Molecular dissection of the roles of phytochrome in photoperiodic flowering in rice

Phytochromes mediate the photoperiodic control of flowering in rice (Oryza sativa), a short-day plant. Recent molecular genetics studies have revealed a genetic network that enables the critical daylength response of florigen gene expression. Analyses using a rice phytochrome chromophore-deficient mutant, photoperiod sensitivity5, have so far revealed that within this network, phytochromes are required for expression of Grain number, plant height and heading date7 (Ghd7), a floral repressor gene in rice. There are three phytochrome genes in rice, but the roles of each phytochrome family member in daylength response have not previously been defined. Here, we revealed multiple action points for each phytochrome in the critical daylength response of florigen expression by using single and double phytochrome mutant lines of rice. Our results show that either phyA alone or a genetic combination of phyB and phyC can induce Ghd7 mRNA, whereas phyB alone causes some reduction in levels of Ghd7 mRNA. Moreover, phyB and phyA can affect Ghd7 activity and Early heading date1 (a floral inducer) activity in the network, respectively. Therefore, each phytochrome gene of rice has distinct roles, and all of the phytochrome actions coordinately control the critical daylength response of florigen expression in rice.     

Growth and Dormancy in Lunularia cruciata (L.) Dum.: III. THE WAVELENGTHS OF LIGHT EFFECTIVE IN PHOTOPERIODIC CONTROL

Growth and dormancy in Lunularia are controlled by daylength,short-day promoting active growth, long-day or light-break treatmentinducing dormancy. Light-breaks of red light are highly effectivein inducing dormancy, while irradiation with other wavebandsis much less inhibitory to growth. Far-red light given afterred irradiation causes substantial reversal of the red-lighteffect, suggesting strongly that phytochrome is involved inthe photoperiodic response mechanism of Lunularia. However,even short(15 sec.) exposures to far-red light alone cause significantgrowth inhibition, and it is considered possible that far-redirradiation also leads to the formation of some of the P 730form of phytochrome.     

Abscisic acid as a potent regulator of the transition from juvenile to mature stage inXanthium strumarium

The cotyledons ofXanthium strumarium plants are of low sensitivity to photoperiodic treatment and contain only trace amounts of ABA under long-day conditions. The first pair of leaves, very sensitive to photoperiodic treatment, contains a higher level ofABA, decreasing with age of the plant. Prolonged short-day photoperiodic treatment increases the ABA level in the cotyledons but this is still 10times lower than in the first two leaves. Exogenous 10^?4M ABA increases the ABA level in the cotyledons to the level corresponding to that in the first leaves, and enhances the photoperiodic sensitivity of cotyledons. In contrast to cotyledons, the photoperiodic treatment affects the ABA level in the first pair of leaves only slightly. The authors propose that a high ABA level supports the transition of plants to flowering, while a low ABA level may be responsible for a low photoperiodic sensitivity of cotyledons inXanthium plants.     

Fluctuation of endogenous cytokinins in leaves and roots of short-day and long-day tobacco associated with photoperiodic induction

As the dynamics of changes in phytohormones may be involved in photoperiodic regulation of the rates of growth and flowering, fluctuation of cytokinins was followed in long-day and short-day tobacco. Zeatin (Z) and zeatin riboside (ZR) were identified in leaves and roots using a GC-MSC system. In plants of the long-day tobaccoNicotiana silvestris increasing the number of long-day inductive for flowering (10, 20, 30, 40 LD) resulted in a rise in ZR activity. Half the plants reached a reproductive stage on the 40th day of induction. In short-day Mam moth tobacco plants, short-day floral induction (10, 20, 30, 40 SD) caused similar but less marked changes in ZR.     

