Abstract
In epidermal and mesophyll cells of Arabidopsis (Arabidopsis thaliana) leaves, nuclei become relocated in response to strong blue light. We previously reported that nuclear positions both in darkness and in strong blue light are regulated by the blue light receptor phototropin2 in mesophyll cells. Here, we investigate the involvement of phototropin and the actin cytoskeleton in nuclear positioning in epidermal cells. Analysis of geometrical parameters revealed that, in darkness, nuclei were distributed near the center of the cell, adjacent to the inner periclinal wall, independent of cell shape. Dividing the anticlinal wall into concave, convex, and intermediate regions indicated that, in strong blue light, nuclei became relocated preferably to a concave region of the anticlinal wall, nearest the center of the cell. Mutant analyses verified that light-dependent nuclear positioning was regulated by phototropin2, while dark positioning of nuclei was independent of phototropin. Nuclear movement was inhibited by an actin-depolymerizing reagent, latrunculin B, but not by a microtubule-disrupting reagent, propyzamide. Imaging actin organization by immunofluorescence microscopy revealed that thick actin bundles, periclinally arranged parallel to the longest axis of the epidermal cell, were associated with the nucleus in darkness, whereas under strong blue light, the actin bundles, especially in the vicinity of the nucleus, became arranged close to the anticlinal walls. Light-dependent changes in the actin organization were clear in phot1 mutant but not in phot2 and phot1phot2 mutants. We propose that, in Arabidopsis, blue-light-dependent nuclear positioning is regulated by phototropin2-dependent reorganization of the actin cytoskeleton.
Highlights
In epidermal and mesophyll cells of Arabidopsis (Arabidopsis thaliana) leaves, nuclei become relocated in response to strong blue light
After dark treatment for 16 h, nuclei were rarely if ever near anticlinal walls (Fig. 1A) and were determined to be adjacent to inner periclinal walls by changing the fluorescence microscope focal plane
S1 and S2), nuclei in epidermal cells adopted the light position somewhat less consistently; they were completely unresponsive to blue light without phototropin2. These results indicate that phototropin2 mediates blue-light-dependent nuclear positioning in epidermal cells and mesophyll cells
Summary
In epidermal and mesophyll cells of Arabidopsis (Arabidopsis thaliana) leaves, nuclei become relocated in response to strong blue light. We previously reported that nuclear positions both in darkness and in strong blue light are regulated by the blue light receptor phototropin in mesophyll cells. We investigate the involvement of phototropin and the actin cytoskeleton in nuclear positioning in epidermal cells. In epidermal and mesophyll cells of darktreated leaves, nuclei are distributed along the inner periclinal wall. Under strong light, they become located along the anticlinal walls. Nuclear movement from inner periclinal to anticlinal walls is induced repeatedly and by blue light of high-fluence rate (more than 50 mmol m22 s21) and is regulated by the blue light receptor phototropin. Phototropin is more sensitive to light than its paralog and mediates low-fluence-rate light responses, whereas phototropin functions predominantly under higher fluence rates (Sakai et al, 2001)
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