The Induction of Seed Germination in Arabidopsis thaliana Is Regulated Principally by Phytochrome B and Secondarily by Phytochrome A

Abstract

We examined whether spectrally active phytochrome A (PhyA) and phytochrome B (PhyB) play specific roles in the induction of seed germination in Arabidopsis thaliana (L.) Heynh., using PhyA- and PhyB-null mutants, fre1-1 (A. Nagatani, J.W. Reed, J. Chory [1993] Plant Physiol 102: 269-277) and hy3-Bo64 (J. Reed, P.Nagpal, D.S. Poole, M. Furuya, J. Chory [1993] Plant Cell 5: 147-157). When dormant seeds of each genotype imbibed in the dark on aqueous agar plates, the hy3 (phyB) mutant did not germinate, whereas the fre1 (phyA) mutant germinated at a rate of 50 to 60%, and the wild type (WT) germinated at a rate of 60 to 70%. By contrast, seeds of all genotypes germinated to nearly 100% when plated in continuous irradiation with white or red light. When plated in continuous far-red light, however, frequencies of seed germination of the WT and the fre1 and hy3 mutants averaged 14, nearly 0, and 47%, respectively, suggesting that PhyB in the red-absorbing form prevents PhyA-dependent germination under continuous far-red light. When irradiated briefly with red or far-red light after imbibition for 1 h, a typical photoreversible effect on seed germination was observed in the fre1 mutant and the WT but not in the hy3 mutant. In contrast, when allowed to imbibe in the dark for 24 to 48 h and exposed to red light, the seed germination frequencies of the hy3 mutant were more than 40%. Immunoblot analyses of the mutant seeds showed that PhyB apoprotein accumulated in dormant seeds of the WT and the fre1 mutant as much as in the seeds that had imbibed. In contrast, PhyA apoprotein, although detected in etiolated seedlings grown in the dark for 5 d, was not detectable in the dormant seeds of the WT and the hy3 mutant. The above physiological and immunochemical evidence indicates that PhyB in the far-red-absorbing form was stored in the Arabidopsis seeds and resulted in germination in the dark. Hence, PhyA does not play any role in dark germination but induces germination under continuous irradiation with far-red light. Finally, we examined seeds from a signal transduction mutant, det1, and a det1/hy3 double mutant. The det1 seeds exhibited photoreversible responses of germination on aqueous agar plates, and the det1/hy3 double mutant seeds did not. Hence, DET1 is likely to act in a distinct pathway from PhyB in the photoregulation of seed germination.

Patterns of Expression and Normalized Levels of the Five Arabidopsis Phytochromes

“…Roles for the other two phytochromes in germination have not been described and, although phyD mutants are defective in early seedling hypocotyl elongation and cotyledon opening responses (Aukerman et al, 1997), little has been described relating to the roles of phyC or phyE in early seedling growth. In earlier studies, phyA was not detectable on immunoblots of freshly sterilized Arabidopsis seeds or after a 24-h imbibition but was detectable in 2-d-old dark-grown seedlings, whereas phyB was present in seeds at a level similar to that in etiolated seedlings (Shinomura et al, 1994(Shinomura et al, , 1996. This induction of phyA synthesis correlated with the development of photoresponsiveness of seed germination in a phyB mutant line (Shinomura et al, 1996).…”

In vitro regeneration through organogenesis and somatic embryogenesis in pigeon pea [Cajanus cajan (L.) Millsp.] cv. JKR105

“…Additionally, the incubation under darkness may also be a key factor in realizing good germination frequency. Such a phenomenon has been attributed to involvement of PhyB noticed in Arabidopsis (Shinomura et al 1994). In contrast, the initial induction of cells with morphogenetic potential requires dark incubation, while plantlet formation resulted under light conditions in pigeon pea (Franklin et al 2000).…”

Flowering Responses to Altered Expression of Phytochrome in Mutants and Transgenic Lines of Arabidopsis thaliana (L.) Heynh

“…Thus, despite its instability in light, there might have been sufficient phytochrome A in 8-h short days to allow it to contribute an EOD R/FR response in mutants lacking phytochrome B (Table 111, whereas phytochrome B would operate normally in the phytochrome A mutant. Such substitution may be an extreme example of the overlapping roles of phytochrome A and B in hypocotyl elongation (Nagatani et al, 1993;Parks and Quail, 1993) and seed germination (Shinomura et al, 1994).…”