Genome-wide analyses of light-regulated genes in Aspergillus nidulans reveal a complex interplay between different photoreceptors and novel photoreceptor functions

Fungi sense light of different wavelengths using blue-, green-, and red-light photoreceptors. Blue light sensing requires the “white-collar” proteins with flavin as chromophore, and red light is sensed through phytochrome. Here we analyzed genome-wide gene expression changes caused by short-term, low-light intensity illumination with blue-, red- or far-red light in Aspergillus nidulans and found that more than 1100 genes were differentially regulated. The largest number of up- and downregulated genes depended on the phytochrome FphA and the attached HOG pathway. FphA and the white-collar orthologue LreA fulfill activating but also repressing functions under all light conditions and both appear to have roles in the dark. Additionally, we found about 100 genes, which are red-light induced in the absence of phytochrome, suggesting alternative red-light sensing systems. We also found blue-light induced genes in the absence of the blue-light receptor LreA. We present evidence that cryptochrome may be part of this regulatory cue, but that phytochrome is essential for the response. In addition to in vivo data showing that FphA is involved in blue-light sensing, we performed spectroscopy of purified phytochrome and show that it responds indeed to blue light.     

Distinct white collar-1 genes control specific light responses in Mucor circinelloides

Light regulates many developmental and physiological processes in a large number of organisms. The best-known light response in the fungus Mucor circinelloides is the biosynthesis of beta-carotene. Here, we show that M. circinelloides sporangiophores also respond to light, exhibiting a positive phototropism. Analysis of both responses to different light wavelengths within the visible spectrum demonstrated that phototropism is induced by green and blue light, whereas carotenogenesis is only induced by blue light. The blue regulation of both responses suggests the existence of blue-light photoreceptors in M. circinelloides. Three white collar-1 genes (mcwc-1a, mcwc-1b and mcwc-1c) coding for proteins showing similarity with the WC-1 photoreceptor of Neurospora crassa have been identified. All three contain a LOV (light, oxygen or voltage) domain, similar to that present in fungal and plant blue-light receptors. When knockout mutants for each mcwc-1 gene were generated to characterize gene functions, only mcwc-1c mutants were defective in light induction of carotene biosynthesis, indicating that mcwc-1c is involved in the light transduction pathway that control carotenogenesis. We have also shown that positive phototropism is controlled by the mcwc-1a gene. It seems therefore that mcwc-1a and mcwc-1c genes control different light transduction pathways, although cross-talk between both pathways probably exists because mcwc-1a is involved in the light regulation of mcwc-1c expression.     

Light-activated heterotrophic growth of the cyanobacterium Synechocystis sp. strain PCC 6803: a blue-light-requiring process.

A glucose-tolerant strain of Synechocystis sp. strain 6803 will not grow on glucose under complete darkness unless given a daily pulse of white light, typically 5 min of 40 mumol m-2 s-1 (light-pulsed conditions). The light pulse is insufficient for photoautotrophy, as glucose is required and growth yield is dependent on glucose concentration. Growth rate is independent of fluence, but growth yield is dependent on fluence, saturating at 40 to 75 mumol m-2 s-1. A Synechocystis strain 6803 psbA mutant strain grows under light-pulsed conditions at rates similar to those for the glucose-tolerant strain, indicating that photosystem II is not required for growth. The relative spectral sensitivity of the growth of light-pulsed cultures (growth only in blue light, 400 to 500 nm, maximum at 450 nm) precludes energetic contribution from cyclic electron transport around photosystem I. Pulses of long-wavelength light (i.e., 550 and 650 nm) did not support the growth of Synechocystis strain 6803 and, when supplied before or after a blue-light pulse, did not inhibit blue-light-stimulated growth of Synechocystis strain 6803. We conclude that the required blue-light pulse does not support growth via photosynthetic electron transport but appears instead to function as an environmental signal regulating heterotrophic metabolism, cell division, or other photomorphogenic processes. We have termed the growth of Synechocystis strain 6803 pulsed with light and kept otherwise in complete darkness light-activated heterotrophic growth. This observation of a blue-light requirement for the growth of Synechocystis strain 6803 represents a novel blue light effect on the growth of a cyanobacterium.     

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.     

Cryptochrome 1 contributes to blue-light sensing in pea

Cryptochromes are widespread in higher plants but their physiological roles as blue-light photoreceptors have been examined in relatively few species. Screening in a phyA null mutant background has identified several blue-light response mutants in pea (Pisum sativum), including one that carries a substitution of a highly conserved glycine residue in the N-terminal photolyase-homologous domain of the pea CRY1 gene. Analyses of cry1, phyA, and phyB mutants show that all three photoreceptors contribute to seedling photomorphogenesis under high-irradiance blue light, whereas phyA is the main photoreceptor active under low irradiances. Triple phyA phyB cry1 mutants grown under high-irradiance blue light are indistinguishable from dark-grown wild-type plants in length and leaf expansion but show a small residual response to higher-irradiance white light. Monogenic cry1 mutants have little discernable phenotype at the seedling stage, but later in development are more elongated than wild-type plants. In addition, the loss of cry1 moderates the short-internode phenotype of older phyA mutants, suggesting an antagonism between phyA and cry1 under some conditions. Pea cry1 has a small inhibitory effect on flowering under long and short days. However, the phyA cry1 double mutant retains a clear promotion of flowering in response to blue-light photoperiod extensions, indicating a role for one or more additional blue-light photoreceptors in the control of flowering in pea.     

