Two Photobiological Pathways of Phytochrome A Activity, Only One of Which Shows Dominant Negative Suppression by Phytochrome B (original) (raw)
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Different Phototransduction Kinetics of Phytochrome A and Phytochrome B in Arabidopsis thaliana1
1998
The kinetics of phototransduction of phytochrome A (phyA) and phytochrome B (phyB) were compared in etiolated Arabidopsis thaliana seedlings. The responses of hypocotyl growth, cotyledon unfolding, and expression of a light-harvesting chlorophyll a/bbinding protein of the photosystem II gene promoter fused to the coding region of -glucuronidase (used as a reporter enzyme) were mediated by phyA under continuous far-red light (FR) and by phyB under continuous red light (R). The seedlings were exposed hourly either to n min of FR followed by 60 minus n min in darkness or to n min of R, 3 min of FR (to back-convert phyB to its inactive form), and 57 minus n min of darkness. For the three processes investigated here, the kinetics of phototransduction of phyB were faster than that of phyA. For instance, 15 min R h ؊1 (terminated with a FR pulse) were almost as effective as continuous R, whereas 15 min of FR h ؊1 caused less than 30% of the effect of continuous FR. This difference is interpreted in terms of divergence of signal transduction pathways downstream from phyA and phyB.
Phytochrome Control of Two Low-Irradiance Responses in Etiolated Oat Seedlings
PLANT PHYSIOLOGY, 1981
Light-induced coleoptile stimulation and mesocotyl suppression in etiolated Avena sativa (cv. Lodi) has been quantitated. Etiolated seedlings showed the greatest response to light when they were illuminated 48 to 56 hours after imbibition. Two low-irradiance photoresponses for each tissue have been described. Red light was 10 times more effective than green and 1,000 times more effective than far red light in evoking these responses. The first response, which resulted in a 45% mesocotyl suppression and 30% coleoptile stimulation, had a threshold at 10-14 einsteins per square centimeter and was saturated at 3.0 x 10-12 einsteins per square centimeter of red light. This very low-irradiance response could be induced by red, green, or far red light and was not photoreversible. Reciprocity failed if the duration of the red illumination exceeded 10 minutes. The low-irradiance response which resulted in 80% mesocotyl suppression and 60% coleoptile stimulation, had a threshold at 10' einsteins per square centimeter and was saturated at 3.0 x 108 einsteins per square centimeter of red light. A complete low-irradiance response could be induced by either red or green light but not by far red light. This response could be reversed by a far red dose 30 times greater than that of the initial red dose for both coleoptiles and mesocotyls. Reciprocity failed if the duration of the red illumination exceeded 170 minutes. Both of these responses can be explained by the action of phytochrome. Blaauw et al. (1) and Vanderhoef et al. (29) have shown that dose-response curves for the effect of red light on elongation growth of etiolated oat and corn tissues are composed of two or more steps. Oat mesocotyl suppression showed three steps with the first detectable response, called here very low irradiance response (VLIR2), at l-5 and final saturation near 4.0 x l0-4 nE cm-2 (1). Blaauw et al (1, 2) characterized the VLIR in oat
Phytochrome-Mediated Phototropism in De-Etiolated Seedlings : Occurrence and Ecological Significance
PLANT PHYSIOLOGY, 1992
Phototropic responses to broadband far red (FR) radiation were investigated in fully de-etiolated seedlings of a long-hypocotyl mutant (Ih) of cucumber (Cucumis sativus L.), which is deficient in phytochrome-B, and its near isogenic wild type (WT). Continuous unilateral FR light provided against a background of white light induced negative curvatures (i.e. bending away from the FR light source) in hypocotyls of WT seedlings. This response was fluencerate dependent and was absent in the Ih mutant, even at very high fluence rates of FR. The phototropic effect of FR light on WT seedlings was triggered in the hypocotyls and occurred over a range of fluence rates in which FR was very effective in promoting hypocotyl elongation. FR light had no effect on elongation of Ihmutant hypocotyls. Seedlings grown in the field showed negative phototropic responses to the proximity of neighboring plants that absorbed blue (B) and red light and back-reflected FR radiation. The bending response was significantly larger in WT than in Ih seedlings. Responses of WT and Ih seedlings to lateral B light were very similar; however, elimination of the lateral B light gradients created by the proximity of plant neighbors abolished the negative curvature only in the case of Ih seedlings. More than 40% of the total hypocotyl curvature induced in WT seedlings by the presence of neighboring plants was present after equilibrating the fluence rates of B light received by opposite sides of the hypocotyl. These results suggest that: (a) phytochrome functions as a phototropic sensor in de-etiolated plants, and (b) in patchy canopy environments, young seedlings actively project new leaves into light gaps via stem bending responses elicited by the B-absorbing photoreceptor(s) and phytochrome.
Current biology : CB, 2016
Plants in dense vegetation perceive their neighbors primarily through changes in light quality. Initially, the ratio between red (R) and far-red (FR) light decreases due to reflection of FR by plant tissue well before shading occurs. Perception of low R:FR by the phytochrome photoreceptors induces the shade avoidance response [1], of which accelerated elongation growth of leaf-bearing organs is an important feature. Low R:FR-induced phytochrome inactivation leads to the accumulation and activation of the transcription factors PHYTOCHROME-INTERACTING FACTORs (PIFs) 4, 5, and 7 and subsequent expression of their growth-mediating targets [2, 3]. When true shading occurs, transmitted light is especially depleted in red and blue (B) wavelengths, due to absorption by chlorophyll [4]. Although the reduction of blue wavelengths alone does not occur in nature, long-term exposure to low B light induces a shade avoidance-like response that is dependent on the cryptochrome photoreceptors and th...
