Inhibition of Acid-Enhanced Elongation of Zea mays Root Segments by Galactose

Abstract

The effect of sugars and metabolic inhibitors on the elongation of Zea mays root segments was analyzed by a rhizometer which records the elongation of each of 32 root segments at the same time. Galactose suppressed the acid-enhanced rapid elongation after a lag period of 1.5 hours, but it did not inhibit the slow elongation at pH 7. Mannose was less inhibitory than galactose.Arabinose, xylose, glucose, sucrose, mannitol, and sorbitol caused no inhibition. When galactose was removed after a 1-hour treatment, the elongation was partially recovered. Cycloheximide and 2-deoxyglucose suppressed acid-enhanced elongation when these were applied at the same time as acid treatments, whereas cordycepin (3'-deoxyadenosine) inhibited elongation only if it was applied prior to acid treatment. Over the 9-hour period of elongation studied, the inhibition by galactose was comparable to that of cycloheximide. Since galactose has been reported to suppress the sugar metabolism necessary for the cell wall synthesis, the later phase of acid-enhanced elongation of root segments may at least partially depend on the synthesis or metabolism of cell wall components. The inhibition of root growth by galactose may be partially ascribed to a direct effect on the elongation process in roots, an effect that is enhanced by the acidification of the cell walls.Galactose is known to inhibit root growth in many plant species, such as wheat (3, 16), maize (1 1, 12), pea (16), and tomato (14). Galactose also inhibits the elongation of excised segments ofoat coleoptiles (15,19,30,31 The purpose ofthis study was to ascertain whether the acidinduced elongation process per se is sensitive to galactose and, if so, whether the senstivity bears a causal relationship to the inhibition of root growth by galactose. In each experiment, the elongation kinetics of large samples of root segments was measured simultaneously using the rhizometer (26-29). MATERIALS AND METHODS Plant MaterialCaryopses of Zea mays cv Sweetom 102 were surfacesterilized in 0.1% sodium hypochlorite solution and imbibed in running tap water at 20°C for 20 h. Swollen caryopses were then placed on the frames of acrylic plates sided by wet filter paper as described previously (25,26), and these frames were kept in a humid box in the dark for 42 h at 23°C.For long-term elongation (20 h), 5 mm segments were excised from selected straight roots (22 ± 2 mm) at 1 mm behind the tip, preincubated in distilled water for 1.5 h, and then treated with or without sugars at pH 4 or at pH 7. Citrate-sodium phosphate (1-2 mM) buffer was used at pH 4 and pH 7. Lots of 30 root segments were gently shaken with 20 mL buffer solution in a Petri dish (12 cm) at 23°C in room light, and the length of the segments was measured with a low power microscope after treatment.For the time course experiments (10 h), 7 mm segments were excised from 22 ± 2-mm long roots 1 mm behind the tip and mounted on the root holders with the apical end down as indicated in Figure 1. The basal end of the holder was filled with 0

Root growth as a function of ammonium and nitrate in the root zone

“…Changes in pH around the cell wall may also mediate the differential effects of NH4 and NO3 upon root growth. Rhizosphere acidity promotes root growth (Tanimoto, Scott & Masuda 1989), but regulation of this phenomenon is poorly understood (Taiz & Zeiger 1991). Absorption of NHt and its subsequent assimilation acidify the rhizosphere whereas absorption of NO3 may alkalinize the rhizosphere (Raven & Smith 1976;Raven 1988).…”

Inhibition of Downy Brome (Bromus tectorum) Root Growth by a Phytotoxin fromPseudomonas fluorescensStrain D7

“…Others have monitored root elonga tion by placing ink marks directly on the roots and measuring subsequent increases in distances between the marks and the root tips (1,5). Very rapid detection of inhibition of root elongation has been achieved by electrically measuring small changes in root length (16).…”