Ectopic expression of oat phytochrome A in hybrid aspen changes critical daylength for growth and prevents cold acclimatization

Survival of temperate-zone tree species under the normal summer-winter cycle is dependent on proper timing of apical growth cessation and cold acclimatization. This timing is primarily based on the perception of daylength, and through evolution many tree species have developed photoperiodic ecotypes which are closely adapted to the local light conditions. The longest photoperiod inducing growth cessation, the critical photoperiod, is inherited as a quantitative character. The phytochrome pigment family is the probable receptor of daylength, but the exact role of phytochrome and the physiological basis for the different responses between photoperiodic ecotypes are not known. This report shows for the first time that over-expression of the oat phytochrome A gene ( PHYA ) in a tree significantly changes the critical daylength and effectively prevents cold acclimatization. While the critical daylength for elongation growth in the wild-type of hybrid aspen ( Populus tremula × tremuloides ) was approximately 15 h, transgenic lines with a strong expression of the oat PHYA gene did not stop growing even under a photoperiod of 6 h. Quantitative analysis of gibberellins (GA) as well as indole-3-acetic acid (IAA) revealed that levels of these were not down-regulated under short days in the transgenic plants expressing high levels of oat PHYA , as in the wild-type. These results indicate that photoperiodic responses in trees might be regulated by the amount of PHYA gene expressed in the plants, and that the amount of phytochrome A (phyA) affects the metabolism of GAs and IAA.     

Diapause and seasonal life cycle strategy in the house spider, Achaearanea tepidariorum (Araneae, Theridiidae)

Abstract In order to elucidate the mechanism regulating its seasonal life cycle, the photoperiodic response of Achaearanea tepidariorum has been analysed. Nymphal development was faster in long-day and slower in short-day photoperiods. The combined action of low temperature, poor food supply and short daylength induced diapause at an earlier developmental stage than short days alone. Thus, photoperiod is a primary factor inducing nymphal diapause, but the diapausing instar is influenced by both temperature and food supply. Hibernating nymphs became unresponsive to photoperiod in late December. After hibernation, however, sensitivity was restored and the nymphs remained sensitive to photoperiod throughout their life. This spider could also enter an imaginal or reproductive diapause. Photoperiod was again a primary inducing factor and temperature modified the photoperiodic response to some extent. The induction of the reproductive diapause was almost temperature-compensated whereas development was not. So the involvement of a photoperiodic counter system was suggested. Irrespective of whether the nymph had experienced diapause or not, the imaginal diapause was induced in response to a short-day photoperiod after adult moult. Based on these observations, the seasonal life cycle and the adaptive significance of nymphal and imaginal diapause are discussed.     

Floral initiation in strawberry: Spectral evidence for the regulation of flowering by long-day inhibition

Summary Floral initiation in strawberry cv. Cambridge Favourite, a facultative short-day plant, was inhibited by a daylength extension with red light (R) during the second half of a 16-hour night but not during the first half, and by far-red light (FR) in the first half but not during the second. Mixed R plus FR light was inhibitory to flowering at both times. This change in sensitivity to R and FR light in the evening and morning resembles the pattern for flower induction in long-day plants but differs from the pattern for flower inhibition in several other short-day plants, examples of which are given. These experiments afford further support for the hypothesis that the control of flower initiation in strawberry depends on the production of a flower inhibitor by leaves exposed to long photoperiods.Abbreviations R red - FR far-red - SD short day - LD long day - SDP short-day plant - LDP long-day plant     

Photoperiodic polyphenisms in rodents: neuroendocrine mechanisms, costs, and functions

Annual changes in daylength figure prominently in the generation of seasonal rhythms in reproduction, and a wide variety of mammals use ambient photoperiod as a proximate cue to time critical reproductive events. Nevertheless, within many reproductively photoperiodic mammalian species, there exist individuals--termed "photoperiod nonresponders"--that fail to adopt a seasonal breeding strategy and instead exhibit reproductive competence at a time of year when their conspecifics are reproductively quiescent. Photoperiod nonresponsiveness has been principally characterized by laboratory observations--over half of the species known to be reproductively photoperiodic contain a proportion of nonresponsive individuals. The study of nonresponders has generated basic insights regarding photic regulation of reproduction in mammals. The neuroendocrine mechanisms by which the short-day photoperiodic signal is degraded or lost in nonresponders varies between species: differences in features of the circadian pacemaker, which provides photoperiodic input to the reproductive neuroendocrine system, have been identified in hamsters; changes in the responsiveness of hypothalamic gonadotrophs to melatonin and as-yet-unspecified inhibitory signals have been implicated in voles and mice. Individuals that continue to breed when their conspecifics refrain might enjoy higher fitness under certain circumstances. Statements regarding the adaptive function of reproductive nonresponsiveness to photoperiod require additional information on the costs (metabolic and fitness) of sustaining reproductive function during the winter months and how these costs vary as a function of environmental conditions. Reproductive nonresponders thus continue to represent a challenge to theories that extol the adaptive function of seasonality. Several nonexclusive hypotheses are proposed to account for the maintenance of nonresponsive individuals in wild rodent populations.     

