Expression of the Cucumber Hydroxypyruvate Reductase Gene Is Down-Regulated by Elevated CO2

We examined the effects of CO2 concentration on the white-light-stimulated expression of the cucumber (Cucumis sativus L.) Hpr gene. Hpr encodes hydroxypyruvate reductase, an enzyme important in the photorespiratory glycolate pathway, which plays an integral role in carbon allocation in C3 plants. Because CO2 is an end product of this pathway and because increased CO2 concentrations lessen the need for photorespiration, we tested whether exposure of plants to elevated CO2 would affect white-light-stimulated Hpr gene expression. Exposure of dark-adapted cucumber seedlings to elevated CO2 (2 to 3 times ambient) during a 4-h white-light irradiation significantly inhibited the accumulation of Hpr mRNA. Increasing the CO2 concentration during irradiation to 6 or 9 times ambient did not further inhibit Hpr mRNA accumulation. The depressing effect of high CO2 on Hpr mRNA accumulation was seen in both high and low light, but was more pronounced in higher light. These results suggest that maximum sensitivity to CO2 occurs in conditions near those normally encountered by the plant (high light, CO2 concentration near ambient) and support a model in which white-light-regulated Hpr expression is modulated in part by environmental CO2 concentration.     

Development of enzymes in the cotyledons of watermelon seedlings

Changes in hypocotyl length, cotyledon weight, lipid content, chlorophyll content, and capacity for photosynthesis have been described in seedlings of Citrullus vulgaris, Schrad. (watermelon) growing at 30 C under various light treatments. Corresponding changes in the levels of 19 enzymes in the cotyledons are described, with particular emphasis on enzymes of microbodies, since during normal greening, enzymes of the glyoxysomes are lost and those of leaf peroxisomes appear. In complete darkness enzymes of the glyoxysomes reach a peak at 4 days and decline as the fat is depleted. Enzymes of mitochondria and of glycolytic pathways also peak at 4 to 5 days and either remain unchanged or decline to a lesser extent. Exposure to light at 4 days, when the cotyledons emerge, results in a selectively greater destruction of enzymes of the glyoxylate cycle; chlorophyll synthesis and capacity for photosynthesis increase in parallel, and there is a striking increase in the activities of chloroplast enzymes and in those of the leaf peroxisomes, hydroxypyruvate reductase and glycolate oxidase. The reciprocal changes in enzymes of the glyoxysomes and of leaf peroxisomes can be temporally dissociated, since even after 10 days in darkness, when malate synthetase and isocitrate lyase have reached very low levels, hydroxypyruvate reductase and glycolate oxidase increase strikingly on exposure to light and the cotyledons become photosynthetic. Furthermore, the parallel development of enzymes of leaf peroxisomes and functional chloroplasts is not immutable, since hydroxypyruvate reductase and glycolate oxidase activity can be elicited in darkness following a 5-minute exposure to light at day 4 while chlorophyll does not develop under these conditions.     

不同光敏色素分子的特异感光性

简要综述两种光敏色素（ＰｈｙＡ、ＰｈｙＢ） 的分子特性、感光性及作用模式等方面的研究进展。光敏色素是一种调节植物中许多光反应的色素蛋白复合体。不同光敏色素分子具有特异的感光性。ＰｈｙＡ负责‘甚低辐照反应’和远红光‘高辐照反应’; 而ＰｈｙＢ则调节‘低辐照反应’及红光‘高辐照反应’。另外, 讨论了ＰｈｙＡ和ＰｈｙＢ在光周期感受中的作用。     

Light- and carbon-signaling pathways. Modeling circuits of interactions

Here, we report the systematic exploration and modeling of interactions between light and sugar signaling. The data set analyzed explores the interactions of sugar (sucrose) with distinct light qualities (white, blue, red, and far-red) used at different fluence rates (low or high) in etiolated seedlings and mature green plants. Boolean logic was used to model the effect of these carbon/light interactions on three target genes involved in nitrogen assimilation: asparagine synthetase (ASN1 and ASN2) and glutamine synthetase (GLN2). This analysis enabled us to assess the effects of carbon on light-induced genes (GLN2/ASN2) versus light-repressed genes (ASN1) in this pathway. New interactions between carbon and blue-light signaling were discovered, and further connections between red/far-red light and carbon were modeled. Overall, light was able to override carbon as a major regulator of ASN1 and GLN2 in etiolated seedlings. By contrast, carbon overrides light as the major regulator of GLN2 and ASN2 in light-grown plants. Specific examples include the following: Carbon attenuated the blue-light induction of GLN2 in etiolated seedlings and also attenuated the white-, blue-, and red-light induction of GLN2 and ASN2 in light-grown plants. By contrast, carbon potentiated far-red-light induction of GLN2 and ASN2 in light-grown plants. Depending on the fluence rate of far-red light, carbon either attenuated or potentiated light repression of ASN1 in light-grown plants. These studies indicate the interaction of carbon with blue, red, and far-red-light signaling and set the stage for further investigation into modeling this complex web of interacting pathways using systems biology approaches.     

