Ultrastructure of trichoblasts in the red alga Osmundea spectabilis var. spectabilis (Rhodomelaceae,Ceramiales)

The apex of the tetrasporangial branches of Osmundea spectabilis var. spectabilis (= Laurencia spectabilis var. spectabilis) exhibits cavities in which tufts of multicellular trichoblasts occur. Trichoblast development in Osmundea spectabilis var. spectabilis begins with the differentiation of an epidermal cell within the crypt. This cell differentiates into a trichoblast mother cell (TMC). The TMC divides to form a two-celled incipient trichoblast. Successive periclinal divisions of the apical cell of the young trichoblast result in the formation of a multicellular developing trichoblast. With the exception of the apical cell all trichoblast cells are at the same developmental stage. They possess a large nucleus, abundant plastids with peripheral and some internal thylakoids and dictyosomes. Daughter chloroplasts result from one constriction or multiple fission of a single chloroplast. Dictyosomal cisternae and mucilage sacs contribute material to wall formation. Each differentiating trichoblast cell is surrounded by a bi-layered wall. The outer wall layer represents the trichoblast mother cell wall and the inner wall layer is the trichoblast cell wall. Mature trichoblast cells have thin walls, probably as a consequence of mucilage extrusion, the most likely function of trichoblasts in Osmundea.     

Cell axiality and polarity in plants — adding pieces to the puzzle

Plant cell polarity is important for cellular function and multicellular development. Classical physiological and cell biological analyses identified cues that orient cell polarity and suggested molecules that translate a cue into intracellular asymmetry. A range of proteins that either mark or are involved in the establishment of a (polar) axis are now available, as are many relevant mutants. These tools are likely to facilitate a dissection of the molecular mechanisms behind cell and organ polarity in the near future.     

Structural aspects of bulge formation during root hair initiation

Using light and electron microscopy, the early stages of root hair initiation were investigated under control conditions and in a situation where F-actin polymerization was effectively inhibited by latrunculin B. Trichoblasts in their early stage of bulge formation possessed large vacuole traversed by cytoplasmic strands and enclosed within a narrow peripheral layer of cytoplasm. The nucleus was settled at the inner periclinal cell wall, typically opposite the site of bulge formation. Within the bulging area, dense cytoplasm and numerous ER elements, and other organelles were accumulated, together with pleiomorphic membrane-bound structures, the identity and nature of which will require further studies. These unusual structures, which were associated with the outer cell wall, contained material similar to that of the cell wall. Similar cell wall-like bodies were observed also in the cytoplasm and sometimes within vacuoles. The possible role of these novel organelles of plant cells in cell wall thinning/degradation or in the turgor pressure maintenance are discussed. Latrunculin B treatment allowed bulge formation but prevented the switch from the slow and diffuse expansion of bulge into the rapid tip-growth characteristic of the emerging root hair. Moreover, the cytoplasm of the bulging domain became extensively vacuolated and lacked abundant ER elements and other organelles including the membrane-bound structures. These results indicate important roles of F-actin in the switch from diffuse to highly polarized tip growth.