Co-action between phytochrome B and HY4 in Arabidopsis thaliana

Abstract

Abstract.A combination of physiological and genetic approaches was used to investigate whether phytochromes and blue light (BL) photoreceptors act in a fully independent manner during photomorphogenesis of Arabidopsis thaliana (L.) Heynh. Wild-type seedlings and phyA, phyB and hy4 mutants were daily exposed to 3 h BL terminated with either a red light (R) or a far-red light (FR) pulse. In wild-type and phyA-mutant seedlings, BL followed by an R pulse inhibited hypocotyl growth and promoted cotyledon unfolding. The effects of BL were reduced if exposure to BL was followed by an FR pulse driving phytochrome to the R-absorbing form (Pr). In the wild type, the effects of R versus FR pulses were small in seedlings not exposed to BL. Thus, maximal responses depended on the presence of both BL and the FR-absorbing form of phytochrome (Pfr) in the subsequent dark period. Impaired responses to BL and to R versus FR pulses were observed in phyB and hy4 mutants. Simultaneous irradiation with orange light indicated that BL, perceived by specific BL photoreceptors (i.e. not by phytochromes), required phytochrome B to display a full effect. These results indicate interdependent co-action between phytochrome B and BL photoreceptors, parti-

The function of phytochrome A

“…Cryptochrome 1 increases the responsivity to phyB Pfr but not the responsivity to the active form(s) of phyA present under continuous or pulsed FR (Casal & Boccalandro 1995). Overexpression of phyA actually impairs stem growth responses to blue light in transgenic tobacco (Casal & Sanchez 1994b).…”

“…Pretreatments with continuous FR perceived by phyA enhance the response to terminal R versus FR pulses perceived by phyB, indicating a synergistic action of the two phototeceptors (Casal 1995;Casal & Boccalandro 1995). Blue light, perceived by cryptochrome 1, also enhances the effect of phyB Pfr on hypocotyl extension growth and cotyledon unfolding in Arabidopsis (Casal & Boccalandro 1995). The ecological significance of responsivity amplification is not clear at present.…”

A Gain-of-Function Mutation of Arabidopsis CRYPTOCHROME1 Promotes Flowering

“…Because the hypersensitivity of cry1-L407F to red pulses could be fully reverted by far-red pulses, at least in this case it was clearly phytochrome signaling that was affected in the cry1-L407F mutant. Interactions of red light-absorbing phytochrome and blue light-absorbing cryptochrome signaling cascades have been reported (Casal and Boccalandro, 1995;Ahmad and Cashmore, 1997;Hennig et al, 1999). Furthermore, nuclear import of phyB was initiated by blue light, but not by light of 695 nm, which establishes a similar phytochrome photoequilibrium as blue light (Gil et al, 2000).…”

“…Even though the functions of the different photoreceptors are assigned to specific segments of the spectrum of light, physiological and mutant analyses have revealed extensive cross talk between blue and red light photoreceptors (Casal, 2000): phyA and phyB display antagonistic and synergistic effects on the action of each other, depending on which responses are studied (Reed et al, 1994;Casal and Boccalandro, 1995), and several of the phytochromes functionally interact with the cryptochromes (Ahmad and Cashmore, 1997;Casal and Mazzella, 1998;Neff and Chory, 1998;Hennig et al, 1999;Mockler et al, 1999;Devlin and Kay, 2000). In addition, a physical interaction of phyA with cry1 (Ahmad et al, 1998b) and of phyB with cry2 (Más et al, 2000) has been reported.…”

Combinatorial interaction of light‐responsive elements plays a critical role in determining the response characteristics of light‐regulated promoters in Arabidopsis

“…In this case, repression by RL required functional PhyB as well as Cry1, and repression in BL required Cry1 as well as PhyB (Figure 5b,c). Thus, it appears that signaling pathways initiated by phytochromes and the Cry1 BL receptor interact to mediate wide range of events including hypocotyl elongation, anthocyanin accumulation (Ahmad and Cashmore, 1997;Casal and Boccalandro, 1995) and gene expression.…”