title: Auxin regulates the initiation and radial position of plant lateral organs.
abstract: Leaves originate from the shoot apical meristem, a small mound of undifferentiated tissue at the tip of the stem. Leaf formation begins with the selection of a group of founder cells in the so-called peripheral zone at the flank of the meristem, followed by the initiation of local growth and finally morphogenesis of the resulting bulge into a differentiated leaf. Whereas the mechanisms controlling the switch between meristem propagation and leaf initiation are being identified by genetic and molecular analyses, the radial positioning of leaves, known as phyllotaxis, remains poorly understood. Hormones, especially auxin and gibberellin, are known to influence phyllotaxis, but their specific role in the determination of organ position is not clear. We show that inhibition of polar auxin transport blocks leaf formation at the vegetative tomato meristem, resulting in pinlike naked stems with an intact meristem at the tip. Microapplication of the natural auxin indole-3-acetic acid (IAA) to the apex of such pins restores leaf formation. Similarly, exogenous IAA induces flower formation on Arabidopsis pin-formed1-1 inflorescence apices, which are blocked in flower formation because of a mutation in a putative auxin transport protein. Our results show that auxin is required for and sufficient to induce organogenesis both in the vegetative tomato meristem and in the Arabidopsis inflorescence meristem. In this study, organogenesis always strictly coincided with the site of IAA application in the radial dimension, whereas in the apical-basal dimension, organ formation always occurred at a fixed distance from the summit of the meristem. We propose that auxin determines the radial position and the size of lateral organs but not the apical-basal position or the identity of the induced structures.
authors.affiliation: Institute of Plant Physiology, University of Berne, CH-3013 Berne, Switzerland.
authors.forename: D
authors.initials: D
authors.lastname: Reinhardt
authors.forename: T
authors.initials: T
authors.lastname: Mandel
authors.forename: C
authors.initials: C
authors.lastname: Kuhlemeier
mesh_terms: D000225:Adenine / Q000031:analogs & derivatives / Q000494:pharmacology; D017360:Arabidopsis / Q000166:cytology / Q000187:drug effects / Q000235:genetics / Q000254:growth & development; D029681:Arabidopsis Proteins; D001692:Biological Transport / Q000187:drug effects; D060406:Brassinosteroids; D002455:Cell Division / Q000187:drug effects; D002777:Cholestanols / Q000494:pharmacology; D046508:Culture Techniques; D005875:Gibberellins / Q000494:pharmacology; D006027:Glycosides / Q000494:pharmacology; D007210:Indoleacetic Acids / Q000378:metabolism / Q000494:pharmacology*; D007701:Kinetin; D018551:Solanum lycopersicum / Q000166:cytology / Q000187:drug effects / Q000254:growth & development / Q000378:metabolism; D008565:Membrane Proteins / Q000235:genetics; D026901:Membrane Transport Proteins; D018519:Meristem / Q000166:cytology / Q000187:drug effects / Q000254:growth & development* / Q000378:metabolism; D009024:Morphogenesis / Q000187:drug effects; D009154:Mutation / Q000235:genetics; D010641:Phenotype; D010797:Phthalimides / Q000494:pharmacology; D018515:Plant Leaves / Q000166:cytology / Q000187:drug effects / Q000254:growth & development* / Q000378:metabolism; D018514:Plant Structures / Q000166:cytology / Q000187:drug effects / Q000254:growth & development / Q000378:metabolism; D013260:Steroids, Heterocyclic / Q000494:pharmacology
chemical_list: D029681:Arabidopsis Proteins; D060406:Brassinosteroids; D002777:Cholestanols; D005875:Gibberellins; D006027:Glycosides; D007210:Indoleacetic Acids; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; C116363:PIN1 protein, Arabidopsis; D010797:Phthalimides; D013260:Steroids, Heterocyclic; C007808:fusicoccin; C002687:alpha-naphthylphthalamic acid; C030737:indoleacetic acid; D000225:Adenine; D007701:Kinetin; C023623:brassinolide