FLOWERING LOCUS T genes control onion bulb formation and flowering

Tuber, or not tuber: Molecular and morphological basis of underground storage organ development

Underground storage organs occur in phylogenetically diverse plant taxa and arise from multiple tissue types including roots and stems. Thickening growth allows underground storage organs to accommodate carbohydrates and other nutrients and requires proliferation at various lateral meristems followed by cell expansion. The WOX-CLE module regulates thickening growth via the vascular cambium in several eudicot systems, but the molecular mechanisms of proliferation at other lateral meristems are not well understood. In potato, onion, and other systems, members of the phosphatidylethanolamine-binding protein (PEBP) gene family induce underground storage organ development in response to photoperiod cues. While molecular mechanisms of tuber development in potato are well understood, we lack detailed mechanistic knowledge for the extensive morphological and taxonomic diversity of underground storage organs in plants.

Evolution of FLOWERING LOCUS T-like genes in angiosperms: a core Lamiales-specific diversification

Plant life history is determined by two transitions, germination and flowering time, in which the phosphatidylethanolamine-binding proteins (PEBPs) FLOWERING LOCUS T (FT) and TERMINAL FLOWER1 (TFL1) play key regulatory roles. Compared with the highly conserved TFL1-like genes, FT-like genes vary significantly in copy numbers in gymnosperms, and monocots within the angiosperms, while sporadic duplications can be observed in eudicots. Here, via a systematic analysis of the PEBPs in angiosperms with a special focus on 12 representative species featuring high-quality genomes in the order Lamiales, we identified a successive lineage-specific but systematic expansion of FT-like genes in the families of core Lamiales. The first expansion event generated FT1-like genes mainly via a core Lamiales-specific whole-genome duplication (cL-WGD), while a likely random duplication produced the FT2-like genes in the lineages containing Scrophulariaceae and the rest of the core Lamiales. Both FT1- and FT2-like genes were further amplified tandemly in some families. These expanded FT-like genes featured highly diverged expression patterns and structural variation, indicating functional diversification. Intriguingly, some core Lamiales contained the relict MOTHER OF FT AND TFL1 like 2 (MFT2) that probably expanded in the common ancestor of angiosperms. Our data showcase the highly dynamic lineage-specific expansion of the FT-like genes, and thus provide important and fresh evolutionary insights into the gene regulatory network underpinning flowering time diversity in Lamiales and, more generally, in angiosperms.

How to awaken a sleeping giant: antagonistic expression of Flowering locus T homologs and elements of the age-related pathway are associated with the flowering transition in Agave tequilana

KEY MESSAGE

Antagonistic expression of Flowering locus T proteins and the ageing pathway via miRNAs and sugar metabolism regulate the initiation of flowering in A. tequilana. Flowering in commercial plantations of Agave tequilana signals that plants are ready to harvest for tequila production. However, time of flowering is often unpredictable and a detailed understanding of the process would be beneficial in the field, for breeding and for the development of future research. This report describes the functional analysis of A. tequilana FLOWERING LOCUS T (FT) genes by heterologous expression in A. thaliana and in situ hybridization in agave plants. The gene structures of the Agave tequilana FT family are also described and putative regulatory promoter elements were identified. Most Agave species have monocarpic, perennial life cycles that can last over 25 years during which plants do not respond to the normal environmental signals which induce flowering, suggesting that the ageing pathway as described in Arabidopsis may play an important role in determining flowering time in these species. Elements of this pathway were analyzed and in silico data is presented that supports the regulation of SQUAMOSA PROMOTER BINDING LIKE proteins (SPL), APETALA2 (AP2) proteins and members of Plant Glycoside Hydrolase Family 32 (PGHF32) by interactions with miRNAs 156, 172 and 164 during the initiation of flowering in A. tequilana.

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.

Temperature-Regulated Flowering Locus T-Like Gene Coordinates the Spike Initiation in Phalaenopsis Orchid

Phalaenopsis aphrodite can be induced to initiate spike growth and flowering by exposure to low ambient temperatures. However, the factors and mechanisms responsible for spike initiation in P. aphrodite remain largely unknown. In this study, we show that a repressor Flowing Locus T-like (FTL) gene, FTL, can act as a negative regulator of spike initiation in P. aphrodite. The mRNA transcripts of PaFTL are consistently high during high ambient temperature, thereby preventing premature spike initiation. However, during low ambient temperature, PaFTL expression falls while FT expression increases, allowing for spike initiation. Knock-down of PaFTL expression through virus-inducing gene silencing promoted spike initiation at 30/28°C. Moreover, PaFTL interacts with FLOWERING LOCUS D in a similar manner to FT to regulate downstream flowering initiation genes. Transgenic P. aphrodite plants exhibiting high expression of PaFTL do not undergo spike initiation, even when exposed to low ambient temperatures. These findings shed light on the flowering mechanisms in Phalaenopsis and provide new insights into how perennial plants govern spike initiation in response to temperature cues.

