1.3.2 Resumption of cell growth

1.3.2 Resumption of cell growth

When red-light grown protonemata are transferred into the dark, cell division occurs in the apical region of the linear protonemata (Wada and Furuya, 1972). If protonemal cells are kept in the dark after cell division for several days, almost all cytoplasm in the apical cells, including chloroplasts and a nucleus, moves toward the cell plate at the basal end, so that the cells are occupied by a large vacuole and become transparent. These cells appear dormant and neither grow nor divide until light is provided. It is not known how long the cells can survive without light. When protonemata are irradiated with red light continuously, the nucleus moves toward the cell tip, the cytoplasm disperses over the entire cell, and then elongation resumes at the apex of the cell (Kadota and Furuya, 1981). When red-light grown protonemata kept in the dark for 3 days were used, 24 hours of irradiation was required for all protonemata to recover and grow normally (Kadota and Furuya, 1981). Resumption of cell growth is controlled in a phytochrome red/far-red light reversible manner. However, full reversibility by far-red light is lost when cells are irradiated by red light for longer than 4 s at 4.6 W m-2,although it is not dependent on an escape reaction (Kadota and Furuya, 1981). This is an unusually rapid and sensitive response compared to other phytochrome-dependent phenomena in fern gametophytes,
e.g., the timing of cell division, which is induced by transferring protonemata from red light to darkness and is still reversible by far-red light after irradiation by red light for 10 minutes (Wada and Furuya, 1972).

Red light controlled protonemal growth, cessation, and resumption of growth discussed in this section are mediated by phytochrome, but it is not yet known which of the three phytochrome genes cloned in A. capillus-veneris mediates these phenomena.

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