The cytoskeleton facilitates a three-dimensional symplasmic continuum in the long-lived ray and axial parenchyma cells of angiosperm trees

The microtubule (MT), microfilament (NIF) and myosin components of the cyto skeleton were studied in the long-lived ray and axial parenchyma cells of t he secondary xylem (wood) and secondary phloem of two angiosperm trees, Aes culus hippocastanum L. (horse-chestnut) and Populus tremula L. x P. tremulo ides Michx. (hybrid aspen), using indirect immunofluorescence localisation and transmission electron microscopy. MTs and MFs were bundled and oriented axially (parallel to the cell's long axis) within all parenchyma cell type s after they had fully differentiated. Additionally, actin and myosin were immunolocalised at the thin-walled membranes of the pits, which linked cell s in neighbouring files of both ray and axial parenchyma, and at the pits b etween axial and ray parenchyma cells themselves. Anti-callose antibody imm unolocated the plasmodesmata at the pit membranes, and in the same pattern as that of anti-myosin. Ray cells are important symplasmic pathways between the xylem and the phloem throughout the life of trees. We hypothesise that the NIT and MF components of the cytoskeleton in the ray and axial parench yma cells are involved in the transport of materials within those cells, an d, in association with the acto-myosin of plasmodesmata at pit fields, are also important in intercellular transport. Thus, the symplasmic coupling be tween ray cells, between axial parenchyma cells, and between axial parenchy ma and ray cells represents an extensive three-dimensional communication pa thway permeating the tree from the phloem through the cambium into the wood . We suggest,that this cytoskeletal pathway has an important role in delive ry of photosynthate, and mobilised reserves, to the actively dividing cambi um, and in the movement of materials to sites of reserve deposition, princi pally within the wood. This pathway could also have an important role in co -ordinating developmental processes throughout the tree.