Mutation in TERMINAL FLOWER1 reverses the photoperiodic requirement for flowering in the wild strawberry Fragaria vesca

Photoperiodic flowering has been extensively studied in the annual short-day and long-day plants rice (Oryza sativa) and Arabidopsis (Arabidopsis thaliana), whereas less is known about the control of flowering in perennials. In the perennial wild strawberry, Fragaria vesca (Rosaceae), short-day and perpetual flowering long-day accessions occur. Genetic analyses showed that differences in their flowering responses are caused by a single gene, SEASONAL FLOWERING LOCUS, which may encode the F. vesca homolog of TERMINAL FLOWER1 (FvTFL1). We show through high-resolution mapping and transgenic approaches that FvTFL1 is the basis of this change in flowering behavior and demonstrate that FvTFL1 acts as a photoperiodically regulated repressor. In short-day F. vesca, long photoperiods activate FvTFL1 mRNA expression and short days suppress it, promoting flower induction. These seasonal cycles in FvTFL1 mRNA level confer seasonal cycling of vegetative and reproductive development. Mutations in FvTFL1 prevent long-day suppression of flowering, and the early flowering that then occurs under long days is dependent on the F. vesca homolog of FLOWERING LOCUS T. This photoperiodic response mechanism differs from those described in model annual plants. We suggest that this mechanism controls flowering within the perennial growth cycle in F. vesca and demonstrate that a change in a single gene reverses the photoperiodic requirements for flowering.     

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.     

Photoperiod and Temperature Interactions in Growth and Flowering of Strawberry

Growth and flowering of strawberry cultivars were studied in controlled environments. Early cultivars adapted to marginal growing areas in Scandinavia initiated flower buds in all photoperiods including continuous light at temperatures of 12 and 18°C. At 24°C they remained vegetative in photoperiods above 14 or 16 h. The later cultivars ‘Senga Sengana’ and ‘Abundance’ did not initiate flower buds in 24-h photoperiods at any of these temperatures. Their critical photoperiod changed from above 16 h at 12°C to about 14 and 13 h at 18 and 24°C, respectively. It is concluded that at high latitudes temperature is as important as photoperiod in controlling flowering in the strawberry. Stolon formation, petiole elongation, and leaf area growth were stimulated by high temperature and long days, usually with optima at 16 h and 18°C for petiole elongation and 16 h and 24°C for stolon formation. Although growth and flowering responses in general were opposite, the results indicate that they are to some extent independent. The photoperiodic growth responses were mainly of morphogenetic nature. Dry weight of stem and leaves was little influenced by photoperiod when the irradiance was kept constant.     

Seed Germination in Amaranthus retroflexus L. as Affected by the Photoperiod and Age During Flower Induction of the Parent Plants

Seed germination in Amaranthtis retroflexus, a facultative shortday plant, was affected by the parental photoperiodic conditions.Seeds from parents grown continuously in short days (SD, 8 h)had a higher dark germination and a greater response (at 30°C) to a short irradiation or low temperature pretreatmentthan seeds from plants grown continuously in long days (LD,16 h). Daily night breaks of 1 h in the middle of the long-nightinhibited the SD induction of flowering as well as the SD promotionof germinability. Germinability of seeds produced by plantsinduced to flower in LD by 1, 2, or 3 SD was lower than thatof seeds produced by plants grown continuously in SD, and decreasedwith the age of the parent plants at the time of flower induction.     

CONSTANS delays Arabidopsis flowering under short days

Long days (LD) promote flowering of Arabidopsis thaliana compared with short days (SD) by activating the photoperiodic pathway. Here we show that growth under very‐SD (3 h) or darkness (on sucrose) also accelerates flowering on a biological scale, indicating that SD actively repress flowering compared with very‐SD. CONSTANS (CO) repressed flowering under SD, and the early flowering of co under SD required FLOWERING LOCUS T (FT). FT was expressed at a basal level in the leaves under SD, but these levels were not enhanced in co. This indicates that the action of CO in A. thaliana is not the mirror image of the action of its homologue in rice. In the apex, CO enhanced the expression of TERMINAL FLOWER 1 (TFL1) around the time when FT expression is important to promote flowering. Under SD, the tfl1 mutation was epistatic to co and in turn ft was epistatic to tfl1. These observations are consistent with the long‐standing but not demonstrated model where CO can inhibit FT induction of flowering by affecting TFL1 expression.     

