pH and auxin-induced growth: A causal relationship?

Penny, Pauline; Dunlop, James; Perley, James E.; Penny, David

doi:10.1016/0304-4211(75)90072-3

Cited by 48 publications

(21 citation statements)

References 23 publications

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“…within the latent period for auxin action on elongation. A pH microelectrode that permits this type of measurement (13) has recently been used to deny the validity of the acid secretion hypothesis (14). Employing here the same type of electrode to examine more critically the pH in the free space of maize coleoptile and pea stem segments we are able to confirm the predictions of the acid secretion hypothesis.…”

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confidence: 63%

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

Jacobs

Ray²

1976

Plant Physiol.

146

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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.

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confidence: 63%

“…Penny et al (14) recently reported attempts to detect auxincaused decrease in extracellular pH in oat coleoptile and lupine hypocotyl using the same type of pH microelectrode that was employed in the present experiments. They detected no marked drop in pH in response to 30 ,uM IAA.…”

Section: Discussionmentioning

confidence: 99%

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

Jacobs

Ray²

1976

Plant Physiol.

146

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“…Acidic solutions promote the growth of plant stem (16,21) and coleoptile sections (5,14), but it is not known whether the protons entered the tissues through the cuticle or simply through the cut ends. The inability to detect significant auxin-induced acidification of the external medium with sections of some dicot stems (12,19) where the direction of proton movement is outside to inside, but we have conducted sufficient experiments in which the direction of proton movement was reversed so as to demonstrate that the direction of proton movement has little effect on its conductance (data not shown).…”

Section: Discussionmentioning

confidence: 99%

“…The proton conductance can be greatly enhanced by removal of cuticular waxes. While this technique has been used by Penny et al (12), in our experience the dewaxing causes unacceptable damage to the underlying cells, and is therefore of limited use in studies on the proton excretion of plant tissues. Alternatively, the proton conductance can be enhanced by abrading the tissue.…”

Section: Discussionmentioning

confidence: 99%

“…The excreted protons might be expected to acidify the external solution, too, but it has been difficult to demonstrate any auxin-induced acidification (11,12,19) unless the cuticle is abraded (3,9,16) or removed (1,13). This led us to suggest (1,13,15) that proton conductance of the cuticle is so low that protons excreted into the wall solution are trapped within the tissue unless they escape through cut surfaces.…”

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Low Proton Conductance of Plant Cuticles and Its Relevance to the Acid-Growth Theory

Dreyer¹,

Seymour²,

Cleland³

1981

Plant Physiol.

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Evidence obtained on the relation between the pH of the medium and the growth of intact stem sections is compatible with the acid-growth theory only if the proton conductance of the cuticle is so low that the cuticle is an effective barrier to the entry or exit of protons from the tissue. By measuring the rate at which protons cross frozen-thawed epidermal strips of sunflower (Helianthus annuus L.) and soybean hypocotyls (Glycine max Morr.) and enzymically isolated cuticles of Berberis aquifoliun Persh. and tomato (Lycopersicum eseadentum Mill.) frlit, we have now demonstrated the low proton conductance of the cuticular layer. Unless the conductance is enhanced by abrasion of the cuticle or by removal of the cuticular waxes, proton movement into and out of a tissue across the cuticle will be signlifcant only over long time periods.The aerial portions of all higher plants are covered by a cuticle, which is believed to act as a barrier to the entry and exit of materials from tissues (8). The ability of the cuticle to limit movement of cations such as K+ (7, 21), organic substances (4), and water (17) has been established, but the proton conductance2 of a cuticle has been examined only after long periods (3). Knowledge about the proton conductance of the cuticle is of importance in regard to the acid-growth theory of auxin-induced growth (15).The acid-growth theory states (3,15) that auxin causes the cells to excrete protons into the wall solution where the resulting increased acidity activates wall-loosening enzymes. The excreted protons might be expected to acidify the external solution, too, but it has been difficult to demonstrate any auxin-induced acidification (11,12,19) unless the cuticle is abraded (3, 9, 16) or removed (1, 13). This led us to suggest (1, 13, 15) that proton conductance of the cuticle is so low that protons excreted into the wall solution are trapped within the tissue unless they escape through cut surfaces. A low proton conductance would also explain why, in order to achieve a maximum rate of acid-induced growth, it is necessary to add a 50-fold greater proton concentration to intact A vena coleoptile sections as compared with sections whose cuticle had been removed (13 the cuticle may not be a barrier to protons. In this study, we have made use of a simple assay that allows us to assess the ability of protons to cross cuticular layers, and we show for the first time that the cuticle is an effective barrier to the entry and exit of protons. The cuticle of some seedlings was abraded by rubbing the hypocotyl five to ten times with a slurry of emory powder (American Optical, Seattle, WA, grade M180) before isolation of the epidermal strips. Other seedlings were wiped twice with a 3:1 (v/ v) mixture of chloroform:ethanol to remove some of the cuticular waxes. MATERIALS AND METHODSLeaves of Berberis aquifolium Dursh. were selected from a plant growing outside the laboratory. To isolate the cuticle, leaf pieces were infiltrated and incubated for at least 2 days with a solution containing...

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Growth

Dörffling¹

1976

Progress in Botany / Fortschritte Der Botanik

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pH and auxin-induced growth: A causal relationship?

Cited by 48 publications

References 23 publications

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

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

Low Proton Conductance of Plant Cuticles and Its Relevance to the Acid-Growth Theory

Growth

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