Evidence for a Phytochrome-Mediated Phototropism in Etiolated Pea Seedlings (original) (raw)
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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.
Red Light Enhancement of the Phototropic Response of Etiolated Pea Stems
Plant Physiology, 1974
In the subapical third internode of 7-day-old etiolated pea seedlings, the magnitude of phototropic curvature in response to continuous unilateral blue illumination is increased when seedlings are pre-exposed to brief red light. The effect of red light on blue light-induced phototropism becomes manifest maximally 4 or more hours after red illumination, and closely parallels the promotive action of red light on the elongation of the subapical cells. Ethylene inhibits phototropic curvature by an inhibitory action on cell elongation without affecting the lateral transport of auxin. Pretreatment of seedlings with gibberellic acid causes increased phototropic curvature, but experiments using "C-gibberellic acid indicate that gibberellic acid itself is not laterally transported under phototropic stimuli.
Photochemistry and Photobiology, 2000
The plant receptor phytochrome A (phyA) mediates responses like hypocotyl growth inhibition and cotyledon unfolding that require continuous far-red (FR) light for maximum expression (high-irradiance responses, HIR), and responses like seed germination that can be induced by a single pulse of FR (very-low-fluence responses, VLFR). It is not known whether this duality results from either phyA interaction with different end-point processes or from the intrinsic properties of phyA activity. Etiolated seedlings of Arabidopsis thaliana were exposed to pulses of FR (3 min) separated by dark intervals of different duration. Hypocotyl-growth inhibition and cotyledon unfolding showed two phases. The first phase (VLFR) between 0.17 and 0.5 pulses•h Ϫ1 , a plateau between 0.5 and 2 pulses•h Ϫ1 and a second phase (HIR) at higher frequencies. Reciprocity between fluence rate and duration of FR was observed within phases, not between phases. The fluence rate for half the maximum effect was 0.1 and 3 mol•m Ϫ2 •s Ϫ1 for hourly pulses of FR (VLFR) and continuous FR (HIR), respectively. Overexpression of phytochrome B caused dominant negative suppression under continuous but not under hourly FR. We conclude that phyA is intrinsically able to initiate two discrete photoresponses even when a single end-point process is considered.
Blue and Green Light-Induced Phototropism in Arabidopsis thaliana and Lactuca sativa L. Seedlings
PLANT PHYSIOLOGY, 1985
Exposure time-response curves for blue and green light-induced phototropic bending in hypocotyls of Arabidopsis thaliana (L.) Heynh. and Lactaa stiva L. seedlings are presented. These seedlings show signlicant phototropic sensitivity up to 540 to 550 nanometers. Since wavelengths longer than 560 nanometers do not induce phototropic bending, it is suggested that the response to 510 to 550 nanometers light is mediated by the specific blue light photoreceptor of phototropism. We www.plant.org on January 31, 2016 -Published by www.plantphysiol.org Downloaded from
Fluorescence and photochemical properties of phytochromes A and B in etiolated pea seedlings
Journal of Photochemistry and Photobiology B: Biology, 1999
~tuorescence and photochemical properties of phytochromes A and B (phyA and phyB) have been investigated in etiolated pea (Pisum sazi~,um L.) seedlings by comparing wild-type (wt) seedlings with mutants lacking phyB (lv-5) or phyA (fun1-1), and with the double mutant. The red-light (R)-absorbing form of phytochrome (Pr) is characterized by four major parameters: position of the emission maximum at 85 K (A ....), total phytochrome content ([Ptot]); extent of the Pr photoransformation into the first product, lumi-R, at 85 K (3'), and into the far-red light (FR)-absorbing form (Pfr) at 273 K (T2). Phytochrome cannot be detected in the lv-5funl-1 double mutant, allowing specific characterization of phyA and phyB in the single mutants. Thus, for the first time we are able to compare in planta the properties of native phyA and phyB. Our results show the phyA population to be heterogenous both in distribution and properties. [PhyAtot] is highest in the tipper stem (5.23 rel. units), with Amax of 687 nm, "yj = 0.43 and ~/2 = 0.72, and 3.5-fold lower in the lower stem and roots, with Amax blueshifted by more than 1 nm, 3'J =0.3 and 3,=0.7. In contrast, phyB is present at a low level throughout the plant (0.3-0.4 rel. units), with A ...... of 683 nm, y~ of 0.05 and "Y2 of 0.3-0.35. It is also characterized by an earlier red drop in the dependence of "Y2 on the wavelength of the act il~ic light compared to phyA. Experiments with light-induced phytochrome destruction confirm previous findings that the majority of phyA, unlike phyB, is light-labile. The variation in properties of phyA is interpreted in terms of two species, phyA' and phyA". PhyA' is relatively abvndant, is distributed mainly in the upper stem, is light-labile and has a high % value (= 0.5), whereas phyA" is less abundant, is distributed more or less evenly throughout the plant, is relatively light-stable and has a low "y~ value (< 0.05). It is suggested that the lower photochemical actJ x ity of phyB at ambient temperatures (low ~/2) and its earlier red drop (which is likely to be due to the blue-shift in the absorption spectra of l't~yB Pr and Pfr) might explain, at least partially, the lower effectiveness of phyB under R and its lack of effect under FR.
The Plant Journal, 2000
Blue light induces extracellular acidi®cation, a prerequisite of cell expansion, in epidermis cells of young pea leaves, by stimulation of the proton pumping-ATPase activity in the plasma membrane. A transient acidi®cation, reaching a maximum 2.5±5 min after the start of the pulse, could be induced by pulses as short as 30 msec. A pulse of more than 3000 mmol m ±2 saturated this response. Responsiveness to a second light pulse was recovered with a time constant of about 7 min. The¯uence rate-dependent lag time and sigmoidal increase of the acidi®cation suggested the involvement of several reactions between light perception and activation of the ATPase. In wild-type pea plants, the¯uence response relation for short light pulses was biphasic, with a component that saturates at low¯uence and one that saturates at high¯uence. The phytochrome-de®cient mutant pcd2 showed a selective loss of the high-¯uence component, suggesting that the high-¯uence component is phytochrome-dependent and the low-¯uence component is phytochrome-independent. Treatment with the calmodulin inhibitor W7 also led to the elimination of the phytochrome-dependent high-¯uence component. Simple models adapted from the one used to simulate blue light-induced guard cell opening failed to explain one or more elements of the experimental data. The hypothesis that phytochrome and a blue light receptor interact in a short-term photoresponse is endorsed by model calculations based upon a three-step signal transduction cascade, of which one component can be modulated by phytochrome.
Effect of Red Light on Geotropism in Pea Epicotyls
PLANT PHYSIOLOGY, 1979
Dose response curves were determined for phytochrome phototransformation and for a phytuchrome-controfled decrease in geotropic curvature in epicotyls of dark-grown Pisum sadvum L. cv. Alaska. Ten times as much light was required to produce a spectrophotometrically detectable transformation of phytochrome as was required to produce a significant change in the geotropic response. The red lgt energy required for a 50% phytochrome transformation caused a 90% change in the physiological response.
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