Identification of flowering genes in strawberry, a perennial SD plant

Background  

We are studying the regulation of flowering in perennial plants by using diploid wild strawberry (Fragaria vesca L.) as a model. Wild strawberry is a facultative short-day plant with an obligatory short-day requirement at temperatures above 15°C. At lower temperatures, however, flowering induction occurs irrespective of photoperiod. In addition to short-day genotypes, everbearing forms of wild strawberry are known. In 'Baron Solemacher' recessive alleles of an unknown repressor, SEASONAL FLOWERING LOCUS (SFL), are responsible for continuous flowering habit. Although flower induction has a central effect on the cropping potential, the molecular control of flowering in strawberries has not been studied and the genetic flowering pathways are still poorly understood. The comparison of everbearing and short-day genotypes of wild strawberry could facilitate our understanding of fundamental molecular mechanisms regulating perennial growth cycle in plants.     

Dual Floral Induction Requirements in Phleum alpinum

Flowering requirements of four Norwegian populations of Phleumalpinum were studied in controlled environments. A dual inductionrequirement was demonstrated in all populations. Inflorescenceinitiation had an obligatory requirement for short days (SD)and/or low temperature, while culm elongation and heading wereenhanced by long days (LD) and higher temperatures. At 3 and6 °C primary induction was almost independent of photoperiod,whereas SD was more effective than LD at higher temperatures.The critical temperature for primary induction was about 15°C in SD and 12 °C in LD. Saturation of induction required12 weeks of exposure to inductive conditions, although someheading and flowering took place with 6 weeks exposure to optimalconditions (9 °C/SD). Inflorescence development also tookplace in 8 h SD although it was delayed and culm elongationwas strongly inhibited compared with LD conditions. Only smalldifferences in flowering response were found between the populations. Phleum alpinum L., alpine timothy, dual floral induction, flowering, photoperiod, temperature     

Responses to stepwise photoperiodic changes for the larval diapause of the Indian meal moth Plodia interpunctella

Abstract The Indian meal moth Plodia interpunctella Hübner (Lepidoptera: Pyralidae) diapauses as a last‐instar (fifth) larva. At 30 °C, no larvae enter diapause under any photoperiodic conditions; at 25 °C, the photoperiodic response curve is a long‐day type with a critical length of approximately 13 h light; at 20 °C, diapause is induced moderately even under long days (> 13 h). Cumulative effects of short days or long days on diapause induction are determined by alternate, stepwise and gradually changing regimes of photoperiod at 25 °C. When the larvae are repeatedly exposed to LD 16 : 8 h and LD 12 : 12 h photoperiods every other day, the incidence of diapause is 37%. When the larvae are placed under an LD 16 : 8 h photoperiod for 2 days and then under an LD 12 : 12 h photoperiod for 1 day, it is 38 %. Exposure to an LD 16 : 8 h photoperiod for 1 day and then to an LD 12 : 12 h photoperiod for 2 days induces only 15% diapause. This may indicate that the photoperiodic information is not accumulated in a simple fashion despite the generally accepted hypothesis (i.e. photoperiodic counter). Larvae exposed to an LD 16 : 8 h photoperiod for 5 days after oviposition express a very high incidence of diapause even under short days between an LD 2 : 22 h and LD 12 : 12 h photoperiod. After 10 days exposure to an LD 16 : 8 h photoperiod, however, the short day does not induce diapause strongly. On the other hand, an LD 12 : 12 h photoperiod in the early larval life is highly effective in the induction of diapause. A gradual increase or decrease of photoperiod (2 min day^?1) shows that the direction of photoperiodic change does not affect the diapause determination.     

