Mechanism of chloroplast movement

The machinery of chloroplast movement has not yet been clarified in any plant species, although an actin-myosin system has been proposed based on experimental results using inhibitors for actin (e.g., cytochalasin B) and myosin (e.g., N-ethylmaleimide) (see Wada et al., 2003). In the moss Physcomitrella patens microtubules as well as actin filaments are involved in movement (Sato et al., 2001), but ferns do not use microtubules (Kadota and Wada, 1992d). The circular structure of F-actin was observed along the edge of chloroplasts on the surface of plasma membranes in A. capillus-veneris protonemata (Kadota and Wada, 1989a, 1989b). The structure was found only when chloroplasts gathered at a light irradiated area and settled there. It could not be located on a chloroplast under darkness or on chloroplasts located far from a microbeam irradiated area (Kadota and Wada, 1992c), suggesting that the structure plays a role in keeping the chloroplast in the light irradiated area. Before chloroplasts retreat from the irradiated area, the structure disappears (Kadota and Wada, 1992c). We are not sure whether a similar structure of microfilaments is involved with chloroplast movement itself.

1.8 Nuclear movement

Nuclei also move towards light, although the speed of movement is very slow compared to that of chloroplasts. Under weak white light conditions, nuclei remain in the central part of the light-facing side of prothallial cells. Under darkness, nuclei move to the anticlinal wall just as chloroplasts do (Figure 1.18) (Kagawa and Wada, 1993). Polarized red and blue light is also effective, in the same way that the chloroplast accumulation response occurs, although the velocity is more than ten times slower than that of chloroplast movement (Kagawa and Wada, 1995). Very recently Tsuboi et al. (2007) found, using AcNEO1 and AcPHOT2 mutants, that neochrome1 is the photoreceptor for red light induction and those for blue light induced nuclear movement are phototropins.

Figure 1.18 Light micrographs of intact gametophytes (a, c) and cross-sections (b, d) showing intracellular nuclear positioning under different light conditions. Nuclei positioned at the cell surface under weak white light (a, b), but at anticlinal walls under darkness (c, d, kept in the dark for 3 and 2 days, respectively) or under strong light conditions (data not shown).

Figure 1.18 Light micrographs of intact gametophytes (a, c) and cross-sections (b, d) showing intracellular nuclear positioning under different light conditions. Nuclei positioned at the cell surface under weak white light (a, b), but at anticlinal walls under darkness (c, d, kept in the dark for 3 and 2 days, respectively) or under strong light conditions (data not shown).

1.9 Reproductive organs

Developmental and genetic studies of fern reproductive organs are interesting and important, not only for fern investigators but also for plant biologists in general. Recent experimental research has been well reviewed by Banks (1999). The isolation and identification of antheridiogens in different species have been of concern for a long time (Yamauchi et al., 1996). However, the effects of light on differentiation or development of reproductive organs have not been studied, except for a few reports (Gemmrich, 1986; Schraudolf, 1967). In the case of Pteris vittata antheridial formation is inhibited by red and blue light and promoted by far-red light. Red and far-red reversibility of this phenomenon indicates a phy-tochrome involvement (Gemmrich, 1986). The photoresponse necessitates Ca2+ and NO- (Gemmrich, 1988), although the reason for requiring these inorganic ions for antheridiogenesis has not been studied.

1.10 Photoreceptors for photomorphogenesis

In A. capillus-veneris, four genes of the phytochrome protein family have been cloned, i.e., two conventional phytochrome sequences, phytochrome!

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