Footprints of natural and artificial selection for photoperiod pathway genes in Oryza

Asian rice, Oryza sativa, consists of two major subspecies, indica and japonica, which are physiologically differentiated and adapted to different latitudes. Genes for photoperiod sensitivity are likely targets of selection along latitude. We examined the footprints of natural and artificial selections for four major genes of the photoperiod pathway, namely PHYTOCHROME B (PhyB), HEADING DATE 1 (Hd1), HEADING DATE 3a (Hd3a), and EARLY HEADING DATE 1 (Ehd1), by investigation of the patterns of nucleotide polymorphisms in cultivated and wild rice. Geographical subdivision between tropical and subtropical O. rufipogon was found for all of the photoperiod genes in plants divided by the Tropic of Cancer (TOC). All of these genes, except for PhyB, were characterized by the existence of clades that split a long time ago and that corresponded to latitudinal subdivisions, and revealed a likely diversifying selection. Ssp. indica showed close affinity to tropical O. rufipogon for all genes, while ssp. japonica, which has a much wider range of distribution, displayed complex patterns of differentiation from O. rufipogon, which reflected various agricultural needs in relation to crop yield. In japonica, all genes, except Hd3a, were genetically differentiated at the TOC, while geographical subdivision occurred at 31°N in Hd3a, probably the result of varying photoperiods. Many other features of the photoperiod genes revealed domestication signatures, which included high linkage disequilibrium (LD) within genes, the occurrence of frequent and recurrent non‐functional Hd1 mutants in cultivated rice, crossovers between subtropical and tropical alleles of Hd1, and significant LD between Hd1 and Hd3a in japonica and indica.     

OsELF3-1, an Ortholog of Arabidopsis EARLY FLOWERING 3, Regulates Rice Circadian Rhythm and Photoperiodic Flowering

Arabidopsis thaliana EARLY FLOWERING 3 (ELF3) as a zeitnehmer (time taker) is responsible for generation of circadian rhythm and regulation of photoperiodic flowering. There are two orthologs (OsELF3-1 and OsELF3-2) of ELF3 in rice (Oryza sativa), but their roles have not yet been fully identified. Here, we performed a functional characterization of OsELF3-1 and revealed it plays a more predominant role than OsELF3-2 in rice heading. Our results suggest OsELF3-1 can affect rice circadian systems via positive regulation of OsLHY expression and negative regulation of OsPRR1, OsPRR37, OsPRR73 and OsPRR95 expression. In addition, OsELF3-1 is involved in blue light signaling by activating EARLY HEADING DATE 1 (Ehd1) expression to promote rice flowering under short-day (SD) conditions. Moreover, OsELF3-1 suppresses a flowering repressor GRAIN NUMBER, PLANT HEIGHT AND HEADING DATE 7 (Ghd7) to indirectly accelerate flowering under long-day (LD) conditions. Taken together, our results indicate OsELF3-1 is essential for circadian regulation and photoperiodic flowering in rice.     

Photoperiodic flowering of Arabidopsis: integrating genetic and physiological approaches to characterization of the floral stimulus

In many plants the transition from vegetative growth to flowering is controlled by environmental cues. One of these cues is day length or photoperiod, which synchronizes flowering of many species with the changing seasons. Recently, advances have been made in understanding the molecular mechanisms that confer photoperiodic control of flowering and, in particular, how inductive events occurring in the leaf, where photoperiod is perceived, are linked to floral evocation that takes place at the shoot apical meristem. We discuss recent data obtained using molecular genetic approaches on the function of regulatory proteins that control flowering time in Arabidopsis thaliana. These data are compared with the results of physiological analyses of the floral transition, which were performed in a range of species and directed towards identification of the transmitted floral singals.     

