A morphometric analysis of the redistribution of organelles in columella cells in primary roots of normal seedlings and agravitropic mutants of Hordeum vulgare.

The redistribution of organelles in columella cells of horizontally-oriented roots of Hordeum vulgare was quantified in order to determine what structural changes in graviperceptive (i.e., columella) cells are associated with the onset of the root gravicurvature. The sedimentation of amyloplasts is the only major change in cellular structure that correlates positively with the onset of root gravicurvature, which begins within 15 min after re-orientation. There is no consistent contact between sedimented amyloplasts and any other organelles. Nuclei are restricted to the proximal ends of columella cells in vertically-oriented roots, and remain there throughout gravicurvature after roots are oriented horizontally. Root gravicurvature does not involve significant changes in (1) the volume of columella cells, (2) the relative or absolute volumes of organelles in columella cells, or (3) the distribution of endoplasmic reticulum (ER). The size, number and sedimentation rates of amyloplasts in columella cells of non-graviresponsive roots of mutant seedlings are not significantly different from those of graviresponsive roots of normal seedlings. Similarly, there is no significant difference in (1) cellular volume, (2) distribution or surface area of ER, (3) patterns or rates of organelle redistribution in horizontally-oriented roots, (4) relative or absolute volumes of organelles in columella cells of graviresponsive and non-graviresponsive roots. These results suggest that the lack of graviresponsiveness by roots of mutant seedlings is probably not due to either (1) structural differences in columella cells, or (2) differences in patterns or rates of organelle redistribution as compared to that characteristic of graviresponsive roots. Thus, the basis of non-graviresponsiveness in this mutant is probably different from other agravitropic mutants so far studied.