Comparison of Auxin-induced and Acid-induced Elongation in Soybean Hypocotyl

Abstract

Acid-induced growth was compared to auxin-induced growth. After a transient pH 4-induced increase in the elongation rate was completed, auxin could still induce an enhanced rate of elongation in soybean (Glycine max) hypocotyl segments. This auxin response occurred both when the medium was changed to pH 6 before auxin addition, and when the auxin was added directly to the pH 4 medium. This postacid response to auxin was persistent, and quite unlike a postacid response to acid, which was again shortlived. One mM N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (pH 7) inhibited the first response to auxin (the first response to auxin being similar to the acid response), but not the second response. This did not appear to be simply a hydrogen ion neutralizing effect, however, since a 50-fold increase in buffer concentration at pH 6 did not inhibit the first response. Decrease in the pH of the external medium, previously shown to occur with excised soybean hypocotyl segments, was not affected by auxin. Furthermore, this pH drop, during which the cells appear to be adjusting their external pH to about 5.4, did not result in an increased rate of elongation. Addition of auxin after the equilibrium pH had been attained did not alter the pH, but it did increase the rate of elongation, eliciting a normal auxin response. It was concluded that hydrogen ions do not mediate in long term auxin-induced elongation in soybean hypocotyl.The hypothesis that H+ ions act as a second messenger for auxin-promoted elongation (4, 13, 25) is supported by evidence that coleoptile (3,8,10,20,25,26), pea (1,7, 18,19), and soybean (29) segments elongate more rapidly at low pH, Avena and Helianthus wall extensibility is increased at low pH (3, 26), auxin-promoted elongation is preceded by auxin-promoted H+ ion extrusion in Avena (5,6,13,25) (29); (c) after acid-stimulated elongation has subsided, auxin is still capable of increasing the rate of elongation in Avena coleoptile (10, 25, 16) and soybean hypocotyl segments (29); (d) wall-bound glycosidases which have low pH optima, described in support of the hypothesis (16), can be inhibited without inhibiting auxin-promoted elongation in lupin (22) and Avena (9); and (e) auxin does not affect proton secretion in lupin (21) and soybean (28).These data have led us to examine the relationship of HI ionpromoted and auxin-promoted elongation. The accompanying manuscript demonstrates the lack of an auxin effect on medium pH adjustment (28). The experiments described herein compare acid-induced elongation to auxin-induced elongation, and lead us to conclude that HI ions do not mediate in long term auxinpromoted elongation in the soybean hypocotyl. MATERIALS AND METHODSSoybean seedlings (Glycine max L. Merr. var. Wayne) were germinated in the dark, and the elongating segment of the hypocotyl was excised as described (29,31). Hypocotyl extension was continuously measured with a linear transducer in an apparatus modified after that reported by Green and Cummins (12), as previously described (29,...

Auxin Has No Effect on Modification of External pH by Soybean Hypocotyl Cells

“…It was determined that pH adjustment is not affected by the hormone. These data and the determination of the accompanying report (20) do not support the acid growth hypothesis of auxin action. …”

“…Soybean hypocotyl segments, with no cuticle removal, readily adjust the pH of the medium (Figs. 1-4), and readily respond to low pH by showing an increased rate of elongation (20,21). These results suggest that perhaps these segments have no cuticle, not an unexpected finding for tissue grown in high humidity.…”

“…Growth-optimum auxin concentrations have no effect on cellular pH adjustment of the external medium, whether added at the beginning of the experiment or after the equilibrium pH is attained. The pH adjustment by the cells occurs rapidly and in spite of the presence of a cutide.The acid growth hypothesis (3, 9, 19) is clearly supported by evidence that coleoptile (2, 7, 8, 18), pea (1, 6, 14, 15), and soybean segments (20) are induced to increase their elongation rate when the pH is decreased, e.g. from pH 6 to pH 4.…”

“…The acid growth hypothesis (3,9,19) is clearly supported by evidence that coleoptile (2,7,8,18), pea (1,6,14,15), and soybean segments (20) are induced to increase their elongation rate when the pH is decreased, e.g. from pH 6 to pH 4.…”

Gradient of Growth, Spontaneous Changes in Growth Rate and Response to Auxin of Excised Hypocotyl Segments of Phaseolus aureus

“…The nature of the spontaneous changes in the growth rate has been analyzed by Evans (10,12,13 (27), in glucanase activities (15 and references therein), and in polysaccharide metabolism and biochemistry (30,36) during growth have been previously accumulated. Precise studies of responses to auxin have been performed using the closely related soybean hypocotyl (33)(34)(35). It is necessary to carry our experiments to define both the time course of endogenous growth (short and long term experiments) and differences in growth characteristics between cells at various stages of elongation.…”