Photoperiodic and lighting treatments for flowering control and its genetic regulation in sugarcane breeding

Improvement of sugarcane is hampered due to its narrow genetic base, and the difficulty in synchronizing flowering further hinders the exploitation of the genetic potential of available germplasm resources. Therefore, the continuous evaluation and optimization of flowering control and induction techniques are vital for sugarcane improvement. In view of this, the review was conducted to investigate the current understanding of photoperiodic and lighting treatment effects on sugarcane flowering and its genetic regulation. Photoperiod facilities have made a significant contribution to flowering control in sugarcane; however, inductive photoperiods are still unknown for some genotypes, and some intended crosses are still impossible to produce because of unresponsive varieties. The effectiveness of lower red/far-red ratios in promoting sugarcane flowering has been widely understood. Furthermore, there is vast potential for utilizing blue, red, and far-red light wavelengths in the flowering control of sugarcane. In this context, light-emitting diodes (LEDs) remain efficient sources of light. Therefore, the combined use of photoperiod regimes with different light wavelengths and optimization of such treatment combinations might help to control and induce flowering in sugarcane parental clones. In sugarcane, FLOWERING LOCUS T (ScFT) orthologues from ScFT1 to ScFT13 have been identified, and interestingly, ScFT3 has evidently been identified as a floral inducer in sugarcane. However, independent assessments of different FT-like gene family members are recommended to comprehensively understand their role in the regulation of flowering. Similarly, we believe this review provides substantial information that is vital for the manipulation of flowering and exploitation of germplasm resources in sugarcane breeding.

Genome-wide identification, subcellular localization, and expression analysis of the phosphatidyl ethanolamine-binding protein family reveals the candidates involved in flowering and yield regulation of Tartary buckwheat (Fagopyrum tataricum)

Background

PEBP (phosphatidyl ethanolamine-binding protein) is widely found in eukaryotes including plants, animals and microorganisms. In plants, the PEBP family plays vital roles in regulating flowering time and morphogenesis and is highly associated to agronomic traits and yields of crops, which has been identified and characterized in many plant species but not well studied in Tartary buckwheat (Fagopyrum tataricum Gaertn.), an important coarse food grain with medicinal value.

Methods

Genome-wide analysis of FtPEBP gene family members in Tartary buckwheat was performed using bioinformatic tools. Subcellular localization analysis was performed by confocal microscopy. The expression levels of these genes in leaf and inflorescence samples were analyzed using qRT-PCR.

Results

Fourteen Fagopyrum tataricum PEBP (FtPEBP) genes were identified and divided into three sub-clades according to their phylogenetic relationships. Subcellular localization analysis of the FtPEBP proteins in tobacco leaves indicated that FT- and TFL-GFP fusion proteins were localized in both the nucleus and cytoplasm. Gene structure analysis showed that most FtPEBP genes contain four exons and three introns. FtPEBP genes are unevenly distributed in Tartary buckwheat chromosomes. Three tandem repeats were found among FtFT5/FtFT6, FtMFT1/FtMFT2 and FtTFL4/FtTFL5. Five orthologous gene pairs were detected between F. tataricum and F. esculentum. Seven light-responsive, nine hormone-related and four stress-responsive elements were detected in FtPEBPs promoters. We used real-time PCR to investigate the expression levels of FtPEBPs among two flowering-type cultivars at floral transition time. We found FtFT1/FtFT3 were highly expressed in leaf and young inflorescence of early-flowering type, whereas they were expressed at very low levels in late-flowering type cultivars. Thus, we deduced that FtFT1/FtFT3 may be positive regulators for flowering and yield of Tartary buckwheat. These results lay an important foundation for further studies on the functions of FtPEBP genes which may be utilized for yield improvement.

The evolution and functional divergence of FT-related genes in controlling flowering time in Brassica rapa ssp. rapa

KEY MESSAGE

The BrrFT paralogues exhibit distinct expression patterns and play different roles in regulating flowering time, and BrrFT4 competes with BrrFT1 and BrrFT2 to interact with BrrFD proteins. Flowering time is an important agricultural trait for Brassica crops, and early bolting strongly affects the yield and quality of Brassica rapa ssp. rapa. Flowering Locus T paralogues play an important role in regulating flowering time. In this study, we identified FT-related genes in turnip by phylogenetic classification, and four BrrFT homoeologs that shared with high identities with BraFT genes were isolated. The different gene structures, promoter binding sites, and expression patterns observed indicated that these genes may play different roles in flowering time regulation. Further genetic and biochemical experiments showed that as for FT-like paralogues, BrrFT2 acted as the key floral inducer, and BrrFT1 seems to act as a mild 'florigen' protein. However, BrrFT4 acts as a floral repressor and antagonistically regulates flowering time by competing with BrrFT1 and BrrFT2 to bind BrrFD proteins. BrrFT3 may have experienced loss of function via base shift mutation. Our results revealed the potential roles of FT-related genes in flowering time regulation in turnip.

FLOWERING LOCUS T genes control floral induction in lotus

The transition to flowering is a vital process in the lotus life cycle that significantly impacts its ornamental value and seed production. However, the molecular basis of floral transition in lotus remains largely unknown. Here, eight homologous FLOWERING LOCUS T (FT) genes were initially characterized in lotus, which were designated as NnFT1-NnFT8. All of these genes were found to possess the conserved PEBP domain and exhibited high transcript levels in both lotus leaves and floral organs. The proNnFT:β-glucuronidase (GUS) assay exhibited GUS staining in the vascular tissues of leaves. Furthermore, subcellular localization revealed that NnFT proteins were present in various cellular organelles, including the nucleus, cytoplasm, and endoplasmic reticulum. Overexpression of two NnFT homologs, NnFT2 and NnFT3, rescued the late flowering phenotype in the Arabidopsis ft-10 mutant, indicating the stimulative roles of NnFTs in floral induction. Moreover, NnFTs demonstrated interactions with a bZIP transcription factor, FLOWERING LOCUS D (NnFD), both in vitro and in vivo. These findings will not only deepen our understanding of the regulatory mechanism underlying lotus floral transition, but also provide valuable genetic resources for creating new lotus varieties with extended blooming periods using molecular strategies in the future.

