Root Graviresponsiveness and Cellular Differentiation in Wild-type and a Starchless Mutant of Arabidopsis thaliana

Primary roots of a starchless mutant of Arabidopsis thalianaL. are strongly graviresponsive despite lacking amyloplastsin their columella cells. The ultrastructures of calyptrogenand peripheral cells in wild-type as compared to mutant seedlingsare not significantly different. The largest difference in cellulardifferentiation in caps of mutant and wild-type roots is therelative volume of plastids in columella cells. Plastids occupy12.3% of the volume of columella cells in wild-type seedlings,but only 3.69% of columella cells in mutant seedlings. Theseresults indicate that: (1) amyloplasts and starch are not necessaryfor root graviresponsiveness; (2) the increase in relative volumeof plastids that usually accompanies differentiation of columellacells is not necessary for root graviresponsiveness; and (3)the absence of starch and amyloplasts does not affect the structureof calyptrogen (i.e. meristematic) and secretory (i.e. peripheral)cells in root caps. These results are discussed relative toproposed models for root gravitropism. Arabidopsis thaliana, gravitropism (root), plastids, root cap, stereology, ultrastructure