Effect of varying lengths of inductive and supplementary non-inductive photoperiods on vegetative growth and flowering of Impatiens balsamina

The photoperiodic requirement for flowering in Impatiens balsaminachanges with the length of the photoperiod. Floral buds wereinitiated with two 8 hr but with four 15 hr photoperiods andflowers opened with four 8 hr but twenty-eight 15 hr photoperiods.A part of the photoperiodic requirement for floral inductionin this plant can be substituted by LDs containing 4 or morehours of darkness (10). It indicates the identical nature ofthe floral stimulus produced during the dark period, whetherit forms a part of the inductive or non-inductive cycles. Theeffect of these supplementary non-inductive photoperiodic cyclesin causing floral bud initiation also depends on the lengthof the first inductive obligatory cycle. More floral buds andflowers were produced on plants exposed to 15 hr than 8 hr photoperiods,probably due to the higher number of leaves that were producedunder the former condition of weaker induction. The shorterthe dark period in the photoperiodic cycle, the weaker the induction,the slower the rate of extension growth but the more differentiationof leaves. ¹ Present address: Department of Biology, Guru Nanak Dev University,Amritsar-143005, India. (Received November 9, 1977; )     

Control of flowering in rice through synthetic microProteins

Photoperiod‐dependent flowering in rice is regulated by HEADING DATE 1 (Hd1), which acts as both an activator and repressor of flowering in a daylength‐dependent manner. To investigate the use of microProteins as a tool to modify rice sensitivity to the photoperiod, we designed a synthetic Hd1 microProtein (Hd1miP) capable of interacting with Hd1 protein, and overexpressed it in rice. Transgenic OX‐Hd1miP plants flowered significantly earlier than wild type plants when grown in non‐inductive long day conditions. Our results show the potential of microProteins to serve as powerful tools for modulating crop traits and unraveling protein function.     

Flowering of strict photoperiodic <Emphasis Type="Italic">Nicotiana</Emphasis> varieties in non-inductive conditions by transgenic approaches

The genus Nicotiana contains species and varieties that respond differently to photoperiod for flowering time control as day-neutral, short-day and long-day plants. In classical photoperiodism studies, these varieties have been widely used to analyse the physiological nature for floral induction by day length. Since key regulators for flowering time control by day length have been identified in Arabidopsis thaliana by molecular genetic studies, it was intriguing to analyse how closely related plants in the Nicotiana genus with opposite photoperiodic requirements respond to certain flowering time regulators. SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) and FRUITFULL (FUL) are two MADS box genes that are involved in the regulation of flowering time in Arabidopsis. SOC1 is a central flowering time pathway integrator, whereas the exact role of FUL for floral induction has not been established yet. The putative Nicotiana orthologs of SOC1 and FUL, NtSOC1 and NtFUL, were studied in day-neutral tobacco Nicotiana tabacum cv Hicks, in short-day tobacco N. tabacum cv Hicks Maryland Mammoth (MM) and long-day N. sylvestris plants. Both genes were similarly expressed under short- and long-day conditions in day-neutral and short-day tobaccos, but showed a different expression pattern in N. sylvestris. Overexpression of NtSOC1 and NtFUL caused flowering either in strict short-day (NtSOC1) or long-day (NtFUL) Nicotiana varieties under non-inductive photoperiods, indicating that these genes might be limiting for floral induction under non-inductive conditions in different Nicotiana varieties.     

Photoperiodism and enzyme rhythms: Kinetic characteristics of the photoperiodic induction of Crassulacean acid metabolism

