Florigen in rice: complex gene network for florigen transcription, florigen activation complex, and multiple functions

Methyltransferase 1 (OsMTS1) interacts with hydroxycinnamoyltransferase 1 (OsHCT1) and promotes heading by upregulating heading date 1 (Hd1)

Heading date determines the distribution and yield potentials of rice, and is an ideal target for crop improvement using CRISPR/Cas9 genome editing system. In this study, we reported the loss-of-function of Methyltransferase 1 (MTS1), which promotes heading in rice. Here, we constructed knockouts and overexpression transgenic plants of OsMTS1 in ZH8015 and Nipponbare (NIP) for the first time to validate its heading date function in rice subspecies Oryza sativa ssp. Indica and O. Sativa ssp. Japonica, respectively. The OsMTS1 knockouts in ZH8015 and NIP rice significantly promoted heading date under both natural short days (NSD) and natural long days (NLD) conditions, while the overexpression of OsMTS1 significantly delayed heading date in ZH8015 and NIP rice under both NSD and NLD conditions. Likewise, the complementation transgenic plants displayed late heading date phenotype. OsMTS1 repressed heading through up-regulating Heading date 1 (Hd1) and down-regulating Early heading date 1 (Ehd1) and Heading date 3a (Hd3a). The OsMTS1 protein interacted with OsHCT1 proteins using a yeast two-hybrid (Y2H) assay. The Y2H and overexpression confirmed that OsMTS1 interacted with OsHCT1, which delayed heading by 4.7 days under NLD. Taken together, CRISPR/Cas9, genetic complementation, and overexpression results validated that OsMTS1 represses heading in Indica and Japonica rice under both NLD and NSD conditions. These results demonstrated that OsMTS1 is a useful target for breeding early maturing rice varieties by CRISPR/Cas9 gene editing of the functional allele.

Exogenous abscisic acid represses rice flowering via SAPK8-ABF1-Ehd1/Ehd2 pathway

INTRODUCTION

Rice flowering is a major agronomic trait, determining yield and ecological adaptability in particular regions. ABA plays an essential role in rice flowering, but the underlying molecular mechanism remains largely elusive.

OBJECTIVES

In this study, we demonstrated a "SAPK8-ABF1-Ehd1/Ehd2" pathway, through which exogenous ABA represses rice flowering in a photoperiod-independent manner.

METHODS

We generated abf1 and sapk8 mutants using the CRISPR-Cas9 method. Using yeast two-hybrid, Pull down, BiFC and kinase assays, SAPK8 interacted and phosphorylated ABF1. ABF1 directly bound to the promoters of Ehd1 and Ehd2 using ChIP-qPCR, EMSA, and LUC transient transcriptional activity assay, and suppressed the transcription of these genes.

RESULTS

Under both long day and short day conditions, simultaneous knock-out of ABF1 and its homolog bZIP40 accelerated flowering, while SAPK8 and ABF1 over-expression lines exhibited delayed flowering and hypersensitivity to ABA-mediated flowering repression. After perceiving the ABA signal, SAPK8 physically binds to and phosphorylates ABF1 to enhance its binding to the promoters of master positive flowering regulators Ehd1 and Ehd2. Upon interacting with FIE2, ABF1 recruited PRC2 complex to deposit H3K27me3 suppressive histone modification on Ehd1 and Ehd2 to suppress these genes transcription, thereby leading to later flowering.

CONCLUSION

Our work highlighted the biological functions of SAPK8 and ABF1 in ABA signaling, flowering control and the involvement of a PRC2-mediated epigenetic repression mechanism in the transcription regulation governed by ABF1 on ABA-mediated rice flowering repression.

Flowering time: From physiology, through genetics to mechanism

Plant species have evolved different requirements for environmental/endogenous cues to induce flowering. Originally, these varying requirements were thought to reflect the action of different molecular mechanisms. Thinking changed when genetic and molecular analysis in Arabidopsis thaliana revealed that a network of environmental and endogenous signaling input pathways converge to regulate a common set of "floral pathway integrators." Variation in the predominance of the different input pathways within a network can generate the diversity of requirements observed in different species. Many genes identified by flowering time mutants were found to encode general developmental and gene regulators, with their targets having a specific flowering function. Studies of natural variation in flowering were more successful at identifying genes acting as nodes in the network central to adaptation and domestication. Attention has now turned to mechanistic dissection of flowering time gene function and how that has changed during adaptation. This will inform breeding strategies for climate-proof crops and help define which genes act as critical flowering nodes in many other species.

The Function of Florigen in the Vegetative-to-Reproductive Phase Transition in and around the Shoot Apical Meristem

Plants undergo a series of developmental phases throughout their life-cycle, each characterized by specific processes. Three critical features distinguish these phases: the arrangement of primordia (phyllotaxis), the timing of their differentiation (plastochron) and the characteristics of the lateral organs and axillary meristems. Identifying the unique molecular features of each phase, determining the molecular triggers that cause transitions and understanding the molecular mechanisms underlying these transitions are keys to gleaning a complete understanding of plant development. During the vegetative phase, the shoot apical meristem (SAM) facilitates continuous leaf and stem formation, with leaf development as the hallmark. The transition to the reproductive phase induces significant changes in these processes, driven mainly by the protein FT (FLOWERING LOCUS T) in Arabidopsis and proteins encoded by FT orthologs, which are specified as 'florigen'. These proteins are synthesized in leaves and transported to the SAM, and act as the primary flowering signal, although its impact varies among species. Within the SAM, florigen integrates with other signals, culminating in developmental changes. This review explores the central question of how florigen induces developmental phase transition in the SAM. Future research may combine phase transition studies, potentially revealing the florigen-induced developmental phase transition in the SAM.

Life-Cycle Multiomics of Rice Shoots Reveals Growth Stage-Specific Effects of Drought Stress and Time-Lag Drought Responses

Field-grown rice plants are exposed to various stresses at different stages of their life cycle, but little is known about the effects of stage-specific stresses on phenomes and transcriptomes. In this study, we performed integrated time-course multiomics on rice at 3-d intervals from seedling to heading stage under six drought conditions in a well-controlled growth chamber. Drought stress at seedling and reproductive stages reduced yield performance by reducing seed number and setting rate, respectively. High temporal resolution analysis revealed that drought response occurred in two steps: a rapid response via the abscisic acid (ABA) signaling pathway and a slightly delayed DEHYDRATION-RESPONSIVE ELEMENT-BINDING PROTEIN (DREB) pathway, allowing plants to respond flexibly to deteriorating soil water conditions. Our long-term time-course multiomics showed that temporary drought stress delayed flowering due to prolonged expression of the flowering repressor gene GRAIN NUMBER, PLANT HEIGHT AND HEADING DATE 7 (Ghd7) and delayed expression of the florigen genes HEADING DATE 3a (Hd3a) and RICE FLOWERING LOCUS T 1 (RFT1). Our life-cycle multiomics dataset on rice shoots under drought conditions provides a valuable resource for further functional genomic studies to improve crop resilience to drought stress.

COL2-dependent photoperiodic floral induction in Nicotiana sylvestris seems to be lost in the N. sylvestris × N. tomentosiformis hybrid N. tabacum

Introduction

Plants are sessile organisms that maximize reproductive success by adapting to their environment. One of the key steps in the reproductive phase of angiosperms is flower development, requiring the perception of multiple endogenous and exogenous signals integrated via a complex regulatory network. Key floral regulators, including the main transcription factor of the photoperiodic pathway (CONSTANS, CO) and the central floral pathway integrator (FLOWERING LOCUS T, FT), are known in many species.

Methods and results

We identified several CO-like (COL) proteins in tobacco (Nicotiana tabacum). The NtCOL2a/b proteins in the day-neutral plant N. tabacum were most closely related to Arabidopsis CO. We characterized the diurnal expression profiles of corresponding genes in leaves under short-day (SD) and long-day (LD) conditions and confirmed their expression in phloem companion cells. Furthermore, we analyzed the orthologs of NtCOL2a/b in the maternal LD ancestor (N. sylvestris) and paternal, facultative SD ancestor (N. tomentosiformis) of N. tabacum and found that they were expressed in the same diurnal manner. NtCOL2a/b overexpression or knock-out using the CRISPR/Cas9 system did not support a substantial role for the CO homologs in the control of floral transition in N. tabacum. However, NsCOL2 overexpression induced flowering in N. sylvestris under typically non-inductive SD conditions, correlating with the upregulation of the endogenous NsFTd gene.

Discussion

Our results suggest that NsFTd is transcriptionally regulated by NsCOL2 and that this COL2-dependent photoperiodic floral induction seems to be lost in N. tabacum, providing insight into the diverse genetics of photoperiod-dependent flowering in different Nicotiana species.