Molecular cloning and expression analyses of FaFT,FaTFL, and FaAP1 genes in cultivated strawberry: their correlation to flower bud formation

In this study, we cloned flowering-related genes FLOWERING LOCUS T (FT) and TERMINAL FLOWER1 (TFL1) from domesticated octaploid strawberries (Fragaria × ananassa) and analyzed their expression patterns in cultivars Tochiotome and Akihime. The floral meristem generation was induced under the short day and low temperature (SDLT), but not under the long day and high temperature (LDHT). We found that FaFT1, which is an orthologue of the Arabidopsis floral activator FT, was highly expressed in leaves under LDHT but not expressed in leaves under SDLT. On the other hand, the expression of FaTFL2, which belongs to the TFL1 family of flowering repressing genes, decreased in crowns (stem tissue including meristem) under SDLT. These results suggest that FaTFL2, as opposed to FvTFL1 in wild diploid strawberry Fragaria vesca, is related to flowering of the cultivated strawberry. Moreover, the FaTFL2 expression might be regulated by temperature rather than by photoperiod. We demonstrated that a reduction of the FaTFL2 expression is a key signal for flowering in domesticated strawberries.     

Interactions of temperature and photoperiod in the control of flowering of latitudinal and altitudinal populations of wild strawberry (Fragaria vesca)

Floral induction and development requirements of a range of latitudinal and altitudinal Norwegian populations of the wild strawberry Fragaria vesca L. have been studied in controlled environments. Rooted runner plants were exposed to a range of photoperiods and temperatures for 5 weeks for floral induction and then transferred to long day (LD) at 20°C for flower development. A pronounced interaction of temperature and photoperiod was shown in the control of flowering. At 9°C, flowers were initiated in both short day (SD) and LD conditions, at 15 and 18°C in SD only, whereas no initiation took place at 21°C regardless of daylength conditions. The critical photoperiod for SD floral induction was about 16 h and 14 h at 15 and 18°C, respectively, the induction being incomplete at 18°C. The optimal condition for floral induction was SD at 15°C. A minimum of 4 weeks of exposure to such optimal conditions was required. Although the populations varied significantly in their flowering performance, no clinal relationship was present between latitude of origin and critical photoperiod. Flower development of SD-induced plants was only marginally advanced by LD conditions, while inflorescence elongation and runnering were strongly enhanced by LD at this stage. The main shift in these responses took place at photoperiods between 16 and 17 h. Unlike all other populations studied, a high-latitude population from 70°N ('Alta') had an obligatory vernalization requirement. Although flowering and fruiting in its native Subarctic environment and after overwintering in the field in south Norway, this population did not flower in the laboratory in the absence of vernalization, even with 10 or 15 weeks of exposure to SD at 9°C. Flowering performance in the field likewise indicated a vernalization requirement of this high-latitude population.     

Natural variation and CRISPR/Cas9‐mediated mutation in GmPRR37 affect photoperiodic flowering and contribute to regional adaptation of soybean

Flowering time is a critical determinant of the geographic distribution and regional adaptability of soybean (Glycine max) and is strongly regulated by photoperiod and temperature. In this study, quantitative trait locus (QTL) mapping and subsequent candidate gene analysis revealed that GmPRR37, encoding a pseudo‐response regulator protein, is responsible for the major QTL qFT12‐2, which was identified from a population of 308 recombinant inbred lines (RILs) derived from a cross between a very late‐flowering soybean cultivar, ‘Zigongdongdou (ZGDD)’, and an extremely early‐flowering cultivar, ‘Heihe27 (HH27)’, in multiple environments. Comparative analysis of parental sequencing data confirmed that HH27 contains a non‐sense mutation that causes the loss of the CCT domain in the GmPRR37 protein. CRISPR/Cas9‐induced Gmprr37‐ZGDD mutants in soybean exhibited early flowering under natural long‐day (NLD) conditions. Overexpression of GmPRR37 significantly delayed the flowering of transgenic soybean plants compared with wild‐type under long photoperiod conditions. In addition, both the knockout and overexpression of GmPRR37 in soybean showed no significant phenotypic alterations in flowering time under short‐day (SD) conditions. Furthermore, GmPRR37 down‐regulated the expression of the flowering‐promoting FT homologues GmFT2a and GmFT5a, and up‐regulated flowering‐inhibiting FT homologue GmFT1a expression under long‐day (LD) conditions. We analysed haplotypes of GmPRR37 among 180 cultivars collected across China and found natural Gmprr37 mutants flower earlier and enable soybean to be cultivated at higher latitudes. This study demonstrates that GmPRR37 controls soybean photoperiodic flowering and provides opportunities to breed optimized cultivars with adaptation to specific regions and farming systems.     