Summary The effect of photoperiod on Crassulacean acid metabolism (CAM) in Kalanchoe blossfeldiana Poellniz, cv. Tom Thumb, has characteristics similar to its effect on flowering in this plant (although these two phenomena are not causally related). The photoperiodic control of CAM is based on (a) dependance on phytochrome, (b) an endogenous circadian rhythm of sensitivity to photoperiodic signals, (c) a balance between specific positive (increase in enzyme capacity) and negative (inhibitory substances) effects of the photoperiod. Variations in malate content, capacity of phosphoenolpyruvate (PEP) carboxylase, and capacity of CAM inhibitors in young leaves were measured under photoperiodic conditions noninductive for CAM and after transfer into photoperiodic conditions inductive for CAM. Essential characteristics of the photoperiodic induction of CAM are: 1) lag time for malate accumulation; 2) after-effect of the inductive photoperiod on the malate accumulation, on the increase in PEP carboxylase capacity, and on the decrease in the level of long-day produced inhibitors; final levels of malate, enzyme capacity and inhibitor are proportional to the number of inductive day-night cycles; 3) cireadian rhythm in PEP carboxylase capacity with a fixed phase under noninductive photoperiods and a continuously shifting phase under inductive photoperiods, after complex advancing and delaying transients. Kinetic similarities indicate that photoperiodic control of different physiological functions, namely, CAM and flowering, may be achieved through similar mechanisms. Preliminary results with species of Bryophyllum and Sedum support this hypothesis. Phase relationships suggest different degrees of coupling between endogenous enzymic rhythm and photoperiod, depending on whether the plants are under long days or short days.     

The flowering response of coleus in relation to photoperiod and the circadian rhythm of leaf movement

The flowering response of Coleus frederici and Coleus blumei x C. frederici is dependent on the photoperiod; both plants have a critical day length of about 12 hr. The inductive phase, defined as the period when light signals inhibit floral development, started 10 hr after the onset of darkness under 4 and 8-hr photoperiods, and 8 hr after the onset of darkness under a 12-hr photoperiod. However, a fixed temporal relationship between the inductive phase and the minimum leaf position was observed for Coleus frederici. The inductive phase always started 5 hr after the minimum leaf position. This evidence supports the theory that a circadian clock participates in the time measurement process of photoperiodic floral induction.     

Changes in organ growth ofChenopodium rubrum due to suboptimal and multiple photoperiodic cycles with and without flowering effect

The growth changes of cotyledons, leaves, hypocotyls and roots due to photoperiodic induction in short day plantChenopodium rubrum were investigated in relation to flowering. Six-day old plants were induced by photoperiods with a different number of dark hours. We found that the degree of inhibition which occurred during induction in the growth of leaves, cotyledons and roots similarly as the stimulation of hypocotyl is proportional to the length of dark period. The photoperiods with 12, 16 and 20 dark hours bring about marked inhibition of growth and at the same time induce flowering in terminal and axillary meristems. The inhibitory effect of critical period for flowering,i.e. 8 dark hours, is not apparent in all criteria used and even the flower differentiation is retarded. The photoperiods of 4 and 6 dark hours did not affect growth and were ineffective in inducing flowering even if their number has been increased. The experiments with inductive photoperiod interrupted by light break have clearly shown that growth pattern characteristic for induced plants can be evoked in purely vegetative ones. Such statement did not exclude the possible importance of growth inhibition as a modifying factor of flower differentiation. We demonstrated that the early events of flower bud differentiation are accompanied by stimulation of leaf growth. The evaluation of growth and development of axillary buds at different nodes of insertion enabled us to quantify the photoperiodic effect and to detect the effects due to differences in dark period length not exceeding 2 hours.     

The influence of photoperiod on drought induction of dormancy in Lotus scoparius

Summary Many species of plants in Mediterranean climate regions have evolved deciduousness, causing reduced leaf area during the long summer drought characteristic of Mediterranean climates. This summer deciduous growth form has been considered a plant adaptation in Mediterranean regions allowing survival during periods of extreme water stress. Many studies have suggested the ecological importance of this growth form but few studies have examined the physiological stimulus for deciduousness.Previous data indicate that abscission in Lotus scoparius (a mediterranean California deciduous species) is influenced by both photoperiod and water stress in a complex manner. Here the physiological basis of long day enhanced leaf fall during water stress is investigated.Examination of water potential components indicate an osmotic adjustment with incresing water stress which enables the maintenance of turgor at lower water potentials. Osmotic adjustment in plants grown under long photoperiods was greater than that in short photoperiods. Therfore, long day enhanced abscission during water stress was not due to a greater susceptability to turgor loss during long days. Rather, long day treatment caused these plants to initiate dormancy (as indicated by soluble protein concentrations) during the onset of water stress. The dormant condition could not be released by subsequent release from water stress. Apparently, Lotus scoparius has evolved a photoperiodic control (presumably through growth regulators) over the initiation of dormancy during water stress. The adaptive significance of this photoperiodic control over the leaf abscission response to water stress relates to the variable climate of Mediterranean regions.     

