title: Bimodal regulation of ICR1 levels generates self-organizing auxin distribution.
abstract: Auxin polar transport, local maxima, and gradients have become an important model system for studying self-organization. Auxin distribution is regulated by auxin-dependent positive feedback loops that are not well-understood at the molecular level. Previously, we showed the involvement of the RHO of Plants (ROP) effector INTERACTOR of CONSTITUTIVELY active ROP 1 (ICR1) in regulation of auxin transport and that ICR1 levels are posttranscriptionally repressed at the site of maximum auxin accumulation at the root tip. Here, we show that bimodal regulation of ICR1 levels by auxin is essential for regulating formation of auxin local maxima and gradients. ICR1 levels increase concomitant with increase in auxin response in lateral root primordia, cotyledon tips, and provascular tissues. However, in the embryo hypophysis and root meristem, when auxin exceeds critical levels, ICR1 is rapidly destabilized by an SCF(TIR1/AFB) [SKP, Cullin, F-box (transport inhibitor response 1/auxin signaling F-box protein)]-dependent auxin signaling mechanism. Furthermore, ectopic expression of ICR1 in the embryo hypophysis resulted in reduction of auxin accumulation and concomitant root growth arrest. ICR1 disappeared during root regeneration and lateral root initiation concomitantly with the formation of a local auxin maximum in response to external auxin treatments and transiently after gravitropic stimulation. Destabilization of ICR1 was impaired after inhibition of auxin transport and signaling, proteasome function, and protein synthesis. A mathematical model based on these findings shows that an in vivo-like auxin distribution, rootward auxin flux, and shootward reflux can be simulated without assuming preexisting tissue polarity. Our experimental results and mathematical modeling indicate that regulation of auxin distribution is tightly associated with auxin-dependent ICR1 levels.
authors.affiliation: Department of Molecular Biology and Ecology of Plants, Tel Aviv University, Tel Aviv 69978, Israel; and.
authors.forename: Ora
authors.initials: O
authors.lastname: Hazak
authors.affiliation: Department of Molecular Biology and Ecology of Plants, Tel Aviv University, Tel Aviv 69978, Israel; and.
authors.forename: Uri
authors.initials: U
authors.lastname: Obolski
authors.affiliation: Institute of Science and Technology (IST Austria), A-3400 Klosterneuburg, Austria.
authors.forename: Tomáš
authors.initials: T
authors.lastname: Prat
authors.affiliation: Institute of Science and Technology (IST Austria), A-3400 Klosterneuburg, Austria.
authors.forename: Jiří
authors.initials: J
authors.lastname: Friml
authors.affiliation: Department of Molecular Biology and Ecology of Plants, Tel Aviv University, Tel Aviv 69978, Israel; and.
authors.forename: Lilach
authors.initials: L
authors.lastname: Hadany
authors.affiliation: Department of Molecular Biology and Ecology of Plants, Tel Aviv University, Tel Aviv 69978, Israel; and shauly@tauex.tau.ac.il.
authors.forename: Shaul
authors.initials: S
authors.lastname: Yalovsky
mesh_terms: D017360:Arabidopsis / Q000235:genetics / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000378:metabolism*; D001692:Biological Transport / Q000502:physiology; D002352:Carrier Proteins / Q000378:metabolism*; D017931:DNA Primers / Q000235:genetics; D005453:Fluorescence; D018506:Gene Expression Regulation, Plant / Q000502:physiology*; D007091:Image Processing, Computer-Assisted; D007210:Indoleacetic Acids / Q000378:metabolism*; D018613:Microscopy, Confocal; D008954:Models, Biological; D030821:Plants, Genetically Modified; D059748:Proteolysis; D015398:Signal Transduction / Q000502:physiology
chemical_list: D029681:Arabidopsis Proteins; D002352:Carrier Proteins; D017931:DNA Primers; C522443:ICR1 protein, Arabidopsis; D007210:Indoleacetic Acids
keywords: ROP GTPases; auxin; pattern formation; root development; transport