Phytochrome function in seed plants, especially in Arabidopsis and in rice, has been studied intensively ever since phytochrome family proteins and genes were identified (Abe et al., 1989; Sharrock and Quail, 1989), because (1) the whole genome sequence revealed that only phyA to E phytochrome family genes exist in Arabidopsis, and only phyA to C in rice, and (2) many phy-tochrome mutants are available for analyzing the function of each phytochrome molecule in these plants, even double, triple, quadruple mutants (Franklin et al., 2003). In contrast, fern phytochrome studies have not advanced greatly, even though several phytochrome genes have been cloned and sequenced (Okamoto et al., 1993; Kanegae and Wada, 2006). Before seed plant phytochrome genes were cloned and sequenced, the intracellular distribution of functional phytochrome was only known in Mougeotia and ferns from microbeam studies and/or dichroic effects found under polarized light irradiation (see review by Wada et al., 1993). However, these dichroic effects in A. capillus-veneris were mediated by Acneo1, not by any conventional phytochrome (Kawai et al., 2003), indicating that the function of conventional phytochrome is not yet clarified in ferns.
Adiantum capillus-veneris has two conventional phytochrome genes AcPHY1 and AcPHY2, and a sequence corresponding to the N-terminus chromophore-binding domain called AcPHY4 (Kanegae and Wada, 2006). cDNA sequences encoding the chromophore-binding domain of the N-terminus like AcPHY4 is also found in several other fern species (Tsuboi and Suetsugu, unpublished data), suggesting that it may have a specific function. In addition to chloroplast movement and phototropism regulated by Acneo1, the red light controlled phenomena known so far in fern gametophytes are spore germination, apical cell growth, and the timing of cell division. Given that these phenomena are normal in Acneo1 mutant lines and that the red light effect can be reversed by far-red light, these phenomena must be controlled by conventional phytochromes (Kadota and Wada, 1999). We do not have any mutant line defective for these phytochrome-controlled phenomena, because our strategy for screening these mutants is not realistic. A mutant deficient in spore germination or cell growth does not germinate or grow under red light. If mutant spores do not germinate, it is almost impossible to distinguish them from immature or dead spores to identify them as a mutant. But still, it must be possible because fern spores germinate if gibberellins or antheridiogen is applied (Sugai et al., 1987).
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