Structural basis of root hair formation: Early development of trichoblasts and atrichoblasts

Abstract

The development of rhizodermis cells — trichoblasts and atrichoblasts — was investigated in cress radicles by light and electron microscopy. In serial transverse sections — progressing acropetally from the area of root hair formation to the apical meristem prospective trichoblasts could be identified by the following characteristics already at the meristematic level: 1. their localization above the anticlinal cell wall of the cortex cells, 2. their wedge-like shape, 3. their remarkable dense cytoplasm, and 4. their cell length. These characteristics indicated that a large number of trichoblasts was alreadey determined during embryogenesis. The development of dermatogen cells into prospective trichoblasts commenced when the subjacent cells of the outer cortex divided longitudinally in anticlinal direction, a process which caused changes in the arrangement of the intercellular system. Dermatogen cells than grew into these newly developed intercellular space and assumed characteristics of prospective trichoblasts.

Thin sectioning electron microscopy demonstrated that prospective trichoblasts possess more ribosomal particles and smaller vacuoles indicating a more meristematic character of this cell type. The cell wall between trichoblasts and cortex cells was characterized by numerous plasmodesmata mainly grouped in large pit fields. In prospective trichoblasts and atrichoblasts a large number of cortical microtubules was observed running perpendicular to the longitudinal axis of the cells.