Department of Biological Sciences, Graduate School of Science, The University of Tokyo, Bunkyo-ku, Tokyo, 113-0033, Japan.
Exploratory Research Center on Life and Living Systems, National Institutes of Natural Sciences, Okazaki, Aichi, 444-8787, Japan.
Plant J. 2019 Nov;100(4):738-753. doi: 10.1111/tpj.14467. Epub 2019 Sep 9.
Plant morphological and physiological traits exhibit plasticity in response to light intensity. Leaf thickness is enhanced under high light (HL) conditions compared with low light (LL) conditions through increases in both cell number and size in the dorsoventral direction; however, the regulation of such phenotypic plasticity in leaf thickness (namely, sun- or shade-leaf formation) during the developmental process remains largely unclear. By modifying observation techniques for tiny leaf primordia in Arabidopsis thaliana, we analysed sun- and shade-leaf development in a time-course manner and found that the process of leaf thickening can be divided into early and late phases. In the early phase, anisotropic cell elongation and periclinal cell division on the adaxial side of mesophyll tissue occurred under the HL conditions used, which resulted in the dorsoventral growth of sun leaves. Anisotropic cell elongation in the palisade tissue is triggered by blue-light irradiation. We discovered that anisotropic cell elongation processes before or after periclinal cell division were differentially regulated independent of or dependent upon signalling through blue-light receptors. In contrast, during the late phase, isotropic cell expansion associated with the endocycle, which determined the final leaf thickness, occurred irrespective of the light conditions. Sucrose production was high under HL conditions, and we found that sucrose promoted isotropic cell expansion and the endocycle even under LL conditions. Our analyses based on this method of time-course observation addressed the developmental framework of sun- and shade-leaf formation.
植物形态和生理特征表现出对光强的可塑性。与低光（LL）条件相比，高光（HL）条件下通过增加背腹方向的细胞数量和大小来增强叶片厚度；然而，叶片厚度这种表型可塑性的调节（即阳生叶或阴生叶的形成）在发育过程中仍然很大程度上不清楚。通过修改拟南芥微小叶片原基的观察技术，我们以时间进程的方式分析了阳生叶和阴生叶的发育，并发现叶片增厚的过程可以分为早期和晚期。在早期，HL 条件下，叶肉组织的近轴侧发生各向异性细胞伸长和垂周细胞分裂，导致阳生叶的背腹生长。栅栏组织中的各向异性细胞伸长是由蓝光照射触发的。我们发现，在垂周细胞分裂之前或之后的各向异性细胞伸长过程被独立地或依赖于蓝光受体信号通路进行差异调节。相比之下，在晚期，与内循环相关的各向同性细胞扩张发生，这决定了最终的叶片厚度，而与光照条件无关。HL 条件下蔗糖产量较高，我们发现即使在 LL 条件下，蔗糖也能促进各向同性细胞扩张和内循环。我们基于这种时间进程观察方法的分析解决了阳生叶和阴生叶形成的发育框架问题。
