Cytoplasmic streaming affects gravity-induced amyloplast sedimentation in maize coleoptiles

Abstract

Living maize (Zea mays L.) coleoptile cells were observed using a horizontal microscope to determine the interaction between cytoplasmic streaming and gravity-induced amyloplast sedimentation. Sedimentation is heavily influenced by streaming which may (1) hasten or slow the velocity of amyloplast movement and (2) displace the plastid laterally or even upwards before or after sedimentation. Amyloplasts may move through transvacuolar strands or through the peripheral cytoplasm which may be divided into fine cytoplasmic strands of much smaller diameter than the plastids. The results indicate that streaming may contribute to the dynamics of graviperception by influencing amyloplast movement.

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

“…Quantitative analysis of a large data set obtained from fine time-lapse imaging revealed that the amyloplasts actually moved toward the direction of gravity 0 to 3 min after gravistimulation by reorientation in the wild-type cell (Figure 2, Table 1). Saltatory movements of amyloplasts in the living graviperceptive cell have also been reported in the dandelion (Taraxacum officinale) flower stalks (Clifford and Barclay, 1980), mung bean (Vigna radiate) hypocotyls (Heathcote, 1981), and the maize (Zea mays) coleoptile (Sack and Leopold, 1985). Rapid movements of amyloplasts toward the gravity were detected (>3 mm/10 s, ;20 mm/min) as previously reported in the living graviperceptive cell; however, amyloplasts undergoing such movements constituted only a fraction of the overall population.…”

“…Sack and Leopold (1985) suggested that amyloplasts may move through transvacuolar strands. This study showed using confocal laser scanning microscopy that amyloplasts in the peripheral area of the cytoplasm were almost entirely covered with a vacuolar membrane.…”

“…Plants have developed many mechanisms during the course of evolution to survive in such circumstances by changing growth direction or architecture of their body shape. Typical of such growth responses, gravitropism is one of most prominent phenomena and has been the focus of studies on many plant species over a long period of time (Knight, 1806;Sack, 1991;Fukaki et al, 1996b;Chen et al, 1999). In higher plants, shoots and roots generally show negative and positive gravitropism, respectively.…”

“…Some classical hypotheses have been supported by recent molecular genetical studies. For example, the starch-statolith hypothesis is involved in the first step and proposes that the sedimenting amyloplasts, which contain dense starch granules, act in sensing the direction of gravity (Sack, 1991;Weise and Kiss, 1999). However, the mechanism functioning in gravity perception has not yet been characterized at the molecular level.…”

Cytoplasmic pH Dynamics in Maize Pulvinal Cells Induced by Gravity Vector Changes,

“…In a similar manner, the dynamics of amyloplast streaming in the maize pulvinus (this study), and amyloplast motility in other gravitropic tissues (Sack and Leopold, 1985), indicate extensive interactions between amyloplasts and the actin cytoskeleton. Thus, it is possible that while moving to the new cell bottom, amyloplasts might interact with actin and/or membranes, thereby eliciting pH c changes in cellular microdomains.…”