Auxin induces exocytosis and the rapid synthesis of a high-turnover pool of plasma-membrane H⁺-ATPase

Abstract

Auxin causes elongation growth of plant cells by increasing the plastic extensibility of the cell wall. Putative cellular events involved in this hormone action were studied using maize (Zea mays L.) coleoptiles with the following results: (i) Auxin enhances membrane flow from the endoplasmic reticulum to the plasma membrane (PM). This effect was demonstrated by pulse-labeling of the endoplasmic reticulum with myo-[³H]inositol in coleoptile segments and by measuring the distribution of the label within isolated and separated microsomal membrane fractions, (ii) Auxin rapidly increases the amount of antibody-detectable H⁺-ATPase in the PM. This augmentation is already significant 10 min after the addition of indole-3-acetic acid (IAA) and reaches a new higher steady-state level after about 30 min. (iii) Cycloheximide, a potent inhibitor of both protein synthesis and extension growth, quickly diminishes the auxin-enhanced level of the PM H⁺-ATPase, indicating an apparent half-life of the enzyme of around 12 min. (iv) Cordycepin, which blocks the synthesis of mRNAs, reduces the auxin-elevated level of the H⁺-ATPase similar to cycloheximide. (v) Changes in the growth rate of coleoptile segments in response to IAA, cycloheximide, and cordycepin exactly reflect the changes of the H⁺-ATPase level in the PM. (vi) The elongation growth induced by fusicoccin, or ester compounds, or by an elevated CO₂ concentration in the incubation medium, is not related to an increased number of H⁺-ATPase molecules within the PM. (vii) The necessity of H⁺ for cell-wall-loosening processes is again demonstrated by growth experiments with abraded coleoptile segments. The adjustment of the cell wall to a pH of ≥6.5 completely abolishes the auxin-induced elongation growth; no inhibition occurs with non-abraded segments. Buffer solutions of pH ≤6.0 induce “acid growth” of abraded segments for several hours. It is suggested that auxin activates a cluster of genes responsible (i) for the induction and acceleration of exocytotic processes (e.g. by the synthesis of either proteins, necessary for the fusion of membranes, or of other effectors); (ii) for the synthesis of PM H⁺-ATPases, increasing the capacity for H⁺-extrusion into the apoplast as a precondition for wall enlargement (“acid growth”); (iii) for a supposed synthesis and exocytosis of certain proteins, enzymes and wall precursors necessary for wall metabolism and the “repair” of the proton-loosened and turgor-stretched cell wall. Both, fusicoccin and auxin affect cell-wall plasticity according to the “acid-growth” theory. However, the mechanisms leading to this event are completely different; the auxinenhanced H⁺-extrusion is a gene-controlled process.