Signal Integration by ABA in the Blue Light-Induced Acidification of Leaf Pavement Cells in Pea (Pisum sativum L. var. Argenteum)

Leaf pavement cell expansion in light depends on apoplastic acidification by a plasma membrane proton-pumping ATPase, modifying cell wall extensibility and providing the driving force for uptake of osmotically active solutes generating turgor. This paper shows that the plant hormone ABA inhibits light-induced leaf disk growth as well as the blue light-induced pavement cell growth in pea (Pisum sativum L.). In the phytochrome chromophore-deficient mutant pcd2, the effect of ABA on the blue light-induced apoplastic acidification response, which exhibits a high fluence phase via phytochrome and a low fluence phase via an unknown blue light receptor, is still present, indicating an interaction of ABA with the blue light receptor pathway. Furthermore, it is shown that ABA inhibits the blue light-induced apoplastic acidification reversibly. These results indicate that the effect of ABA on apoplastic acidification can provide a mechanism for short term, reversible adjustment of leaf growth rate to environmental change.Key Words: ABA, apoplastic acidification, blue light, epidermal pavement cell growth, leaf growth, pea (Pisum sativum L.), signal integration     

Two distinct blue-light responses regulate epicotyl elongation in pea

Blue light induces a long-term suppression of epicotyl elongation in red-light-grown pea (Pisum sativum L.) seedlings. The fluence-response characteristics are bell-shaped, indicating the possibility of two different blue-light responses: a lower fluence response causing suppression and a higher fluence response alleviating the suppression. To determine if two responses are in effect, we have grown pea seedlings under dark conditions hoping to eliminate one or the other response. Under these growth conditions, only the lower fluence portion of the response (suppression of elongation) is apparent. The kinetics of suppression are similar to those observed for the lower fluence response of red-light-grown seedlings. The response to blue light in the dark-grown seedlings is not due to the excitation of phytochrome because a pulse of far-red light large enough to negate phytochrome-induced suppression has no effect on the blue-light-induced suppression. Furthermore, treatment of the dark-grown seedlings with red light immediately prior to treatment with high fluence blue light does not elicit the higher fluence response, indicating that the role of red light in the blue high fluence response is to allow the plant to achieve a specific developmental state in which it is competent to respond to the higher fluences of blue light.     

Desensitization by red and blue light of phototropism in maize coleoptiles

The effects of pretreatments with red and blue light (RL, BL) on the fluence-response curve for the phototropism induced by a BL pulse (first positive curvature) were investigated with darkadapted maize (Zea mays L.) coleoptiles. A pulse of RL, giving a fluence sufficient to saturate phytochrome-mediated responses in this material, shifted the bell-shaped phototropic fluence-response curve to higher fluences and increased its peak height. A pulse of high-fluence BL given immediately prior to this RL treatment temporarily suppressed the phototropic fluence-response curve, and shifted the curve to higher fluences than induced by RL alone. The shift by BL progressed rapidly compared to that by RL. The results indicate (1) that first positive curvature is desensitized by both phytochrome and a BL system, (2) that desensitization by BL occurs with respect to both the maximal response and the quantum efficiency, and (3) that the desensitization responses mediated by phytochrome and the BL system can be induced simultaneously but develop following different kinetics. It is suggested that theses desensitization responses contribute to the induction of second positive curvature, a response induced by prolonged irradiation.Abbreviations BL blue light - RL red light CIW-DPB Publication No. 1001     

Light-Stimulated Apical Hook Opening in Wild-Type Arabidopsis thaliana Seedlings

Apical hook opening and cotyledon unfolding are characteristic responses that occur during deetiolation of dicotyledonous seedlings. Light-stimulated apical hook opening and cotyledon unfolding in etiolated Arabidopsis thaliana seedlings appears to involve the activities of multiple photosensory systems. Red, far-red, and blue light are all effective in stimulating these responses in Arabidopsis. Stimulation of hook opening by red light and low fluence blue light is inductive, far-red reversible, and exhibits reciprocity, as is characteristic of many low fluence-dependent phytochrome-mediated responses. Far-red and high-fluence blue light appear to stimulate hook opening and cotyledon unfolding through high-irradiance-response systems during long-term light treatments. Although a phytochrome high-irradiance-response system presumably mediates the responses in far-red light, the responses to high-fluence blue light may be mediated by a blue light-specific photosensory system.     