Cultivating potential: Harnessing plant stem cells for agricultural crop improvement

Meristems are stem cell-containing structures that produce all plant organs and are therefore important targets for crop improvement. Developmental regulators control the balance and rate of cell divisions within the meristem. Altering these regulators impacts meristem architecture and, as a consequence, plant form. In this review, we discuss genes involved in regulating the shoot apical meristem, inflorescence meristem, axillary meristem, root apical meristem, and vascular cambium in plants. We highlight several examples showing how crop breeders have manipulated developmental regulators to modify meristem growth and alter crop traits such as inflorescence size and branching patterns. Plant transformation techniques are another innovation related to plant meristem research because they make crop genome engineering possible. We discuss recent advances on plant transformation made possible by studying genes controlling meristem development. Finally, we conclude with discussions about how meristem research can contribute to crop improvement in the coming decades.

Onion cryptochrome 1 (AcCRY1) regulates photomorphogenesis and photoperiod flowering in Arabidopsis and exploration of its functional mechanisms under blue light

Cryptochromes (CRYs), as blue-light photoreceptors, play a crucial role in regulating flowering time and hypocotyl and cotyledon development. Their physiological functions have been extensively studied in various plant species. However, research on onions remains limited. In this study, we identified AcCRY1 and conducted preliminary investigations into its function. Our results demonstrate that AcCRY1 possesses a conserved domain typical of cryptochromes with high homology to those found in monocots. Furthermore, we examined the expression level of AcCRY1 in onion. The green tissues is significantly higher compared to non-green tissues, and it exhibits a significant response to blue-light induction. AcCRY1 demonstrates cytoplasmic localization under blue-light conditions, while it localizes in the nucleus during darkness, indicating a strong dependence on blue-light for its subcellular distribution. In comparison to cry1, overexpression of AcCRY1 leads to a significant shorten in seedling hypocotyl length, notable expansion of cotyledons, and acceleration of flowering time. The yeast two-hybrid experiment demonstrated the in vitro interaction between AcCRY1, AcCOP1, and AcSPA1. Additionally, BIFC analysis confirmed their interaction in Onion epidermis. Notably, under blue-light conditions, a significantly enhanced binding activity was observed compared to dark conditions. These findings establish a functional foundation for the regulatory role of AcCRY1 in important physiological processes of onion and provide initial insights into the underlying molecular mechanisms.

Melica as an emerging model system for comparative studies in temperate Pooideae grasses

BACKGROUND AND AIMS

Pooideae grasses contain some of the world's most important crop and forage species. Although much work has been conducted on understanding the genetic basis of trait diversification within a few annual Pooideae, comparative studies at the subfamily level are limited by a lack of perennial models outside 'core' Pooideae. We argue for development of the perennial non-core genus Melica as an additional model for Pooideae, and provide foundational data regarding the group's biogeography and history of character evolution.

METHODS

Supplementing available ITS and ndhF sequence data, we built a preliminary Bayesian-based Melica phylogeny, and used it to understand how the genus has diversified in relation to geography, climate and trait variation surveyed from various floras. We also determine biomass accumulation under controlled conditions for Melica species collected across different latitudes and compare inflorescence development across two taxa for which whole genome data are forthcoming.

KEY RESULTS

Our phylogenetic analyses reveal three strongly supported geographically structured Melica clades that are distinct from previously hypothesized subtribes. Despite less geographical affinity between clades, the two sister 'Ciliata' and 'Imperfecta' clades segregate from the more phylogenetically distant 'Nutans' clade in thermal climate variables and precipitation seasonality, with the 'Imperfecta' clade showing the highest levels of trait variation. Growth rates across Melica are positively correlated with latitude of origin. Variation in inflorescence morphology appears to be explained largely through differences in secondary branch distance, phyllotaxy and number of spikelets per secondary branch.

CONCLUSIONS

The data presented here and in previous studies suggest that Melica possesses many of the necessary features to be developed as an additional model for Pooideae grasses, including a relatively fast generation time, perenniality, and interesting variation in physiology and morphology. The next step will be to generate a genome-based phylogeny and transformation tools for functional analyses.

An overview of floral regulatory genes in annual and perennial plants

The floral initiation in angiosperms is a complex process influenced by endogenous and exogenous signals. With this approach, we aim to provide a comprehensive review to integrate this complex floral regulatory process and summarize the regulatory genes and their functions in annuals and perennials. Seven primary paths leading to flowering have been discovered in Arabidopsis under several growth condition that include; photoperiod, ambient temperature, vernalization, gibberellins, autonomous, aging and carbohydrates. These pathways involve a series of interlinked signaling pathways that respond to both internal and external signals, such as light, temperature, hormones, and developmental cues, to coordinate the expression of genes that are involved in flower development. Among them, the photoperiodic pathway was the most important and conserved as some of the fundamental loci and mechanisms are shared even by closely related plant species. The activation of floral regulatory genes such as FLC, FT, LFY, and SOC1 that determine floral meristem identity and the transition to the flowering stage result from the merging of these pathways. Recent studies confirmed that alternative splicing, antisense RNA and epigenetic modification play crucial roles by regulating the expression of genes related to blooming. In this review, we documented recent progress in the floral transition time in annuals and perennials, with emphasis on the specific regulatory mechanisms along with the application of various molecular approaches including overexpression studies, RNA interference and Virus-induced flowering. Furthermore, the similarities and differences between annual and perennial flowering will aid significant contributions to the field by elucidating the mechanisms of perennial plant development and floral initiation regulation.

