The characean internode has been established as a valuable experimental system for investigating local wound responses. The cells readily react to injury by exocytosis of wall material. The structure, chemical composition and the function of the wound wall vary according to the type of wounding (light, chemicals or mechanical injury) and to the extent of damage. We are especially interested in the reorganization of the cytoskeleton, in actin-microtubule interactions, and in visualizing exocytotic stages in living cells. The results obtained with characean algae are compared with wound responses and defence mechanisms in other plant cells.
Cortical and subcortical endoplasmic reticulum (ER) in internodal cells are separated from each other by a layer of stationary chloroplasts. The inner ER tubes are coated with myosin molecules which generate cytoplasmic streaming by interaction with subcortical actin filament bundles. In contrast to this fast moving system the cortical ER forms a more stable meshwork close to the plasma membrane. We showed recently that the morphology and dynamics of this cortical ER depends on interaction with microtubules during early stages of cell elongation. We currently investigate how the microtubule-dependent dynamics of the cortical ER affects the deposition of cellulose microfibrils.
The surface of internodal cells is characterized by alternating bands of acid and alkaline pH which differ in photosynthetic activity in spite of a homogenous distribution of chloroplasts. The pH banding pattern is reflected by spatiotemporal variations in the number and density of cortical mitochondria required for recycling and detoxification of photosynthetic metabolites. The uneven distribution of mitochondria is controlled by interaction with cortical actin filaments and microtubules. Dual interaction with both fibrillar systems also determines the dynamic morphology of giant mitochondria formed in thalli grown under low light conditions. Current research focuses on the identification of signalling pathways between chloroplasts, cortical cytoskeleton and mitochondria.
The plasma membrane of characean internodes contains discrete plasma membrane domains, up to 3 µm in diametre, which can be stained with FM-dyes and with filipin suggesting that they are enriched in sterols. These plasma membrane domains are heterogeneously distributed over the cell surface and are especially abundant in internodal cells of the genus Chara. We investigate their possible involvement in the pH-banding pattern and in endocytosis.
We combine light and electron microscopical techniques with immunocytochemistry, perfusion and microinjection.