Effect of Gibberellic Acid and Phosfon on the Critical Dark Period Reqnirement for Flowering of Impatiens balsamina cv. Rose

The critical dark period requirement for flowering of Impatiens balsamina L. cv. Rose, an obligate short day plant, is about 8.5 hours. While GA₃ completely substituted for the dark period requirement, Phosfon prolonged it to 9.5 hours. GA₃ hastened and Phosfon delayed the initiation of floral buds under all photoperiods. Floral buds opened into flowers only during 8 and 14 hour photoperiods in control and Phosfon-treated plants but during all photoperiods in GA₃-treated ones. The delay in floral bud initiation and flowering was correlated with shifting up of the node bearing the first floral bud and flower respectively. While GA₃ increased the numher of floral buds and flowers in all photoperiods except 8-hour, Phosfon increased their number in the 14-hour photoperiod only. The number of flowering plants decreased with increasing photoperiod regardless of GA₃ and Phosfon application. The effect of Phosfon was completely or partially overcome, depending upon the photoperiod, by simultaneous application of GA₃.     

INDUCER OF CBF EXPRESSION 1 integrates cold signals into FLOWERING LOCUS C‐mediated flowering pathways in Arabidopsis

Plants constantly monitor changes in photoperiod and temperature throughout the year to synchronize flowering with optimal environmental conditions. In the temperate zones, both photoperiod and temperature fluctuate in a somewhat predictable manner through the seasons, although a transient shift to low temperature is also encountered during changing seasons, such as early spring. Although low temperatures are known to delay flowering by inducing the floral repressor FLOWERING LOCUS C (FLC), it is not fully understood how temperature signals are coordinated with photoperiodic signals in the timing of seasonal flowering. Here, we show that the cold signaling activator INDUCER OF CBF EXPRESSION 1 (ICE1), FLC and the floral promoter SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) constitute an elaborate signaling network that integrates cold signals into flowering pathways. The cold‐activated ICE1 directly induces the gene encoding FLC, which represses SOC1 expression, resulting in delayed flowering. In contrast, under floral promotive conditions, SOC1 inhibits the binding of ICE1 to the promoters of the FLC gene, inducing flowering with a reduction of freezing tolerance. These observations indicate that the ICE1‐FLC‐SOC1 signaling network contributes to the fine‐tuning of flowering during changing seasons.     

The E3 Ubiquitin Ligase HOS1 Regulates Arabidopsis Flowering by Mediating CONSTANS Degradation Under Cold Stress

The timing of flowering is coordinated by a web of gene regulatory networks that integrates developmental and environmental cues in plants. Light and temperature are two major environmental determinants that regulate flowering time. Although prolonged treatment with low nonfreezing temperatures accelerates flowering by stable repression of FLOWERING LOCUS C (FLC), repeated brief cold treatments delay flowering. Here, we report that intermittent cold treatments trigger the degradation of CONSTANS (CO), a central activator of photoperiodic flowering; daily treatments caused suppression of the floral integrator FLOWERING LOCUS T (FT) and delayed flowering. Cold-induced CO degradation is mediated via a ubiquitin/proteasome pathway that involves the E3 ubiquitin ligase HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENE 1 (HOS1). HOS1-mediated CO degradation occurs independently of the well established cold response pathways. It is also independent of the light signaling repressor CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) E3 ligase and light wavelengths. CO has been shown to play a key role in photoperiodic flowering. Here, we demonstrated that CO served as a molecular hub, integrating photoperiodic and cold stress signals into the flowering genetic pathways. We propose that the HOS1-CO module contributes to the fine-tuning of photoperiodic flowering under short term temperature fluctuations, which often occur during local weather disturbances.     

