This article will cover historical and recent aspects of reactions and mechanisms involved in the auxin-induced signalling cascade that terminates in the dramatic elongation growth of cells and plant organs. Massive evidence has accumulated that the final target of auxin action is the plasma membrane H(+)-ATPase, which excretes H(+) ions into the cell wall compartment and, in an antiport, takes up K(+) ions through an inwardly rectifying K(+) channel. The auxin-enhanced H(+) pumping lowers the cell wall pH, activates pH-sensitive enzymes and proteins within the wall, and initiates cell-wall loosening and extension growth. These processes, induced by auxin or by the "super-auxin" fusicoccin, can be blocked instantly and specifically by a voltage inhibition of the H(+)-ATPase due to removal of K(+) ions or the addition of K(+)-channel blockers. Vice versa, H(+) pumping and growth are immediately switched on by addition of K(+) ions. Furthermore, the treatment of segments either with auxin or with fusicoccin (which activates the H(+)-ATPase irreversibly) or with acid buffers (from outside) causes an identical transformation and degradation pattern of cell wall constituents during cell-wall loosening and growth. These and other results described below are in agreement with the acid-growth theory of elongation growth. However, objections to this theory are also discussed.

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-[(3)H]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.

1. Sections of auxin-starved hypocotyls of Helianthus annuus do not show any significant growth rate in water of buffers of pH\>-6. However, in buffers with pH-values of approximately 4, elongation growth is observed; its rate is similar to the rate of auxin-stimulated growth (after 6 h incubation). \3- This phenomenon of acid-induced growth occurs also under anaerobic conditions in contrast to auxin-induced growth (Hager 1962). 2. Intact cell wall aggregates of Helianthus hypocotyls were obtained by complete plasmolysis of hypocotyls in 50% glycerol; cell wall associated enzymes were still active after this treatment, at least in part. While cell walls in solutions of pH\>-6 show only a small plastic extension during the first minute in response to a 50 g stretching force, a constant rate of elongation over longer periods of time (measured up to 1 h) is observed in weakly acid buffers. The highest rate of elongation is observed at about pH 4. This acid-induced plastic extension is completely inhibited by Cu(2+)-ions (5mM); the elongation of cell walls is apparently the consequence of an enzyme-catalysed increase in plasticity having a pH optimum of about 4. The pH optimum of acid-induced cell wall extension observed during stretching of plasmolysed hypocotyls coincides with the pH optimum of acid-induced growth of intact hypocotyl sections (around pH 4). 3. Under anaerobic conditions the growth rate of intact coleoptiles stays unchanged (at a low value) if the sections are incubated in a buffer of pH 5.0. Higher proton concentrations, however, stimulate growth immediately, whereas low proton concentrations are inhibitory (Fig. 7 and 8). The strongest initial growth response is elicited by buffers or acids of pH 3.9 (Fig. 9). Acid-induced growth of coleoptiles with a similar pH optimum is also found under anaerobic conditions. The growth of coleoptile cylinders can be switched on or off by repeatedly changing to acid or basic medium, respectively (under conditions of anaerobiosis) (Fig. 10). IAA-induced growth (aerobic conditions, pH≥5) can also be inhibited immediately by basic buffers or NaOH-solutions, and resumes after the pH value is lowered (Fig. 11). This pH-dependency may be taken as an indication that auxin affects the same reaction which is stimulated by high proton concentrations and which may be the last step in the process of cell elongation. CCCP, known to make membranes permeable for protons, rapidly inhibits the auxin-induced elongation growth (pH 6,5) when applied at a concentration which does not influence respiration; removal of CCCP shows that the growth inhibition by CCCp is partly reversible (Fig. 12). In contrast, acid-induced elongation growth (pH 4) shows inhibition by CCCP not before 10 min after application.-These findings suggest that auxin induces a proton accumulation in a cell wall compartment and, as a consequence, enzymatic cell wall softening. Such an accumulation could be brought about by an auxin-activated, membrane-bound, anisotropic ATPase or ion pump. The notion that ATPases...

Cell-wall components from the ectomycorrhizal fungi Amanita muscaria and Hebeloma crustuliniforme and from the spruce pathogen Heterobasidion annosum elicited a transient release of active oxygen species from cultured spruce cells (Picea abies (L.) Karst.). Since the detection of active oxygen was suppressed by catalase, H2O2 was assumed to be the prevailing O2 species. On the other hand, superoxide dismutase enhanced the concentration of detectable H2O2 indicating that the superoxide anion was formed before dismutating to H2O2. The elicitors induced the formation of active oxygen in a dose-dependent manner. Interestingly, elicitors from mycorrhizal fungi had a lower H2O2-inducing activity than equal amounts of cell-wall preparations from the pathogen H. annosum. In Ca(2+)-depleted medium the production of active oxygen by elicitor-treated spruce cells was suppressed. Additionally, the ionophore A 23187 induced active oxygen formation in a medium with Ca(2+) but not in a Ca(2+)-depleted medium. Furthermore, the protein-kinase inhibitor staurosporine inhibited the oxidative burst. At a concentration of 34 nM the effect was diminished to 50%. From these results it is suggested that the release of active oxygen species from cultured spruce cells triggered by cell-wall-derived fungal elicitors depends on external Ca(2+) and a protein-kinase activity. In these respects the effect shows similarities with the well-studied respiratory burst of mammalian neutrophils.

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