The Tryptophan Conjugates of Jasmonic And Indole-3-Acetic Acids Are Endogenous Auxin Inhibitors

Paul E. Staswick ^*

Department of Agronomy and Horticulture, University of Nebraska, Lincoln, Nebraska 68583-0915

^* Corresponding author; email: pstaswick1{at}unl.edu.

Most conjugates of plant hormones are inactive and some functionto reduce the active hormone pool. This study characterizedthe activity of the Trp conjugate of jasmonic acid (JA-Trp)in Arabidopsis thaliana. Unexpectedly, JA-Trp caused agravitropicroot growth in seedlings, unlike JA or nine other JA-amino acidconjugates. The response was dose dependent from 1 to100 µM,was independent of the COI1 jasmonate signaling locus, and unlikethe jasmonate signal JA-Ile, JA-Trp minimally inhibited rootgrowth. The Trp conjugate with indole-3-acetic acid (IAA-Trp)produced a similar response while Trp alone and conjugates withbenzoic and cinnamic acids did not. JA-Trp and IAA-Trp at 25µM nearly eliminated seedling root inhibition caused by2 µM IAA. The TIR1 auxin receptor is required for activitybecause roots of tir1-1 grew only $~$ 60% of wild type length onIAA plus JA-Trp, even though tir1-1 is auxin resistant. However,neither JA-Trp nor IAA-Trp interfered with IAA-dependent interactionbetween TIR1 and Aux/IAA7 in cell-free assays. Trp conjugatesinhibited IAA-stimulated lateral root production and DR5-GUSgene expression. JA deficient mutants were hypersensitive toIAA and a Trp over accumulating mutant was less sensitive, suggestingendogenous conjugates affect auxin sensitivity. Conjugates werepresent at 5.8 pmoles g^-1 FW or less in roots, seedlings, leavesand flowers and the values increased $~$ 10-fold in roots incubatedin 25 µM Trp and IAA or JA at 2 µM. These resultsshow that JA-Trp and IAA-Trp constitute a previously unrecognizedmechanism to regulate auxin action.