Light regulates various aspects of plant growth, and the photoreceptor phytochrome B (phyB) mediates many responses to red light. In a screen for Arabidopsis mutants with phenotypes similar to those of phyB mutants, we isolated two new elf3 mutants. One has weaker morphological phenotypes than previously identified elf3 alleles, but still abolishes circadian rhythms under continuous light. Like phyB mutants, elf3 mutants have elongated hypocotyls and petioles, flower early, and have defects in the red light response. However, we found that elf3 mutations have an additive interaction with a phyB null mutation, with phyA or hy4 null mutations, or with a PHYB overexpression construct, and that an elf3 mutation does not prevent nuclear localization of phyB. These results suggest that either there is substantial redundancy in phyB and elf3 function, or the two genes regulate distinct signaling pathways.
Plants adjust their growth and development according to diurnal, seasonal, and local variations in their light environment. Light can induce leaf formation, leaf expansion, and chloroplast differentiation; inhibit stem elongation; induce bending toward or away from light; and induce or repress flowering. Light can also phase the circadian rhythm. Several photoreceptors sense light, including red/far-red light receptors called phytochromes, blue light receptors called cryptochromes, the NPH1 photoreceptor required for phototropism, and hypothesized UV light receptors (Fankhauser and Chory, 1997 ; Deng and Quail, 1999 ).
Genetic analyses in Arabidopsis have been particularly helpful in dissecting the roles of the various photoreceptors. Arabidopsis has five phytochromes, phyA to phyE, and two cryptochromes, cry1 (also known as HY4) and cry2. Analyses of the effects of mutations in genes encoding PHYA, PHYB, PHYD, PHYE, CRY1, CRY2, and NPH1 and transgenic plants overexpressing PHYA, PHYB, PHYC, CRY1, or CRY2 have revealed the developmental functions and capabilities of each of these photoreceptors (Fankhauser and Chory, 1997 ; Deng and Quail, 1999 ). The various phytochromes and cryptochromes share some functions, but are also specialized to some degree. For example, different photoreceptors contribute to inhibition of hypocotyl elongation under different light conditions. In white light, phyB and cry1 play the largest roles and phyA, phyD, and cry2 play lesser roles (Reed et al., 1994 ; Aukerman et al., 1997 ; Smith et al., 1997 ; Lin et al., 1998 ). Signal transduction pathways downstream of these photoreceptors probably interact. For example, under some light conditions phyB and cry1 require each other's activity for maximum inhibition of hypocotyl elongation (Casal and Boccalandro, 1995 ; Casal and Mazzella, 1998 ). Conversely, whereas phyB normally inhibits flowering, phyA and cry2 each promote flowering under certain light conditions (Johnson et al., 1994 ; Reed et al., 1994 ; Guo et al., 1998 ). cry2 mutant plants flower later than wild-type plants in light containing both red and blue frequencies, and a phyB mutation suppresses this effect, indicating that cry2 antagonizes phyB-mediated inhibition of flowering (Guo et al., 1998 ; Mockler et al., 1999 ). Thus, signal transduction pathways downstream of different photoreceptors may reinforce or antagonize each other, depending on the response.
Phytochromes exist in two photointerconvertible forms called Pr and Pfr. Red light converts Pr to Pfr, which absorbs far-red light. Far-red light reconverts Pfr to Pr. For most responses it is thought that Pfr is the active form, because most phytochrome-mediated responses are induced by red light (Furuya, 1993 ; Quail et al., 1995 ). However, phyA mediates far-red light responses, and therefore it is possible that the Pr form of phyA is active (Shinomura et al., 2000 ). Recent biochemical results have shown that phytochromes act as kinases (Yeh et al., 1997 ; Yeh and Lagarias, 1998 ; for review, see Reed, 1999 ). Both phyA and phyB proteins localize to the nucleus under light conditions when they mediate light responses, suggesting that nuclear localization may be important for phytochrome signaling (Sakamoto and Nagatani, 1996 ; Kircher et al., 1999 ; Yamaguchi et al., 1999 ).
Other recent work has aimed to identify downstream targets of phytochromes. Several mutations cause phenotypes similar to those caused by mutations in phytochrome genes (Whitelam et al., 1993 ; Ahmad and Cashmore, 1996 ; Barnes et al., 1996 ; Lin and Cheng, 1997 ; Wagner et al., 1997 ; Soh et al., 1998 ; Hudson et al., 1999 ) or confer hypersensitive red and/or far-red light responses (Genoud et al., 1998 ; Hoecker et al., 1998 ). These mutations may affect genes encoding immediate targets of phytochrome action or downstream regulators of phytochrome signaling. Other potential phytochrome signaling components have been identified in yeast two-hybrid screens. The PIF3 and PKS1 proteins can interact with both phyA and phyB, and NDPK2 interacts with phyA (Ni et al., 1998 ; Choi et al., 1999 ; Fankhauser et al., 1999 ). Studies with plants that overexpress or underexpress these genes suggest that PIF3 and NDPK2 activate phytochrome responses, whereas PKS1 may repress phytochrome responses (Ni et al., 1998 ; Choi et al., 1999 ; Fankhauser et al., 1999 ; Halliday et al., 1999 ). Given the complexity of light responses and the relatively subtle phenotypes of these transgenic plants, these proteins are probably just a subset of actual phytochrome signaling components.
