Ultrastructure and movements of cell organelles in the root cap of agravitropic mutants and normal seedlings of Arabidopsis thaliana

Abstract

The root anatomy and ultrastructure of the agravitropic A. thaliana L. mutants Dwf and aux-1 were compared with the gravitropic mutant aux-2 and the wild type (WT) in an attempt to find an explanation for the lack of response to gravity. No differences were found in the organization of the root cap. The central part of the cap (columella) contains 5 storeys of developing, functioning and degenerating statocytes. Their ultrastructure is very similar in all 4 types of plant. Particular attention was paid to the distribution of rough endoplasmic reticulum (ER). Both in the WT and the mutants the ER is concentrated in the distal part at the floor of the cell. Light micrographs were used to compare the sedimentation rates of movable cell structures in normal and agravitropic root statocytes. A longitudinal movement of amyloplasts and nuclei was observed when the roots were inverted. In WT and aux-2 the rates were on average 6.3 .mu.m h-1 (amyloplasts) and 2.1 .mu.m h-1 (nucleus). In aux-1 the sedimentation rates were significantly lower: 2.4 and 0.6 .mu.m h-1, respectively. Based on magnified electron micrographs of normal and inverted statocytes a morphometrical analysis of the distribution and redistribution of amyloplasts, nuclei, mitochondria, vacuoles and ER was made. The only significant differences was found in the redistribution of amyloplasts between aux-1 and the gravitropical normal types.