Kinetics of amyloplast sedimentation in gravistimulated maize coleoptiles

Abstract

Inner mesophyll cells from coleoptiles of Zea mays L. cv. Merit were fixed after varying periods of gravistimulation. A statistically significant amount (17-21%) of amyloplast sedimentation occurred in these cells after 30 s of gravistimulation. The presentation time is approx. 40 s or less. The accumulation of amyloplasts near the new lower wall shows a linear relationship to the logarithm of the gravistimulation time (r=0.92 or higher). The intercept of this line with the baseline value of amyloplasts in vertical coleoptiles indicates that the number of amyloplasts on the new lower wall begins increasing 11-15 s after the onset of gravistimulation. Direct observations of living cells confirm that many amyloplasts sediment within less than 15-30 s. These rapid kinetics are consistent with the classical statolith hypothesis of graviperception involving the sedimentation of amyloplasts to the vicinity of the new lower wall.

Amyloplasts and Vacuolar Membrane Dynamics in the Living Graviperceptive Cell of the Arabidopsis Inflorescence Stem

“…That population is very small, a few percent, but they may retain a large amount of kinetic energy, taking into account that the kinetic energy is proportional to the square of the velocity, and this large amount of kinetic energy may be enough to overcome the threshold to elicit the signal for a downstream response. This kind of movement may correspond with the sedimentation of amyloplast ''faster and to a greater magnitude,'' as discussed by MacCleery and Kiss (1999) or ''the leading amyloplasts'' (Sack et al, 1984). The mean values of Dx direction in some time periods (in wild type, 2 to 3 and 3 to 4 min, and in the latrunculin B-treated samples 0 to 1 min and 3 to 4 min are reorientation) are also statistically significantly increased compared with the mean values before reorientation.…”

The Onset of Gravisensitivity in the Embryonic Root of Flax

“…Actindependent cellular motion (noise) may explain why starchless mutants are capable of proper orientation in response to gravity even though they do not have the benefit of massive, sedimenting amyloplasts (Caspar and Pickard, 1989). This concept of dynamic sensing is supported by saltatory movements of amyloplasts (Sack et al, 1984;Saito et al, 2005) and assumes that the cellular activity regularly probes the cells' orientation and contributes to the static signal that is likely to originate from sedimented statoliths.…”

How to Activate a Plant Gravireceptor. Early Mechanisms of Gravity Sensing Studied in Characean Rhizoids during Parabolic Flights

“…ciliates (Hemmersbach et al, 1996), flagellates (Häder et al, 1995), and higher plant statocytes (Brown et al, 1995). In conclusion, the mechanisms of gravity sensing might be overbuilt (Bjö rkman, 1988;Sack, 1997); however, only a high sensitivity ensures that the sensing system can operate efficiently and can correct even the smallest deviations from the gravitropic set-point angle.…”

“…There are several reports indicating that the actomyosin system is involved in the positioning of statoliths and in the modulation of the sedimentation process in order to meet specific requirements for a most beneficial gravitropic response (Sievers et al, 1991a;Volkmann et al, 1991Volkmann et al, , 1999Driss-Ecole et al, 2000;Perbal et al, 2004). Although the kinetics of statolith movements have been analyzed in gravisensing cells of shoots (Sack et al, 1984;Saito et al, 2005) and roots (Sack et al, 1985(Sack et al, , 1986MacCleery and Kiss, 1999;Yoder et al, 2001), interpretation of the results with respect to gravity sensing is difficult because the interaction between statoliths and the cytoskeleton and its role in the graviperception mechanism is far from being understood in these cell types.…”