Interactive effects of photoperiod and temperature on diapause induction and termination in the yellow-spotted longicorn beetle, Psacothea hilaris

Abstract. The interactive effects of temperature (20 °C or 25 °C) and photoperiod (LD 12 : 12 h or LD 15 : 9 h) on diapause induction and termination are investigated in the west‐Japan type yellow‐spotted longicorn beetle, Psacothea hilaris (Pascoe) (Coleoptera: Cerambycidae). Larval diapause of P. hilaris is induced under three diapause‐inducing conditions (20 °C–SD, 20 °C–LD and 25 °C–SD), and the diapause larvae are transferred to one of four conditions (20 °C–SD, 20 °C–LD, 25 °C–SD or 25 °C–LD) for observation of pupation, which indicates termination of diapause. The intensity of diapause induced under the three conditions increases in the order 20 °C–SD < 25 °C–SD < 20 °C–LD, when assessed by the time course of pupation after the transfer. On the other hand, the effectiveness of the temperature–photoperiod combinations to terminate diapause is in the order 25 °C–SD (ineffective) < < 20 °C–LD < 25 °C–LD < 20 °C–SD. Among the temperatures (5, 10, 15 and 20 °C) examined, 15 °C is the most effective in terminating diapause under the short day; diapause in most larvae appears to have been completed in 15 days.     

Changes in mRNA level rhythmicity in the leaves ofSinapis alba during a lengthening of the photoperiod which induces flowering

A previous study has shown that mRNAs exhibit complex patterns of diurnal rhythms in their quantity in the leaves ofSinapis alba during an 8 h light/16 h dark short day (SD). In order to determine whether this situation is rapidly modified in plants subjected to an extended light treatment, we have usedin vitro translation and two-dimensional polyacrylamide gel electrophoresis, together with a strict gel comparison procedure giving aP=0.03 certitude level, to analyse the mRNA complement at different times during a 22 h light/2 h dark long day (LD).During this LD, complex changes affected about 10% of the mRNAs. Thirty-four different patterns were observed. Some diurnal rhythms present in SD are not modified by the lengthening of the light period, but most are affected. Moreover, we have shown that some mRNAs presenting a constant quantity under a SD regime show an increase or a decrease during the first hours of the photoperiod lengthening.InSinapis, this LD also induces flowering. All the changes in mRNA quantity detected thus parallel the photoperiodic induction of flowering in the leaves and are quantitative; no mRNA was shown to appear or to disappear.     

Effect of Photoperiod on the Regulation of Wheat Vernalization Genes VRN1 and VRN2

Wheat is usually classified as a long day (LD) plant because most varieties flower earlier when exposed to longer days. In addition to LD, winter wheats require a long exposure to low temperatures (vernalization) to become competent for flowering. Here we show that in some genotypes this vernalization requirement can be replaced by interrupting the LD treatment by 6 weeks of short day (SD), and that this replacement is associated with the SD down-regulation of the VRN2 flowering repressor. In addition, we found that SD down-regulation of VRN2 at room temperature is not followed by the up-regulation of the meristem identity gene VRN1 until plants are transferred to LD. This result contrasts with the VRN1 up-regulation observed after the VRN2 down-regulation by vernalization, suggesting the existence of a second VRN1 repressor. Analysis of natural VRN1 mutants indicated that a CArG-box located in the VRN1 promoter is the most likely regulatory site for the interaction with this second repressor. Up-regulation of VRN1 under SD in accessions carrying mutations in the CArG-box resulted in an earlier initiation of spike development, compared to other genotypes. However, even the genotypes with CArG box mutations required LD for a normal and timely spike development. The SD acceleration of flowering was observed in photoperiod sensitive winter varieties. Since vernalization requirement and photoperiod sensitivity are ancestral traits in Triticeae species we suggest that wheat was initially a SD–LD plant and that strong selection pressures during domestication and breeding resulted in the modification of this dual regulation. The down-regulation of the VRN2 repressor by SD is likely part of the mechanism associated with the SD–LD regulation of flowering in photoperiod sensitive winter wheat. These authors contributed equally to this work     