The circadian system controls biological rhythms with a period of roughly 24 h (Lumsden and Millar, 1998 ). Circadian-regulated outputs in Arabidopsis include expression of many genes, leaf movements, and hypocotyl growth (Millar and Kay, 1991 ; Hicks et al., 1996 ; Dowson-Day and Millar, 1999 ). Both red and blue light signals control the phase, period, and amplitude of circadian rhythms in higher plants (Lumsden and Millar, 1998 ). In Arabidopsis, phyA, phyB, and cry1 have all been shown to participate in these responses of the circadian system (Millar et al., 1995 ; Anderson et al., 1997 ; Somers et al., 1998 ). Light regulation and circadian control may allow more flexible responses together than either does alone; the two modes of regulation are frequently associated, sometimes in a complex manner. The processes that are directly regulated by phyB, for example, overlap with those controlled by circadian rhythms. For CAB gene activation, the amplitude of the light response is modulated by the circadian clock (Millar and Kay, 1996 ; Anderson et al., 1997 ).
Arabidopsis phyB mutants have several defects in red light responses, including reduced seed germination, reduced induction of CAB gene expression, elongated hypocotyls and stems, and a longer circadian rhythm period, and they also flower early (Koornneef et al., 1980 ; Reed et al., 1993 , 1994 ; Halliday et al., 1994 ; Shinomura et al., 1994 ; Somers et al., 1998 ). In a screen for mutants with phenotypes similar to those of the phyB mutants, we have discovered two alleles of a previously known locus called ELF3. elf3 mutants were first identified based on their early flowering phenotype, but also have elongated hypocotyls and lack circadian rhythms in constant light (Hicks et al., 1996 ; Zagotta et al., 1996 ; Dowson-Day and Millar, 1999 ). In constant darkness, elf3-1 plants retained a circadian rhythm, and it was proposed that ELF3 mediates an interaction between light and the circadian clock, rather than being a component of the clock itself (Hicks et al., 1996 ). We report genetic and physiological experiments that explore the relationship between ELF3 and phyB. We have compared several phenotypes of elf3 and phyB mutants, and have assayed light responsiveness of hypocotyl elongation in elf3 mutants and several double mutants between elf3 and photoreceptor mutations. These phenotypic analyses showed that elf3 mutants resemble phyB mutants in several respects, and we have therefore tested whether the elf3-1 null mutation affects either the ability of overexpressed phyB to confer a phenotype or phyB nuclear localization. Finally, we have examined the effects of a weak elf3 allele on circadian rhythms. We find that ELF3 and phyB can act independently to control a common set of phenotypes.
Plants adjust their growth and development according to diurnal, seasonal, and local variations in their light environment. Light can induce leaf formation, leaf expansion, and chloroplast differentiation; inhibit stem elongation; induce bending toward or away from light; and induce or repress flowering. Light can also phase the circadian rhythm. Several photoreceptors sense light, including red/far-red light receptors called phytochromes, blue light receptors called cryptochromes, the NPH1 photoreceptor required for phototropism, and hypothesized UV light receptors (Fankhauser and Chory, 1997 ; Deng and Quail, 1999 ).
Genetic analyses in Arabidopsis have been particularly helpful in dissecting the roles of the various photoreceptors. Arabidopsis has five phytochromes, phyA to phyE, and two cryptochromes, cry1 (also known as HY4) and cry2. Analyses of the effects of mutations in genes encoding PHYA, PHYB, PHYD, PHYE, CRY1, CRY2, and NPH1 and transgenic plants overexpressing PHYA, PHYB, PHYC, CRY1, or CRY2 have revealed the developmental functions and capabilities of each of these photoreceptors (Fankhauser and Chory, 1997 ; Deng and Quail, 1999 ). The various phytochromes and cryptochromes share some functions, but are also specialized to some degree. For example, different photoreceptors contribute to inhibition of hypocotyl elongation under different light conditions. In white light, phyB and cry1 play the largest roles and phyA, phyD, and cry2 play lesser roles (Reed et al., 1994 ; Aukerman et al., 1997 ; Smith et al., 1997 ; Lin et al., 1998 ). Signal transduction pathways downstream of these photoreceptors probably interact. For example, under some light conditions phyB and cry1 require each other's activity for maximum inhibition of hypocotyl elongation (Casal and Boccalandro, 1995 ; Casal and Mazzella, 1998 ). Conversely, whereas phyB normally inhibits flowering, phyA and cry2 each promote flowering under certain light conditions (Johnson et al., 1994 ; Reed et al., 1994 ; Guo et al., 1998 ). cry2 mutant plants flower later than wild-type plants in light containing both red and blue frequencies, and a phyB mutation suppresses this effect, indicating that cry2 antagonizes phyB-mediated inhibition of flowering (Guo et al., 1998 ; Mockler et al., 1999 ). Thus, signal transduction pathways downstream of different photoreceptors may reinforce or antagonize each other, depending on the response.
