Organelle Sedimentation in Gravitropic Roots of Limnobium is Restricted to the Elongation Zone

Roots of the aquatic angiosperm Limnobium spongia (Bosc) Steud.were evaluated by light and electron microscopy to determinethe distribution of organelle sedimentation towards gravity.Roots of Limnobium are strongly gravitropic. The rootcap consistsof only two layers of cells. Although small amyloplasts arepresent in the central cap cells, no sedimentation of any organelle,including amyloplasts, was found. In contrast, both amyloplastsand nuclei sediment consistently and completely in cells ofthe elongation zone. Sedimentation occurs in one cell layerof the cortex just outside the endodermis. Sedimentation ofboth amyloplasts and nuclei begins in cells that are in theirinitial stages of elongation and persists at least to the levelof the root where root hairs emerge. This is the first modernreport of the presence of sedimentation away from, but not in,the rootcap. It shows that sedimentation in the rootcap is notnecessary for gravitropic sensing in at least one angiosperm.If amyloplast sedimentation is responsible for gravitropic sensing,then the site of sensing in Limnobium roots is the elongationzone and not the rootcap. These data do not necessarily conflictwith the hypothesis that sensing occurs in the cap in otherroots, since Limnobium roots are exceptional in rootcap originand structure, as well as in the distribution of organelle sedimentation.Similarly, if nuclear sedimentation is involved in gravitropicsensing, then nuclear mass would function in addition to, notinstead of, that of amyloplasts.Copyright 1994, 1999 AcademicPress Limnobium spongia, gravitropism, roots, sedimentation, cortex