Effect of brief treatments with benzyladenine on growth and development of watermelon cotyledons

Abstract Watermelon (Citrullus vulgaris Schrad.) cotyledons were excised from the seed after 24 h of imbibition and incubated for different times (ranging from 1 to 240 min) on filter paper saturated with 0.1 mM BA (benzyladenine) solution. After the incubation in BA the cotyledons were thoroughly rinsed in water and grown for 5 days in darkness on filter paper saturated with distilled water. Continuous presence of BA is not a prerequisite for optimal growth. Several effects of the hormone (stimulation of expansion growth, acceleration of lipid breakdown, enhancement of hydroxypyruvate reductase activity, increase in carotenoid content) are more intensive if the cotyledons are treated with BA for a relatively short time and then grown in water. Maximal effects are observed after a 4 h exposure to BA. A single 1 min exposure is sufficient to elicit measurable effects on growth, hydroxypyruvate reductase and carotenoid content. The sensitivity to very short exposures is considerably different for different hormone effects. Carotenoid content and hydroxypyruvate reductase activity are enhanced much more than expansion growth.     

Changes in cotyledon mRNA during floral induction of Pharbitis nil CV. Violet

Floral induction in seedlings of Pharbitis nil Choisy cv. Violet, with one cotyledon removed, was manipulated by applying various photoperiodic treatments to the remaining cotyledon. Populations of polyadenylated RNA from treated cotyledons were examined to identify messages specifically involved in floral induction. The RNA was translated in vitro using a wheat-germ system, and the resulting translation products were analysed by two-dimensional polyacrylamide gel electrophoresis. Substantial qualitative and quantitative differences were found between mRNA from cotyledons of seedlings kept in continuous light (non-induced) and of seedlings given a 16-h dark period (induced). In contrast, inhibition of flowering with a night-break resulted only in one detectable, quantitative difference in mRNA.Abbreviations CL continuous light - kDa kilodalton - NB 16 h darkness+10 min red-light break, 8 h into the dark period - poly(A)⁺ RNA polyadenylated RNA (isolated by binding to a cellulose oligodeoxythymidine affinity column) - SD short day (16 h dark) - SDP short-day plant - SDS sodium dodecyl sulfate     

Isolation and Initial Characterization of Arabidopsis Mutants That Are Deficient in Phytochrome A

Phytochrome, a red/far-red-light photoreceptor protein of plants, is encoded by a small gene family. Phytochrome A (PHYA), the product of the PHYA gene, is the predominant molecular species of phytochrome in etiolated tissue and has been best characterized biochemically. To define a role for PHYA, we isolated new mutants, designated fre1 (far-red elongated), in Arabidopsis thaliana that were specifically deficient in PHYA spectral activity and protein accumulation. These mutants were identified on the basis of their long hypocotyl phenotype under continuous far-red light. Although the fre1 mutants lacked the hypocotyl response to continuous far-red light, their responses to continuous white light and to end-of-day far-red-light treatments were normal. Thus, PHYA appears to play only a minor role in the regulation of hypocotyl elongation under natural conditions. In contrast, the fre1 mutation affected greening a fre1 mutant was less able than the wild type to deetiolate after growth in the dark. However, the potentiation effect of a red-light pulse on accumulation of chlorophyll was not changed significantly in the fre1 mutants. Thus, the function of PHYA might be highly specialized and restricted to certain phases of Arabidopsis development.     

Benzyladenine-induced stimulation of two components of chlorophyII formation in etiolated cucumber cotyledons

Excised etiolated cotyledons of cucumber ( Cucumis sativus L. cv. Aonagajibai) were continuously irradiated under various intensities of white light. The rate of chlorophyII (Chi) formation during the lag phase reaches a plateau at fluence rates above 1.4 urmol m⁻² s⁻¹. This is true in both water-control and benzyladenine (BA)-pretreated cotyledons. In cotyledons pretreated for 14 h with BA in darkness (in which case, Chl formation is stimulated by BA during both the lag and the steady-state phases), the increase in the steady-state rate of Chl formation with increasing light in tensity is stimulated compared to that of the water control over the range of fluence rates, 0. 25-43 urmol m⁻² s⁻¹. In cotyledons pretreated for 6 h with BA in darkness (only Chl formation during the lag phase is stimulated), only an increase in fluence rate from 0.25 to 1.4 umol m⁻² s⁻¹ causes a higher increase in the Chl formation in the BA-treated cotyledons than in the water control. The time course of Chl formation shows that the BA-induced late-appearing effect (stimulation of the steady-state rate) is almost absent at low intensity illumination, but the BA-induced fast-appearing effect (elimination of the lag phase) is effective at all intensities. From this evidence, the Chl-forming process apparently consists of two components, whose periods of operation or light-intensity requirements are different. BA stimulates the rates of the respective components in both the fast and the late-appearing effects.     

