The Cytoskeleton and Secretory Vesicles in Root Hairs of Equisetum and Limnobium and Cytoplasmic Streaming in Root Hairs of Equisetum

Abstract

The ultrastructure of root hairs of Equisetum hyemale (L.) and Limnobium stoloniferum (L.) was studied by means of freeze-substitution. Attention was given to secretory vesicles and the cytoskeleton. Rate of cytoplasmic streaming in developmental stages of root hairs of E. hyemale was measured. ‘Secretory vesicles’, smooth vesicles primarily located in the hair tip, measure approx. 300 nrm in diameter. The vesicles at the outside of the trans-face of the dictyosomes are similar in appearance and size to the ‘secretory vesicles’. Coated pits occur in abundance at the plasma membrane and are occasionally seen at the transcisternae of the dictyosomes; they are hypothesized to function in membrane turn-over. Besides microtubules, two distinguishable classes of filaments can be identified in the root hairs. Single filaments occur in the cortical cytoplasm, and are in association with microtubules. Bundles of filaments are found throughout the cytoplasm with the exception of the tip region of the hair. The bundles of microfilaments stained with rhodamine-labelled phalloidin and are presumably F-actin. Microfilament bundles are hypothesized to function in cytoplasmic streaming, while secretory vesicles might be transported via microtubules or the single filaments that accompany them. At a temperature of 25 °C cytoplasmic streaming is absent from the tip region of short hairs, but gradually increases up to 3.5 μm s⁻¹ in the base of the hair. In long hairs cytoplasmic streaming reaches up to the tip and is up to 7μm s⁻¹ in the tube of the hair.