Phytochromes exist in two photointerconvertible forms called Pr and Pfr. Red light converts Pr to Pfr, which absorbs far-red light. Far-red light reconverts Pfr to Pr. For most responses it is thought that Pfr is the active form, because most phytochrome-mediated responses are induced by red light (Furuya, 1993 ; Quail et al., 1995 ). However, phyA mediates far-red light responses, and therefore it is possible that the Pr form of phyA is active (Shinomura et al., 2000 ). Recent biochemical results have shown that phytochromes act as kinases (Yeh et al., 1997 ; Yeh and Lagarias, 1998 ; for review, see Reed, 1999 ). Both phyA and phyB proteins localize to the nucleus under light conditions when they mediate light responses, suggesting that nuclear localization may be important for phytochrome signaling (Sakamoto and Nagatani, 1996 ; Kircher et al., 1999 ; Yamaguchi et al., 1999 ).
Other recent work has aimed to identify downstream targets of phytochromes. Several mutations cause phenotypes similar to those caused by mutations in phytochrome genes (Whitelam et al., 1993 ; Ahmad and Cashmore, 1996 ; Barnes et al., 1996 ; Lin and Cheng, 1997 ; Wagner et al., 1997 ; Soh et al., 1998 ; Hudson et al., 1999 ) or confer hypersensitive red and/or far-red light responses (Genoud et al., 1998 ; Hoecker et al., 1998 ). These mutations may affect genes encoding immediate targets of phytochrome action or downstream regulators of phytochrome signaling. Other potential phytochrome signaling components have been identified in yeast two-hybrid screens. The PIF3 and PKS1 proteins can interact with both phyA and phyB, and NDPK2 interacts with phyA (Ni et al., 1998 ; Choi et al., 1999 ; Fankhauser et al., 1999 ). Studies with plants that overexpress or underexpress these genes suggest that PIF3 and NDPK2 activate phytochrome responses, whereas PKS1 may repress phytochrome responses (Ni et al., 1998 ; Choi et al., 1999 ; Fankhauser et al., 1999 ; Halliday et al., 1999 ). Given the complexity of light responses and the relatively subtle phenotypes of these transgenic plants, these proteins are probably just a subset of actual phytochrome signaling components.
The circadian system controls biological rhythms with a period of roughly 24 h (Lumsden and Millar, 1998 ). Circadian-regulated outputs in Arabidopsis include expression of many genes, leaf movements, and hypocotyl growth (Millar and Kay, 1991 ; Hicks et al., 1996 ; Dowson-Day and Millar, 1999 ). Both red and blue light signals control the phase, period, and amplitude of circadian rhythms in higher plants (Lumsden and Millar, 1998 ). In Arabidopsis, phyA, phyB, and cry1 have all been shown to participate in these responses of the circadian system (Millar et al., 1995 ; Anderson et al., 1997 ; Somers et al., 1998 ). Light regulation and circadian control may allow more flexible responses together than either does alone; the two modes of regulation are frequently associated, sometimes in a complex manner. The processes that are directly regulated by phyB, for example, overlap with those controlled by circadian rhythms. For CAB gene activation, the amplitude of the light response is modulated by the circadian clock (Millar and Kay, 1996 ; Anderson et al., 1997 ).
Arabidopsis phyB mutants have several defects in red light responses, including reduced seed germination, reduced induction of CAB gene expression, elongated hypocotyls and stems, and a longer circadian rhythm period, and they also flower early (Koornneef et al., 1980 ; Reed et al., 1993 , 1994 ; Halliday et al., 1994 ; Shinomura et al., 1994 ; Somers et al., 1998 ). In a screen for mutants with phenotypes similar to those of the phyB mutants, we have discovered two alleles of a previously known locus called ELF3. elf3 mutants were first identified based on their early flowering phenotype, but also have elongated hypocotyls and lack circadian rhythms in constant light (Hicks et al., 1996 ; Zagotta et al., 1996 ; Dowson-Day and Millar, 1999 ). In constant darkness, elf3-1 plants retained a circadian rhythm, and it was proposed that ELF3 mediates an interaction between light and the circadian clock, rather than being a component of the clock itself (Hicks et al., 1996 ). We report genetic and physiological experiments that explore the relationship between ELF3 and phyB. We have compared several phenotypes of elf3 and phyB mutants, and have assayed light responsiveness of hypocotyl elongation in elf3 mutants and several double mutants between elf3 and photoreceptor mutations. These phenotypic analyses showed that elf3 mutants resemble phyB mutants in several respects, and we have therefore tested whether the elf3-1 null mutation affects either the ability of overexpressed phyB to confer a phenotype or phyB nuclear localization. Finally, we have examined the effects of a weak elf3 allele on circadian rhythms. We find that ELF3 and phyB can act independently to control a common set of phenotypes.
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