Gibberellic Acid Stimulation of Cucumber Hypocotyl Elongation

Abstract

Recently developed techniques have been used to reinvestigate the mechanism by which gibberellic acid (GA3) stimulates elongation of light-grown cucumber (Cucumis sativus L.) seedlings. Osmotic pressure and turgor pressure were slightly reduced in GA3-treated seedlings, which elongated 3.5 times faster than control seedlings. This indicated that GA3 enhancement of growth was not controlled by changes in the osmotic properties of the tissues. Stress/strain (Instron) analysis revealed that plastic extension of the cell walls of GA3-treated seedlings increased by up to 35% above the control values. Stress-relaxation measurements on frozen-thawed tissue showed that To, the minimum relaxation time, was reduced following application of GA3. In vivo wall relaxation (measured by the pressure block technique)showed that the wall yield coefficient was increased, and the yield threshold was slightly reduced. Thus GA3 affected both the mechanical (viscoelastic) and biochemical (chemorheological) properties of the cell walls of light-grown cucumber. The previous hypothesis, that GA3 stimulates cucumber hypocotyl growth by increasing osmotic pressure and cell turgor, is contradicted by our results.GA3 can reverse the inhibition of stem elongation caused by light in many plant species (1,12,17,18,21,23,24). The mechanism by which GA3 enhances the rate of elongation has been investigated by several groups, many of whom have concluded that GA3 acts on cell wall mechanical properties to increase growth (2,13,15,18,20).Cucumber is an often-cited exception to this general trend, in that GA3 is thought to stimulate hypocotyl elongation by causing increases in cell turgor pressure and osmotic pressure. This hypothesis, however, is based on meager direct evidence. Rather, it is indirectly supported by studies which found little effect of GA3 on the mechanical properties of growing walls (5, 14), and it assumes that wall growth properties are well correlated with wall mechanical properties (4). This assumption, however, is sometimes invalid. For example, a recent study (8) Therefore, this study was undertaken to reexamine the mechanism by which GA stimulated elongation in lightgrown cucumber. Osmotic and turgor pressures were measured directly, and in vivo wall relaxation properties were assessed in living tissue using the pressure block technique. We also assessed wall mechanical properties in frozen-thawed tissue by stress-relaxation and Instron analyses. MATERIALS AND METHODS Plant Material

Mechanical Properties of Plant Cell Walls Probed by Relaxation Spectra

“…Taylor and Cosgrove (1989) reported stress relaxation spectra for cell wall specimens from killed cucumber (Cucumis sativus) hypocotyls, which showed a broad, but not box-like, maximum centered between 0.2 and 0.6 s (in different spectra) and a minimum at near 100 s, the increase above which did not reach a maximum by the spectrum's upper limit of 200 s but presumably would have given a second peak beyond this point. These spectra thus appear to be two peaked, like ours, but right shifted to higher t values (relatively slower rates of relaxation) than those of our peaks.…”

Microtubule-Associated Proteins MAP65-1 and MAP65-2 Positively Regulate Axial Cell Growth in EtiolatedArabidopsisHypocotyls

“…Gibberellic acid (GA 4 ) increases the rate of hypocotyl growth in Arabidopsis seedlings (Taylor and Cosgrove, 1989;Collett et al, 2000) and has been shown to bias cortical MT orientation toward transverse coalignment . We assayed darkgrown wild-type and double tDNA map65-1-1 map65-2-2 mutant seedlings for hypocotyl growth effects across a range of GA 4 concentrations ( Figure 3D).…”

Phytochrome, Gibberellins, and Hypocotyl Growth (A Study Using the Cucumber (Cucumis sativus L.) long hypocotyl Mutant)

“…early steps in GA biosynthesis should be examined (Talón et al, 1991 the wall yield coefficient is increased and the yield threshold reduced (Taylor and Cosgrove, 1989). In peas, GA,-dependent growth is brought about by a similar biophysical mechanism (Cosgrove and Sovonick-Dunford, 1989), whereas R lowers the wall yield coefficient alone (Kigel and Cosgrove, 1991).…”