Deleterious mutations predicted in the sorghum (Sorghum bicolor) Maturity (Ma) and Dwarf (Dw) genes from whole-genome resequencing

In sorghum [Sorghum bicolor (L.) Moench] the Maturity (Ma1, Ma2, Ma3, Ma4, Ma5, Ma6) and Dwarf (Dw1, Dw2, Dw3, Dw4) loci, encode genes controlling flowering time and plant height, respectively, which are critical for designing sorghum ideotypes for a maturity timeframe and a harvest method. Publicly available whole-genome resequencing data from 860 sorghum accessions was analyzed in silico to identify genomic variants at 8 of these loci (Ma1, Ma2, Ma3, Ma5, Ma6, Dw1, Dw2, Dw3) to identify novel loss of function alleles and previously characterized ones in sorghum germplasm. From ~ 33 million SNPs and ~ 4.4 million InDels, 1445 gene variants were identified within these 8 genes then evaluated for predicted effect on the corresponding encoded proteins, which included newly identified mutations (4 nonsense, 15 frameshift, 28 missense). Likewise, most accessions analyzed contained predicted loss of function alleles (425 ma1, 22 ma2, 40 ma3, 74 ma5, 414 ma6, 289 dw1, 268 dw2 and 45 dw3) at multiple loci, but 146 and 463 accessions had no predicted ma or dw mutant alleles, respectively. The ma and dw alleles within these sorghum accessions represent a valuable source for manipulating flowering time and plant height to develop the full range of sorghum types: grain, sweet and forage/biomass.

Identification, characterization, and comprehensive expression profiling of floral master regulators in pigeon pea (Cajanus cajan [L.] Millspaugh)

Pigeon pea is an important protein-rich pulse crop. Identification of flowering master regulators in pigeon pea is highly imperative as indeterminacy and late flowering are impediments towards yield improvement. A genome-wide analysis was performed to explore flowering orthologous groups in pigeon pea. Among the 412 floral orthologs identified in pigeon pea, 148 genes belong to the meristem identity, photoperiod-responsive, and circadian clock-associated ortholog groups. Our comparative genomics study revealed purifying selection pressures (ka/ks) on floral orthologs, and duplication patterns and evolution through synteny with other model species. Phylogenetic analysis of floral genes substantiated a connection between pigeon pea plant architecture and flowering time as all the PEBP domain-containing genes belong to meristem identity floral networks of pigeon pea. Expression profiling of eleven major orthologs in contrasting determinate and indeterminate genotypes indicated that these orthologs might be involved in flowering regulation. Expression of floral inducer, FT, and floral repressor, TFL1, was non-comparable in indeterminate genotypes across all the developmental stages of pigeon pea. However, dynamic FT/TFL1 expression ratio detected in all tissues of both the genotypes suggested their role in floral transition. One TFL1 ortholog having high sequence conserveness across pigeon pea genotypes showed differential expression indicating genotype-dependent regulation of this ortholog. Presence of conserved 6mA-methylation patterns in light-responsive elements and in other cis-regulatory elements of FT and TFL1 across different plant genotypes indicated possible involvement of epigenetic regulation in flowering.

Rice florigens control a common set of genes at the shoot apical meristem including the F-BOX BROADER TILLER ANGLE 1 that regulates tiller angle and spikelet development

Rice flowering is triggered by transcriptional reprogramming at the shoot apical meristem (SAM) mediated by florigenic proteins produced in leaves in response to changes in photoperiod. Florigens are more rapidly expressed under short days (SDs) compared to long days (LDs) and include the HEADING DATE 3a (Hd3a) and RICE FLOWERING LOCUS T1 (RFT1) phosphatidylethanolamine binding proteins. Hd3a and RFT1 are largely redundant at converting the SAM into an inflorescence, but whether they activate the same target genes and convey all photoperiodic information that modifies gene expression at the SAM is currently unclear. We uncoupled the contribution of Hd3a and RFT1 to transcriptome reprogramming at the SAM by RNA sequencing of dexamethasone-inducible over-expressors of single florigens and wild-type plants exposed to photoperiodic induction. Fifteen highly differentially expressed genes common to Hd3a, RFT1, and SDs were retrieved, 10 of which still uncharacterized. Detailed functional studies on some candidates revealed a role for LOC_Os04g13150 in determining tiller angle and spikelet development and the gene was renamed BROADER TILLER ANGLE 1 (BRT1). We identified a core set of genes controlled by florigen-mediated photoperiodic induction and defined the function of a novel florigen target controlling tiller angle and spikelet development.

OsFTL12, a member of FT-like family, modulates the heading date and plant architecture by florigen repression complex in rice

FLOWERING LOCUS T (FT), a florigen in Arabidopsis, plays critical roles in floral transition. Among 13 FT-like members in rice, OsFTL2 (Hd3a) and OsFTL3 (RFT1), two rice homologues of FT, have been well characterized to act as florigens to induce flowering under short-day (SD) and long-day (LD) conditions, respectively, but the functions of other rice FT-like members remain largely unclear. Here, we show that OsFTL12 plays an antagonistic function against Hd3a and RFT1 to modulate the heading date and plant architecture in rice. Unlike Hd3a and RFT1, OsFTL12 is not regulated by daylength and highly expressed in both SD and LD conditions, and delays the heading date under either SD or LD conditions. We further demonstrate that OsFTL12 interacts with GF14b and OsFD1, two key components of the florigen activation complex (FAC), to form the florigen repression complex (FRC) by competing with Hd3a for binding GF14b. Notably, OsFTL12-FRC can bind to the promoters of the floral identity genes OsMADS14 and OsMADS15 and suppress their expression. The osmads14 osmads15 double mutants could not develop panicles and showed erect leaves. Taken together, our results reveal that different FT-like members can fine-tune heading date and plant architecture by regulating the balance of FAC and FRC in rice.

Two interacting basic helix-loop-helix transcription factors control flowering time in rice

Flowering time is one of the most important agronomic traits affecting the adaptation and yield of rice (Oryza sativa). Heading date 1 (Hd1) is a key factor in the photoperiodic control of flowering time. In this study, two basic helix-loop-helix (bHLH) transcription factors, Hd1 Binding Protein 1 (HBP1) and Partner of HBP1 (POH1) were identified as transcriptional regulators of Hd1. We generated knockout mutants of HBP1 and ectopically expressed transgenic lines of the two bHLH transcription factors and used these lines to investigate the roles of these two factors in regulating flowering time. HBP1 physically associated with POH1 forming homo- or heterodimers to perform their functions. Both HBP1 and POH1 bound directly to the cis-acting elements located in the promoter of Hd1 to activate its expression. CRISPR/Cas9-generated knockout mutations of HBP1, but not POH1 mutations, promoted earlier flowering time; conversely, HBP1 and POH1 overexpression delayed flowering time in rice under long-day and short-day conditions by activating the expression of Hd1 and suppressing the expression of Early heading date 1 (Ehd1), Heading date 3a (Hd3a), and Rice Flowering locus T 1 (RFT1), thus controlling flowering time in rice. Our findings revealed a mechanism for flowering time control through transcriptional regulation of Hd1 and laid theoretical and practical foundations for improving the growth period, adaptation, and yield of rice.

High ambient temperature impacts on flowering time in Brassica napus through both H2A.Z-dependent and independent mechanisms

Knowledge concerning the integration of genetic pathways mediating the responses to environmental cues controlling flowering initiation in crops is scarce. Here, we reveal the diversity in oilseed rape (OSR) flowering response to high ambient temperature. Using a set of different spring OSR varieties, we found a consistent flowering delay at elevated temperatures. Remarkably, one of the varieties assayed exhibited the opposite behaviour. Several FT-like paralogs are plausible candidates to be part of the florigen in OSR. We revealed that BnaFTA2 plays a major role in temperature-dependent flowering initiation. Analysis of the H2A.Z histone variant occupancy at this locus in different Brassica napus varieties produced contrasting results, suggesting the involvement of additional molecular mechanisms in BnaFTA2 repression at high ambient temperature. Moreover, BnARP6 RNAi plants showed little accumulation of H2A.Z at high temperature while maintaining temperature sensitivity and delayed flowering. Furthermore, we found that H3K4me3 present in BnaFTA2 under inductive flowering conditions is reduced at high temperature, suggesting a role for this hallmark of transcriptionally active chromatin in the OSR flowering response to warming. Our work emphasises the plasticity of flowering responses in B. napus and offers venues to optimise this process in crop species grown under suboptimal environmental conditions.

