gene, strongly suggesting that trp2-1 is a mutation in TSB1. The trp2-1 mutants are conditional: they require tryptophan for growth under standard illumination but not under very low light conditions. Presumably, under low light the poorly expressed gene, TSB2, is capable of supporting growth. Genetic redundancy may be common to many aromatic amino acid biosynthetic enzymes in plants because mutants defective in two other genes (TRP1 and TRP3) also exhibit a conditional tryptophan auxotrophy. The existence of two tryptophan pathways has important consequences for tissue-specific regulation of amino acid and secondary metabolite biosynthesis.

A deletion in an indole synthase gene is responsible for the DIMBOA-deficient phenotype ofbxbx maize

“…There are several examples of duplication of tryptophan pathway genes (39)(40)(41)(42) with substantial evidence that one acts in tryptophan production while the other affords intermediates for production of defensive secondary metabolites (43)(44)(45). In Ruta graveolens, one of the duplicate genes for anthranilate synthase alpha (AS␣2) is constitutive and the corresponding enzyme is feedback-inhibited by L-tryptophan.…”

The ArabidopsisTUMOR PRONE5Gene Encodes an Acetylornithine Aminotransferase Required for Arginine Biosynthesis and Root Meristem Maintenance in Blue Light

“…The His biosynthesis mutant hpa1 resembles tup5-1 insofar as it has a short root when grown in vitro, but there is no obvious phenotype in the aerial part and hpa1 mutants are fully fertile plants (Mo et al, 2006). Mutants for several Trp biosynthetic pathway genes are generally impaired in development under high-light but not under low-light conditions (Last et al, 1991;Radwanski et al, 1996). It has been suggested that the residual activity of the mutant enzyme might be able to cover the lower Trp demand of slowgrowing plants in low light (Last et al, 1991).…”

“…Mutants for several Trp biosynthetic pathway genes are generally impaired in development under high-light but not under low-light conditions (Last et al, 1991;Radwanski et al, 1996). It has been suggested that the residual activity of the mutant enzyme might be able to cover the lower Trp demand of slowgrowing plants in low light (Last et al, 1991). In contrast, the roots and shoots of soil-grown tup5-1 plants grown under standard light develop normally, though high metabolic activity can be expected under these conditions.…”

“…One such example is the characterization of a Trp biosynthesis mutant displaying a severe growth defect under high light by Last et al (1991); another one is the description of a Lys biosynthesis mutant showing altered leaf morphology and mild dwarfism by Song et al (2004). Amino acid metabolism might also influence root meristem maintenance, as well as root growth and development, as was indicated by mutants of Asp and His biosynthesis (Miesak and Coruzzi, 2002;Mo et al, 2006).…”

Variations in CYP74B2 (Hydroperoxide Lyase) Gene Expression Differentially Affect Hexenal Signaling in the Columbia and Landsberg erecta Ecotypes of Arabidopsis

“…Together with the higher expression levels of CYP74B2 (HPL) transcripts and volatiles produced, these data suggest that the levels of JAs are also elevated in Ler leaves compared to Col leaves. Also interesting among the set more highly expressed in the Ler ecotype are several loci in aliphatic glucosinolate synthesis, including CYP79F1, which exists upstream of CYP83A1 (Hansen et al, 2001;Reintanz et al, 2001;Chen et al, 2003), CYP83A1 itself, and 2-oxoglutarate-dependent dioxygenase (AOP3), which exists downstream of CYP83A1 (Kliebenstein et al, 2001b); Trp synthetase (TSB2; Last et al, 1991) also potentially impacts aliphatic glucosinolate synthesis because it is postulated to feed substrates into the aliphatic glucosinolate pathway via CYP79B2 and CYP79B3 . Contrasting with these glucosinolate synthetic enzymes that are more highly expressed in the Ler ecotype, one locus more highly expressed in the Col ecotype codes for thioglycosyl hydrolase (TGG2, myrosinase), which degrades glucosinolates to release toxic derivatives (Xue et al, 1995).…”