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.

Cell division pattern influences gene expression in the shoot apical meristem

“…Superimposed on this molecular engine for meristem maintenance is a mechanism by which particular regions of the meristem become determined for leaf formation. How this occurs is still unclear, but elements of hormone action, cell wall extensibility, and transcription factor activity have all been implicated (7,25,26). Understanding how these different levels of meristem maintenance and function interact and are coordinated is a major target for research in plant development.…”

Computer simulations reveal properties of the cell-cell signaling network at the shoot apex in Arabidopsis

“…(i) Auxin passively diffuses via all walls (edges of the individual cells in the graph) and is actively transported via oriented connections only (1)(2)(3)(4)(8)(9)(10)(11). We only consider net auxin flux from cell to cell, without taking into account the molecular mechanisms involved (5,(12)(13)(14).…”

Virtual phyllotaxis and real plant model cases

“…The latest developments in plant biology point out the role of auxin, transported acropetally, in the superficial cellular layer of SAM (Reinhardt et al 2000(Reinhardt et al , 2003Heisler et al 2005). The polarity of the cells transporting the hormone alters periodically in the vicinity of emerging primordium, with the flux of auxin being once oriented towards the primordium tip and, after a while, away from it, towards the tip of the apex itself.…”