MODULATION OF STATOLITH MASS AND GROUPING IN WHITE CLOVER (TRIFOLIUM-REPENS) GROWN IN 1-G, MICROGRAVITY AND ON THE CLINOSTAT

Current models of gravity perception in higher plants focus on the buo yant weight of starch-filled amyloplasts as the initial gravity signal susceptor (statolith). However, no tests have yet determined if stato lith mass is regulated to increase or decrease gravity stimulus to the plant. To this end, the root caps of white clover (Trifolium repens) grown in three gravity environments with three different levels of gra vity stimulation have been examined: (i) 1-g control with normal stati c gravistimulation, (ii) on a slow clinostat with constant gravistimul ation, and (iii) in the stimulus-free microgravity aboard the Space Sh uttle. Seedlings were germinated and grown in the BioServe Fluid Proce ssing Apparatus and root cap structure was examined at both light and electron microscopic levels, including three-dimensional cell reconstr uction from serial sections. Quantitative analysis of the electron mic rographs demonstrated that the starch content of amyloplasts varied: w ith seedling age but not gravity condition. It was also discovered tha t, unlike in starch storage amyloplasts, all of the starch granules of statolith amyloplasts were encompassed by a fine filamentous, ribosom e-excluding matrix. From light micrographic 3-D cell reconstructions, the absolute volume, number, and positional relationships between amyl oplasts showed (i) that individual amyloplast volume increased in micr ogravity but remained constant in seedlings grown for up to three days on the clinostat, (ii) the number of amyloplasts per cell remained un changed in microgravity but decreased on the clinostat, and (iii) the three-dimensional positions of amyloplasts were not random. Instead am yloplasts in microgravity were grouped near the cell centers while tho se from the clinostat appeared more dispersed. Taken together, these o bservations suggest that changing gravity stimulation can elicit feedb ack control over statolith mass by changing the size, number, and grou ping of amyloplasts. These results support the starch-statolith theory of graviperception in higher plants and add to current models with a new feedback control loop as a mechanism for modulation of statolith r esponsiveness to inertial acceleration.