The current status of the acid‐growth hypothesis

Abstract

SUMMARY Excised coleoptile segments that were depleted of endogenous auxin after pre‐incubation in water respond to the phytotoxin fusicoccin (FC) and the plant hormone auxin (indole‐3‐acetic acid, IAA) with an enhancement of growth. This promotion of organ elongation is caused by an increase in the extensibility of the thick, growth‐limiting outer epidermal wall. The acid‐growth hypothesis postulates that both FC and IAA cause wall‐loosening and the concomitant induction of growth by rapid acidification of the extension‐limiting cell wall. At saturating concentrations FC and IAA cause very similar growth responses. However, the equilibrium‐pH in the incubation medium (∼ pH of the peripheral cell wall) of abraded FC‐treated segments is 3.5–4.0, whereas in the presence of IAA a pH of 4.8–5.0 is established. The FC‐ and IAA‐mediated induction of growth can be stimulated by an external buffer of pH 3.5–4.0. Acid buffers of pH 5, however, cause almost no enhancement of growth compared WJth the water‐control. FC‐mediated growth can be inhibited by neutral buffers infiltrated into the outer epidermal wall, but a substantial growth response occurs after addition of IAA under identical conditions. A sub‐optimal concentration of FC was used to mimic the effect of IAA on acid secretion. When a pH of 4.8–5.0 was established by FC in the peripheral wall no promotion of growth occurred. Hence, IAA‐induced proton excretion is insufficient to cause cell‐wall loosening in the coleoptile. Abraded segments that rapidly elongate in acid buffers (pH 3.5–4.0) respond to IAA with an additional enhancement of growth, but under identical conditions FC is without effect. The growth‐promoting effects of acid and IAA are additive, i.e. IAA and acid act via separate mechanisms. These results are consistent with the acid‐growth hypothesis of FC action. However, they indicate that IAA induces growth by a mechanism independent of cell‐wall acidification. Alternative hypotheses of IAA‐mediated cell‐wall loosening are discussed.

A comparison of the effects of 1,4-naphthoquinone and 2-hydroxy-1,4-naphthoquinone (lawsone) on indole-3-acetic acid (IAA)-induced growth of maize coleoptile cells

“…3b, 4) and the hyperpolarisation of the membrane potential (Table 1). These data are in good agreement with the results obtained previously and recently by others investigators (Burdach et al 2014;Felle et al 1991;Karcz and Burdach 2002;Keller and Van Volkenburgh 1996;Kutschera and Schopfer 1985a, b;Kutschera 1994Kutschera , 2006Lüthen et al 1990;Rücke et al 1993;Siemieniuk and Karcz 2015). It 1 3 is also well established that the plasma membrane hyperpolarisation that was recorded in the presence of IAA is a consequence of stimulated proton extrusion through PM H + -ATPase (Hedrich et al 1995;Rücke et al 1993).…”

“…In addition, the hyperpolarisation of E m that was induced by IAA was suppressed in the presence of both naphthoquinones. The lack of a correlation between IAA-induced growth and medium pH, which was simultaneously measured with growth (see also Kutschera and Schopfer 1985a, b;Kutschera 1994Kutschera , 2006, indicates that the loosening of the cell wall probably depends on the ROS that are liberated during the action of quinones as was shown here and previously by Schopfer et al (2002) and Kurtyka et al (2016). In addition, the obtained results may also indicate that NQ and NQ-2-OH at higher concentrations undergo both redox cycling and nucleophilic substitution reactions in maize coleoptile cells.…”

“…In agreement with this hypothesis, at least in maize coleoptile cells, auxin causes the acidification of the cell wall (Rayle and Cleland 1992) and the hyperpolarisation of the membrane potential by stimulating the activity and/or amount of the plasma membrane H + -ATPase. Acidification of the cell wall either directly lowers the yield threshold of the wall or optimises the activity of the cell wall-localised proteins that loosen the wall, whereas the hyperpolarisation of the membrane potential causes the activation of voltagedependent, inwardly rectifying K + channels, the activity of which contributes to the water uptake that is necessary for cell expansion (reviewed in Kutschera 1994Kutschera , 2006Hager 2003;Kutschera and Wang 2016). In this study, the redox cycling properties of two naphthoquinones, 1,4-naphthoquinone and 2-hydroxy-1,4-naphthoquinone (estimated by H 2 O 2 production, catalase activity, MDA content and hexacyanoferrate III reduction), were also compared in experiments that were performed on maize coleoptile segments incubated with or without the addition of IAA.…”

Physiological regulation and functional significance of shade avoidance responses to neighbors

“…[93][94][95][96] Auxin plays a role in the acidification of the apoplast, where auxin can lower the apoplastic pH within minutes. 97 This rapid acidification probably sets the optimal pH for cell wall modifying proteins such as expansins 98 and XTHs. 99 These genes are thus likely activated during SAS as the downstream targets of the interacting network of hormones mentioned above.…”

Localization of Germin Genes and Their Products in Developing Wheat Coleoptiles

“…Tel: +905326562209; Fax: +903262455867 E-mail: caliskan@mku.edu.tr elongation of cells other then division of cells. The mechanism of this elongation is a matter of debate (Fry, 1989;Schopfer, 1993;Kutschera, 1994;Hohl and Schopfer, 1995). A few genes were considered to be involved in the elongation of coleoptiles (McQueen-Mason et al, 1992;McQueenMason, 1995).…”