Functional Modules in the Meristems: "Tinkering" in Action

BACKGROUND

A feature of higher plants is the modular principle of body organisation. One of these conservative morphological modules that regulate plant growth, histogenesis and organogenesis is meristems-structures that contain pools of stem cells and are generally organised according to a common principle. Basic content: The development of meristems is under the regulation of molecular modules that contain conservative interacting components and modulate the expression of target genes depending on the developmental context. In this review, we focus on two molecular modules that act in different types of meristems. The WOX-CLAVATA module, which includes the peptide ligand, its receptor and the target transcription factor, is responsible for the formation and control of the activity of all meristem types studied, but it has its own peculiarities in different meristems. Another regulatory module is the so-called florigen-activated complex, which is responsible for the phase transition in the shoot vegetative meristem (e.g., from the vegetative shoot apical meristem to the inflorescence meristem).

CONCLUSIONS

The review considers the composition and functions of these two functional modules in different developmental programmes, as well as their appearance, evolution and use in plant breeding.

RD29A promoter constitutively drives a rice Hd3a expression to promote early-flowering in Saussurea involucrate Kar. et Kir. ex Maxim

S. involucratae, an endemic and endangered plant, is a valuable and traditional Chinese medicinal herb. In order to control the flowering time of S. involucratae, we used the well-known stress inducible RD29A promoter to drive Hd3a (a FT ortholog from rice) expression in S. involucratae. Unexpectedly, the majority of regenerated buds in RD29A::Hd3a transgenic lines (S-RH) produced flowers in tissue culture stage under normal growth (25 ± 2 °C) condition. Their flowering time was not further influenced by salt treatment. Hd3a in S-RH was strongly expressed in MS media supplemented with or without 50 mM NaCl. RD29A::GUS transgenic experiments further revealed that RD29A constitutively promoted GUS expression in both S. involucrate and halophyte Thellungiella halophile, in contrast to glycophic plants Oryza sativa L. 'Zhonghua 11', in which its expression was up-regulated by cold, salinity, and drought stress. The results supported the hypothesis that RD29A promoter activity is inducible in stress-sensitive plants, but constitutive in stress-tolerant ones. Importantly, S-RH plants produced pollen grains and seeds under normal conditions. Additionally, we found that OsLEA3-1::Hd3a and HSP18.2::Hd3a could not promote S. involucrate to flower under either normal conditions or abiotic stresses. Taken together, we demonstrated the potential of RD29A::Hd3a might be served as a feasible approach in breeding S. involucrate under normal condition.

Their C-termini divide Brassica rapa FT-like proteins into FD-interacting and FD-independent proteins that have different effects on the floral transition

Members of the FLOWERING LOCUS T (FT)-like clade of phosphatidylethanolamine-binding proteins (PEBPs) induce flowering by associating with the basic leucine zipper (bZIP) transcription factor FD and forming regulatory complexes in angiosperm species. However, the molecular mechanism of the FT-FD heterocomplex in Chinese cabbage (Brassica rapa ssp. pekinensis) is unknown. In this study, we identified 12 BrPEBP genes and focused our functional analysis on four BrFT-like genes by overexpressing them individually in an FT loss-of-function mutant in Arabidopsis thaliana. We determined that BrFT1 and BrFT2 promote flowering by upregulating the expression of floral meristem identity genes, whereas BrTSF and BrBFT, although close in sequence to their Arabidopsis counterparts, had no clear effect on flowering in either long- or short-day photoperiods. We also simultaneously genetically inactivated BrFT1 and BrFT2 in Chinese cabbage using CRISPR/Cas9-mediated genome editing, which revealed that BrFT1 and BrFT2 may play key roles in inflorescence organogenesis as well as in the transition to flowering. We show that BrFT-like proteins, except for BrTSF, are functionally divided into FD interactors and non-interactors based on the presence of three specific amino acids in their C termini, as evidenced by the observed interconversion when these amino acids are mutated. Overall, this study reveals that although BrFT-like homologs are conserved, they may have evolved to exert functionally diverse functions in flowering via their potential to be associated with FD or independently from FD in Brassica rapa.

Anatomical observation and transcriptome analysis of buds reveal the association between the AP2 gene family and reproductive induction in hybrid larch (Larix kaempferi × Larix olgensis)

Hybrid larch is an excellent afforestation species in northern China. The instability of seed yield is an urgent problem to be solved. The biological characteristics related to seed setting in larch are different from those in angiosperms and other gymnosperms. Studying the developmental mechanism of the larch sporophyll can deepen our understanding of conifer reproductive development and help to ensure an adequate supply of seeds in the seed orchard. The results showed that the formation of microstrobilus primordia in hybrid larch could be observed in anatomical sections collected in the middle of July. The contents of endogenous gibberellin 3 (GA3) and abscisic acid (ABA) were higher and the contents of GA4, GA7, jasmonic acid and salicylic acid were lower in multiseeded larch. Transcriptome analysis showed that transcription factors were significantly enriched in the AP2 family. There were 23 differentially expressed genes in the buds of the multiseeded and less-seeded types, and the expression of most of these genes was higher in the buds than in the needles. We conclude that mid-July is the early stage of reproductive organ development in hybrid larch and is suitable for the study of reproductive development. GA3 and ABA may be helpful for improving seed setting in larch, and 23 AP2/EREBP family genes are involved in the regulation of reproductive development in larch.

