No abstract

The cold stability of microtubules during seasons of active and dormant cambium was analyzed in the conifers Abies firma, Abies sachalinensis and Larix leptolepis by immunofluorescence microscopy. Samples were fixed at room temperature and at a low temperature of 2-3°C to examine the effects of low temperature on the stability of microtubules. Microtubules were visible in cambium, xylem cells and phloem cells after fixation at room temperature during seasons of active and dormant cambium. By contrast, fixation at low temperature depolymerized microtubules in cambial cells, differentiating tracheids, differentiating xylem ray parenchyma and phloem ray parenchyma cells during the active season. However, similar fixation did not depolymerize microtubules during cambial dormancy in winter. Our results indicate that the stability of microtubules in cambial cells and cambial derivatives at low temperature differs between seasons of active and dormant cambium. Moreover, the change in the stability of microtubules that we observed at low temperature might be closely related to seasonal changes in the cold tolerance of conifers. In addition, low-temperature fixation depolymerized microtubules in cambial cells and differentiating cells that had thin primary cell walls, while such low-temperature fixation did not depolymerize microtubules in differentiating secondary xylem ray parenchyma cells and tracheids that had thick secondary cell walls. The stability of microtubules at low temperature appears to depend on the structure of the cell wall, namely, primary or secondary. Therefore, we propose that the secondary cell wall might be responsible for the cold stability of microtubules in differentiating secondary xylem cells of conifers.

Cambial cells dierentiate into secondary xylem cells through a process of expansion or elongation, cell wall thickening, cell wall sculpturing, lignification, and cell death (formation of wood). The secondary xylem cells develop modifications of the cell wall such as pits, helical thickenings, perforations and warts, through the localized deposition of cell wall materials. Recent observations have revealed that the localized appearance or disappearance of cortical microtubules is related to the localized deposition of cellulose microfibrils in secondary xylem cells. Cortical mi-crotubules play an important role in the morphogenesis of secondary xylem cells, thereby controlling the structure of wood. Therefore, cortical microtubules provide a target for biotechnological applications to change the quality of wood.

The initial uptake of water by small fragments of compressed and drying- set wood of Cryptomeria japonica D. Don was monitored by confocal laser scanning microscopy (CLSM) using an aqueous solution of the fluorescent dye acridine orange. CLSM allowed visualization of the recovery over time of compressed and drying-set wood and the uptake of water by the specimens. Furthermore, CLSM allowed us to monitor the structure of deformed tracheids under atmospheric conditions. Increases in the compression ratio increased the time required for the uptake of water. The uptake of water was detected first between deformed and undeformed regions of compressed and drying-set wood at all compression ratios tested.

The aim of the present study was to investigate the orientation and localization of actin filaments and cortical microtubules in wood-forming tissues in conifers to understand wood formation. Small blocks were collected from the main stems of Abies firma, Pinus densiflora, and Taxus cuspidata during active seasons of the cambium. Bundles of actin filaments were oriented axially or longitudinally relative to the cell axis in fusiform and ray cambial cells. In differentiating tracheids, actin filaments were oriented longitudinally relative to the cell axis during primary and secondary wall formation. In contrast, the orientation of well-ordered cortical microtubules in tracheids changed from transverse to longitudinal during secondary wall formation. There was no clear relationship between the orientation of actin filaments and cortical microtubules in cambial cells and cambial derivatives. Aggregates of actin filaments and a circular band of cortical microtubules were localized around bordered pits and cross-field pits in differentiating tracheids. In addition, rope-like bands of actin filaments were observed during the formation of helical thickenings at the final stage of formation of secondary walls in tracheids. Actin filaments might not play a major role in changes in the orientation of cortical microtubules in wood-forming tissues. However, since actin filaments were co-localized with cortical microtubules during the formation of bordered pits, cross-field pits and helical thickenings at the final stage of formation of the secondary wall in tracheids, it seems plausible that actin filaments might be closely related to the localization of cortical microtubules during the development of these modifications of wood structure.