Ultrastructure of Medicago sativa rhizodermis cells over their early development

Abstract

Ultrastructure was investigated along the files of developing epidermal cells in the root tip of a model plant Medicago sativa, in which all rhizodermal cells are potential hair-forming trichoblasts. Differentiation at subcellular level was observed up to the stage of bulge initiation in the trichoblasts. Root hair initiation indicated by the emergence of bulges from trichoblasts was detected at various distances from the root tip and, it was independent of the trichoblast size. During rhizodermal cell differentiation, starch grains accumulated in the plastids. Nuclei located in the central part of the young, meristematic cells moved towards the inner periclinal wall as the central vacuole enlarged. The bulging region of the trichoblasts located opposite the nucleus and was rich in mitochondria, ER, ribosomes, and Golgi bodies, and contained also vesicles enclosing fibrillar material. This material responded positively to phosphotungstic acid, which was used for detection of cell wall polysaccharides. The cell wall thickness within the bulging domain was significantly lower than in other parts of trichoblasts. We suggest that internalization of cell wall polysaccharides occurs within the bulging area, contributing to local thinning of the cell wall and providing a source of osmotically active compounds for maintaining turgor in the trichoblast. Thus, the internalization process might be necessary for root hair outgrowth.