When protonemal cells cultured under continuous red light are transferred to darkness, the growth rate is reduced and ultimately cell growth stops (Kadota and Furuya, 1977). Simultaneously, the nucleus moves a short distance towards the cell base and after a period of time cell division occurs (Figure 1.5) (Wada et al., 1980). The timing of cell division is controlled by phytochrome (Wada and Furuya, 1972), as will be discussed in Section 1.5. The timing of growth retardation and the length of cell growth during the dark period before cell division occurs are also controlled reversibly by red/far-red light irradiation just before transferring to the dark (Kadota and Furuya, 1977). Protonemal cell growth may be controlled in conjunction with the timing of cell division by the same phytochrome system. It is curious, however, that the timing of cell division is delayed by far-red light whereas growth retardation is advanced and, consequently, growth is reduced. This result indicates that cell cycle advancement and cell growth retardation (i.e., cell growth) are not really parallel. Protonemal cell growth is also controlled by blue light; this will be discussed in Section 1.5.
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