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

The phytohormone indole-3-acetic acid (IAA) plays a vital role in plant gro wth and development as a regulator of numerous biological processes. Its bi osynthetic pathways have been studied for decades. Recent genetic and in vi tro 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 biosynthes is 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 sm all rosettes and reduced fertility. Protein gel blot analysis indicated tha t IGS expression was greatly reduced in the antisense lines. Quantitative a nalyses of IAA and Trp content in antisense IGS transgenic plants and Trp b iosynthetic mutants revealed striking differences. Compared with wild-type plants, the Trp content in all the transgenic and mutant plants decreased s ignificantly. However, total IAA levels were significantly decreased in ant isense IGS transgenic plants, but remarkably increased in trp3-1 and trp2-1 plants. These results suggest that indole-3-glycerol phosphate (IGP) in th e Arabidopsis Trp biosynthetic pathway serves as a branchpoint compound in the Trp-independent IAA de novo biosynthetic pathway.