Changes in the Ultrastructure of Cell Walls, Cellulose Synthesis, and Glucan Synthase Activity from Gravistimulated Pulvini of Oat (Avena sativa)

Abstract

Ultrastructural analyses of the cell walls from top and bottom halves of gravistimulated pulvini from oat leaves show a decrease in the density of material within the cell walls from the lower halves of pulvini after 24 h of gravistimulation. Assays of cellulose synthesis with a 14C-sucrose pulse-chase experiment indicate no difference in the amount of new cellulose synthesized in top compared with bottom halves of gravistimulated pulvini. The highest rate of cellulose synthesis occurs with 12-24 h of gravistimulation. Treatment of graviresponding pulvini with 2,6-dichlorobenzonitrile (DCBN) had only a minor effect on segment gravitropic curvature. We also found that there is no difference in the activities of either glucan synthase I or glucan synthase II in top halves as compared with bottom halves of gravistimulated pulvini. We conclude that the graviresponse in oat stems is not driven by new cell wall synthesis but, rather, by changes in cell wall plasticity and osmotic potential.

Cell Wall Modifications in Maize Pulvini in Response to Gravitational Stress

“…Levels of heteroxylans increased about 35% in the lower pulvinus of gravistimulated maize plants (Table II). Cellulose, which together with arabinoxylan accounts for 70% to 80% by weight of the walls (Table II), did not change significantly in either the upper or lower pulvini of the gravistimulated maize stalks and this is consistent with the observations of Lu et al (1992) in gravistimulated pulvini of oats. It is noteworthy that the absolute levels of cellulose assayed via the acetic acid/nitric acid method are in the range 35% to 40% by weight (Fig.…”

Unequal distribution of osmiophilic particles in the epidermal periplasmic space of upper and lower flanks of gravi-responding rye coleoptiles

“…The speculated "chemical asymmetry which may give rise to a counter-reaction leading to a diminished and finally abolished asymmetry when the autotropic reactions come to an end" (Chapman et al 1994) may therefore be equivalent to the asymmetric distribution of OPs. After all, it is conceivable that the principal mechanism underlying the repeatedly described apparent decrease of IAA-sensitivity in the UFs of graviresponding plant organs (Salisbury et al 1988;Rorabough and Salisbury 1989;Lu et al 1992) may be similar to the one proposed in the present study.…”

Hormonal and Gravitropic Specificity in the Regulation of Growth and Cell Wall Synthesis in Pulvini and Internodes from Shoots of Avena sativa L. (Oat)

“…The only exception was a somewhat greater increase in P-1,3-glucan in the downward-facing halves, in which higher levels of glucan synthase activity (product uncharacterized) were found as well. In contrast, Lu et al (1992) used ultrastructural analysis to show a decrease in the density of material within the cell walls from the lower halves of oat pulvini after 24 h of gravistimulation. Moreover, even using ['4C]Suc pulse labeling (apparently chased with continuously supplied 0.1 M SUC), they were unabie to demonstrate enhanced cell wall synthesis in the bottom (downward facing) halves compared with the top halves of gravistimulated pulvini.…”