Large-scale analyses of angiosperm Flowering Locus T genes reveal duplication and functional divergence in monocots

FLOWERING LOCUS T (FT) are well-known key genes for initiating flowering in plants. Delineating the evolutionary history and functional diversity of FT genes is important for understanding the diversification of flowering time and how plants adapt to the changing surroundings. We performed a comprehensive phylogenetic analysis of FT genes in 47 sequenced flowering plants and the 1,000 Plant Transcriptomes (1KP) database with a focus on monocots, especially cereals. We revealed the evolutionary history of FT genes. The FT genes in monocots can be divided into three clades (I, II, and III), whereas only one monophyletic group was detected in early angiosperms, magnoliids, and eudicots. Multiple rounds of whole-genome duplications (WGD) events followed by gene retention contributed to the expansion and variation of FT genes in monocots. Amino acid sites in the clade II and III genes were preferentially under high positive selection, and some sites located in vital domain regions are known to change functions when mutated. Clade II and clade III genes exhibited high variability in important regions and functional divergence compared with clade I genes; thus, clade I is more conserved than clade II and III. Genes in clade I displayed higher expression levels in studied organs and tissues than the clade II and III genes. The co-expression modules showed that some of the FT genes might have experienced neofunctionalization and subfunctionalization, such as the acquisition of environmental resistance. Overall, FT genes in monocots might form three clades by the ancient gene duplication, and each clade was subsequently subjected to different selection pressures and amino acid substitutions, which eventually led to different expression patterns and functional diversification. Our study provides a global picture of FT genes' evolution in monocots, paving a road for investigating FT genes' function in future.

Allele mining, amplicon sequencing and computational prediction of Solanum melongena L. FT/TFL1 gene homologs uncovers putative variants associated to seed dormancy and germination

The FT/TFL1 gene homolog family plays a crucial role in the regulation of floral induction, seed dormancy and germination in angiosperms. Despite its importance, the FT/TFL1 gene homologs in eggplant (Solanum melongena L.) have not been characterized to date. In this study, we performed a genome-wide identification of FT/TFL1 genes in eggplant using in silico genome mining. The presence of these genes was validated in four economically important eggplant cultivars (Surya, EP-47 Annamalai, Pant Samrat and Arka Nidhi) through Pacbio RSII amplicon sequencing. Our results revealed the presence of 12 FT/TFL1 gene homologs in eggplant, with evidence of diversification among FT-like genes suggesting their possible adaptations towards various environmental stimuli. The amplicon sequencing also revealed the presence of two alleles for certain genes (SmCEN-1, SmCEN-2, SmMFT-1 and SmMFT-2) of which SmMFT-2 was associated with seed dormancy and germination. This association was further supported by the observation that seed dormancy is rarely reported in domesticated eggplant cultivars, but is commonly observed in wild species. A survey of the genetic regions in domesticated cultivars and a related wild species, S. incanum, showed that the alternative allele of S. incanum was present in some members of the Pant Samrat cultivar, but was absent in most other cultivars. This difference could contribute to the differences in seed traits between wild and domesticated eggplants.

Omics Profiles of Non-GM Tubers from Transgrafted Potato with a GM Scion

"Transgrafting" is a grafting procedure whereby a transgenic plant body is grafted to a non-transgenic plant body. It is a novel plant breeding technology that allows non-transgenic plants to obtain benefits usually conferred to transgenic plants. Many plants regulate flowering by perceiving the day-length cycle via expression of FLOWERING LOCUS T (FT) in the leaves. The resulting FT protein is translocated to the shoot apical meristem via the phloem. In potato plants, FT is involved in the promotion of tuber formation. Here we investigated the effects of a genetically modified (GM) scion on the edible parts of the non-GM rootstock by using potato plants transformed with StSP6A, a novel potato homolog of the FT gene. Scions prepared from GM or control (wild-type) potato plants were grafted to non-GM potato rootstocks; these were designated as TN and NN plants, respectively. After tuber harvest, we observed no significant differences in potato yield between TN and NN plants. Transcriptomic analysis revealed that only one gene-with unknown function-was differentially expressed between TN and NN plants. Subsequent proteomic analysis indicated that several members of protease inhibitor families, known as anti-nutritional factors in potato, were slightly more abundant in TN plants. Metabolomic analysis revealed a slight increase in metabolite abundance in NN plants, but we observed no difference in the accumulation of steroid glycoalkaloids, toxic metabolites found in potato. Finally, we found that TN and NN plants did not differ in nutrient composition. Taken together, these results indicate that FT expression in scions had a limited effect on the metabolism of non-transgenic potato tubers.

GIGANTEA regulates lateral root formation by modulating auxin signaling in Arabidopsis thaliana

Lateral root (LR) formation is a vital organogenetic process that determines the root architecture in plants. The number of root branches governs the degree of anchorage, efficiency of nutrients acquisition, and water uptake. The molecular pathways involved in LR formation have been extensively studied in Arabidopsis thaliana (At). A plant hormone, Auxin, is a key regulator of root development and promotes LR formation in plants. A plethora of Arabidopsis genes have been identified to regulate LR initiation, patterning, and emergence processes. Recently, the involvement of flowering time control pathways and circadian clock pathways in LR development has come to light, but the connecting link between these processes is still missing. We have established that GIGANTEA (GI), a key component of photoperiodic flowering, can regulate the formation of LRs in Arabidopsis. GI is known to be involved in red light signaling and circadian clock signaling pathways. Here, we report that over-expression of GI enhances LR formation in red light in At. Real-time PCR analysis shows that GI positively regulates the transcription of TRANSPORT INHIBITOR RESPONSE 1 (TIR1) which is an upstream component of auxin signaling. Furthermore, gi-100 mutant downregulates the LR initiation signaling gene, AUXIN RESPONSE FACTOR 7 (ARF7), and its downstream target gene, LATERAL ORGAN BOUNDARIES-DOMAIN 16 (LBD16). Hence, GI acts as a positive regulator of IAA14-ARF7-LBD16 modules during LR initiation. We have also checked the effect of GI on the expression of NAC1 and AIR3 genes which are controlled by TIR1 during LR formation. Our results show that GI induces the NAC1 transcription and its downstream gene, AIR3 expression, which leads to the enhancement of LR initiation. Taken together, our results suggest that GI controls the expression of TIR1 to govern auxin signaling during LR formation in presence of red light and GI can act as a link between circadian clock signaling, flowering time control pathways, light signaling, and lateral root development pathways.

