Gibberellin Response in Lettuce Hypocotyl Sections

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The elongation response elicited by incubating excised hypocotyl sections of lettuce (Lactuca sativa L.) in Ught in gibberellin (GA) can be enhanced by the addition of Cl-, Br-, and NO3-salts of K+ and Na+. Sections incubated in light in the absence of GA do not elongate in response to the addition of salts. In contrast, excised hypocotyls incubated in darkness elongate equally in both GA and water, and their elongation can also be enhanced by KCI treatment. Growth stimulation by the salts of K+ and Na+ occurs opimally at 10 mM and the magnitude of the response is proportionad to the duration of siat treatment. Although the growth of sections incubated in light in the absence of GA is not enhanced by various salts of K+ and Na+, the concentration of these cations exceeds that in GA-treated sections. In dark-grown tissue, uptake of K+ also occurs in both GA-and H20-treated sections incubated in 10 mm KCI. Since increased osmotic potential resulting from cation uptake does not correlate with growth stimulation resulting from salt treatments, we condude that increased cell turgor is not the principal driving force for growth in hypocotyl sections. Changes in the extensibility of GA-treated, light-grown tissue and dark-rown tissue incubated with and without GA correlate with the increased growth rate of these sections. Incubation of sections in KCI results only in changes in water potential of sections without having a significant effect on extensibilit. When changes in water potential are accompanied by increased extensibility, however, a marked increase in growth rate is observed.The effects of salts of the alkali metals on hormone-stimulated growth responses have been described for a number of plant systems. Of the roles proposed for the action of salts on growth, the most notable effects are on osmotic pressure, cell wall extensibility, or on the general metabolism, of the tissue (12,13,16,17). Thimann and Schneider (17) found that in Avena coleoptile sections K+ and, to a lesser extent, Na+ and Rb+ chlorides stimulated a 2-fold increase in auxin-induced growth, while growth of H20 controls was unaffected. Addition of KCl to Avena coleoptile segments previously treated with CaC12 also caused an increase in growth which was attributed to an increase in turgor pressure caused by KCl uptake (6). On the other hand, Tagawa and Bonner (16) period of 14 hr (13). Potassium effects have been attributed to its role as an enzyme cofactor and to its general, nonspecific effect on metabolism. The curvature of the hypocotyl hook of lettuce can be modified by numerous salts including K+ after treatment with either gibberellin or ethylene (12). This effect of salt treatment was attributed to salting-in and salting-out effects on macromolecules in the cell wall (12).A stimulatory effect of 10 mm KCI on the elongation growth of excised lettuce hypocotyl sections was reported by Silk and Jones (15). Potassium chloride causes at least a 2-fold increase in the length of dark-grown sections and of GA-treated, lightgrown se...

Section: Resultsmentioning

confidence: 55%

“…A stimulatory effect of 10 mm KCI on the elongation growth of excised lettuce hypocotyl sections was reported by Silk and Jones (15). Potassium chloride causes at least a 2-fold increase in the length of dark-grown sections and of GA-treated, lightgrown sections although it has no effect on the growth of lightgrown sections incubated in H20.…”

mentioning

confidence: 96%

Roles of Extensibility and Turgor in Gibberellin- and Dark-stimulated Growth

Stuart

Jones²

1977

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“…Cell wall plasticity of Avena internode segments, measured by instron extension and microextension of living internode segments, was increased 150% by treatment with GA, with a lag of about 1 h (1). ' Supported by a grant from Monsanto Co., St Excised hypocotyls oflettuce, incubated in the light, elongated in response to GA (21). The addition of 10 mM KCl doubled elongation for segments preincubated with GA but had no effect on the controls.…”

mentioning

confidence: 99%

Gibberellic Acid Regulates Cell Wall Extensibility in Wheat (Triticum aestivum L.)

Keyes¹,

Sorrells²,

Setter³

1990

Mutations (Rht genes) blocking sensitivity to gibberellic acid (GA) were used to examine phytohormone mediated cell wall expansion in wheat (Triticum aestivum L.). Irreversible extensibility of immature leaf segments, as determined by stress/strain (instron) measurements, declined with Rht gene dose. Exogenous GA3 significantly increased wall extensibility in the nonmutant controls but had no effect on the near-isogenic GA-insensitive genotypes. Furthermore, ancymidol, an inhibitor of gibberellin biosynthesis, diminished wall extensibility in the nonmutant control. Extensibility of immature segments was highly correlated with mature leaf sheath length (R = +0.95). The results indicate that wall yielding properties of expanding wheat leaves are associated with leaf cell expansion potential and that GA is involved in the determination of those properties.The plane of cell division together with the rate and orientation of irreversible expansion determine cell size and shape. The rate of plant cell expansion may be further reduced to a few physical components including: (a) cell wall extensibility, which is the capacity of the wall to undergo irreversible (plastic) extension in response to a load; (b) the yield threshold of the wall, which is the minimum stress required to initiate extension; (c) hydraulic conductance, the capacity ofthe tissue to transport water to the expanding cell; and (d) the difference in water potential between the cell and extracellular space (16). Lockhart's (16) equations show that a change in the growth rate can take place with changes in wall structural/ mechanical properties which determine extensibility and/or changes in hydraulic processes and/or turgor changes.Heyn (9)

“…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).…”

mentioning

confidence: 99%

Gibberellic Acid Stimulation of Cucumber Hypocotyl Elongation

Taylor

Cosgrove²

1989

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