Kinetics of gravity-induced amyloplast sedimentation in statocytes of cress roots grown under fast clino-rotation, 1g and after 180° inversion

In order to detail the relation between the initial positioning of statoliths and root graviresponsiveness, the movement of amyloplasts within root statocytes of cress (Lepidium sativum L.) seedlings grown under fast clino-rotation (50 rpm) and vertically at 1g or 180° inverted after growth at 1g has been analyzed during a subsequent 6-min period of lateral (reoriented 90°) gravistimulation. The main parameters of statolith motion kinetics such as relative positions and motion velocities in transverse and longitudinal directions have been evaluated taking linear measurements of organelle position with respect to the lower longitudinal and distal wall in the cells of the 2nd-5th columella storeys. A significant discrepancy in the direction and velocity of statolith movement has been determined within the first and second minutes of root horizontal placement. Centrally or proximally located statoliths in clino-rotated or inverted root statocytes moved transversely to the lower longitudinal cell wall more quickly as compared with distally located ones in 1-g root cells. At the same time, the longitudinal displacement of proximally located statoliths towards the cell center has been determined, while this displacement of distally located plastids was significantly more pronounced. The longitudinal motion of amyloplasts grouped in the central cell part was negligible within this 2-min period. After the 6th minute of stimulation, the total downward shift of the mean relative position of statoliths amounted to 16.3% from the total cell width in clino-rotated root statocytes and 17.1% in the cells of inverted roots, while it equalled 13.3% in 1-g root statocytes. The total change of the relative longitudinal position of amyloplasts with respect to the distal cell wall remained almost unchanged in roots grown on the clinostat. In inverted roots, the initial longitudinal position of 56.2% from the total cell length has been shifted significantly to 47.8% and from 27.7% to 38.8% in 1-g roots. The data imply that a more pronounced sedimentation of centrally or proximally located amyloplasts during the first minutes of root stimulation could be responsible for the earlier generation and transmission of a directional signal which triggers the subsequent events of gravitropic reaction in clino-rotated or inverted roots as compared to 1-g roots.