不同波长光源对新蚜虫疠霉产孢能力的影响及霉菌蓝光受体基因的克隆和表达分析

光在调控真菌的多种生理过程中发挥着重要作用。为探究光照对新蚜虫疠霉Pandora neoaphidis产孢的影响,本文研究了不同波长光源(蓝光、绿光、白光、红光、黄光)和无光黑暗条件对新蚜虫疠霉分生孢子弹射能力的影响,通过cDNA末端的快速扩增(RACE)对新蚜虫疠霉的蓝光受体蛋白基因pnwc-1进行克隆并对其进行生物信息学分析,利用qRT-PCR对蓝光光源不同照射时长下pnwc-1的表达量进行了定量分析。结果表明,蓝色光源(波长460–465nm)照射后的新蚜虫疠霉菌丝产生的分生孢子数量显著高于其他波长光源,排序为:蓝光>绿光>白光>红光>黄光>无光。另外,分析克隆获得的全长为2 423bp的pnwc-1基因发现,其编码的蛋白具有蓝光受体蛋白典型的保守结构域,同源比对结果显示新蚜虫疠霉与接合菌门真菌归为一类但相对独立。qRT-PCR的定量分析结果表明随着照射时长增加,蓝光处理能显著提高pnwc-1的表达量,而且pnwc-1的相对表达量与累积产孢量存在正相关(R2=0.9798)。本研究为后续蓝光及其受体基因功能的深入研究提供了实验基础,并促进以新蚜虫疠霉为代表的虫霉目真菌在害虫生物防治中的应用。     

Bacterial bilin- and flavin-binding photoreceptors

The growing number of sequenced prokaryotic genomes reveals a wide distribution of open reading frames (ORFs) that putatively encode for red- and blue light sensing photoreceptors. They comprise the bilin-binding phytochromes and the flavin-binding cryptochromes, LOV and BLUF proteins, indicating that about 1/4 of bacteria do possess at least one of these photosensory proteins. The distribution of red- and blue-light sensors among different prokaryotic phyla and classes, and their functional activity as light-switched systems are the subject of this perspective. These photoreceptors were originally found in plants by following the associated physiological responses induced by the respective spectral irradiation. Genome-based approaches now require the assignment of a photochemical/physiological function to the heterologously expressed gene product. Database searches demonstrate in some cases several genes of one category in a certain prokaryot, indicating the presence of more than one type of red- or blue-light sensing properties, but also show a combination of proteins with both spectral sensitivities. Another interesting feature now "comes into light": according to their nature as biological sensors, these photoreceptors are equipped with signalling domains, initiating a cellular response, thereby constituting modular systems switchable by light. It is seen that many of these signalling domains, now found together with light-inducible sensing domains, were already described for other stimuli, e.g., osmo-regulation, oxygen, hydrogen, chemicals, or pH. In some cases, the same type of signalling domain can be found in a red- or a blue-light sensing photoreceptor. Following the characterization of their photochemistry, for several of these bacterial photoreceptors physiological functions are now assigned.     

Role of the Alternaria alternata Blue-Light Receptor LreA (White-Collar 1) in Spore Formation and Secondary Metabolism

Alternaria alternata is a filamentous fungus that causes considerable loss of crops of economically important feed and food worldwide. It produces more than 60 different secondary metabolites, among which alternariol (AOH) and altertoxin (ATX) are the most important mycotoxins. We found that mycotoxin production and spore formation are regulated by light in opposite ways. Whereas spore formation was largely decreased under light conditions, the production of AOH was stimulated 2- to 3-fold. ATX production was even strictly dependent on light. All light effects observed could be triggered by blue light, whereas red light had only a minor effect. Inhibition of spore formation by light was reversible after 1 day of incubation in the dark. We identified orthologues of genes encoding the Neurospora crassa blue-light-perceiving white-collar proteins, a cryptochrome, a phytochrome, and an opsin-related protein in the genome of A. alternata. Deletion of the white-collar 1 (WC-1) gene (lreA) resulted in derepression of spore formation in dark and in light. ATX formation was strongly induced in the dark in the lreA mutant, suggesting a repressing function of LreA, which appears to be released in the wild type after blue-light exposure. In addition, light induction of AOH formation was partially dependent on LreA, suggesting also an activating function. A. alternata ΔlreA was still able to partially respond to blue light, indicating the action of another blue-light receptor system.     

