Hypergravity can reduce but not enhance the gravitropic response ofChara globularis protonemata

Abstract

The relationship between the position of the statoliths and the direction and rate of tip growth in negatively gravitropic protonemata of Chara globularis was studied with a centrifuge video microscope. Cells placed perpendicularly to the acceleration vector (stimulation angle 90 degrees) showed a gradual reduction of the gravitropic curvature with increasing accelerations from 1 g to 8 g despite complete sedimentation of all statoliths on the centrifugal cell flank. It is argued that the increased weight of the statoliths in hypergravity impairs their acropetal transport which is induced when the cell axis deviates from the normal upright orientation. When the statoliths were centrifuged deep into the apical dome at 6 g and a stimulation angle of 170 degrees the gravitropic curvature after 1 h was identical to that determined for the same cells at 1 g and the same stimulation angle. This indicates that gravitropism in Chara protonemata is either independent of the pressure exerted by the statoliths on an underlying structure or is already saturated at 1 g. When the statoliths were moved along the apical cell wall at 8 g and the stimulation angle was gradually increased from 170 degrees to 220 degrees the gravitropic curvature reverted sharply when the cluster of statoliths passed over the cell pole. This experiment supports the hypothesis that in Chara protonemata asymmetrically distributed statoliths inside the apical dome displace the Spitzenkorper and thus the centre of growth, resulting in gravitropic bending. In contrast to the positively gravitropic Chara rhizoids, no modifications either in the transport of statoliths during basipetal acceleration (6 g, stimulation angle 0 degree, 5 h) or in the subsequent gravitropic response could be detected in the protonemata. The different effects of centrifugation on the positioning of statoliths in Chara protonemata and rhizoids indicate subtle differences in the function of the cytoskeleton in both types of cells.

Gravitropism in Phycomyces: violation of the so‐called resultant law – evidence for two response components

“…The postulate of a cosine component is, as such, not entirely novel (see Introduction), but is applied in this work for the first time over a large dynamic range which enabled us to estimate the threshold for the cosine response. Our results are in agreement with data that were obtained with horizontally centrifuged coleoptiles of Avena (Bremekamp ) and with protonemata of Chara globularis (Hodick & Sievers ). In both of these organisms a large increase of the terrestrial gravity vector negatively affected the bending.…”

Amyloplast displacement is necessary for gravisensing in Arabidopsis shoots as revealed by a centrifuge microscope

“…Furthermore, hypergravitropism was enhanced by 30 g in the gravitropic mutants tested in this study, suggesting that the observed attenuation in wild type is unlikely to be a result of mechanical bending/damage during application of hypergravity. Gravitropism in Arabidopsis roots and hypocotyls was also inhibited by hypergravity of more than 10 g (Fitzelle and Kiss, ), and hypergravity‐induced inhibition of gravitropism has also been observed in other plant species (Hodick and Sievers, ). These phenomena may reflect receptor desensitization or adaptation.…”

The cytoskeleton in plant and fungal cell tip growth

“…The reaction consists either of a change of growth direction and/or velocity or in differentiation, all of which involve a functioning cytoskeleton. Such responses to environmental stimuli are gravitropism (moss protonemata, Schwuchow et al ., 1990; Walker & Sack, 1995; and references therein, algal rhizoids and protonemata, Sievers & Schnepf, 1981 and references therein, Braun & Sievers, 1994; Hodick, 1994; Leitz et al ., 1995; Braun, 1997; Hodick & Sievers, 1998: fungi, Monzer, 1996), phototropism (moss, Meske et al ., 1996: fern, Kadota & Wada, 1989, 1992: algal zygotes, Fowler & Quatrano, 1997 and references therein) and reaction to physical features, chemical gradients or electrical fields (review Sievers & Schnepf, 1981). In moss protonemata MTs are involved in the positioning of chloroplasts according to the light circumstances (review Schnepf, 1986; Abel et al ., 1989, and references therein).…”