Oscillation of stem polarity expression in transport of natural auxin of pine cambium

Abstract

Simultaneous measurements of acropetal and basipetal efflux of natural auxin from the cambial region of a series of successive sections from <em>Pinus silvestris</em> stems revealed an unknown until now variation of polarity expression in the transport of auxin, which occurs in short cycles along the stem. It is suggested that the direction of phase synchronization of this oscillation between the neighboring cambial cells determines the fronts of the auxin waves. Modulations of the auxin-wave caused by apical application of synthetic IAA were shown to depend upon changes in the polarity expression in respect to natural auxin transport. Some evidence of a direct involvement of the exogenous auxin in the mechanism responsible for oscillations of the polarity expression was also obtained in experiments, in which <sup>14</sup>C-IAA was applied directly into the phloem of living trees of <em>Pinus taeda</em> L. Transport of labelled auxin was very slow however, and its direct involvement in polar transport (the oscillation of which was measured) extended only for a short distance from the place of aplication. Simultaneously, the natural auxin wave measured by bioassays extended to further distances. The results corroborate the hypothesis that oscillations of the polarity expression responsible for generation of the auxin wave in cambium depend primarily upon a system autonomous to cells of the cambial region, independent of the direct contribution of the molecules of auxin produced at distant sources. On the other hand, such external to cambium sources of auxin modulate the auxin-wave parameters affecting the system responsible for expression of the polarity in the regions close to their synthesis. These modulations are than propagated in the cambial zone.