Nitrate reductase in cotyledons of cucumber seedlings as affected by nitrate, phytochrome and calcium

Regulation by the active form of phytochrome (P_FR) and the effect of Ca²⁺ was examined with nitrate reductase (NR) in etiolated cucumber ( Cucumis sativus cv. Beilpuig). Nitrate reductase activity (NRA) was studied in excised cotyledons of cucumber seedlings grown in distilled water and in darkness for seven days at 24 ± 0.5°C. All experiments were performed in the dark and a dim green safelight was used during analyses. In etiolated cucumber cotyledons NRA was induced by nitrate and a brief irradiation (15 min) with red light (R) resulted in 62% increase in NRA. This effect was nullified when R was followed immediately by a brief (5 min) far-red light (FR). NRA also showed a semidian (12 h) rhythmicity. Both P_FR, and nitrate effects were age dependent. Calcium seemed to be involved since the phytochrome effect was only observed when calcium was supplied in the external solution. The effect of R on NRA depended on the period of calcium nitrate incubation. An external supply of calcium ionophore mimicked the effect of R and, if supplied to R-irradiated cotyledons, produced a higher NR level than that caused by R alone. This suggested that intracellular free calcium was involved.     

Far-red light blocks greening of Arabidopsis seedlings via a phytochrome A-mediated change in plastid development.

We have characterized a far-red-light response that induces a novel pathway for plastid development in Arabidopsis seedlings. This response results in the inability of cotyledons to green upon subsequent white light illumination, and the response is suppressed by exogenous sucrose. Studies with mutants showed that this far-red block of greening is phytochrome A dependent and requires an intact downstream signaling pathway in which FHY1 and FHY3 may be components but in which HY5 is not. This highlights a previously undefined branchpoint in the phytochrome signaling pathway. Ultrastructural analysis showed that the far-red block correlates with both the failure of plastids to accumulate prolamellar bodies and the formation of vesicles in the stroma. We present evidence that the far-red block of greening is the result of severe repression of protochlorophyllide reductase (POR) genes by far-red light coupled with irreversible plastid damage. This results in the temporal separation of phytochrome-mediated POR; repression from light-dependent protochlorophyllide reduction, two processes that normally occur in coordination in white light.     

Photocontrol of hypocotyl elongation in light-grown Cucumis sativus L.

An interaction is demonstrated between the effects of phytochrome and cryptochrome (the specific blue-light photoreceptor) in the inhibition of hypocotyl elongation of light-grown cucumber (Cucumis sativus L.) cv. Ridge Greenline seedlings. At certain fluence rates of blue light the total inhibition response is greater than the sum of the separate responses to each photoreceptor. The threshold for response to blue light is reduced at least 30-fold by additional red-light irradiation. The synergistic effect is demonstrated for two different fluence rates of red light. Synergism is mediated by phytochrome in both the cotyledons and the hypocotyl.Abbreviations and symbols BL blue light - FR far-red light - Pfr far-red-absorbing form of phytochrome - R red light - photostationary state of phytochrome - _c calculated      

Phytochrome-mediated light regulation of nitrate reductase expression in squash cotyledons

In etiolated squash (Cucurbita maxima L.) cotyledons, nitrate-inducible NADH:nitrate reductase activity and protein were increased in darkness by red light pulses with red/far-red photoreversibility. Continuous far-red light also led to increased levels of nitrate reductase activity and protein. Poly(A)+RNA, which hybridizes to squash nitrate reductase cDNA, was also increased by light treatments. Thus, we found that after nitrate triggering, nitrate reductase expression appears to be regulated by light via phytochrome.     

Development of the Casparian strip is delayed by blue light in pea stems

To understand the regulatory mechanisms involved in tissue development by light, the kinetics of regulation of Casparian strip (CS) development in garden pea stems was studied. We found that short-term irradiation with white light delayed the development of the CS and used this delay to assess the quantitative effect of light on CS development. We examined the effect of the duration and fluence rates of white light treatment on CS development and observed a significant relationship between fluence and the delay in CS development indicating that the Bunsen–Roscoe law of reciprocity holds for this response. The effect of white light irradiation was not inhibited in the presence of a photosynthetic inhibitor, DCMU, or a carotenoid biosynthesis inhibitor, Norflurazon, indicating that the delay in CS development by light is a photomorphogenetic response rather than a subsidiary effect mediated by photosynthetic activity. An action spectrum for the response displayed a major peak in the blue-light region, suggesting a dominant role for blue-light receptors. A minor peak in the red-light region also suggested the possible involvement of phytochromes. Although phytochromes are known to contribute to blue-light responses, phytochrome-deficient mutants showed a normal delay of CS development in response to blue light, indicating that the response is not mediated by phytochrome and suggesting a role for one or more specific blue-light receptors.     

Transient down-regulation of phytochrome mRNA abundance in etiolated cucumber cotyledons in response to continuous white light

Phytochrome mRNA abundance decreased to 20% of the initial level in etiolated cucumber (Cucumis sativus L.) cotyledons exposed to continuous white light. Unexpectedly, by 12 hours of continuous white light, phytochrome mRNA had reaccumulated to 60% of the control level. High stringency RNA blot analyses suggest that it is the mRNA encoding type I phytochrome that reaccumulates.