Genome-wide analysis of gene expression to distinguish photoperiod-dependent and -independent flowering in Brassicaceae

Photoperiod is the most important environmental cue for the regulation of flowering time, a highly important agronomic trait for crop productivity. To help elucidate the photoperiodic control of flowering in Brassicaceae, we performed microarray experiments using species-specific oligo-arrays with the long day (LD) plant Arabidopsis thaliana and the photoperiod-independent plant rapid cycling Brassica rapa (RCBr). Enrichment analysis of the gene ontologies of differentially expressed genes (DEGs) did not uncover clear differences in gene expression between photoperiod-dependent and -independent plants. Most genes that were up-regulated under LD conditions in Arabidopsis were also up-regulated in RCBr. In addition, most genes associated with light signaling and the circadian clock showed similar expression patterns between Arabidopsis and RCBr, implying that most components known to be key regulators in the photoperiodic flowering pathway are not responsible for the photoperiod independence of RCBr. Nonetheless, we identified one clock-associated gene, PSEUDO-RESPONSE REGULATOR9 (PRR9), as a candidate gene explaining the photoperiod independence of RCBr. The mechanism underlying the role of PRR9 in photoperiodic control and genomic polymorphisms should be further explored using different B. rapa species.     

Role of VIN3-LIKE 2 in facultative photoperiodic flowering response in Arabidopsis

In Arabidopsis, expression of FLC and FLC-related genes (collectively called FLC clade) contributes to flowering time in response to environmental changes, such as day length and temperature, by acting as floral repressors. VIN3 is required for vernalization-mediated FLC repression and a VIN3 related protein, VIN3-LIKE 1/VERNALIZATION 5 (VIL1/VRN5), acts to regulate FLC and FLM in response to vernalization.^1–3 VIN3 also exists as a small family of PHD finger proteins in Arabidopsis, including VIL1/VRN5, VIL2/VEL1, VIL3/VEL2 and VIL4/VEL3. We showed that the PHD finger protein, VIL2, is required for proper repression of MAF5, an FLC clade member, to accelerate flowering under non-inductive photoperiods. VIL2 acts together with POLYCOMB REPRESSIVE COMPLEX 2 (PRC2) to repress MAF5 in a photoperiod dependent manner.Key words: photoperiod, chromatin, floweringThe decision to flower is critical to the survival of flowering plants. Thus, plants sense environmental cues to initiate floral transition at a time that both ensures and optimizes their own reproductive fitness. Using a model plant, Arabidopsis thaliana, genetic studies have shown that the regulation of floral transition mainly consists of four genetic pathways: the inductive photoperiod pathway, the autonomous pathway, the vernalization pathway and the gibberellin pathway.⁴ In Arabidopsis, these four flowering pathways eventually merge into a group of genes called floral integrators, including FLOWERING LOCUS T (FT), SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) and LEAFY (LFY). Based on the response to specific photoperiod conditions, the flowering behaviors of plants can be classified into three groups: long day (LD), short day (SD) and day neutral response.^5,6 Depending on the requirement of day length, plants show either obligate or facultative responses. For example, henbane, carnation and ryegrass are obligate long day (LD) flowering plants which flower under increasing inductive photoperiod but do not flower at all under non-inductive photoperiod.⁵ On the other hand, plants including Arabidopsis, wheat, lettuce and barley, are considered to be facultative flowering plants. Thus, these plants exhibit early flowering under LD and late-flowering under non-inductive short days (SD). Studies on photoperiodic flowering time mainly focus on the inductive LD-photoperiod pathway in Arabidopsis.