BOS1 is a basic helix-loop-helix transcription factor involved in regulating panicle development in rice

Panicle development is crucial to increase the grain yield of rice (Oryza sativa). The molecular mechanisms of the control of panicle development in rice remain unclear. In this study, we identified a mutant with abnormal panicles, termed branch one seed 1-1 (bos1-1). The bos1-1 mutant showed pleiotropic defects in panicle development, such as the abortion of lateral spikelets and the decreased number of primary panicle branches and secondary panicle branches. A combined map-based cloning and MutMap approach was used to clone BOS1 gene. The bos1-1 mutation was located in chromosome 1. A T-to-A mutation in BOS1 was identified, which changed the codon from TAC to AAC, resulting in the amino acid change from tyrosine to asparagine. BOS1 gene encoded a grass-specific basic helix-loop-helix transcription factor, which is a novel allele of the previously cloned LAX PANICLE 1 (LAX1) gene. Spatial and temporal expression profile analyses showed that BOS1 was expressed in young panicles and was induced by phytohormones. BOS1 protein was mainly localized in the nucleus. The expression of panicle development-related genes, such as OsPIN2, OsPIN3, APO1, and FZP, was changed by bos1-1 mutation, suggesting that the genes may be the direct or indirect targets of BOS1 to regulate panicle development. The analysis of BOS1 genomic variation, haplotype, and haplotype network showed that BOS1 gene had several genomic variations and haplotypes. These results laid the foundation for us to further dissect the functions of BOS1.

Genome-wide identification and expression analysis disclose the pivotal PHOSPHATIDYLETHANOLAMINE BINDING PROTEIN members that may be utilized for yield improvement of Chenopodium quinoa

Quinoa (Chenopodium quinoa) is a prospective orphan crop that needs yield improvement. Previous studies indicate PHOSPHATIDYLETHANOLAMINE BINDING PROTEIN (PEBP) family genes are highly associated with the key agronomic traits of crops. Characterizing the pivotal PEBP genes will speed up the domestication and yield improvement of quinoa. Previous investigations on PEBP genes of Chenopodium species indicated that, the PEBP genes, despite in the same subclade, may have experienced functional diversification. Especially, the allotetraploidy (AABB) and numerous segmental duplications and chromosomal rearrangements in quinoa make it more difficult to understand the functions of PEBP genes. More recently, 6 quinoa FT subfamily genes were predicted to be related to flowering of quinoa. However, investigation on the whole PEBP family members is still lacking. In this study, we obtained 23 PEBP genes, including 5 MFT, 11 FTL and 7 TFL genes. We found 7 orthologous gene pairs, from sub-genome A and sub-genome B, respectively, showing collinearities with sugar beet. Evolution analysis on PEBP genes of two quinoa sub-genomes, sugar beet and relatives of diploid ancestors indicated that, the reasons for gene duplication events varied and 4 tandem duplications are the major reason for PEBP family expansion. Tissue-specific expression analysis suggested that expression patterns are mostly differing between orthologous gene pairs. Analysis on gene expressions at 6 stages suggested the possible positive roles of CqFTL1/CqFTL2, CqFTL5, CqFTL8, CqFTL6/CqFTL9 and CqTFL6/CqTFL7, and negative roles of CqTFL1/CqTFL2/CqTFL3, CqTFL4/CqTFL5 in inflorescence branching. Expression analysis in ABA-treated seed, in combination with the cis-acting element distribution analysis, indicated that CqMFT2, CqMFT3 and CqMFT4 may regulate seed germination via ABA signaling. Observations on responses to night break and photoperiod changes highlighted the roles of CqFTL5 and CqFTL8 under short day, and CqFTL6 under long day for quinoa flowering. Further, co-expression network analysis indicated that 64 transcription factors may act upstream of CqFTL5 and CqFTL8 to regulate flowering. Together, this study will help us identify the pivotal PEBP genes that may be utilized for quinoa breeding in future.

Comparative phylogenomic analyses and co-expression gene network reveal insights in flowering time and aborted meiosis in woody bamboo, Bambusa oldhamii 'Xia Zao' ZSX

Woody bamboos have peculiar flowering characteristics with intervals ranging from several years to more than 100 years. Elucidating flowering time and reproductive development in bamboo could be beneficial for both humans and wildlife. To identity the mechanisms responsible for flowering time and embryo abortion in Bambusa oldhamii 'Xia Zao' ZSX, a transcriptome sequencing project was initiated to characterize the genes involved in developing flowers in this bamboo species. Morphological studies showed that pollen abortion in this bamboo species was mainly caused by a delay in tapetum degradation and abnormal meiotic process. Differential expression (DE) and optimized hierarchical clustering analyses identified three of nine gene expression clusters with decreasing expression at the meiosis of flowering stages. Together with enriched Gene Ontology Biological Process terms for meiosis, this suggests that their expression pattern may be associated with aborted meiosis in B. oldhamii 'Xia Zao'. Moreover, our large-scale phylogenomic analyses comparing meiosis-related transcripts of B. oldhamii 'Xia Zao' with well annotated genes in 22 representative angiosperms and sequence evolution analyses reveal two core meiotic genes NO EXINE FORMATION 1 (NFE1) and PMS1 with nonsense mutations in their coding regions, likely providing another line of evidence supporting embryo abortion in B. oldhamii 'Xia Zao'. Similar analyses, however, reveal conserved sequence evolution in flowering pathways such as LEAFY (LFY) and FLOWERING LOCUS T (FT). Seventeen orthogroups associated with flowering were identified by DE analyses between nonflowering and flowering culm buds. Six regulators found primarily in several connected network nodes of the photoperiod pathway were confirmed by mapping to the flowering time network in rice, such as Heading date (Hd3a) and Rice FT-like 1 (RFT1) which integrate upstream signaling into the downstream effectors. This suggests the existence of an intact photoperiod pathway is likely the key regulators that switch on/off flowering in B. oldhamii 'Xia Zao'.

The tetratricopeptide repeat protein OsTPR075 promotes heading by regulating florigen transport in rice

Rice (Oryza sativa) is one of the most important crops worldwide. Heading date is a vital agronomic trait that influences rice yield and adaption to local conditions. Hd3a, a proposed florigen that primarily functions under short-day (SD) conditions, is a mobile flowering signal that promotes the floral transition in rice. Nonetheless, how Hd3a is transported from leaves to the shoot apical meristem (SAM) under SDs remains elusive. Here, we report that FT-INTERACTING PROTEIN9 (OsFTIP9) specifically regulates rice flowering time under SDs by facilitating Hd3a transport from companion cells (CCs) to sieve elements (SEs). Furthermore, we show that the tetratricopeptide repeat (TPR) protein OsTPR075 interacts with both OsFTIP9 and OsFTIP1 and strengthens their respective interactions with Hd3a and the florigen RICE FLOWERING LOCUS T1 (RFT1). This in turn affects the trafficking of Hd3a and RFT1 to the SAM, thus regulating flowering time under SDs and long-day conditions, respectively. Our findings suggest that florigen transport in rice is mediated by different OsFTIPs under different photoperiods and those interactions between OsTPR075 and OsFTIPs are essential for mediating florigen movement from leaves to the SAM.

Modified photoperiod response of CsFT promotes day neutrality and early flowering in cultivated cucumber

Map-based cloning and photoperiod response detection suggested that CsFT is the critical gene for cucumber photoperiod domestication. Photoperiod sensitivity is important for sensing seasonal changes and local adaptation. However, day-length sensitivity limits crop geographical adaptation and it should be modified during domestication. Cucumber was domesticated in southern Asia and is currently cultivated worldwide across a wide range of latitudes, but its photoperiod sensitivity and its change during cucumber domestication are unknown. Here, we confirmed wild cucumber (Hardwickii) was a short-day plant, and its flowering depends on short-day (SD) conditions, while the cultivated cucumber (9930) is a day-neutral plant that flowers independently of day length. A photoperiod sensitivity locus (ps-1) was identified by the 9930 × Hardwickii F₂ segregating populations, which span a ~ 970 kb region and contain 60 predicted genes. RNA-seq analysis showed that the critical photoperiod pathway gene FLOWERING LOCUS T (CsFT) within the ps-1 locus exhibits differential expression between 9930 and Hardwickii, which was confirmed by qRT-PCR detection. CsFT in Hardwickii was sensitive to day length and could be significantly induced by SD conditions, whereas CsFT was highly expressed in 9930 and was insensitive to day length. Moreover, the role of CsFT in promoting flowering was verified by overexpression of CsFT in Arabidopsis. We also identified the genetic variations existing in the promoter of CsFT among the different geographic cucumbers and suggest they have possible roles in photoperiod domestication. The results of this study suggest that a variation in photoperiod sensitivity of CsFT is associated with day neutrality and early flowering in cultivated cucumber and could contribute to cucumber cultivation in diverse regions throughout the world.

