Dynamics of cytoskeleton in differentiation of xylem cells of trees grown under the environmental stress

树木形成层细胞的细胞分裂及其衍生物的分化产生可持续的木材生物量。木材的数量和质量取决于树木生长的环境条件。树木生长过程中常受到高温、低温、干旱、酸雨、臭氧等多种环境胁迫的影响，这些胁迫通过抑制形成层的活动而导致木材产量的减少。本课题以低温胁迫为研究对象，分析了低温胁迫对形成层活性和木质部分化的影响机制。活性形成层细胞中的微管对低温作出反应，因此，在样品固定过程中通常需要避免低温。然而，我们发现在仲冬，当温度低于零度时，在休眠的形成层细胞中有清晰的微管束。此外，形成层休眠细胞中的微管不因低温固定而脱落。这些观察结果表明，微管在休眠形成层在低温下的稳定性。这种稳定性可能与形成层细胞中的内源性因素有关，因为微管的冷稳定性可以被植物激素如赤霉素和脱落酸改变。此外，很可能是活跃和休眠的形成层细胞中的微管由不同的微管蛋白同种型组成，其聚合表现出不同程度的冷稳定性。因此，微管的冷稳定性与树木对环境条件的适应密切相关，从而控制着木材生物量的数量和质量。

The cell division of cambial cells and differentiation of their derivatives in trees produce sustainable wood biomass. The quantity and quality of wood vary depending on the environmental conditions of growth of trees. Trees usually grow under the several environmental stresses such as high or low temperature, drought, acid rain, and ozone etc. Such stresses result in decrease in wood quantity through the inhibition of cambial activity. In this project, we analyzed the cellular mechanism how low temperature, one of environmental stresses, affects the cambial activity and xylem differentiation. Microtubules in active cambial cells depolymerize in response to low temperatures and, thus, it is generally necessary to avoid low temperatures during the fixation of samples. However, we found clear bundles of microtubules in dormant cambial cells in mid-winter, when the temperature was below zero. In addition, microtubules in dormant cambial cells did not depolymerize by the fixation of samples under low temperature. These observations suggest the stability of microtubules in dormant cambium at low temperatures. Such stability might be related to endogenous factors in cambial cells since the cold stability of microtubules can be modified by plant hormones, such as gibberellin and abscisic acid. In addition, it is likely that microtubules in active and dormant cambial cells consist of different isotypes of tubulin whose polymerization exhibits different degrees of cold stability. We conclude that the cold stability of microtubules is closely related to the adaptation to environmental conditions of trees, therefore controlling the quantity and quality of wood biomass.