Evolutionary Conservation and Transcriptome Analyses Attribute Perenniality and Flowering to Day-Length Responsive Genes in Bulbous Barley (Hordeum bulbosum)

Rapid population growth and dramatic climatic turnovers are challenging global crop production. These challenges are spurring plant breeders to enhance adaptation and sustainability of major crops. One intriguing approach is to turn annual systems into perennial ones, yet long-term classical breeding efforts to induce perenniality have achieved limited success. Here, we report the results of our investigation of the genetic basis of bulb formation in the nonmodel organism Hordeum bulbosum, a perennial species closely related to barley. To identify candidate genes that regulate bulb formation in H. bulbosum, we applied two complementary approaches. First, we explored the evolutionary conservation of expressed genes among annual Poaceae species. Next, we assembled a reference transcriptome for H. bulbosum and conducted a differential expression (DE) analysis before and after stimulating bulb initiation. Low conservation was identified in genes related to perenniality in H. bulbosum compared with other species, including bulb development and sugar accumulation genes. We also inspected these genes using a DE analysis, which enabled identification of additional genes responsible for bulb initiation and flowering regulation. We propose a molecular model for the regulation of bulb formation involving storage organ development and starch biosynthesis genes. The high conservation observed along a major part of the pathway between H. bulbosum and barley suggests a potential for the application of biotechnological techniques to accelerate breeding toward perenniality in barley.

Integrated Genomic and Transcriptomic Elucidation of Flowering in Garlic

Commercial cultivars of garlic are sterile, and therefore efficient breeding of this crop is impossible. Recent restoration of garlic fertility has opened new options for seed production and hybridization. Transcriptome catalogs were employed as a basis for garlic genetic studies, and in 2020 the huge genome of garlic was fully sequenced. We provide conjoint genomic and transcriptome analysis of the regulatory network in flowering garlic genotypes. The genome analysis revealed phosphatidylethanolamine-binding proteins (PEBP) and LEAFY (LFY) genes that were not found at the transcriptome level. Functions of TFL-like genes were reduced and replaced by FT-like homologs, whereas homologs of MFT-like genes were not found. The discovery of three sequences of LFY-like genes in the garlic genome and confirmation of their alternative splicing suggest their role in garlic florogenesis. It is not yet clear whether AsLFY1 acts alone as the "pioneer transcription factor" or AsLFY2 also provides these functions. The presence of several orthologs of flowering genes that differ in their expression and co-expression network advocates ongoing evolution in the garlic genome and diversification of gene functions. We propose that the process of fertility deprivation in garlic cultivars is based on the loss of transcriptional functions of the specific genes.

Transposon insertions within alleles of BnaFT.A2 are associated with seasonal crop type in rapeseed

We identified two new transposon insertions within the promoter of BnaFT.A2 in addition to an existing 288 bp MITE within the second intron. Each insertion event corresponds to a distinct BnaFT.A2 haplotype and is closely associated with established crop seasonal ecotypes. Florigen, encoded by FLOWERING LOCUS T (FT), plays key roles not only as a flowering hormone, but also a universal growth factor affecting several aspects of plant architecture. In rapeseed, BnaFT.A2 has been revealed as one of the major loci associated with flowering time and different ecotypes. However, it is unclear how allelic variations of BnaFT.A2 affect its function in flowering time regulation and beyond. In this study, we confirmed an existing 288 bp miniature inverted-repeat transposable element (MITE) insertion within the second intron and identified two new insertions within the promoter of BnaFT.A2-a 3971 bp CACTA and a 1079 bp Helitron. Each insertion event corresponds to a distinct BnaFT.A2 haplotype and is closely associated with established crop seasonal ecotypes. These alleles have similar tissue-specific expression patterns but discrete transcriptional patterns tightly associated with rapeseed flowering time and ecotype. RNAi lines and mutants of BnaFT.A2 flowered significantly later than controls. Differentially expressed genes (DEGs), identified in transcriptomic profiling of seedling leaves from two loss-of-function mutants (Bnaft.a2-L1 and Bnaft.a2-L2) compared with controls, indicated significant enrichment for hormone metabolic genes and roles related to plant cell wall synthesis and photosynthesis. Plants with loss-of-function BnaFT.A2 had smaller leaves and lower net photosynthetic rate compared to controls. These findings not only further clarify the genetic basis of flowering time variation and ecotype formation in B. napus, but also provide an additional toolbox for genetic improvement of seasonal adaptation and production.

Structural changes and adaptative evolutionary constraints in FLOWERING LOCUS T and TERMINAL FLOWER1-like genes of flowering plants

Regulation of flowering is a crucial event in the evolutionary history of angiosperms. The production of flowers is regulated through the integration of different environmental and endogenous stimuli, many of which involve the activation of different genes in a hierarchical and complex signaling network. The FLOWERING LOCUS T/TERMINAL FLOWER 1 (FT/TFL1) gene family is known to regulate important aspects of flowering in plants. To better understand the pivotal events that changed FT and TFL1 functions during the evolution of angiosperms, we reconstructed the ancestral sequences of FT/TFL1-like genes and predicted protein structures through in silico modeling to identify determinant sites that evolved in both proteins and allowed the adaptative diversification in the flowering phenology and developmental processes. In addition, we demonstrate that the occurrence of destabilizing mutations in residues located at the phosphatidylcholine binding sites of FT structure are under positive selection, and some residues of 4th exon are under negative selection, which is compensated by the occurrence of stabilizing mutations in key regions and the P-loop to maintain the overall protein stability. Our results shed light on the evolutionary history of key genes involved in the diversification of angiosperms.