The high concentrations of abscisic, jasmonic, and salicylic acids produced under long days do not accelerate flowering in Chenopodium ficifolium 459

The survival and adaptation of angiosperms depends on the proper timing of flowering. The weedy species Chenopodium ficifolium serves as a useful diploid model for comparing the transition to flowering with the important tetraploid crop Chenopodium quinoa due to the close phylogenetic relationship. The detailed transcriptomic and hormonomic study of the floral induction was performed in the short-day accession C. ficifolium 459. The plants grew more rapidly under long days but flowered later than under short days. The high levels of abscisic, jasmonic, and salicylic acids at long days were accompanied by the elevated expression of the genes responding to oxidative stress. The increased concentrations of stress-related phytohormones neither inhibited the plant growth nor accelerated flowering in C. ficifolium 459 at long photoperiods. Enhanced content of cytokinins and the stimulation of cytokinin and gibberellic acid signaling pathways under short days may indicate the possible participation of these phytohormones in floral initiation. The accumulation of auxin metabolites suggests the presence of a dynamic regulatory network in C. ficifolium 459.

A secretory phospholipase D hydrolyzes phosphatidylcholine to suppress rice heading time

Phospholipase D (PLD) hydrolyzes membrane phospholipids and is crucial in various physiological processes and transduction of different signals. Secretory phospholipases play important roles in mammals, however, whose functions in plants remain largely unknown. We previously identified a rice secretory PLD (spPLD) that harbors a signal peptide and here we reported the secretion and function of spPLD in rice heading time regulation. Subcellular localization analysis confirmed the signal peptide is indispensable for spPLD secretion into the extracellular spaces, where spPLD hydrolyzes substrates. spPLD overexpression results in delayed heading time which is dependent on its secretory character, while suppression or deficiency of spPLD led to the early heading of rice under both short-day and long-day conditions, which is consistent with that spPLD overexpression/suppression indeed led to the reduced/increased Hd3a/RFT1 (Arabidopsis Flowing Locus T homolog) activities. Interestingly, rice Hd3a and RFT1 bind to phosphatidylcholines (PCs) and a further analysis by lipidomic approach using mass spectrometry revealed the altered phospholipids profiles in shoot apical meristem, particularly the PC species, under altered spPLD expressions. These results indicate the significance of secretory spPLD and help to elucidate the regulatory network of rice heading time.

Secretory phospholipases play essential roles in physiological processes of mammals, while functions of them in plants remain unknown. We identified a rice secretory PLD (spPLD) harboring a signal peptide which is indispensable for secretion of spPLD. Functional studies showed that altered spPLD expression resulted in the changed heading time of rice under both short-day and long-day conditions, which is dependent on the secretory character of spPLD. Rice Hd3a and RFT1, the homologs of Arabidopsis Flowing Locus T (FT), bind to phosphatidylcholine (PC) to promote heading. Analysis of phospholipids profiles in shoot apical meristem by using a mass spectrometry-based lipidomic approach demonstrated that spPLD regulates heading time by hydrolyzing the light period-predominant PC species, further revealing the crucial role of secretory proteins in regulating plant growth and development.

Comprehensive Analysis of Five Phyllostachys edulis SQUA-like Genes and Their Potential Functions in Flower Development

Bamboo is one of the most important non-timber forest resources worldwide. It has considerable economic value and unique flowering characteristics. The long juvenile phase in bamboo and unpredictable flowering time limit breeding and genetic improvement and seriously affect the productivity and application of bamboo forests. Members of SQUA-like subfamily genes play an essential role in controlling flowering time and floral organ identity. A comprehensive study was conducted to explain the functions of five SQUA-like subfamily genes in Phyllostachys edulis. Expression analysis revealed that all PeSQUAs have higher transcript levels in the reproductive period than in the juvenile phase. However, PeSQUAs showed divergent expression patterns during inflorescence development. The protein–protein interaction (PPI) patterns among PeSQUAs and other MADS-box members were analyzed by yeast two-hybrid (Y2H) experiments. Consistent with amino acid sequence similarity and phylogenetic analysis, the PPI patterns clustered into two groups. PeMADS2, 13, and 41 interacted with multiple PeMADS proteins, whereas PeMADS3 and 28 hardly interacted with other proteins. Based on our results, PeSQUA might possess different functions by forming protein complexes with other MADS-box proteins at different flowering stages. Furthermore, we chose PeMADS2 for functional analysis. Ectopic expression of PeMADS2 in Arabidopsis and rice caused early flowering, and abnormal phenotype was observed in transgenic Arabidopsis lines. RNA-seq analysis indicated that PeMADS2 integrated multiple pathways regulating floral transition to trigger early flowering time in rice. This function might be due to the interaction between PeMADS2 and homologous in rice. Therefore, we concluded that the five SQUA-like genes showed functional conservation and divergence based on sequence differences and were involved in floral transitions by forming protein complexes in P. edulis. The MADS-box protein complex model obtained in the current study will provide crucial insights into the molecular mechanisms of bamboo’s unique flowering characteristics.

Phosphorylation of OsFD1 by OsCIPK3 promotes the formation of RFT1-containing florigen activation complex for long-day flowering in rice

Heading date is a critical trait that determines the regional adaptability and grain productivity of many crops. Although rice is a facultative short-day plant, its domestication led to the Ghd7-Ehd1-Hd3a/RFT1 pathway for adaptation to long-day conditions (LDs). The formation of the "florigen activation complex" (FAC) containing florigen Hd3a has been characterized. However, the molecular composition of the FAC that contains RFT1 for long-day flowering is unclear. We show here that RFT1 forms a ternary FAC with 14-3-3 proteins and OsFD1 to promote flowering under LDs. We identified a calcineurin B-like-interacting protein kinase, OsCIPK3, which directly interacts with and phosphorylates OsFD1, thereby facilitating the localization of the FAC to the nucleus. Mutation in OsCIPK3 results in a late heading date under LDs but a normal heading date under short-day conditions. Collectively, our results suggest that OsCIPK3 phosphorylates OsFD1 to promote RFT1-containing FAC formation and consequently induce flowering in rice under LDs.

Identification of the specific long-noncoding RNAs involved in night-break mediated flowering retardation in Chenopodium quinoa

Background

Night-break (NB) has been proven to repress flowering of short-day plants (SDPs). Long-noncoding RNAs (lncRNAs) play key roles in plant flowering. However, investigation of the relationship between lncRNAs and NB responses is still limited, especially in Chenopodium quinoa, an important short-day coarse cereal.

Results

In this study, we performed strand-specific RNA-seq of leaf samples collected from quinoa seedlings treated by SD and NB. A total of 4914 high-confidence lncRNAs were identified, out of which 91 lncRNAs showed specific responses to SD and NB. Based on the expression profiles, we identified 17 positive- and 7 negative-flowering lncRNAs. Co-expression network analysis indicated that 1653 mRNAs were the common targets of both types of flowering lncRNAs. By mapping these targets to the known flowering pathways in model plants, we found some pivotal flowering homologs, including 2 florigen encoding genes (FT (FLOWERING LOCUS T) and TSF (TWIN SISTER of FT) homologs), 3 circadian clock related genes (EARLY FLOWERING 3 (ELF3), LATE ELONGATED HYPOCOTYL (LHY) and ELONGATED HYPOCOTYL 5 (HY5) homologs), 2 photoreceptor genes (PHYTOCHROME A (PHYA) and CRYPTOCHROME1 (CRY1) homologs), 1 B-BOX type CONSTANS (CO) homolog and 1 RELATED TO ABI3/VP1 (RAV1) homolog, were specifically affected by NB and competed by the positive and negative-flowering lncRNAs. We speculated that these potential flowering lncRNAs may mediate quinoa NB responses by modifying the expression of the floral homologous genes.

Conclusions

Together, the findings in this study will deepen our understanding of the roles of lncRNAs in NB responses, and provide valuable information for functional characterization in future.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07605-2.