Plant blue-light receptors

Plants have several blue-light receptors, which regulate different aspects of growth and development. Recent studies have identified three such receptors: cryptochrome 1, cryptochrome 2 and phototropin. Cryptochromes 1 and 2 are photolyase-like receptors that regulate hypocotyl growth and flowering time; phototropin mediates phototropism in response to blue light. In addition, phytochrome A has also been found to mediate various blue-light responses. Although the signal-transduction mechanisms of blue-light receptors remain largely unclear, phototropin is probably a protein kinase that regulates cytoplasmic calcium concentrations, whereas the cryptochromes might regulate anion-channel activity and changes in gene expression.     

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.     

Second positive phototropism results from coordinated co-action of the phototropins and cryptochromes

Phototropism and hypocotyl growth inhibition are modulated by the coaction of different blue-light photoreceptors and their signaling pathways. How seedlings integrate the activities of the different blue-light photoreceptors to coordinate these hypocotyl growth responses is still unclear. We have used time-lapse imaging and a nontraditional mathematical approach to conduct a detailed examination of phototropism in wild-type Arabidopsis and various blue-light photoreceptor mutants. Our results indicate that high fluence rates of blue light (100 micro mol m(-)(2) s(-)(1)) attenuate phototropism through the coaction of the phototropin and cryptochrome blue-light photoreceptors. In contrast, we also demonstrate that phototropins and cryptochromes function together to enhance phototropism under low fluence rates (<1.0 micro mol m(-)(2) s(-)(1)) of blue light. Based on our results, we hypothesize that phototropins and cryptochromes regulate phototropism by coordinating the balance between stimulation and inhibition of growth of the hypocotyl depending on the fluence rate of blue light.     

Effects of blue and red light on expression of nuclear genes encoding chloroplast glyceraldehyde-3-phosphate dehydrogenase of Arabidopsis thaliana.

We have characterized the effects of different light spectra on expression of the nuclear genes (GapA and GapB) encoding chloroplast glyceraldehyde-3-phosphate dehydrogenase in Arabidopsis thaliana. Steady-state mRNA levels for both genes in etiolated seedlings increased after a short exposure to red or blue light. However, these increases could not be reversed by immediate far-red light following the initial light treatment. In mature plants, a short light pulse, regardless of its spectrum, had no apparent effect on GapA or GapB mRNA levels in dark-adapted plants. In contrast, continuous exposure to red, blue, or white light resulted in increases of GapA and GapB mRNA levels, with blue and white light being far more efficient than red light. Similarly, continuous exposure of etiolated seedlings to red, blue, or white light also resulted in increased GapA and GapB mRNA levels. In addition, we show that illumination of red light-saturated Arabidopsis plants with continuous blue light results in further increases of GapA and GapB mRNA levels. Based on these results, we conclude that both blue light photoreceptor- and phytochrome-mediated pathways are involved in light regulation of GapA and GapB genes in Arabidopsis, with blue light acting as the dominant regulator.     

The coevolution of blue-light photoreception and circadian rhythms

Sunlight is a primary source of energy for life. However, its UV component causes DNA damage. We suggest that the strong UV component of sunlight contributed to the selective pressure for the evolution of the specialized photoreceptor cryptochrome from photolyases involved in DNA repair and propose that early metazoans avoided irradiation by descending in the oceans during the daytime. We suggest further that it is not coincidental that blue-light photoreception evolved in an aquatic environment, since only blue light can penetrate to substantial depths in water. These photoreceptors were then also critical for sensing the decreased luminescence that signals the coming of night and the time to return to the surface. The oceans and the 24-h light-dark cycle therefore provided an optimal setting for an early evolutionary relationship between blue-light photoreception and circadian rhythmicity.     

A single flavoprotein,AppA, integrates both redox and light signals in Rhodobacter sphaeroides

Anoxygenic photosynthetic proteobacteria exhibit various light responses, including changing levels of expression of photosynthesis genes. However, the underlying mechanisms are largely unknown. We show that expression of the puf and puc operons encoding structural proteins of the photosynthetic complexes is strongly repressed by blue light under semi-aerobic growth in Rhodobacter sphaeroides but not in the related species Rhodobacter capsulatus. At very low oxygen tension, puf and puc expression is independent of blue light in both species. Photosynthetic electron transport does not mediate the blue light repression, implying the existence of specific photoreceptors. Here, we show that the flavoprotein AppA is likely to act as the photoreceptor for blue light-dependent repression during continuous illumination. The FAD cofactor of AppA is essential for the blue light-dependent sensory transduction of this response. AppA, which is present in R. sphaeroides but not in R. capsulatus, is known to participate in the redox-dependent control of photosynthesis gene expression. Thus, AppA is the first example of a protein with dual sensing capabilities that integrates both redox and light signals.