Flowering in cultivars of sesame (Sesamum indicum) Differing in photoperiodic sensitivity

Studies of flowering in sesame (Sesamum indicum L.) under different photoperiods revealed that cultivars Glauca, Venezuela-52 and Oro were short day strains while a fourth cultivar, Aceitera, proved to be day length neutral. Sensitivity to varying photoperiods, then, does not seem to be a universal characteristic of all sesame cultivars. Also, no correlation was found between photoperiodic class and latitude of origin in the cultivars studied. Inheritance studies involved crosses between the day neutral cultivar and two of the short day cultivars. In all cases, the F₂ populations demonstrated continuous variation. Most of the F₂ variability was of genetic origin and heritability values ranged from 86 to 96%. Additionally, transgressive segregation was observed for both early and late flowering. It is proposed that a minimum of three loci are involved in the inheritance of photoperiodic response in the sesame cultivars studied.     

Unravelling proximate cues of mass flowering in the tropical forests of South‐East Asia from gene expression analyses

Elucidating the physiological mechanisms of the irregular yet concerted flowering rhythm of mass flowering tree species in the tropics requires long‐term monitoring of flowering phenology, exogenous and endogenous environmental factors, as well as identifying interactions and dependencies among these factors. To investigate the proximate factors for floral initiation of mast seeding trees in the tropics, we monitored the expression dynamics of two key flowering genes, meteorological conditions and endogenous resources over two flowering events of Shorea curtisii and Shorea leprosula in the Malay Peninsula. Comparisons of expression dynamics of genes studied indicated functional conservation of FLOWERING LOCUS T (FT) and LEAFY (LFY) in Shorea. The genes were highly expressed at least 1 month before anthesis for both species. A mathematical model considering the synergistic effect of cool temperature and drought on activation of the flowering gene was successful in predicting the observed gene expression patterns. Requirement of both cool temperature and drought for floral transition suggested by the model implies that flowering phenologies of these species are sensitive to climate change. Our molecular phenology approach in the tropics sheds light on the conserved role of flowering genes in plants inhabiting different climate zones and can be widely applied to dissect the flowering processes in other plant species.     

Sorghum Phytochrome B Inhibits Flowering in Long Days by Activating Expression of SbPRR37 and SbGHD7, Repressors of SbEHD1, SbCN8 and SbCN12

Light signaling by phytochrome B in long days inhibits flowering in sorghum by increasing expression of the long day floral repressors PSEUDORESPONSE REGULATOR PROTEIN (SbPRR37, Ma1) and GRAIN NUMBER, PLANT HEIGHT AND HEADING DATE 7 (SbGHD7, Ma6). SbPRR37 and SbGHD7 RNA abundance peaks in the morning and in the evening of long days through coordinate regulation by light and output from the circadian clock. 58 M, a phytochrome B deficient (phyB-1, ma3^R) genotype, flowered ∼60 days earlier than 100 M (PHYB, Ma3) in long days and ∼11 days earlier in short days. Populations derived from 58 M (Ma1, ma3^R, Ma5, ma6) and R.07007 (Ma1, Ma3, ma5, Ma6) varied in flowering time due to QTL aligned to PHYB/phyB-1 (Ma3), Ma5, and GHD7/ghd7-1 (Ma6). PHYC was proposed as a candidate gene for Ma5 based on alignment and allelic variation. PHYB and Ma5 (PHYC) were epistatic to Ma1 and Ma6 and progeny recessive for either gene flowered early in long days. Light signaling mediated by PhyB was required for high expression of the floral repressors SbPRR37 and SbGHD7 during the evening of long days. In 100 M (PHYB) the floral activators SbEHD1, SbCN8 and SbCN12 were repressed in long days and de-repressed in short days. In 58 M (phyB-1) these genes were highly expressed in long and short days. Furthermore, SbCN15, the ortholog of rice Hd3a (FT), is expressed at low levels in 100 M but at high levels in 58 M (phyB-1) regardless of day length, indicating that PhyB regulation of SbCN15 expression may modify flowering time in a photoperiod-insensitive manner.