Ion fluxes, auxin and the induction of elongation growth in Nicotiana tabacum cells

Immobilized cultured tobacco cells become polarized upon the addition of na phthalene-1-acetic acid and start to elongate from an initial spherical sha pe. The question as to how a diffuse-growing cell forms a polar axis is add ressed here with approaches successfully applied to the study of tip growth . With two kinds of vibrating probes the electric current flow and proton f luxes were mapped around such elongating cells. No consistent polar pattern of ion fluxes, which is typical for actively tip-growing cells, was detect ed. Therefore, other signals must provide the positional information needed for polar axis formation. Furthermore, neither a specific pattern of intra cellular Ca2+ concentration nor a polar distribution of putative ion-channe l antagonist-binding sites were found in elongating tobacco cells. Auxin fl ux, on the other hand, was found to be important as TIBA, an inhibitor of p olar auxin transport, clearly inhibited elongation in a concentration-depen dent way. Cross-linking of arabinogalactan-proteins with the beta -Yariv re agent also resulted in inhibition of elongation. A model is proposed for th e induction of polar growth where localized auxin efflux starts a signal ca scade that triggers molecules that reorient microtubules. These then guide cellulose deposition in the cell wall, which in turn alters cell wall mecha nics and leads to elongation. In this scheme, arabinogalactan-proteins are not causal agents but are probably important regulators of growth and survi val of the cell.