Rapid Auxin-induced Decrease in Free Space pH and Its Relationship to Auxin-induced Growth in Maize and Pea

Abstract

A pH microelectrode has been used to investigate the auxin effect on free space pH and its correbtion with auxin-stimulated elongation in segments of pea (Pisum sativum) stem According to the currently popular acid secretion hypothesis of auxin action (2, 7), auxins stimulate growth by causing acidification of the cell walls, which at low pH undergo an increase in extensibility that leads to rapid cell enlargement. Several lines of evidence are consistent with this hypothesis (7,9,19,24), but published measurements of auxin-induced release of acid from tissue into an external bathing medium (3,10,11,18) have not shown that auxin stimulates H+ secretion quickly enough to account for the rapid response (17) of elongation to auxin. If the acid secretion theory is correct it must be demonstrable that following exposure to an auxin the pH in the cell wall space falls to an elongation-stimulating value by the time that observable elongation in response to auxin actually commences, i.e. within the latent period for auxin action on elongation. A pH microelectrode that permits this type of measurement (13) Arbor, Mich.) of exposed tip length of 5 Aim and tip diameter of 2 ,um. The reference electrode was a segment of glass tubing (80 x 1 mm) the end of which was drawn into a capillary with a tip diameter of a few ,um. This tube was filled with 3 M KCI, in which was immersed an AgCI-coated silver wire that connected to the reference terminal of the pH meter. The electrode was read using the mv mode of operation of the meter and was calibrated against phosphate-citrate buffers (50 mm in K-phosphate and Na-citrate) of pH 4 and 6 before and after each experiment.The relation between pH of a buffer solution and mv reading by the pH microelectrodes was checked for several microelectrodes with a series of solutions varying in pH from 4 to 7. Because this relation was always found to be linear over the pH range, mv readings from experiments were converted to pH values by linear interpolation using the mv values measured for the calibration buffers of pH 4 and 6.The sensitivity of a pH microelectrode varied, for different experiments, from 50 to 58 mv/pH unit, but it was always constant through a given experiment. Between the beginning and the end of a typical experiment, however, the absolute mv values recorded by the microelectrode for the two calibration buffers would often change by a few mv. This drift in value was always of the same magnitude and in the same direction for both calibration buffers. Thus, the sensitivity of the microelectrode did not drift. Although calibration of the microelectrode against buffers of known pH before and after each experiment allowed us to monitor microelectrode drift, we used only the results of the postexperiment calibration for conversion of microelectrode mv readings to pH values.

pH-Dependence of Extension Growth in Avena Coleoptiles and Its Implications for the Mechanism of Auxin Action

“…Thus, on a relative scale, the responses of nonpeeled (abraded) and peeled coleoptile investigating acid-induced extension (e.g. 10,13,17,21). In contrast, a preincubation in distilled water (+ osmoticum) was used both in the present investigation (Figs.…”

“…pH 4) results in wall loosening and thereby in turgor-driven cell extension (3,12,14,15,23). It is equally accepted that auxin induces these tissues to excrete protons which acidify the cell-wall solution to a pH as low as about 5 (4,5,(12)(13)(14)(15)23). The decisive question, however, is whether this pH is sufficiently low to account, at least initially, for the increase in growth rate caused by auxin in these cells.…”

Production of organic acids and adsorption of Cd on roots of durum wheat (Triticum turgidum L. var. durum)

“…In a review by Kurkdjian and Guern (1989) the pH of the cytoplasm ranged from 6.8 to 7.5 in roots and was 7.5 in leaves, and the pH of the vacuole ranged from 4.6 to 6.0 in roots and was 5.3 in leaves. Cell wall free space pH ranges from 4.5 to 6.5 (Jacobs and Ray 1976). We recognize that other metal-binding sites (such as the cell wall matrix) exist within plant tissue, the model simply gives an estimate of the potential for Cd to bind to the various LMWOAs.…”

Auxin action and auxin‐binding proteins

“…For soybean hypocotyls the first, rapid response has a lag time of 12 min followed by a second elongation response with a lag time of 35 min (Vanderhoef et al, 1976). In pea epicotyl the first lag time was slightly longer (21 min) but the associated apoplastic pH drop was measured after 15 min (Jacobs & Ray, 1976). In the monocotyledenous species maize {Zea mays L.) response times were 12 min to an apoplastic pH drop and 18 min to measurable elongation growth (Jacobs & Ray, 1976).…”

“…Many workers have measured the increased rate of H"^-efRux induced by auxin, correlating it with increased elongation (early work was reviewed by Cleland, 1971;Rayle, 1973;Jacobs & Ray, 1976;Kutschera & Schopfer, 1985 a, b;Senn & Goldsmith, 1988;Luthen, Bigdon & Bottger, 1990;Peters & Fele, 1991 a). In most cases increased H+-efflux has been found to rise in concert with elongation or just to precede it.…”