A Molecular Framework for the Control of Adventitious Rooting by the TIR1/AFB2-Aux/IAA-Dependent Auxin Signaling in Arabidopsis
Mol Plant. 2019 Sep 11 [Epub ahead of print]
Lakehal A, Chaabouni S, Cavel E, Le Hir R, Ranjan A, Raneshan Z, Novák O, Păcurar DI, Perrone I, Jobert F, Gutierrez L, Bakò L, Bellini C

Abstract
In Arabidopsis thaliana, canonical auxin-dependent gene regulation is mediated by 23 transcription factors from the AUXIN RESPONSE FACTOR (ARF) family interacting with 29 auxin/indole acetic acid repressors (Aux/IAA), themselves forming coreceptor complexes with one of six TRANSPORT INHIBITOR1/AUXIN-SIGNALLING F-BOX (TIR1/AFB) PROTEINS. Different combinations of co-receptors drive specific sensing outputs, allowing auxin to control a myriad of processes. Considerable efforts have been made to discern the specificity of auxin action. However, owing to a lack of obvious phenotype in single loss-of-function mutants in Aux/IAA genes, most genetic studies have relied on gain-of-function mutants, which are highly pleiotropic. ARF6 and ARF8 are positive regulators of adventitious root initiation upstream of jasmonate, but the exact auxin co-receptor complexes controlling the transcriptional activity of these proteins was still unknown. Here using loss-of-function mutants we show that IAA6, IAA9 and IAA17 genes act additively in the control of AR initiation, and by performing protein-protein interaction analysis, we show that the corresponding proteins interact with ARF6 and/or ARF8 and likely repress their activity. We also demonstrate that TIR1 and AFB2 are positive regulators of adventitious root formation and suggest a dual role for TIR1 in the control of JA biosynthesis and conjugation, as revealed by upregulation of several JA biosynthesis genes in the tir1-1 mutant. We propose that in the presence of auxin, TIR1 and AFB2 form specific sensing complexes with IAA6, IAA9 and/or IAA17 that modulate JA homeostasis to control AR initiation.

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