Genomic insights into the evolutionary history and diversification of bulb traits in garlic

Background

Garlic is an entirely sterile crop with important value as a vegetable, condiment, and medicine. However, the evolutionary history of garlic remains largely unknown.

Results

Here we report a comprehensive map of garlic genomic variation, consisting of amazingly 129.4 million variations. Evolutionary analysis indicates that the garlic population diverged at least 100,000 years ago, and the two groups cultivated in China were domesticated from two independent routes. Consequently, 15.0 and 17.5% of genes underwent an expression change in two cultivated groups, causing a reshaping of their transcriptomic architecture. Furthermore, we find independent domestication leads to few overlaps of deleterious substitutions in these two groups due to separate accumulation and selection-based removal. By analysis of selective sweeps, genome-wide trait associations and associated transcriptomic analysis, we uncover differential selections for the bulb traits in these two garlic groups during their domestication.

Conclusions

This study provides valuable resources for garlic genomics-based breeding, and comprehensive insights into the evolutionary history of this clonal-propagated crop.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13059-022-02756-1.

Identification of QTLs and a putative candidate gene involved in rhizome enlargement of Asian lotus (Nelumbo nucifera)

QTL mapping studies identified three reliable QTLs of rhizome enlargement in lotus. NnBEL6 located within the confidence interval of the major QTL cqREI-LG2 is a key candidate gene enhancing rhizome enlargement. Lotus (Nelumbo) is perennial aquatic plant with nutritional, pharmacological, and ornamental significance. Rhizome is an underground lotus stem that acts as a storage organ and as a reproductive tissue for asexual production. The enlargement of lotus rhizome is an important adaptive strategy for surviving the cold winter. The aims of this study were to identify quantitative trait loci (QTLs) for rhizome enlargement traits including rhizome enlargement index (REI) and number of enlarged rhizome (NER), and to uncover their associated candidate genes. A high-density genetic linkage map was constructed, consisting of 2935 markers binned from 236,840 SNPs. A total of 14 significant QTLs were detected for REI and NER, which explained 6.7-22.3% of trait variance. Three QTL regions were repeatedly identified in at least 2 years, and a major QTL, designated cqREI-LG2, with a rhizome-enlargement effect and about 20% of the phenotypic contribution was identified across the 3 climatic years. A candidate NnBEL6 gene located within the confidence interval of cqREI-LG2 was considered to be putatively involved in lotus rhizome enlargement. The expression of NnBEL6 was exclusively induced by rhizome swelling. Sequence comparison of NnBEL6 among lotus cultivars revealed a functional Indel site in its promoter that likely initiates the rhizome enlargement process. Transgenic potato assay was used to confirm the role of NnBEL6 in inducing tuberization. The successful identification QTLs and functional validation of NnBEL6 gene reported in this study will enrich our knowledge on the genetic basis of rhizome enlargement in lotus.

Role of FaSOC1 and FaCO in the seasonal control of reproductive and vegetative development in the perennial crop Fragaria × ananassa

The diploid woodland strawberry (F. vesca) represents an important model for the genus Fragaria. Significant advances in the understanding of the molecular mechanisms regulating seasonal alternance of flower induction and vegetative reproduction has been made in this species. However, this research area has received little attention on the cultivated octoploid strawberry (F. × ananassa) despite its enormous agronomical and economic importance. To advance in the characterization of this intricated molecular network, expression analysis of key flowering time genes was performed both in short and long days and in cultivars with seasonal and perpetual flowering. Analysis of overexpression of FaCO and FaSOC1 in the seasonal flowering 'Camarosa' allowed functional validation of a number of responses already observed in F. vesca while uncovered differences related to the regulation of FaFTs expression and gibberellins (GAs) biosynthesis. While FvCO has been shown to promote flowering and inhibit runner development in the perpetual flowering H4 accession of F. vesca, our study showed that FaCO responds to LD photoperiods as in F. vesca but delayed flowering to some extent, possibly by induction of the strong FaTFL1 repressor in crowns. A contrasting effect on runnering was observed in FaCO transgenic plants, some lines showing reduced runner number whereas in others runnering was slightly accelerated. We demonstrate that the role of the MADS-box transcription factor FaSOC1 as a strong repressor of flowering and promoter of vegetative growth is conserved in woodland and cultivated strawberry. Our study further indicates an important role of FaSOC1 in the photoperiodic repression of FLOWERING LOCUS T (FT) genes FaFT2 and FaFT3 while FaTFL1 upregulation was less prominent than that observed in F. vesca. In our experimental conditions, FaSOC1 promotion of vegetative growth do not require induction of GA biosynthesis, despite GA biosynthesis genes showed a marked photoperiodic upregulation in response to long days, supporting GA requirement for the promotion of vegetative growth. Our results also provided insights into additional factors, such as FaTEM, associated with the vegetative developmental phase that deserve further characterization in the future.

Functional diversification and molecular mechanisms of FLOWERING LOCUS T/TERMINAL FLOWER 1 family genes in horticultural plants

Flowering is an important process in higher plants and is regulated by a variety of factors, including light, temperature, and phytohormones. Flowering restriction has a considerable impact on the commodity value and production cost of many horticultural crops. In Arabidopsis, the FT/TFL1 gene family has been shown to integrate signals from various flowering pathways and to play a key role in the transition from flower production to seed development. Studies in several plant species of the FT/TFL1 gene family have revealed it harbors functional diversity in the regulation of flowering. Here, we review the functional evolution of the FT/TFL1 gene family in horticulture plants and its unique regulatory mechanisms; in addition, the FT/TFL1 family of genes as an important potential breeding target is explored.

