Abstract
Chloroplasts change their positions in the cell depending on the light conditions. In the dark, chloroplasts in fern prothallia locate along the anticlinal wall (dark position). However, chloroplasts become relocated to the periclinal wall (light position) when the light shines perpendicularly to the prothallia. Red light is effective in inducing this relocation in Adiantum capillus-veneris, and neochrome1 (neo1) has been identified as the red light receptor regulating this movement. Nevertheless, we found here that chloroplasts in neo1 mutants still become relocated from the dark position to the light position under red light. We tested four neo1 mutant alleles (neo1-1, neo1-2, neo1-3, and neo1-4), and all of them showed the red-light-induced chloroplast relocation. Furthermore, chloroplast light positioning under red light occurred also in Pteris vittata, another fern species naturally lacking the neo1-dependent phenomenon. The light positioning of chloroplasts occurred independently of the direction of red light, a response different to that of the neo1-dependent movement. Photosynthesis inhibitors 3-(3,4 dichlorophenyl)-1,1-dimethylurea or 2,5-dibromo-3-isopropyl-6-methyl-p-benzoquinone blocked this movement. Addition of sucrose (Suc) or glucose to the culture medium induced migration of the chloroplasts to the periclinal wall in darkness. Furthermore, Suc could override the effects of 3-(3,4 dichlorophenyl)-1,1-dimethylurea. Interestingly, the same light positioning was evident for nuclei under red light in the neo1 mutant. The nuclear light positioning was also induced in darkness with the addition of Suc or glucose. These results indicate that photosynthesis-dependent nondirectional movement contributes to the light positioning of these organelles in addition to the neo1-dependent directional movement toward light.
Highlights
Chloroplasts change their positions in the cell depending on the light conditions
When prothallia from the neo1-1 mutant and wildtype A. capillus-veneris kept in the dark for 3 d, with their chloroplasts in the dark position, were continuously irradiated with red light (4.1 mmol m22 s21) coming from above, chloroplasts of the wild-type prothallia showed relocation movement toward the periclinal wall facing the incident light via the neo1-dependent response, as reported previously (Kadota and Wada, 1999; Kawai et al, 2003)
When the direction of the red light was changed and the prothallial cells were irradiated from the side with red light of 6.4 mmol m22 s21, chloroplasts in the wildtype cells showed movement toward the light, and the chloroplasts became relocated to the anticlinal walls facing the red light (Fig. 1, A and B; Supplemental Movie S1)
Summary
Chloroplasts change their positions in the cell depending on the light conditions. In the dark, chloroplasts in fern prothallia locate along the anticlinal wall (dark position). While in the dark chloroplasts locate along the anticlinal wall or the bottom of the cell depending on plant species (dark position), they move toward and accumulate along the periclinal cell wall that face incident illumination under weak or moderate light conditions (light position) This response is thought to optimize photosynthetic activity (Zurzycki, 1955). In the fern Adiantum capillus-veneris, the noncanonical phytochrome neochrome (neo; formerly known as phytochrome; Suetsugu and Wada, 2007), which is a chimera of N-terminal chromophorecontaining half of a canonical phytochrome plus a fulllength phototropin downstream from it, has been identified as the receptor for the red-light-induced chloroplast and nuclei relocation (Kawai et al, 2003; Tsuboi et al, 2007). Sugiyama and Kadota lacks neo1-dependent responses (Kadota et al, 1989), still show light positioning of chloroplasts under red light regulated by photosynthesis and that relocation of nuclei accompanies the response
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