Regulation of phytochrome A mRNA abundance in parsley seedlings and cell-suspension cultures

A cDNA clone encoding the apoprotein of a parsley phytochrome was isolated and classified as parsley PHYA phytochrome, on the basis of a sequence homology comparison with all available phytochrome sequences. Red light pulses led to a phytochrome-dependent down-regulation of PHYA mRNA abundance in etiolated parsley seedlings to a level of 10–20% compared with the dark control. The PHYA mRNA abundance in a parsley cell suspension culture was also down-regulated by light pulses. Transient expression assays in parsley protoplasts showed light regulation of a chimeric pea PHYA promoter uidA-gene construct.     

Chloroplast-avoidance response induced by high-fluence blue light in prothallial cells of the fern adiantum capillus-veneris as analyzed by microbeam irradiation

Chloroplast movement was induced by partial cell illumination using a high-fluence blue microbeam in light-grown and dark-adapted prothallial cells of the fern Adiantum capillus-veneris. Chloroplasts inside the illuminated area moved out (high-fluence response [HFR]), whereas those outside moved toward the irradiated area (low-fluence response [LFR]), although they stopped moving when they reached the border. These results indicate that both HFR and LFR signals are generated by high-fluence blue light of the same area, and that an LFR signal can be transferred long-distance from the beam spot, although an HFR signal cannot. The lifetime of the HFR signal was calculated from the traces of chloroplast movement induced by a brief pulse from a high-fluence blue microbeam to be about 6 min. This is very short compared with that of the LFR (30–40 min; T. Kagawa, M. Wada [1994] J Plant Res 107: 389–398). These data indicate that the signal transduction pathways of the HFR and the LFR must be distinct.     

Multiple phytochromes are involved in red-light-induced enhancement of first-positive phototropism in Arabidopsis thaliana.

The amplitude of phototropic curvature to blue light is enhanced by a prior exposure of seedlings to red light. This enhancement is mediated by phytochrome. Fluence-response relationships have been constructed for red-light-induced enhancement in the phytochrome A (phyA) null mutant, the phytochrome B- (phyB) deficient mutant, and in two transgenic lines of Rabidopsis thaliana that overexpress either phyA or phyB. These fluence-response relationships demonstrate the existence of two response in enhancement, a response in the very-low-to-low-fluence range, and a response in the high-fluence range. Only the response in the high-fluence range is present in the phyA null mutant. In contrast, the phyB-deficient mutant is indistinguishable from the wild-type parent in red-light responsiveness. These data indiacate that phyA is necessary for the very-low-to-low but not the high-influence response, and that phyB is not necessary for either response range. Based on these results, the high-fluence response, if controlled by a single phytochrome, must be controlled by aphytochorme other than phyA of phyB. Overexpression of phyA has a negative effect and overexpression of phyB has an enhancing effect in the high-fluence range. These results suggest that overexpression of either phytochrome perturbs the function of the endogenous photoreceptor system in an unpredictable fashion.     

Specificity and Photomorphogenic Nature of Ultraviolet-B-Induced Cotyledon Curling in Brassica napus L

Three general classes of photomorphogenic photoreceptors have been characterized in higher plants: phytochrome, a blue light/ultraviolet (UV)-A photoreceptor(s), and a UV-B sensory system(s). Although a great deal is known about phytochrome and the blue light/UV-A photoreceptor(s), little is known about UV-B detection processes. One reason for this is the lack of readily quantifiable morphogenic responses that are specifically induced by UV-B radiation. We have discovered a response to UV-B, upward curling of Brassica napus L. cotyledons, that may be useful for probing the mechanism of UV-B photoreception. The process was initially observed when B. napus seeds were germinated under visible light plus UV-B radiation, but did not occur under visible light alone or visible light plus UV-A. When 5-d-old seedlings grown in visible light were given relatively short exposures of UV-B (100 min of 5.5 [mu]mol m-2 s-1), the curling response was also observed. Development of curling was separated from the application of this UV-B pulse by a 14-h latent period. Pulses of red light, blue light, farred light, and UV-A (100 min of 5.5 [mu]mol m-2 s-1) did not induce curling, indicating UV-B specificity Additionally, these other spectral regions did not reverse or enhance the UV-B-triggered response. The degree of curling showed a log-linear dependence on UV-B fluence (6-40 mmol m-2) and reciprocity with respect to length of exposure and fluence rate. The data indicate that curling is photomorphogenic in nature and may be triggered by a single photoreceptor species.     

