title: The tryptophan conjugates of jasmonic and indole-3-acetic acids are endogenous auxin inhibitors.
abstract: Most conjugates of plant hormones are inactive, and some function to reduce the active hormone pool. This study characterized the activity of the tryptophan (Trp) conjugate of jasmonic acid (JA-Trp) in Arabidopsis (Arabidopsis thaliana). Unexpectedly, JA-Trp caused agravitropic root growth in seedlings, unlike JA or nine other JA-amino acid conjugates. The response was dose dependent from 1 to 100 microm, was independent of the COI1 jasmonate signaling locus, and unlike the jasmonate signal JA-isoleucine, JA-Trp minimally inhibited root growth. The Trp conjugate with indole-3-acetic acid (IAA-Trp) produced a similar response, while Trp alone and conjugates with benzoic and cinnamic acids did not. JA-Trp and IAA-Trp at 25 microm nearly eliminated seedling root inhibition caused by 2 microm IAA. The TIR1 auxin receptor is required for activity because roots of tir1-1 grew only approximately 60% of wild-type length on IAA plus JA-Trp, even though tir1-1 is auxin resistant. However, neither JA-Trp nor IAA-Trp interfered with IAA-dependent interaction between TIR1 and Aux/IAA7 in cell-free assays. Trp conjugates inhibited IAA-stimulated lateral root production and DR5-beta-glucuronidase gene expression. JA-deficient mutants were hypersensitive to IAA and a Trp-overaccumulating mutant was less sensitive, suggesting endogenous conjugates affect auxin sensitivity. Conjugates were present at 5.8 pmol g(-1) fresh weight or less in roots, seedlings, leaves, and flowers, and the values increased approximately 10-fold in roots incubated in 25 microm Trp and IAA or JA at 2 microm. These results show that JA-Trp and IAA-Trp constitute a previously unrecognized mechanism to regulate auxin action.
authors.affiliation: Department of Agronomy and Horticulture, University of Nebraska, Lincoln, Nebraska 68583-0915, USA. pstaswick1@unl.edu
authors.forename: Paul E
authors.initials: PE
authors.lastname: Staswick
mesh_terms: D017360:Arabidopsis / Q000187:drug effects* / Q000254:growth & development / Q000378:metabolism; D029681:Arabidopsis Proteins / Q000235:genetics / Q000502:physiology; D001692:Biological Transport; D003517:Cyclopentanes / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology*; D044783:F-Box Proteins / Q000235:genetics / Q000502:physiology; D015870:Gene Expression / Q000187:drug effects; D018522:Gravitropism / Q000187:drug effects; D007210:Indoleacetic Acids / Q000037:antagonists & inhibitors* / Q000737:chemistry / Q000494:pharmacology; D054883:Oxylipins / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology*; D010937:Plant Growth Regulators / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D018517:Plant Roots / Q000187:drug effects / Q000254:growth & development; D011956:Receptors, Cell Surface / Q000235:genetics / Q000502:physiology; D014364:Tryptophan / Q000378:metabolism*
chemical_list: D029681:Arabidopsis Proteins; D003517:Cyclopentanes; D044783:F-Box Proteins; D007210:Indoleacetic Acids; D054883:Oxylipins; D010937:Plant Growth Regulators; D011956:Receptors, Cell Surface; C501405:TIR1 protein, Arabidopsis; C011006:jasmonic acid; C030737:indoleacetic acid; D014364:Tryptophan