Mechanisms of Auxin Regulation of Structural and Physiological Polarity in Plants, Tissues, Cells and Embryos

Abstract

New experiments and insights extend current concepts to yield four integrated hypotheses on polarization. (1) In tissues that lack polarity, as in meristems and proliferating callus, short-distance diffusive movement of auxin between spatially separate sources and sinks can gradually induce polar transport of auxin and associated proton currents in the direction of the initially diffusive movement. (2) The formation of auxin at the shoot tip, on which maintenance of the plant's polarity depends, occurs by hydrolysis of auxin precursors such as IAA-myo-inositol, which readily hydrolyses (even non-enzymically) at rates that increase with pH, so that higher pHs arising in leaf primordia through auxin-proton cotransport can promote local formation of auxin. (3) The long axes of cells and the secondary wall banding of tracheary elements tend respectively to develop parallel and perpendicular to the direction of polar transport of auxin through tissue, especially vascular tissue; these orientations are mediated by the bioelectric current associated with polar auxin transport, through orientation of cortical microtubules transverse to the current. (4) Initiation of polarity in zygotic embryos is controlled by the direction of auxin movement in the surrounding parental tissues; in lower plants the exoscopic embryo's polarity is similar to that of the surrounding tissues, but in seed plants the endoscopic embryo's polarity is inverted as a result of physiological isolation, localised auxin breakdown, and suspensor formation.