From seed germination to flowering, light controls plant development via the pigment phytochrome
Proceedings of the National Academy of Sciences, 1996
Plant growth and development are regulated by interactions between the environment and endogenous developmental programs. Of the various environmental factors controlling plant development, light plays an especially important role, in photosynthesis, in seasonal and diurnal time sensing, and as a cue for altering developmental pattern. Recently, several laboratories have devised a variety of genetic screens using Arabidopsis thaliana to dissect the signal transduction pathways of the various photoreceptor systems. Genetic analysis demonstrates that light responses are not simply endpoints of linear signal transduction pathways but are the result of the integration of information from a variety of photoreceptors through a complex network of interacting signaling components. These signaling components include the red/far-red light receptors, phytochromes, at least one blue light receptor, and negative regulatory genes (DET, COP, and FUS) that act downstream from the photoreceptors in the nucleus. In addition, a steroid hormone, brassinolide, also plays a role in light-regulated development and gene expression in Arabidopsis. These molecular and genetic data are allowing us to construct models of the mechanisms by which light controls development and gene expression inArabidopsis. In the future, this knowledge can be used as a framework for understanding how all land plants respond to changes in their environment.
The Plant Journal, 1997
et al., 1995a; Malhotra et al.,1995), and overexpression of USA CRY1 protein in transgenic plants conferred a blue-light hypersensitive phenotype (Lin et al.,1995b), consistent with its role as photoreceptor. Blue-light-dependent phenotypes Summary shown to be under the control of CRY1 include inhibition Blue-light responses in higher plants are mediated by of hypocotyl elongation and anthocyanin production in specific photoreceptors, which are thought to be flavoseedlings (Ahmad et al., 1995; Jackson and Jenkins, 1995; proteins; one such flavin-type blue-light receptor, CRY1 Koornneef et al., 1980). In spite of its striking homology to (for cryptochrome), which mediates inhibition of hypocotyl the DNA photolyases, CRY1 shows no demonstrable DNA elongation and anthocyanin biosynthesis, has recently binding or photoreactivating activity (Lin et al., 1995a; been characterized. Prompted by classical photobiological Malhotra et al., 1995). The structure of CRY1 suggests a studies suggesting possible co-action of the red/far-red mechanism of action involving electron transfer; the reacabsorbing photoreceptor phytochrome with blue-light tion partners and downstream transduction apparatus photoreceptors in certain plant species, the role of phytoremain to be identified. chrome in CRY1 action in Arabidopsis was investigated. A recurring theme in plant blue-light research has been The activity of the CRY1 photoreceptor can be substantially an involvement of the red/far-red-absorbing photoreceptor altered by manipulating the levels of active phytochrome phytochrome in physiological responses to blue-light treat-(Pfr) with red or far-red light pulses subsequent to bluements. Experiments in a number of monocot and dicot light treatments. Furthermore, analysis of severely phytoplant species have shown that blue-light responses such chrome-deficient mutants showed that CRY1-mediated as inhibition of hypocotyl elongation or anthocyanin accublue-light responses were considerably reduced, even mulation can be partially reversed if the blue-light pulses though Western blots confirmed that levels of CRY1 photoare followed by, or given in the presence of, saturating receptor are unaffected in these phytochrome-deficient pulses of far-red light (Casal, 1994; Gaba et al., 1984; mutant backgrounds. It was concluded that CRY1-medi-Mancinelli et al., 1991; Mohr, 1994). Such far-red reversiated inhibition of hypocotyl elongation and anthocyanin bility had been taken as evidence that phytochrome, or the production requires active phytochrome for full expresphytochrome signal transduction pathway, was somehow sion, and that this requirement can be supplied by low implicated in blue-light responses. However, interpretation levels of either phyA or phyB. of these studies has been complicated by the fact that the phytochrome photoreceptor itself directly absorbs blue light. It is therefore difficult to unequivocally distinguish
Planta, 1995
Arena phytochrome A (phyA) overexpressed in tobacco (Nicotiana tabacum L.) and tomato (Lycopersicon sculentum Mill) was functionally characterised by comparing wild-type (WT) and transgenic seedlings. Different proportions of phytochrome in its far-red-absorbing form (Pfr/P) were provided by end-of-day (EOD) light pulses. Stem-length responses occurred largely in the range of low Pfr/P (3-61%) for WT seedlings and in the range of high Pfr/P (61-87%) for transgenic seedlings. A similar shift was observed when the photoperiod was interrupted by short light pulses providing different Pfr/P ratios and followed by 1 h dark incubation. In other experiments, Arena phyA was allowed to re-accumulate in darkness and subsequently phototransformed to Pfr but no extra inhibition of stem extension growth was observed. In transgenic tomato seedlings the response to EOD far-red light was faster and the response to a far-red light pulse delayed into darkness was larger than in the WT. Arena phyA Pfr remaining at the end of the photoperiod appears intrinsically unable to sustain growth inhibition in subsequent darkness. Arena phyA modifies the sensitivity and the kinetics of EOD responses mediated by native phytochrome.