Gravisensing in hypocotyls and roots of garden cress seedlings

Abstract

Gravisensing in negatively and positively gravitropic plant organs was studied by analysing the location and motion of amyloplasts in hypocotyl endodermal and root columella cells of Lepidium sativum L. seedlings during a subsequent 6-min period of lateral (reoriented 90°) gravitropic stimulation. The amyloplast positioning has been evaluated taking linear measurements of each plastid position with respect to the longitudinal wall and original bottom of the gravity sensing cells. After growth for 30 h at 1 g, the amyloplasts are distributed symmetrically across the celės and located closer to the bottom cell wall in the endodermis than in root columella (15.2% and 27.7%, respectively). A more rapid plastid displacement (approximately 3 μm/min) towards the gravity and simultaneous sliding along the cells were determined within the first minute of gravitropic stimulation in hypocotyl endodermal cells as compared with that (1 μm/min) in root statocytes. During the second minute, the amyloplast location remained almost unchanged in endodermal cells, while they continued to slide intensively along the columella cells shifting slightly downwards. After a 6-min period, the final relative statolith distances from the Po longitudinal wall and bottom of the statocytes equalled to 31.1% and 28.9% in hypocotyls and to 36.5% and 38.8% in roots. The data show that, in agreement with the gravity force, the elastic forces of the cytoskeleton act actively transporting the amyloplasts along the statocytes into both organs within a 6-min gravitropic stimulation. However, the magnitude relation of tiese forces changes differently in gravisensing cells of hypocotyls and roots during the first minutes of gravitropic stimulation. Keywords: gravisensing, statocyte, amyloplast, root, hypocotyl, garden cress