| Abstract [eng] |
The peculiarities of gravisensing of hypocotyls and roots of the same seedling for the first time were compared under different gravistimulation conditions. Indirect experimental method, i.e. quantitative analysis of the dependence of amyloplast statics and kinetics on the direction and magnitude of gravitational force, was applied to assess the cytoskeleton role in the maintenance of polarized root and hypocotyl statocyte structure and amyloplast movements during seedling gravistimulation. The experiments were performed with garden cress (Lepidium sativum L.) seedlings in the dark by using original devices ‒ in-flight centrifuge ‘Neris-5’ (biosatellite ‘Bion-10’) in space and centrifuge-clinostat (a device with two orthogonal axes) in the Earth for modelling of altered gravity and gravitropic stimulation conditions. The magnitude of the gravitational force was changed from microgravity (real in space or simulated by horizontal clinostat) to 1-g (simulated by centrifugation in space or natural Earth's gravity) and its action direction was changed at 90° or 180° inversion with respect to the longitudinal axis of the seedlings. It was determined that the action of gravitational force is not an essential factor for the formation of gravisensing tissues in hypocotyl and root of garden cress seedlings; however, under real and simulated microgravity the growth of endodermal cells in hypocotyls is slower, the location of amyloplasts changes significantly with respect to the morphological bottom of statocytes in roots and hypocotyls. The logarithmic dependence between the positioning of amyloplasts along statocytes of roots and hypocotyls and the magnitude of gravitational force, which acted permanently on seedlings in root-tip direction, was determined. It was found that the unimodal regularity is characteristic of amyloplast distribution in root statocytes under tested gravitational loads. In hypocotyl statocytes, unimodal regularity of amyloplast distribution changes into bimodal when the gravitational force is reduced to 0.02 g. The magnitude of intracellular forces, which lift up the amyloplasts from the morphological cell bottom, is in the range from 0.1 g to 1 g in root statocytes. In hypocotyl endodermal cells, it is in the range from 0.5 g to 1 g. The magnitude of intracellular forces maintaining the positioning of amyloplasts under microgravity conditions is in the range from 0.004 g to 0.02 g in roots and in the range from 0.008 g to 0.02 g in hypocotyls. Short-term exposition to simulated microgravity or 180° inversion of seedlings grown vertically under natural gravitational force provoked the lifting of amyloplasts from the morphological bottom of gravisensing cells. In root statocytes, within the first two minutes the plastids moved at comparable rate towards the cell centre, but later on their motion slowed down under microgravity and proceeded under inversion. In hypocotyl statocytes, the amyloplasts lifted up slowly during the entire period of exposition to microgravity, but much more rapidly during the first two minutes of inversion. Short-term longwise stimulation of seedlings grown in simulated microgravity by the gravitational force in root-tip or opposite direction provoke the movement of amyloplasts in gravisensing cells parallel to its action. In root statocytes, the amyloplasts sediment intensively by a comparable extent during the first four minutes of both stimulations. In hypocotyl statocytes, the sedimentation of amyloplasts proceeds about two times faster towards the morphological cell bottom than in opposite direction already during the first minute. It was determined that transverse stimulation of seedlings grown vertically under natural gravitational conditions by the gravity of 1 g induced the sedimentation of amyloplasts with simultaneous longwise sliding towards the centre of gravisensing cells. In root statocytes, during the first minute, the amyloplasts moved by the comparable rate in both directions, during the second – faster longwise than downwards. In hypocotyl statocytes, within the first minute of stimulation, the movement of statoliths occurred more quickly downwards than longwise, and it proceeded by the comparable rate in both directions during the second minute. The obtained data on the statics and kinetics of amyloplasts confirm that gravity-dependent positioning and movement of amyloplasts both in hypocotyl endodermal cells and root statocytes of garden cress seedlings are modulated by intracellular forces, firstly generated by the cytoskeleton. |