A Set of Three Genes Regulates Photoperiodic Responses of Flowering in Rice (Oryza sativa)

Differentiation in photoperiodic response of flowering has been key to the evolution and wide geographic distribution of rice, an essentially short-day plant. Crosses were made such that the hybrid F1 plants flower later than the late-flowering parents to investigate the genetic basis underlying this differentiation. From initial experiments, three major genes for flowering time were identified from four naturally occurring variants under natural long-day conditions. An F2-derived trigenic mutant line bred-true for a day-neutral response was selected and used as the recipient to synchronize the genetic background for the major genes. Experiments conducted under various daylengths indicated that these genes are responsible for photoperiodic sensitivity and the trihybrid has a critical daylength between 13.5 and 14 h. The three genes regulate photoperiodic responses qualitatively and quantitatively through complementary and other epistatic effects, respectively. The complementation suggests that the three genes act in a linear manner to repress the transition from the vegetative to reproductive phases under long daylengths. This set of genes also provides a model to understand the genetic mechanism underlying the elongated vegetative growth period in the F1 generation, which is usually an obstacle to the use of heterosis, and the selection for early maturation in rice breeding.     

Photoperiodic history affects the critical daylength of the short-day plant Acrosymphyton purpuriferum (Rhodophyta)

The crustose tetrasporophyte of the red alga Acrosymphyton purpuriferum is a qualitative short-day plant in the formation of its tetrasporangia. The critical daylength for the response was determined in plants precultured in various long-day regimes [20:4, 18:6, 16:8 and 14:10 (L:D, h)]. There was a strong influence of photoperiodic history. The sharper the decrease in daylength the stronger and faster the plants responded. The critical daylength (daylength inducing 50% response) increased from 9·5 h for plants precultured in 14 h days to 12·5 h for plants precultured in 20 h days. Acrosymphyton thus responds to a change in daylength, rather than to a fixed critical value. This is of adaptive significance in synchronising the onset of reproduction throughout its broad depth range in the subtidal region.     

PHYTOCHROME CHANGES IN TISSUES OF DARK-GROWN SEEDLINGS REPRESENTING VARIOUS PHOTOPERIODIC CLASSES

Excised tissues of dark-grown seedlings representing long day, short day and daylength indifferent photoperiodic classes were assayed for nonphotochemical changes in phytochrome. In all tissues tested, these changes were qualitatively the same. A brief irradiation with red light was followed in darkness by a decrease in total phytochrome, the disappearance of P_FR, and an increase in detectable P_R. Within the limits of the tissues tested, the kinetics of phytochrome change can be assigned to three groups on the basis of rates. These groups are represented by coleoptiles, hypocotyls and epicotyls, and mesocotyls. The kinetics could not be distinguished on the basis of the photoperiodic class of the mature plant. The significance of these kinetics with respect to the photochemistry of phytochrome conversion is discussed.     

A photoperiodic response mediated by blue light in the brown alga Scytosiphon lomentaria

Summary The crustose phase of Scytosiphon lomentaria (Lyngb.) J. Ag. persists indefinitely in 16 h of white light per day, but produces erect, cylindrical thalli vegetatively in 8-h days. The critical daylength for this short-day (SD) response is sharply defined, and, between 12 and 13 h, differences in daylength of only 15 min produce substantial differences in response. A significant response to SD can be induced by as few as 4 SD cycles, but 10–12 cycles are required to saturate the response and induce more than 90% of the plants to form thalli. The response to SD is completely inhibited by a 1-min light-break with a low irradiance of blue light, given in the middle of a 16-h dark period, but is unaffected by longer periods and higher irradiances of red or far-red light. There is good reciprocity between the irradiance and the length of a light-break with blue light, and 50% inhibition of the response to SD is induced by about 2 nE cm^-2 at 449 nm. All attempts to reverse the inhibitory effects of blue light by subsequent irradiation with another wavelength have so far failed. These results indicate that phytochrome is not the photoreceptor pigment for this response, in spite of the similarity of the response in all other respects to the photoperiodic responses of flowering plants and other algae.Abbreviations and Symbols SD short day - LD long day. Horizontal above number of hours indicates dark period     