The involvement of indole-3-acetic acid in the control of stem elongation in dark- and light-grown pea (Pisum sativum) seedlings

We investigated the role of auxin on stem elongation in pea (Pisum sativum L.) grown for 10d in continuous darkness or under low-irradiance blue, red, far red and white light. The third internode of treated seedlings was peeled and the tissues (epidermis and cortex+central cylinder) were separately analyzed for the concentration of free and conjugated indole-3-acetic acid (IAA). Under red, far red and white light internode elongation was linearly related with the free IAA content of all internode tissues, suggesting that phytochrome-dependent inhibition of stem growth may be mediated by a decrease of free IAA levels in pea seedlings. The correlation between IAA and internode elongation, however, did not hold for blue light-grown seedlings. The hypothesis that the growth response under low-irradiance blue light might be correlated with the lack of phytochrome B signalling and changes in gibberellin metabolism is discussed in view of current knowledge on hormonal control of stem growth.     

Regulation of pea epicotyl elongation by blue light : fluence-response relationships and growth distribution

Irradiation with blue light causes a rapid decrease in stem elongation in Pisum sativum. Growing plants under continuous red light allowed us to study the fluence dependence and spatial distribution of blue-induced growth effects without interference from large changes in the ratio of the far-red absorbing form of phytochrome to total phytochrome. The magnitude of the inhibition generated by a 30-second pulse of blue light was linearly related to the log of the fluence applied over two orders of magnitude. Reciprocity held for irradiations with a pulse length shorter than the lag time for the response. The spatial distribution of inhibition was studied by marking the growing zone and photographing the stem at 10-minute intervals before, during, and after a 1-hour exposure to blue light. The region just below the hook does not undergo any perceptible change in growth rate while growth is nearly 100% inhibited in the base of the third internode.     

Evidence for a phytochrome-mediated phototropism in etiolated pea seedlings

Entirely etiolated pea seedlings (Pisum sativum, L. cv Alaska) were tested for a phototropic response to short pulses of unilateral blue light. They responded with small curvatures resembling in fluence-dependence and kinetics of development a phytochrome-mediated phototropic response previously described in maize mesocotyls. Irradiations from above with saturating red or far-red light, either immediately before or after the unilateral phototropic stimulus, strongly reduced or eliminated subsequent positive phototropic curvature. Only blue light from above, however, entirely eliminated curvature at all fluences of stimulus. It is concluded that the phototropism is primarily a result of phytochrome action.     

Far-red light-insensitive, phytochrome A-deficient mutants of tomato

We have selected two recessive mutants of tomato with slightly longer hypocotyls than the wild type, one under low fluence rate (3 mol/m²/s) red light (R) and the other under low fluence rate blue light. These two mutants were shown to be allelic and further analysis revealed that hypocotyl growth was totally insensitive to far-red light (FR). We propose the gene symbol fri (far-red light insensitive) for this locus and have mapped it on chromosome 10. Immunochemically detectable phytochrome A polypeptide is essentially absent in the fri mutants as is the bulk spectrophotometrically detectable labile phytochrome pool in etiolated seedlings. A phytochrome B-like polypeptide is present in normal amounts and a small stable phytochrome pool can be readily detected by spectrophotometry in the fri mutants. Inhibition of hypocotyl growth by a R pulse given every 4 h is quantitatively similar in the fri mutants and wild type and the effect is to a large extent reversible if R pulses are followed immediately by a FR pulse. After 7 days in darkness, both fri mutants and the wild type become green on transfer to white light, but after 7 days in FR, the wild-type seedlings that have expanded their cotyledons lose their capacity to green in white light, while the fri mutants de-etiolate. Adult plants of the fri mutants show retarded growth and are prone to wilting, but exhibit a normal elongation response to FR given at the end of the daily photoperiod. The inhibition of seed germination by continuous FR exhibited by the wild type is normal in the fri mutants. It is proposed that these fri mutants are putative phytochrome A mutants which have normal pools of other phytochromes.     

Effects of light fluence and wavelength on expression of the gene encoding cucumber hydroxypyruvate reductase.

