Auxin regulates the initiation and radial position of plant lateral organs

Leaves originate from the shoot apical meristem, a small mound of undiffere ntiated tissue at the tip of the stem, Leaf formation begins with the selec tion of a group of founder cells in the so-called peripheral zone at the fl ank of the meristem, followed by the initiation of local growth and finally morphogenesis of the resulting bulge into a differentiated leaf, Whereas t he mechanisms controlling the switch between meristem propagation and leaf initiation are being identified by genetic and molecular analyses, the radi al positioning of leaves, known as phyllotaxis, remains poorly understood. Hormones, especially auxin and gibberellin, are known to influence phyllota xis, but their specific role in the determination of organ position is not clear. We show that inhibition of polar auxin transport blocks leaf formati on at the vegetative tomato meristem, resulting in pinlike naked stems with an intact meristem at the tip. Microapplication of the natural auxin indol e-3-acetic acid (IAA) to the apex of such pins restores leaf formation. Sim ilarly, exogenous IAA induces flower formation on Arabidopsis pin-formed1-1 inflorescence apices, which are blocked in flower formation because of a m utation in a putative auxin transport protein. Our results show that auxin is required for and sufficient to induce organogenesis both in the vegetati ve tomato meristem and in the Arabidopsis inflorescence meristem. In this s tudy, organogenesis always strictly coincided with the site of IAA applicat ion in the radial dimension, whereas in the apical-basal dimension, organ f ormation always occurred at a fixed distance from the summit of the meriste m, We propose that auxin determines the radial position and the size of lat eral organs but not the apical-basal position or the identity of the induce d structures.