The Correlation of Profiles of Surface pH and Elongation Growth in Maize Roots

Abstract

High-resolution profiles of surface pH and growth along vertically growing maize (Zea mays) primary root tips were determined simultaneously by pH-sensitive microelectrodes and marking experiments. Methodological tests were carried out that proved the reliability of our kinematic growth analysis, while questioning the validity of an alternative technique employed previously. A distal acidic zone around the meristematic region and a proximal one around the elongation zone proper were detected. This pattern as such persisted irrespective of the bulk pH value. The proximal acidic region coincided with maximum relative elemental growth rates (REGR), and both characters reacted in a correlated manner to auxin and cyanide. The distal acidic band was unrelated to growth, but was abolished by cyanide treatment. We conclude that: (a) the pattern of surface pH as such is a regulated feature of growing root tips; (b) the correlation of extracellular pH and growth rate suggests a functional relationship only along proximal portions of the growing root tip; and (c) the distal acidic band is not caused by pH buffering by root cap mucilage, as suggested previously, but rather is controlled by cellular activity.

The Spatially Variable Inhibition by Water Deficit of Maize Root Growth Correlates with Altered Profiles of Proton Flux and Cell Wall pH

“…Means 6 SE, n 5 6. of surface proton flux determined by us with pH profiles determined by carefully moving microelectrodes along the surface of the maize root elongation zone (Peters and Felle, 1999) reveals close similarities. In both cases, acidification peaked at around 4 mm from the root apex and then declined; in both cases, there appeared to be a small subsidiary peak of enhanced acidification centered at around 1 mm from the apex.…”

“…Moreover, additions of 1 mM KCN, in order to inhibit oxidative phosphorylation, inhibited wall acidification measured in this way within 15 min, even though it would not be expected to affect the pH of mucilage at the root surface (data not shown). Similarly, Peters and Felle (1999) showed that cyanide rapidly dissipated acidification measured along the surface of the maize root elongation zone with pH-sensitive microelectrodes and also inhibited growth. Such findings suggest that wall acidification is an actively ongoing process.…”

“…Moreover, inhibitors of proton pumping, such as cyanide, orthovanadate, and erythrosin B, have been shown to rapidly inhibit growth in expanding roots and leaves (e.g. Bogoslavsky and Neumann, 1998;Peters and Felle, 1999) In this context, recent supportive evidence indicates that a gene encoding a modulator of plasma membrane proton-pumping ATPase activity, which binds to a site different from the 14-3-3 binding site, is preferentially expressed in the accelerating growth region of the maize root elongation zone under control and water-deficit conditions (M. Bassani, P.M. Neumann, and S. Gepstein, unpublished data).…”

“…Recent findings that cell wall acidification is associated with auxin effects on seed embryo growth (Rober-Kleber et al, 2003) and with early growth responses to gravistimulation of root cells (Taylor et al, 1996;Fasano et al, 2001) support the existence of a close relationship between wall acidification and cell expansion. Similarly, the spatial profile of growth rate along the elongation zone of maize roots has been shown to coincide with the spatial profile of rootsurface acidification (Pilet et al, 1983;Peters and Felle, 1999). Changes in cell wall acidification profiles could result from regulated changes in the activity of outward proton pumping H1 ATPase in the plasma membrane.…”

“…Processes regulating the wall loosening, water uptake, and solute uptake that are involved in maintaining cell expansion have each been associated with pH regulation in cell walls and/or cytoplasm (Rayle and Cleland, 1992;Taylor and Bloom, 1998;Peters and Felle, 1999;Tournaire-Roux et al, 2003). For example, according to the acid growth hypothesis of Rayle and Cleland (1992), maintenance of sufficiently acid pH in the cell wall facilitates turgor-driven expansion by directly loosening the polysaccharide matrix or by optimizing the activity of cell wall proteins and enzymes associated with loosening (Cosgrove, 1997;Tanimoto et al, 2000;Wu and Cosgrove, 2000).…”