We have investigated the regulation of cucumber (Cucumis sativus) hydroxypyruvate reductase mRNA abundance in response to white-, red-, and far-red-light treatments. Following irradiation of dark-adapted cucumber seedlings with 15 min to 4 h of either white or red light and return to darkness, the mRNA level for the gene encoding hydroxypyruvate reductase (Hpr) in cotyledons peaks in the darkness 16 to 20 h later. The response of the Hpr mRNA level to total fluence of white light depends more directly on irradiation time than on fluence rate. In addition to this time-dependent component, a phytochrome-dependent component is involved in Hpr regulation in dark-adapted green cotyledons as shown by red-light induction and partial far-red-light reversibility. Parallel measurements of mRNA levels for the ribulose bisphosphate carboxylase/oxygenase small subunit and for the chlorophyll a/b-binding protein show that Hpr is the most responsive to short (about 60 min) white- and red-light treatments and that each mRNA has a characteristic pattern of accumulation in dark-adapted cotyledons in response to light.     

Temporal separation of two components of phytochrome action

Abstract In germinating seedlings of Sinapis alba nitrate reductase activity as assayed in vivo becomes accessible to phytochrome control between 15 and 17 h after sowing. Phytochrome operates via the high irradiance reaction to control nitrate reductase activity in the period 15 to 20 h after sowing. Both continuous red light and far-red light elicit this response with a strong fluence rate dependency being apparent in each case. The induction of nitrate reductase activity by light pulses at 20 h after sowing is greatly influenced by red light pre-treatments (operating through phytochrome) given between 0 and 15 h after sowing. Low fluence rate pre-treatments reduce the effectiveness of a subsequent pulse to below the level of a dark control whilst high fluence rate pre-treatments greatly increase the effectiveness of a subsequent pulse.     

Red-light and blue-light photoreceptors controlling chlorophyll a synthesis in the red alga Porphyra umbilicalis and in the green alga Ulva rigida

Chlorophyll synthesis is stimulated by red light in the green alga Ulva rigida C. Ag. and in the red alga Porphyra umbilicalis (L.) Kützing. Because the effect of red light showed some far-red reversibility in successive red and far-red light treatments, the involvement of phytochrome or a phytochrome-like photoreceptor is suggested. The extent of the response is dependent on exposure and photon fluence rate of red-light pulses. In addition to the effect of red light, a strong stimulation of chlorophyll synthesis by blue light was only observed in Ulva rigida. The effect of blue light shows also some far-red reversibility. In the green alga the accumulated chlorophyll is higher after blue light pulses than after red light pulses. In Porphyra umbilicalis , however, the contrary is observed. In Ulva rigida the involvement of a blue light photoreceptor in addition to phytochrome or a phytochrome-like photoreceptor is proposed. The different responses to red and blue light in both algae are explained in terms of their adaptation to the natural light environment.     

Genetic analysis of the roles of phytochromes A and B1 in the reversed gravitropic response of the lz-2 tomato mutant

The lz-2 mutation in tomato ( Lycopersicon esculentum ) causes conditional reversal of shoot gravitropism by light. This response is mediated by phytochrome. To further elicit the mechanism by which phytochrome regulates the lz-2 phenotype, phytochrome-deficient lz-2 plants were generated. Introduction of au alleles, which severely block chromophore biosynthesis, eliminated the reversal of hypocotyl gravitropism in continuous red and far-red light. The fri ¹ and tri ¹ alleles were introduced to specifically deplete phytochromes A and B1, respectively. In dark-grown seedlings, phytochrome A was necessary for response to high-irradiance far-red light, a complete response to low fluence red light, and also mediated the effects of blue light in a far-red reversible manner. Loss of phytochrome B1 alone did not significantly affect the behaviour of lz-2 plants under any light treatment tested. However, dark-grown lz-2 plants lacking both phytochrome A and B1 exhibited reduced responses to continuous red and were less responsive to low fluence red light and high fluence blue light than plants that were deficient for phytochrome A alone. In high light, full spectrum greenhouse conditions, lz-2 plants grew downward regardless of the phytochrome deficiency. These results indicate that phytochromes A and B1 play significant roles in mediating the lz-2 phenotype and that at least one additional phytochrome is involved in reversing shoot gravitropism in this mutant.