Photoperiodic time signals during twilight

Abstract. Although daylength has a major effect on flowering and several other aspects of plant development, the actual environmental time signals for the beginning and the end of day are obscure. An intensive spectroradiometric study was carried out in three contrasting environments: namely, unshaded sites, a mature oak woodland and a sugar beet crop. Spectral photon distributions were obtained describing numerous twilight phases and intervening photoperiods throughout the year. From each, absolute photon fluence rates, photon fluence rate ratios and phytochrome photoequilibria were calculated. Although substantial changes in spectral composition occurred during twilight, they were less capable of providing reliable and accurate time signals than the absolute fluence rate; this was especially apparent beneath the canopies. Thus, spectral changes are unlikely to be valuable in photoperiodic perception. The results are discussed in relation to the possible involvement of the known plant photoreceptors in photoperiodism.     

CsFTL3, a chrysanthemum FLOWERING LOCUS T-like gene, is a key regulator of photoperiodic flowering in chrysanthemums

Chrysanthemum is a typical short-day (SD) plant that responds to shortening daylength during the transition from the vegetative to the reproductive phase. FLOWERING LOCUS T (FT)/Heading date 3a (Hd3a) plays a pivotal role in the induction of phase transition and is proposed to encode a florigen. Three FT-like genes were isolated from Chrysanthemum seticuspe (Maxim.) Hand.-Mazz. f. boreale (Makino) H. Ohashi & Yonek, a wild diploid chrysanthemum: CsFTL1, CsFTL2, and CsFTL3. The organ-specific expression patterns of the three genes were similar: they were all expressed mainly in the leaves. However, their response to daylength differed in that under SD (floral-inductive) conditions, the expression of CsFTL1 and CsFTL2 was down-regulated, whereas that of CsFTL3 was up-regulated. CsFTL3 had the potential to induce early flowering since its overexpression in chrysanthemum could induce flowering under non-inductive conditions. CsFTL3-dependent graft-transmissible signals partially substituted for SD stimuli in chrysanthemum. The CsFTL3 expression levels in the two C. seticuspe accessions that differed in their critical daylengths for flowering closely coincided with the flowering response. The CsFTL3 expression levels in the leaves were higher under floral-inductive photoperiods than under non-inductive conditions in both the accessions, with the induction of floral integrator and/or floral meristem identity genes occurring in the shoot apexes. Taken together, these results indicate that the gene product of CsFTL3 is a key regulator of photoperiodic flowering in chrysanthemums.     

CIRCANNUAL GROWTH RHYTHM AND PHOTOPERIODIC SORUS INDUCTION IN THE KELP LAMINARIA SETCHELLII (PHAEOPHYTA)1

Growth and reproduction of laboratory-grown sporophytes of Laminaria setchellii Silva were investigated in a tank system with controlled conditions of daylength, temperature, and nutrients (N and P). A circannual growth rhythm of the frond was detected under constant laboratory conditions. In continuous long-day and night-break conditions the period τ of the free-running rhythm varied between 11.3 and 17.3 months; in short-day conditions the frond grew indefinitely. The growth rhythm of individual plants could be synchronized by a simulated annual cycle of day-length with a period of T = 12 months. The four seasons of the year were simultaneously simulated by phase shifting the annual cycle of daylength by 3, 6, or 9 months in three out of four tanks. The annual growth cycle followed these phase shifts, and initiation of the new blade always started just after the winter daylength minimum. The formation of sori was induced by a genuine photoperiodic short-day reaction in 1- to 2-year-old plants. Sori became, visible 9–14 weeks after transfer of individual plants from long-day to short-day conditions, whereas plants cultured in continuous long-day or night-break conditions remained sterile. Sporophytes with or without blades were able to continue growth or produce new blades in continuous darkness.     

What photoperiodic signal is provided by a continuous-release melatonin implant?