Genetic regulation of flowering time and inflorescence architecture by MtFDa and MtFTa1 in Medicago truncatula

Regulation of floral transition and inflorescence development is crucial for plant reproductive success. FLOWERING LOCUS T (FT) is one of the central players in the flowering genetic regulatory network, whereas FLOWERING LOCUS D (FD), an interactor of FT and TERMINAL FLOWER 1 (TFL1), plays significant roles in both floral transition and inflorescence development. Here we show the genetic regulatory networks of floral transition and inflorescence development in Medicago truncatula by characterizing MtFTa1 and MtFDa and their genetic interactions with key inflorescence meristem (IM) regulators. Both MtFTa1 and MtFDa promote flowering; the double mutant mtfda mtfta1 does not proceed to floral transition. RNAseq analysis reveals that a broad range of genes involved in flowering regulation and flower development are up- or downregulated by MtFTa1 and/or MtFDa mutations. Furthermore, mutation of MtFDa also affects the inflorescence architecture. Genetic analyses of MtFDa, MtFTa1, MtTFL1, and MtFULc show that MtFDa is epistatic to MtFULc and MtTFL1 in controlling IM identity. Our results demonstrate that MtFTa1 and MtFDa are major flowering regulators in M. truncatula, and MtFDa is essential both in floral transition and secondary inflorescence development. The study will advance our understanding of the genetic regulation of flowering time and inflorescence development in legumes.

What is going on with the hormonal control of flowering in plants?

Molecular genetic studies using Arabidopsis thaliana as a model system have overwhelmingly revealed many important molecular mechanisms underlying the control of various biological events, including floral induction in plants. The major genetic pathways of flowering have been characterized in-depth, and include the photoperiod, vernalization, autonomous and gibberellin pathways. In recent years, novel flowering pathways are increasingly being identified. These include age, thermosensory, sugar, stress and hormonal signals to control floral transition. Among them, hormonal control of flowering except the gibberellin pathway is not formally considered a major flowering pathway per se, due to relatively weak and often pleiotropic genetic effects, complex phenotypic variations, including some controversial ones. However, a number of recent studies have suggested that various stress signals may be mediated by hormonal regulation of flowering. In view of molecular diversity in plant kingdoms, this review begins with an assessment of photoperiodic flowering, not in A. thaliana, but in rice (Oryza sativa); rice is a staple crop for human consumption worldwide, and is a model system of short-day plants, cereals and breeding crops. The rice flowering pathway is then compared with that of A. thaliana. This review then aims to update our knowledge on hormonal control of flowering, and integrate it into the entire flowering gene network.

Studies of rice Hd1 haplotypes worldwide reveal adaptation of flowering time to different environments

Rice domestication/adaptation is a good model for studies of the development and spread of this important crop. Mutations that caused morphological and physiological change, followed by human selection/expansion, finally led to the improvement of phenotypes suitable for different kinds of environments. We used the sequence information for Heading date 1 (Hd1) gene to reveal the association between sequence changes and flowering phenotypes of rice in different regions. Seven loss-of-function hd1 haplotypes had been reported. By data-mining the genome sequencing information in the public domain, we discovered 3 other types. These loss-of-function allele haplotypes are present in subtropical and tropical regions, which indicates human selection. Some of these haplotypes are present locally. However, types 7 and 13 are present in more than one-third of the world's rice accessions, including landraces and modern varieties. In the present study, phylogenetic, allele network and selection pressure analyses revealed that these two haplotypes might have occurred early in Southeastern Asia and then were introgressed in many local landraces in nearby regions. We also demonstrate that these haplotypes are present in weedy rice populations, which again indicates that these alleles were present in rice cultivation for long time. In comparing the wild rice sequence information, these loss-of-function haplotypes occurred in agro but were not from wild rice.

Late flowering in F1 hybrid rice brought about by the complementary effect of quantitative trait loci

Late flowering sometimes occurs in F₁ hybrids between rice varieties (Oryza sativa L.), although the parental varieties show similar days-to-flowering (DTF). The genetic architecture prompting the occurrence of such late flowering is poorly understood. To clarify the genetic architecture of late flowering in F₁ hybrids from a cross between rice varieties, 'Koshihikari' and 'IR64', we performed quantitative trait locus (QTL) analysis using an F₂ population (selfed progeny of an F₁ plant), in which heterozygous genotypes should segregate in a certain proportion in a Mendelian manner. The QTL analysis detected three significant QTLs. At one QTL (putatively Heading date 1), the 'Koshihikari' allele increased DTF, and at the other two QTLs (putatively Heading date 6 and Oryza sativa Pseudo-Response Regulator 37/Heading date 2), the 'IR64' alleles increased DTF. All alleles at these three QTLs showed partial dominance. The combination of the QTLs explained 82.2% of the total phenotypic variance of DTF in the F₂ population, with contribution from epistasis between QTLs. There was no difference between DTFs of F₁ hybrids and heterozygous genotypes for the three QTLs. Our results demonstrated that the complementary effects accompanied by epistasis of at least three QTLs were responsible for late flowering in F₁ hybrids.

Sieve element biology provides leads for research on phytoplasma lifestyle in plant hosts

Phytoplasmas reside exclusively in sieve tubes, tubular arrays of sieve element-companion cell complexes. Hence, the cell biology of sieve elements may reveal (ultra)structural and functional conditions that are of significance for survival, propagation, colonization, and effector spread of phytoplasmas. Electron microscopic images suggest that sieve elements offer facilities for mobile and stationary stages in phytoplasma movement. Stationary stages may enable phytoplasmas to interact closely with diverse sieve element compartments. The unique, reduced sieve element outfit requires permanent support by companion cells. This notion implies a future focus on the molecular biology of companion cells to understand the sieve element-phytoplasma inter-relationship. Supply of macromolecules by companion cells is channelled via specialized symplasmic connections. Ca2+-mediated gating of symplasmic corridors is decisive for the communication within and beyond the sieve element-companion cell complex and for the dissemination of phytoplasma effectors. Thus, Ca2+ homeostasis, which affects sieve element Ca2+ signatures and induces a range of modifications, is a key issue during phytoplasma infection. The exceptional physical and chemical environment in sieve elements seems an essential, though not the only factor for phytoplasma survival.

Maturity2, a novel regulator of flowering time in Sorghum bicolor, increases expression of SbPRR37 and SbCO in long days delaying flowering

Sorghum bicolor is a drought-resilient facultative short-day C4 grass that is grown for grain, forage, and biomass. Adaptation of sorghum for grain production in temperate regions resulted in the selection of mutations in Maturity loci (Ma1 -Ma6) that reduced photoperiod sensitivity and resulted in earlier flowering in long days. Prior studies identified the genes associated with Ma1 (PRR37), Ma3 (PHYB), Ma5 (PHYC) and Ma6 (GHD7) and characterized their role in the flowering time regulatory pathway. The current study focused on understanding the function and identity of Ma2. Ma2 delayed flowering in long days by selectively enhancing the expression of SbPRR37 (Ma1) and SbCO, genes that co-repress the expression of SbCN12, a source of florigen. Genetic analysis identified epistatic interactions between Ma2 and Ma4 and located QTL corresponding to Ma2 on SBI02 and Ma4 on SBI10. Positional cloning and whole genome sequencing identified a candidate gene for Ma2, Sobic.002G302700, which encodes a SET and MYND (SYMD) domain lysine methyltransferase. Eight sorghum genotypes previously identified as recessive for Ma2 contained the mutated version of Sobic.002G302700 present in 80M (ma2) and one additional putative recessive ma2 allele was identified in diverse sorghum accessions.

Switching genetic effects of the flowering time gene Hd1 in LD conditions by Ghd7 and OsPRR37 in rice

Flowering time control in plants is a major limiting factor on the range of species. Day length, perceived via the photoperiodic pathway, is a critical factor for the induction of flowering. The module of GIGANTEA (GI)-CONSTANS (CO)-FLOWERING LOCUS T in the long day (LD) plant Arabidopsis is conserved in diverse plant species including the short day (SD) plant rice, where this module comprises OsGI-Heading date 1 (Hd1)-Heading date 3a. Hd1, the rice ortholog of Arabidopsis CO, has dual functions in the regulation of flowering time, promoting flowering in SD conditions and delaying it in LD conditions. We herein show genetic interactions among three LD repressor genes: Hd1, Grain number, plant height and heading date 7 (Ghd7), and Oryza sativa Pseudo-Response Regulator37 (OsPRR37). Genetic analyses, including segregation analyses, evaluations of near isogenic lines, and transformation for flowering time demonstrated that Hd1 promoted flowering time in inductive SD and non-inductive LD conditions in genetic condition of loss-of-function Ghd7 and OsPRR37 (ghd7osprr37) in rice. Functional Ghd7 or OsPRR37 may switch the genetic effects of Hd1 from the promotion to the delay of flowering times in LD conditions.

