Dynamics of cytoskeleton myosin during the differentiation of xylem cells in trees

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Cambial cells in tress differentiate into secondary xylem cells through a process of cell expansion, cell wall thickenings, lignification and cell death. During the differentiation, xylem, cells develop modifications of the cell wall such as pits and helical thickenings by the localized deposition of cell wall materials. Our previous results indicated that the localization of cell wall formation was related to the localized appearance and disappearance of components of the cytoskeleton, namely cortical microtubules and actin filaments, indicating that components of the cytoskeleton control the localized deposition of cell wall materials. However, we have only a few papers showing how the localization of components of the cytoskeleton is controlled. We analyzed here the role of myosins, so called motor-proteins, in the differentiation of xylem cells in tress because myosins utilize ATP to generate mechanical force along actin filaments. Firstly, we established the method of visualization of myosin in xylem cells. Myosins were immuno-stained with a monoclonal antibody against myosins and then FITC-conjugated secondary antibody. They were observed by confocal laser scanning microscopy. Fluorescence derived from myosins was observed in differentiating xylem cells. Myosins were aggregated at the sites of apertures of intertracheal bordered pits. As xylem cells differentiated, the aggregation of myosins decreased in diameter. The reduction in diameter of myosins was associated with the development of bordered pits. Such a localization of myosins was a similar to that of actin filaments in differentiating xylem cells. Our results indicate that an acto-myosin contractile system, being a similar manner to the contraction of mammalian muscle, might be present at the site of pit formation. We conclude that myosins play an important role in the formation of modified structure, therefore controlling the morphogenesis of secondary xylem cells.