Auxin signal transduction into a quantitative change in natural auxin polar transport in pine cambium

Abstract

Results of experiments performed with 6-mm high stem sections of <em>Pinus sylvestris</em> L. confirmed the hypothesis that the fusiform cells of the cambial region respond to the arrival of indole-3-acetic-acid (IAA) at the signalling concentration in the apoplastic space around their apical ends by increasing the basipetal efflux of endogenous natural auxin. Thus, the auxin signal propagation along the stem cambial region could be a chain of reactions between the axially neighbouring cells, each capable of responding and contributing to the change of auxin concentration in the apoplast which requires only transduction of the foreign auxin signal by each of the cells to increase the basipetal efflux of their own endogenous auxin. The rate of auxin signal propagation in this system is not limited by the rate of auxin molecular transport, and if it functions in conjunction with feedback inhibition, it may produce oscillations of the auxin basipetal efflux generating supracellular auxin waves. Inhibitors of the proteinaceous auxin efflux carrier associated with the plasmalemma (NPA and TIBA), although reducing total basipetal transport of the natural auxin, did not prevent the stimulated additional efflux of this phytohormone. The studied auxin-signal transduction processes seem to be intracellular but not mediated by the Ca-calmodulin complex. The natural auxin basipetal efflux increased significantly within 45 min following the period when it had been strongly reduced by successive collections to agar receivers replaced several times at the basal ends of 6-mm high stem sections in which the intact fusiform cells of the cambial region in about 90% are arranged in only one axial row. Such exhaustion of the cellular reserve of auxin did not prevent the additional auxin basipetal efflux stimulated by the IAA apical treatment. The results may suggest an effect of the auxin signal upon the supply of newly-synthesised auxin directly to the system responsible for its basipetal efflux.