DELAYED HEADING DATE1 interacts with OsHAP5C/D, delays flowering time and enhances yield in rice

Heading date is an important agronomic trait affecting crop yield. The GRAS protein family is a plant-specific super family extensively involved in plant growth and signal transduction. However, GRAS proteins are rarely reported have a role in regulating rice heading date. Here, we report a GRAS protein DHD1 (Delayed Heading Date1) delays heading and enhances yield in rice. Biochemical assays showed DHD1 physically interacts with OsHAP5C/D both in vitro and in vivo. DHD1 and OsHAP5C/D located in the nucleus and showed that rhythmic expression. Both DHD1 and OsHAP5C/D affect heading date by regulating expression of Ehd1. We propose that DHD1 interacts with OsHAP5C/D to delay heading date by inhibiting expression of Ehd1.

Functional validation of the Pi54 gene by knocking down its expression in a blast-resistant rice line using RNA interference and its effects on other traits

Rice blast disease caused by Magnaporthe oryzae is one of the major diseases affecting the rice (Oryza sativa L.) crop. A major blast resistance gene, Pi54, has already been cloned and deployed in different rice varieties. To understand the role of Pi54 in providing rice blast resistance, we used the RNA interferences (RNAi) approach to knock down the expression of this gene. We showed a high frequency of Agrobacterium tumefaciens-mediated transformation of rice line Taipei 309 containing a single gene (Pi54) for blast resistance. Pi54 RNAi leads to a decreased level of Pi54 transcripts, leading to the susceptibility of otherwise M. oryzae-resistant rice lines. However, among the RNAi knockdown plants, the severity of blast disease varied between the lines. Histochemical analysis of the leaves of knockdown plants inoculated with M. oryzae spores also showed typical cell death and blast lesions. Additionally, Pi54 RNAi also showed an effect on the Hda3 gene, a florigen gene playing a role in rice flowering. By using the RNAi technique, for the first time, we showed that the directed degradation of Pi54 transcripts results in a significant reduction in the rice blast resistance response, suggesting that RNAi is a powerful tool for functional validation of genes.

A WUSCHEL Homeobox Transcription Factor, OsWOX13, Enhances Drought Tolerance and Triggers Early Flowering in Rice

Plants have evolved strategies to cope with drought stress by maximizing physiological capacity and adjusting developmental processes such as flowering time. The WOX13 orthologous group is the most conserved among the clade of WOX homeodomain-containing proteins and is found to function in both drought stress and flower development. In this study, we isolated and characterized OsWOX13 from rice. OsWOX13 was regulated spatially in vegetative organs but temporally in flowers and seeds. Overexpression of OsWOX13 (OsWOX13-ov) in rice under the rab21 promoter resulted in drought resistance and early flowering by 7-10 days. Screening of gene expression profiles in mature leaf and panicles of OsWOX13-ov showed a broad spectrum of effects on biological processes, such as abiotic and biotic stresses, exerting a cross-talk between responses. Protein binding microarray and electrophoretic mobility shift assay analyses supported ATTGATTG as the putative cis-element binding of OsWOX13. OsDREB1A and OsDREB1F, drought stress response transcription factors, contain ATTGATTG motif(s) in their promoters and are preferentially expressed in OsWOX13-ov. In addition, Heading date 3a and OsMADS14, regulators in the flowering pathway and development, were enhanced in OsWOX13-ov. These results suggest that OsWOX13 mediates the stress response and early flowering and, thus, may be a regulator of genes involved in drought escape.

TFL1-Like Proteins in Rice Antagonize Rice FT-Like Protein in Inflorescence Development by Competition for Complex Formation with 14-3-3 and FD

Hd3a, a rice homolog of FLOWERING LOCUS T (FT), is a florigen that induces flowering. Hd3a forms a ternary 'florigen activation complex' (FAC) with 14-3-3 protein and OsFD1 transcription factor, a rice homolog of FD that induces transcription of OsMADS15, a rice homolog of APETALA1 (AP1), which leads to flowering. TERMINAL FLOWER 1 (TFL1) represses flowering and controls inflorescence architecture. However, the molecular basis for floral repression by TFL1 remains poorly understood. Here we show that RICE CENTRORADIALIS (RCN), rice TFL1-like proteins, compete with Hd3a for 14-3-3 binding. All four RCN genes are predominantly expressed in the vasculature, and RCN proteins are transported to the shoot apex to antagonize florigen activity and regulate inflorescence development. The antagonistic function of RCN to Hd3a is dependent on its 14-3-3 binding activity. Our results suggest a molecular basis for regulation of the balance between florigen FT and anti-florigen TFL1.

Differential Regulation of Starch-synthetic Gene Expression in Endosperm Between Indica and Japonica Rice Cultivars

BACKGROUND

Grain filling rates (GFRs) of indica rice cultivars are often higher than those of japonica cultivars. Although GFR is mainly determined by the starch accumulation rate (SAR) in endosperm, the genetic basis for SAR during the ripening period has not been well studied in rice. To elucidate the factors influencing the differing SARs between typical indica and japonica cultivars, we focused on differences in sink potentials, especially on starch synthesis in the endosperm.

RESULTS

SAR in indica rice cultivar IR36 was significantly higher than in japonica cultivar T65. Although enzymes for both amylose and amylopectin syntheses had higher activity in IR36, amylopectin synthesis was seemingly more important for accelerating SAR because an elevation of amylose synthesis ability alone in the T65 genetic background did not result in the same level of SAR as IR36. In IR36, most starch-synthetic genes (SSGs) in the endosperm were more highly expressed during ripening than in T65. In panicle culture experiments, the SSGs in rice endosperm were regulated in either sucrose-dependent or -independent manners, or both. All SSGs except SSI and BEIIa were responsive to sucrose in both cultivars, and GBSSI, AGPS2b and PUL were more responsive to sucrose in IR36. Interestingly, the GBSSI gene (Wx ^a ) in IR36 was highly activated by sucrose, but the GBSSI gene (Wx ^b ) in T65 was insensitive. In sucrose-independent regulation, AGPL2, SSIIIa, BEI, BEIIb and ISA1 genes in IR36 were upregulated 1.5 to 2 times more than those in T65. Additionally, at least SSI and BEIIa might be regulated by unknown signals; that regulation pathway should be more activated in IR36 than T65.

CONCLUSIONS

In this study, at least three regulatory pathways seem to be involved in SSG expression in rice endosperm, and all pathways were more active in IR36. One of the factors leading to the high SAR of IR36 seemed to be an increase in the sink potential.

Constitutive expression of OsDof4, encoding a C2-C2 zinc finger transcription factor, confesses its distinct flowering effects under long- and short-day photoperiods in rice (Oryza sativa L.)

BACKGROUND

Dof (DNA binding with one finger) proteins, a class of plant-specific transcription factors which contain a conserved C2-C2-type zinc finger domain, are involved in many fundamental processes. In the Arabidopsis photoperiod response pathway, CDF (CYCLING DOF FACTOR) proteins have a primary role as acting via transcriptional repression of the direct FLOWERING LOCUS T (FT) activator CONSTANS (CO). Our previous study indicated that one of CDF homologs, OsDOf12, was involved in photoperiodic flowering. However, the functional characterization of other rice CDF like genes is still in progress. Here, we characterized the function of OsDof4 in rice.

RESULTS

Phylogenic analysis indicated that OsDof4 is closely clustered into the same subgroup with CDFs and OsDof12. The subcellular localization experiment and transcriptional activity assay suggested that OsDof4 may function as a transcription factor. The diurnal expression pattern indicated that OsDof4 was regulated by endogenous circadian clock. Overexpression of OsDof4 led to earlier flowering under natural long-day field conditions (NLDs) and late flowering under natural short-day field conditions (NSDs), respectively. We compared the expression level of key floral genes in vector line and OsDof4-ox lines grown under long-day conditions (LDs) and short-day conditions (SDs). Real-time q-PCR results demonstrated that under LDs, Hd3a, RFT1 and Ehd1 were up-regulated whereas under SDs they were down-regulated. Hd1 was down-regulated at dusk period independent of photoperiods.

CONCLUSIONS

Taken these results together, we may speculate that the abnormal flowering responses in OsDof4-ox plants under LDs and SDs might be mediated by Ehd1 and Hd1.

