Indole‐3‐glycerol phosphate, a branchpoint of indole‐3‐acetic acid biosynthesis from the tryptophan biosynthetic pathway in Arabidopsis thaliana

Abstract

The phytohormone indole-3-acetic acid (IAA) plays a vital role in plant growth and development as a regulator of numerous biological processes. Its biosynthetic pathways have been studied for decades. Recent genetic and in vitro labeling evidence indicates that IAA in Arabidopsis thaliana and other plants is primarily synthesized from a precursor that is an intermediate in the tryptophan (Trp) biosynthetic pathway. To determine which intermediate(s) acts as the possible branchpoint for the Trp-independent IAA biosynthesis in plants, we took an in vivo approach by generating antisense indole-3-glycerol phosphate synthase (IGS) RNA transgenic plants and using available Arabidopsis Trp biosynthetic pathway mutants trp2-1 and trp3-1. Antisense transgenic plants display some auxin deficient-like phenotypes including small rosettes and reduced fertility. Protein gel blot analysis indicated that IGS expression was greatly reduced in the antisense lines. Quantitative analyses of IAA and Trp content in antisense IGS transgenic plants and Trp biosynthetic mutants revealed striking differences. Compared with wild-type plants, the Trp content in all the transgenic and mutant plants decreased significantly. However, total IAA levels were significantly decreased in antisense IGS transgenic plants, but remarkably increased in trp3-1 and trp2-1 plants. These results suggest that indole-3-glycerol phosphate (IGP) in the Arabidopsis Trp biosynthetic pathway serves as a branchpoint compound in the Trp-independent IAA de novo biosynthetic pathway.

Indole-3-acetic acid: A widespread physiological code in interactions of fungi with other organisms

“…Consequently, information on the biochemical processes involved in Trp-independent IAA production in plants is limited. 20,21 In plants, 4 Trp-dependent pathways have been proposed: indole-3-acetamide (IAM), indole-3-pyruvic acid (IPA), tryptamine (TRA), and indole-3-acetaldoxime pathways. 18 Although different plant species might use specific strategies or modifications to optimize synthetic pathways, plants would be expected to share evolutionarily conserved core mechanisms for IAA biosynthesis.…”

Tryptophan-independent auxin biosynthesis contributes to early embryogenesis in Arabidopsis

“…The TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS (TAA)/YUCCA (YUC) linear pathway has been considered as a predominant Trp-dependent auxin biosynthetic pathway (4,(6)(7)(8). However, labeling studies and analyses of Trp auxotrophic mutants have long predicted the existence of a Trp-independent IAA biosynthetic pathway (9)(10)(11)(12)(13). When the Arabidopsis and maize seedlings were fed with isotope-labeled precursor, IAA was more enriched than Trp, and the incorporation of label into IAA from Trp is low, suggesting that IAA can be produced de novo without Trp as an intermediate (11,12).…”

“…When the Arabidopsis and maize seedlings were fed with isotope-labeled precursor, IAA was more enriched than Trp, and the incorporation of label into IAA from Trp is low, suggesting that IAA can be produced de novo without Trp as an intermediate (11,12). The Trp biosynthetic mutant trp1, defective in phosphoribosylanthranilate transferase (PAT), and indole-3-glycerol phosphate synthase (IGS) antisense plants are deficient in steps earlier than indole-3-glycerol phosphate (IGP) formation and display decreased levels of both IAA and Trp (10,14). However, the trp3 and trp2 mutants, defective in tryptophan synthase α (TSA) and tryptophan synthase β (TSB) subunits, respectively, accumulate higher levels of IAA than the wild type despite containing lower Trp levels (10,11,15,16).…”

“…The Trp biosynthetic mutant trp1, defective in phosphoribosylanthranilate transferase (PAT), and indole-3-glycerol phosphate synthase (IGS) antisense plants are deficient in steps earlier than indole-3-glycerol phosphate (IGP) formation and display decreased levels of both IAA and Trp (10,14). However, the trp3 and trp2 mutants, defective in tryptophan synthase α (TSA) and tryptophan synthase β (TSB) subunits, respectively, accumulate higher levels of IAA than the wild type despite containing lower Trp levels (10,11,15,16). These data strongly suggested the existence of the Trp-independent IAA biosynthetic pathway that might branch from IGP and/or indole (10).…”

“…However, the trp3 and trp2 mutants, defective in tryptophan synthase α (TSA) and tryptophan synthase β (TSB) subunits, respectively, accumulate higher levels of IAA than the wild type despite containing lower Trp levels (10,11,15,16). These data strongly suggested the existence of the Trp-independent IAA biosynthetic pathway that might branch from IGP and/or indole (10).…”