The ability for plant growth to be optimized, either in the light or dark, depends on the intricate balance between cell division and differentiation in specialized regions called meristems. When Arabidopsis (Arabidopsis thaliana) seedlings are grown in the dark, hypocotyl elongation is promoted, whereas root growth is greatly reduced as a result of changes in hormone transport and a reduction in meristematic cell proliferation. Previous work showed that the microtubule-associated protein CLASP sustains root apical meristem size by influencing microtubule organization and by modulating the brassinosteroid signaling pathway. Here, we investigated whether CLASP is involved in light-dependent root growth promotion, since dark-grown seedlings have reduced root apical meristem activity, as observed in the clasp-1 null mutant. We showed that CLASP protein levels were greatly reduced in the root tips of dark-grown seedlings, which could be reversed by exposing plants to light. We confirmed that removing seedlings from the light led to a discernible shift in microtubule organization from bundled arrays, which are prominent in dividing cells, to transverse orientations typically observed in cells that have exited the meristem. Brassinosteroid receptors and auxin transporters, both of which are sustained by CLASP, were largely degraded in the dark. Interestingly, we found that despite the lack of protein, CLASP transcript levels were higher in dark-grown root tips. Together, these findings uncover a mechanism that sustains meristem homeostasis through CLASP, and they advance our understanding of how roots modulate their growth according to the amount of light and nutrients perceived by the plant.

无论是在明暗环境中，植物生长的最佳能力取决于在称为分生组织的特殊区域中细胞分裂与分化之间的复杂平衡。当拟南芥幼苗在黑暗中生长时，会促进下胚轴伸长，而由于激素转运的变化和分生细胞增殖的减少，根的生长会大大降低。先前的工作表明，微管相关蛋白CLASP通过影响微管（MT）的组织和调节油菜素类固醇（BR）信号通路来维持根尖分生组织（RAM）的大小。在这里，我们调查了CLASP是否参与光依赖性根的生长促进，因为深色生长的幼苗具有降低的clap-1无效突变体中观察到的RAM活性。我们表明，在黑暗生长的幼苗的根尖，CLASP蛋白水平大大降低，这可以通过将植物暴露在光下逆转。我们证实，从光中移走幼苗会导致MT组织从捆绑阵列（在分裂细胞中突出显示）向通常在离开分生组织的细胞中观察到的横向方向明显转移。CLASP维持的BR受体和生长素转运蛋白在黑暗中被大大降解。有趣的是，我们发现尽管缺乏蛋白质，但深色生长的根尖的CLASP转录水平较高。在一起，这些发现揭示了一种通过CLASP维持分生组织稳态的机制，并加深了我们对根系如何根据植物感知到的光和养分来调节其生长的理解。