Analysis of Genes with Alternatively Spliced Transcripts in the Leaf, Root, Panicle and Seed of Rice Using a Long Oligomer Microarray and RNA-Seq

Pre-mRNA splicing further increases protein diversity acquired through evolution. The underlying driving forces for this phenomenon are unknown, especially in terms of gene expression. A rice alternatively spliced transcript detection microarray (ASDM) and RNA sequencing (RNA-Seq) were applied to differentiate the transcriptome of 4 representative organs of Oryza sativa L. cv. Ilmi: leaves, roots, 1-cm-stage panicles and young seeds at 21 days after pollination. Comparison of data obtained by microarray and RNA-Seq showed a bell-shaped distribution and a co-lineation for highly expressed genes. Transcripts were classified according to the degree of organ enrichment using a coefficient value (CV, the ratio of the standard deviation to the mean values): highly variable (CVI), variable (CVII), and constitutive (CVIII) groups. A higher index of the portion of loci with alternatively spliced transcripts in a group (IAST) value was observed for the constitutive group. Genes of the highly variable group showed the characteristics of the examined organs, and alternatively spliced transcripts tended to exhibit the same organ specificity or less organ preferences, with avoidance of 'organ distinctness'. In addition, within a locus, a tendency of higher expression was found for transcripts with a longer coding sequence (CDS), and a spliced intron was the most commonly found type of alternative splicing for an extended CDS. Thus, pre-mRNA splicing might have evolved to retain maximum functionality in terms of organ preference and multiplicity.

Molecular function of florigen

Florigen is a mobile flowering signal in plants that has a strong impact on plant reproduction and is considered one of the important targets for crop improvement. At the molecular level, florigen is represented as a protein product encoded by the FLOWERING LOCUS T (FT) gene, which is highly conserved across flowering plants and thus the understanding of this protein is expected to be applied to the improvement of many crops. Recent advances in molecular genetics, cell biology and structural biology in plants revealed the presence of intercellular receptors for florigen, a transcriptional complex essential for florigen to function, and also shed light on the molecular basis of pleiotropic function of florigen beyond flowering. Furthermore, cutting-edge technologies, such as live cell imaging and next generation sequencing revealed the precise distribution of florigen and transcriptional targets of the florigen activation complex (FAC) during early stages of floral transition. These understandings will help future crop improvement through the regulation of flowering and other plant developmental processes.

Flowering time in banana (Musa spp.), a day neutral plant, is controlled by at least three FLOWERING LOCUS T homologues

Banana is an important day neutral food crop with a long flowering/fruiting cycle that is affected by hot summers or cold winters in different places. Manipulating its life cycle requires an understanding of its flowering time machinery to bypass these stresses. Twelve FLOWERING LOCUS T (FT) and two TWIN SISTER OF FT (TSF) members were isolated from banana and their organization and expression pattern studied during development in two varieties that differ in flowering time namely Grand Nain (AAA genotype) and Hill banana (AAB genotype). The expression of at least 3 genes namely MaFT1, MaFT2 and MaFT5 (and to some extent MaFT7) increases just prior to initiation of flowering. These four genes and five others (MaFT3, MaFT4, MaFT8, MaFT12 and MaTSF1 could suppress the delayed flowering defect in the Arabidopsis ft-10 mutant and induce early flowering upon over-expression in the Col-0 ecotype. Most genes are diurnally regulated and differentially expressed during development and in various vegetative and reproductive tissues suggesting roles besides flowering. Subtle amino acid changes in these FT/TSF-like proteins provide interesting insights into the structure/function relationships of banana FTs vis-à-vis Arabidopsis. The studies provide a means for manipulation of flowering in banana for better management of resources and to reduce losses through abiotic stresses.

Rice Flowering Locus T 1 plays an important role in heading date influencing yield traits in rice

Important role of flowering genes in enhancing grain productivity in rice has become well recognized for a number of key genes regulating the florigen production, but little has been known for the two florigen genes themselves. In this study, pleiotropism of Rice Flowering Locus T 1 (RFT1), one of the two florigen genes in rice, was firstly evaluated using near isogenic lines (NILs) carrying RFT1 alleles from the indica rice cultivars Zhenshan 97 (ZS97) and Milyang 46, respectively, and then determined by transformation of the RFT1^ZS97 allele into a japonica rice variety, Zhonghua 11. The RFT1^ZS97 allele was shown to delay heading and increase plant height, grain weight, grain number and grain yield, indicating that RFT1 plays an important role in the growth and development of rice. This study has also validated the potential of using a new type of genetic resource, sequential residual heterozygotes (SeqRHs), for QTL fine-mapping. A step-by-step approach was employed for SeqRHs identification, NIL development and QTL fine-mapping. The heterozygous segments and candidate QTL regions were gradually narrowed down. Eventually, the QTL region was delimited to a 1.7 kb region containing a single gene.

OsCOL16, encoding a CONSTANS-like protein, represses flowering by up-regulating Ghd7 expression in rice

Flowering time is an important agronomic trait that coordinates the plant life cycle with regional adaptability and thereby impacts yield potentials for cereal crops. The CONSTANS (CO)-like gene family plays vital roles in the regulation of flowering time. CO-like proteins are typically divided into four phylogenetic groups in rice. Several genes from groups I, III, and IV have been functionally characterized, though little is known about the genes of group II in rice. We report the functional characterization in rice of a constitutive floral inhibitor, OsCOL16, encoding a group-II CO-like protein that delays flowering time and increases plant height and grain yield. Overexpression of OsCOL16 resulted in late heading under both long-day and short-day conditions. OsCOL16 expression exhibits a diurnal oscillation and serves as a transcription factor with transcriptional activation activity. We determined that OsCOL16 up-regulates the expression of the floral repressor Ghd7, leading to down-regulation of the expression of Ehd1, Hd3a, and RFT1. Moreover, genetic diversity and evolutionary analyses suggest that remarkable differences in flowering times correlate with two major alleles of OsCOL16. Our combined molecular biology and phylogeographic analyses revealed that OsCOL16 plays an important role in regulating rice photoperiodic flowering, allowing for environmental adaptation of rice.

The function of OsbHLH068 is partially redundant with its homolog, AtbHLH112, in the regulation of the salt stress response but has opposite functions to control flowering in Arabidopsis

KEY MESSAGE

The homologous genes OsbHLH068 and AtbHLH112 have partially redundant functions in the regulation of the salt stress response but opposite functions to control flowering in Arabidopsis. The transcription factor (TF) basic/Helix-Loop-Helix (bHLH) is important for plant growth, development, and stress responses. OsbHLH068, which is a homologous gene of AtbHLH112 that is up-regulated under drought and salt stresses, as indicated by previous microarray data analysis. However, the intrinsic function of OsbHLH068 remains unknown. In the present study, we characterized the function and compared the role of OsbHLH068 with that of its homolog, AtbHLH112. Histochemical GUS staining indicated that OsbHLH068 and AtbHLH112 share a similar expression pattern in transgenic Arabidopsis during the juvenile-to-adult phase transition. Heterologous overexpression of OsbHLH068 in Arabidopsis delays seed germination, decreases salt-induced H2O2 accumulation, and promotes root elongation, whereas AtbHLH112 knock-out mutant displays an opposite phenotype. Both OsbHLH068-overexpressing transgenic Arabidopsis seedlings and the Atbhlh112 mutant display a late-flowering phenotype. Moreover, the expression of OsbHLH068-GFP driven by an AtbHLH112 promoter can compensate for the germination deficiency in the Atbhlh112 mutant, but the delayed-flowering phenotype tends to be more severe. Further analysis by microarray and qPCR indicated that the expression of FT is down-regulated in both OsbHLH068-overexpressing Arabidopsis plants and Atbhlh112 mutant plants, whereas SOC1 but not FT is highly expressed in AtbHLH112-overexpressing Arabidopsis plants. A comparative transcriptomic analysis also showed that several stress-responsive genes, such as AtERF15 and AtPUB23, were affected in both OsbHLH068- and AtbHLH112-overexpressing transgenic Arabidopsis plants. Thus, we propose that OsbHLH068 and AtbHLH112 share partially redundant functions in the regulation of abiotic stress responses but have opposite functions to control flowering in Arabidopsis, presumably due to the evolutionary functional divergence of homolog-encoded proteins.

Suppressor of rid1 (SID1) shares common targets with RID1 on florigen genes to initiate floral transition in rice

The transition from vegetative to reproductive growth is a critical process in the life cycle of higher plants. Previously, we cloned Rice Indeterminate 1 (RID1), which acts as the master switch for the transition from the vegetative to reproductive phase in rice. Although the photoperiod pathway of RID1 inducing expression of the florigen genes Hd3a and RFT1 via Ehd1 has been established, the alternative pathways for the essential flowering transition need to be further examined. Here, we identified a Suppressor of rid1 (SID1), which rescues the never-flowering phenotype of rid1. SID1 encodes an INDETERMINATE DOMAIN (IDD) transcription factor. Mutation in SID1 showed the delayed flowering phenotype. Gain-of-function of SID1, OsIDD1, or OsIDD6 could restore the rid1 to flowering. Further analyses showed SID1 and RID1 directly target the promoter regions of Hd3a and RFT1, two florigen genes in rice. Taken together, our results reveal an autonomous flowering pathway might be mediated by RID1, thereby controlling the phase transition from vegetative to reproductive development in rice.