The purpose of this study was to evaluate whether the insertion of a continuous-release melatonin implant into ewes provides a short-day photoperiodic signal or acts as a functional pinealectomy (provides no specific photoperiodic signal but renders ewes incapable of responding to changes in photoperiod). Ewes primed with 60 long days (18L:6D) during the spring were moved to intermediate day length (13L:11D) for 66 days and then given one of five treatments: 1) short-day control, second drop in photoperiod to 8L:16D; 2) intermediate-photoperiod control, kept on 13L:11D; 3) pinealectomy and kept on 13L:11D; 4) melatonin implant and kept on 13L:11D; 5) melatonin implant and moved to 8L:16D. Mean number of estrous cycles per group and total duration of reproductive activity were determined. Ewes in all groups began to exhibit estrous cycles after the initial reduction in photoperiod. The number of estrous cycles and duration of reproductive activity differed among groups. The number of estrous cycles and duration of reproductive activity was extended in ewes receiving the second drop in photoperiod compared to that of the intermediate-photoperiod controls. Pinealectomized ewes had a number of estrous cycles and duration of reproductive activity similar to those of ewes maintained on the intermediate photoperiod. Melatonin implants increased the number of estrous cycles and prolonged reproductive activity in ewes maintained on the intermediate photoperiod; melatonin implants did not prevent the extension of reproductive activity in ewes receiving the second photoperiodic drop to the short daylength.(ABSTRACT TRUNCATED AT 250 WORDS)     

Gibberellins in the control of photoperiodic flower transition in Pharbitis nil

The role of gibberellins in the photoperiodic flower induction of short-day plant Pharbitis nil has been investigated. It has been found that the endogenous content of gibberellins in the cotyledons of P. nil is low before and after a 16-h-long inductive dark period. During the inductive night the content of gibberellins is high at the beginning of darkness and about the middle of the dark period. Exogenous GA₃ when applied to the cotyledons of non-induced plants does not replace the effect of the inductive night but it can stimulate the intensity of flowering in plants cultivated on suboptimal photoperiods. GA₃ could also reverse the inhibitory effect of end-of-day far-red light irradiation on P. nil flowering. 2-Chloroethyltri-methylammonium chloride (CCC) applied to the cotyledons during the inductive night also inhibited flowering. GA₃ could reverse the inhibitory effect of CCC. The obtained results strongly suggest that gibberellins are involved in the phytochrome controlled transition of P. nil to flowering. Their effect could be additive to that of photoperiodic induction.     

Phytochromes confer the photoperiodic control of flowering in rice (a short-day plant)

The photoperiodic sensitivity 5 (se5) mutant of rice, a short-day plant, has a very early flowering phenotype and is completely deficient in photoperiodic response. We have cloned the SE5 gene by candidate cloning and demonstrated that it encodes a putative heme oxygenase. Lack of responses of coleoptile elongation by light pulses and photoreversible phytochromes in crude extracts of se5 indicate that SE5 may function in phytochrome chromophore biosynthesis. Ectopic expression of SE5 cDNA by the CaMV 35S promoter restored the photoperiodic response in the se5 mutant. Our results indicate that phytochromes confer the photoperiodic control of flowering in rice. Comparison of se5 with hy1, a counterpart mutant of Arabidopsis, suggests distinct roles of phytochromes in the photoperiodic control of flowering in these two species.     

Photocontrol of petiole elongation in light-grown strawberry plants

Summary The mode of phytochrome control of elongation growth was studied in fully-green strawberry (Fragaria x Ananassa Duch.) plants. Petiole growth showed two distinct types of response to light. In one, the end-of-day response, petioles were lengthened by low-intensity far-red irradiation for 1 h immediately following the 8 h photoperiod. The response was little or no greater with prolonged exposure and less when the start of far-red was delayed. It was already evident in the first leaf to emerge after treatment began. With the development of successive leaves a second, photoperiodic, type of response appeared, in which petioles lengthened following only prolonged exposure to red, far-red, mixtures of the two, or tungsten lighting, all at low levels of intensity. As with the inhibition of flowering in previous experiments, irradiation with red light during the second half of the otherwise long dark period gave the greatest response.Abbreviations and Symbols FR far-red light - HIR high irradiance response - R red light - P_r phytochrome in the red light absorbing form - P_fr phytochrome in the far-red light absorbing form - SDP short-day plant - LDP long-day plant - PAR photosynthetically active radiation