RESUMO
GIGANTEA (GI) is a nuclear protein involved in the promotion of flowering by long days, in light input to the circadian clock, and in seedling photomorphogenesis under continuous red light but not far-red light (FR). Here, we report that in Arabidopsis (Arabidopsis thaliana) different alleles of gi have defects in the hypocotyl-growth and cotyledon-unfolding responses to hourly pulses of FR, a treatment perceived by phytochrome A (phyA). This phenotype is rescued by overexpression of GI. The very-low-fluence response of seed germination was also reduced in gi. Since the circadian clock modulates many light responses, we investigated whether these gi phenotypes were due to alterations in the circadian system or light signaling per se. In experiments where FR pulses were given to dark-incubated seeds or seedlings at different times of the day, gi showed reduced seed germination, cotyledon unfolding, and activity of a luciferase reporter fused to the promoter of a chlorophyll a/b-binding protein gene; however, rhythmic sensitivity was normal in these plants. We conclude that while GI does not affect the high-irradiance responses of phyA, it does affect phyA-mediated very-low-fluence responses via mechanisms that do not obviously involve its circadian functions.
Assuntos
Proteínas de Arabidopsis/fisiologia , Arabidopsis/crescimento & desenvolvimento , Arabidopsis/metabolismo , Ritmo Circadiano , Luz , Fitocromo A/metabolismo , Alelos , Arabidopsis/genética , Proteínas de Arabidopsis/genética , Proteínas de Arabidopsis/metabolismo , Cotilédone/genética , Cotilédone/crescimento & desenvolvimento , Cotilédone/metabolismo , Flores/genética , Flores/crescimento & desenvolvimento , Flores/metabolismo , Germinação , Hipocótilo/genética , Hipocótilo/crescimento & desenvolvimento , Hipocótilo/metabolismo , Fenótipo , Sementes/crescimento & desenvolvimento , Sementes/metabolismo , Transdução de SinaisRESUMO
The phytochromes is a family of plant photoreceptors that control growth and development in response to environmental cues. Red and far-red light are the most efficient wavebands to induce conformational changes of phytochromes and consequently modify their kinetics, nuclear/cytoplasmic partitioning, ability to phosphorylate substrates, and physical interaction with proteins that bind DNA. Many players in phytochrome signalling have been identified and a complex, highly regulated network is envisaged. Here we describe the connection between different features of the phytochrome signalling network and the versatile relationship between light signals and physiological outputs shown by phytochromes.
Assuntos
Arabidopsis/fisiologia , Arabidopsis/efeitos da radiação , Fitocromo/fisiologia , Fitocromo/efeitos da radiação , Proteínas de Arabidopsis/química , Proteínas de Arabidopsis/fisiologia , Proteínas de Arabidopsis/efeitos da radiação , Luz , Fotobiologia , Fitocromo/química , Transdução de Sinais/efeitos da radiação , Espectrofotometria/métodosRESUMO
Several mutants with altered phytochrome A (phyA) signalling have been identified in screenings under continuous far-red light (FR). The latter protocol could preclude the identification of mutants affected in the signalling pathway that operates even under transient phyA activation, compared to the high-irradiance response (HIR) pathway that requires continuous FR. Since some photomorphogenic mutants show shoot-height phenotypes, the screening was conducted on dwarf mutants of Arabidopsis thaliana (L.) Heynh. from the ABRC stocks grown under hourly FR pulses. The dwarf mutant cp3 (compacta 3) showed normal hypocotyl length and folded cotyledons in darkness but enhanced hypocotyl-growth inhibition and cotyledon unfolding under pulsed FR. The HIR and the response mediated by phyB were not affected. Under pulsed FR, seed germination and blocking of greening upon transfer to white light were enhanced in cp3. PHYA levels were normal in cp3. The phenotype under pulsed FR but not the adult phenotype required phyA. We propose that CP3 is involved in the negative regulation of the signalling pathway that saturates with transient activation of phyA.
Assuntos
Arabidopsis/crescimento & desenvolvimento , Células Fotorreceptoras , Fitocromo/metabolismo , Transdução de Sinais/fisiologia , Fatores de Transcrição , Adaptação Fisiológica/genética , Antocianinas/biossíntese , Antocianinas/efeitos da radiação , Arabidopsis/genética , Arabidopsis/efeitos da radiação , Proteínas de Arabidopsis , Clorofila/biossíntese , Clorofila/efeitos da radiação , Cotilédone/genética , Cotilédone/crescimento & desenvolvimento , Cotilédone/efeitos da radiação , Escuridão , Regulação da Expressão Gênica de Plantas/efeitos da radiação , Hipocótilo/genética , Hipocótilo/crescimento & desenvolvimento , Hipocótilo/efeitos da radiação , Luz , Mutação , Fenótipo , Fotoperíodo , Fotossíntese/genética , Fotossíntese/fisiologia , Fotossíntese/efeitos da radiação , Fitocromo A , Fitocromo B , Sementes/genética , Sementes/crescimento & desenvolvimento , Sementes/efeitos da radiação , Transdução de Sinais/efeitos da radiaçãoRESUMO
Phytochromes (phy) A and B provide higher plants the ability to perceive divergent light signals. phyB mediates red/far-red light reversible, low fluence responses (LFR). phyA mediates both very-low-fluence responses (VLFR), which saturate with single or infrequent light pulses of very low fluence, and high irradiance responses (HIR), which require sustained activation with far-red light. We investigated whether VLFR, LFR, and HIR are genetically coregulated. The Arabidopsis enhanced very-low-fluence response1 mutant, obtained in a novel screening under hourly far-red light pulses, showed enhanced VLFR of hypocotyl growth inhibition, cotyledon unfolding, blocking of greening, and anthocyanin synthesis. However, eve1 showed reduced LFR and HIR. eve1 was found allelic to the brassinosteroid biosynthesis mutant dim/dwarf1. The analysis of both the brassinosteroid mutant det2 in the Columbia background (where VLFR are repressed) and the phyA eve1 double mutant indicates that the negative effect of brassinosteroid mutations on LFR requires phyA signaling in the VLFR mode but not the expression of the VLFR. Under sunlight, hypocotyl growth of eve1 showed little difference with the wild type but failed to respond to canopy shadelight. We propose that the opposite regulation of VLFR versus LFR and HIR could be part of a context-dependent mechanism of adjustment of sensitivity to light signals.