Transition from vegetative to reproductive phase is a critical developmental switch in the life cycle of higher plants. In rice, our previous work suggested Rice Indeterminate 1 (RID1) acts as the master switch for the transition to flowering. Mutation in RID1 results in a never-flowering phenotype. In order to uncover the molecular network regulated by RID1, a Suppressor of rid1 (SID1) was identified in this study. Both SID1 and RID1 encode a plant-specific INDETERMINATE DOMAIN (IDD) transcription factor. Overexpression of SID1, OsIDD1, or OsIDD6 could rescue the never-flowering phenotype of rid1. Molecular data indicate both SID1 and RID1 physically bind the promoters of the florigen genes Hd3a and RFT1 in rice. Thus, we propose that the transition to flowering could be regulated by RID1 through the autonomous pathway, in addition to the photoperiod pathway.

Potato Tuber Induction is Regulated by Interactions Between Components of a Tuberigen Complex

Photoperiod-regulated flowering and potato tuber formation involve leaf-produced mobile signals, florigen and tuberigen, respectively. The major protein component of florigen has been identified as the FLOWERING LOCUS T (FT) protein. In rice, an FT-like protein, Heading date 3a (Hd3a), induces flowering by making the florigen activation complex (FAC) through interactions with 14-3-3 and OsFD1, a rice FD-like protein. In potato, StSP6A, an FT-like protein, was identified as a major component of tuberigen. However, the molecular mechanism of how StSP6A triggers tuber formation remains elusive. Here we analyzed the significance of the formation of a complex including StSP6A, 14-3-3 and FD-like proteins in tuberization. Yeast two-hybrid, bimolecular fluorescence complementation and in vitro pull-down assays showed that StSP6A and StFDL1, a potato FD-like protein, interact with St14-3-3s. StSP6A overexpression induced early tuberization in a 14-3-3-dependent manner, and suppression of StFDL1 delayed tuberization. These results strongly suggest that an FAC-like complex, the tuberigen activation complex (TAC), comprised of StSP6A, St14-3-3s and StFDL1, regulates potato tuber formation.

Seed maturation: Simplification of control networks in plants

Networks controlling developmental or metabolic processes in plants are often complex as a consequence of the duplication and specialisation of the regulatory genes as well as the numerous levels of transcriptional and post-transcriptional controls added during evolution. Networks serve to accommodate multicellular complexity and increase robustness to environmental changes. Mathematical simplification by regrouping genes or pathways in a limited number of hubs has facilitated the construction of models for complex traits. In a complementary approach, a biological simplification can be achieved by using genetic modification to understand the core and singular ancestral function of the network, which is likely to be more prevalent within the plant kingdom rather than specific to a species. With this viewpoint, we review examples of simplification successfully undertaken in yeast and other organisms. A strategy of progressive complementation of single, double and triple mutants of seed maturation confirmed the fundamental role of the AFL sub-family of B3 transcription factors as master regulators of seed maturation, illustrating that biological simplification of complex networks could be more widely applied in plants. Defining minimal control networks will facilitate evolutionary comparisons of regulatory processes and the identification of an essential gene set for synthetic biology.

Density effects on late flowering mutants of Arabidopsis thaliana under continuous light

In general, plant growth is inhibited under high-density conditions, while it is promoted under low-density conditions. This is known as the "density effect". Growing plants at high densities is often associated with an accelerated flowering time. Three major pathways [the long day (LD), gibberellic acid (GA), and autonomous/vernalization pathways] are known to play important roles in the control of flowering time. Circadian clock genes, namely, LHY, CCA1, GI, and ELF3, regulate the LD pathway. GAI and FCA control flowering via GA and autonomous pathways, respectively. The density effect on plant size is caused by specific factors such as the amount of nutrition obtained from the soil and touch frequency among plants. However, the molecular mechanism underlying the acceleration of flowering time due to density effects remains unclear. Here, we show the density effects on three Brassicaceae plants, namely, Brassica rapa var. nipposinica, Brassica napus, and Brassica chinensis f. honsaitai. They showed shorter stems and leaves when grown at high densities on soil under continuous light (LL). Shorter stems and leaves, as well as accelerated flowering times, were observed when a model plant, Arabidopsis thaliana, was grown under the same conditions. Unexpectedly, ethylene insensitive 2 (ein2) showed no differences in density effects in our experiments. The acceleration of flowering at higher densities was largely suppressed by gai, but not by gi, lhy;cca1, or fca. These results suggest that the promotion of flowering (as a density effect) is likely dependent on the GA pathway, but not the LD or autonomous pathways.

FT and florigen long-distance flowering control in plants

The great hunt for florigen, the universal, long distance flowering regulator proposed by Chailakhan in the 1930s, resulted in the discovery a decade ago that FT-like proteins fulfilled the predictions for florigen. They are small (∼175 amino acids), globular, phosphatidylethanolamine-binding (PEBP) proteins, phloem-expressed, graft-transmissible and able to move to the shoot apex to act as potent stimulators of flowering in many plants. Genes that regulate Arabidopsis FT protein movement and some features of Arabidopsis FT protein that make it an effective florigen have recently been identified. Although floral promotion via graft transmission of FT has not been demonstrated in trees, FT-like genes have been successfully applied to reducing the long juvenile (pre-flowering) phase of many trees enabling fast track breeding.

Control of floral transition in the bioenergy crop switchgrass

Switchgrass (Panicum virgatum L.), a perennial warm season bunchgrass native to North America, has been a target in the U.S. as a renewable bioenergy crop because of its ability to produce moderate to high biomass yield on marginal soils. Delaying flowering can increase vegetative biomass production by allowing prolonged growth before switching to the reproductive phase. Despite the identification of flowering time as a biomass trait in switchgrass, the molecular regulatory factors involved in controlling floral transition are poorly understood. Here we identified PvFT1, PvAPL1-3 and PvSL1, 2 as key flowering regulators required from floral transition initiation to development of floral organs. PvFT1 expression in leaves is developmentally regulated peaking at the time of floral transition, and diurnally regulated with peak at approximately 2 h into the dark period. Ectopic expression of PvFT1 in Arabidopsis, Brachypodium and switchgrass led to extremely early flowering, and activation of FT downstream target genes, confirming that it is a strong activator of flowering in switchgrass. Ectopic expression of PvAPL1-3 and PvSL1, 2 in Arabidopsis also activated early flowering with distinct floral organ phenotypes. Our results suggest that switchgrass has conserved flowering pathway regulators similar to Arabidopsis and rice.

Three FLOWERING LOCUS T-like genes function as potential florigens and mediate photoperiod response in sorghum

Sorghum is a typical short-day (SD) plant and its use in grain or biomass production in temperate regions depends on its flowering time control, but the underlying molecular mechanism of floral transition in sorghum is poorly understood. Here we characterized sorghum FLOWERING LOCUS T (SbFT) genes to establish a molecular road map for mechanistic understanding. Out of 19 PEBP genes, SbFT1, SbFT8 and SbFT10 were identified as potential candidates for encoding florigens using multiple approaches. Phylogenetic analysis revealed that SbFT1 clusters with the rice Hd3a subclade, while SbFT8 and SbFT10 cluster with the maize ZCN8 subclade. These three genes are expressed in the leaf at the floral transition initiation stage, expressed early in grain sorghum genotypes but late in sweet and forage sorghum genotypes, induced by SD treatment in photoperiod-sensitive genotypes, cooperatively repressed by the classical sorghum maturity loci, interact with sorghum 14-3-3 proteins and activate flowering in transgenic Arabidopsis plants, suggesting florigenic potential in sorghum. SD induction of these three genes in sensitive genotypes is fully reversed by 1 wk of long-day treatment, and yet, some aspects of the SD treatment may still make a small contribution to flowering in long days, indicating a complex photoperiod response mediated by SbFT genes.

Multi-hierarchical profiling: an emerging and quantitative approach to characterizing diverse biological networks

Multi-hierarchical profiling may offer valuable insights into the structural stability and functional direction of biological networks in cellular development, pathological process and disease variation. Owing to the emergence of several new techniques, such as bioinformatics for omics data, structural biology and structural bioinformatics, the pace of network hierarchical research has accelerated a lot in recent years. Here, we discuss and compare the techniques available for quantifying multilevel hierarchies, with a focus on their features, capabilities and drawbacks when used for different applications. Then, we classify these methods into three types: topological spatial-scales, multilevel hierarchical control and feature ordering. We observe that challenges and limitations do exist in functional hierarchical identification. And, we also provide useful suggestions on how to analyze the dynamic data of complex network studies.