Expression Patterns of Key Genes in the Photoperiod and Vernalization Flowering Pathways in Lilium longiflorum with Different Bulb Sizes

Lilium longiflorum is a wild Lilium, and its flowering transition requires a long period of cold exposure to meet the demand of vernalization. The responses of different sized bulbs to cold exposure and photoperiod are different, and the floral transition pathways of small and large bulbs are different. In this study, small and large bulbs were placed in cold storage for different weeks and then cultured at a constant ambient temperature of 25 °C under long day (LD) and short day (SD) conditions. Then, the flowering characteristics and expression patterns of key genes related to the vernalization and photoperiod pathways in different groups were calculated and analyzed. The results showed that the floral transition of Lilium longiflorum was influenced by both vernalization and photoperiod, that vernalization and LD conditions can significantly improve the flowering rate of Lilium longiflorum, and that the time from planting to visible flowering buds’ appearance was decreased. The flowering time and rate of large bulbs were greatly influenced by cold exposure, and the vernalization pathway acted more actively at the floral transition stage. The floral transition of small bulbs was affected more by the photoperiod pathway. Moreover, it was speculated that cold exposure may promote greater sensitivity of the small bulbs to LD conditions. In addition, the expression of LlVRN1, LlFKF1, LlGI, LlCO5, LlCO7, LlCO16, LlFT1, LlFT3 and LlSOC1 was high during the process of floral transition, and LlCO13, LlCO14 and LlCO15 were highly expressed in the vegetative stage. The expression of LlCO13 and LlCO14 was different under different lighting conditions, and the flowering induction function of LlCO9 and LlFT3 was related to vernalization. Moreover, LlFKF1, LlGI, LlCO5, LlCO16, LlSOC1 and LlFT2 were involved in the entire growth process of plants, while LlCO6, LlCO16 and LlFT1 are involved in the differentiation and formation of small bulblets of plants after the inflorescence stage, and this process is also closely related to LD conditions. This study has great significance for understanding the molecular mechanisms of the vernalization and photoperiod flowering pathways of Lilium longiflorum.

Comparative metabolome analysis unravels a close association between dormancy release and metabolic alteration induced by low temperature in lily bulbs

The correlation between dormancy release and metabolic metabolic changes in lily bulbs during low temperature storage was investigated. Low temperature is a major environmental factor required for dormancy release in lily bulbs. Although great advances in plant metabolomics have been achieved, knowledge about the molecular basis of lily bulb metabolomes at different developmental stages in response to low temperature is still limited. In this work, the dormancy release, vegetative growth, flowering, metabolic profile and gene expression in the less dormant cultivar Lilium longiforum × Oriental hybrid 'Triumphator' (T) and the more dormant cultivar Lilium Asiatic hybrid 'Honesty' (H) were compared. Exposure to low temperature (LT) successfully promoted stem elongation, floral transition and flowering of both T and H bulbs. However, exposure to room temperature (RT) restricted stalk elongation of both T and H bulbs, and prohibited floral transition and flowering of H bulbs. Correspondingly, higher antioxidant enzyme activity and total primary metabolite contents were observed in the apical bud of T bulbs. Gene expression analysis revealed that expressions of LiFT, LiFLK, LiSOC1 and LiCBF were decreased, whereas the expression of LiSVP and LiFLC were increased, in the apical bud of H bulbs under RT storage condition. Our findings reveal that the growth and dormancy breaking of lily bulbs are closely associated with the metabolic changes in the apical buds during postharvest storage.

The New Variation in the Promoter Region of FLOWERING LOCUS T Is Involved in Flowering in Brassica rapa

Flowering time is an important agronomic trait in Brassica rapa and has a wide range of variation. The change from vegetative to reproductive development is a major transition period, especially in flowering vegetable crops. In this study, two non-heading Chinese cabbage varieties with significantly different flowering times, Pak-choi (B. rapa var. communis Tesn et Lee) and Caitai (B. rapa var. tsaitai Hort.), were used to construct segregated F₂ populations. The bulk-segregant approach coupled with whole genome re-sequencing was used for QTL sequencing (QTL-seq) analysis to map flowering time traits. The candidate genes controlling flowering time in B. rapa were predicted by homologous gene alignment and function annotation. The major-effect QTL ft7.1 was detected on chromosome A07 of B. rapa, and the FT family gene BrFT was predicted as the candidate gene. Moreover, a new promoter regional difference of 1577 bp was revealed by analyzing the sequence of the BrFT gene. The promoter region activity analysis and divergent gene expression levels indicated that the difference in the promoter region may contribute to different flowering times. These findings provide insights into the mechanisms underlying the flowering time in Brassica and the candidate genes regulating flowering in production.

Genomic Survey of PEBP Gene Family in Rice: Identification, Phylogenetic Analysis, and Expression Profiles in Organs and under Abiotic Stresses

Phosphatidylethanolamine-binding-protein (PEBP) domain-containing proteins play important roles in multiple developmental processes of plants; however, functions of few members in the PEBP gene family have been elucidated in rice and other crops. In this study, we found that twenty OsPEBPs genes identified in rice are not evenly distributed on the chromosomes. Four colinear pairs are identified, suggesting the duplication of OsPEBPs during evolution. The OsPEBPs are classified into six subgroups by phylogenetic analysis. The structure of all the OsPEBP genes and encoded proteins are similar. The 262 PEBP domain-containing proteins from crops are divided into six groups. The number of colinear pairs varies between rice and other crops. More than thirty cis-acting elements in the promoter region of OsPEBPs are discovered. Expression profiles of OsPEBP genes are differential. Most of the OsPEBPs expression can be regulated by NaCl, ABA, JA, and light, indicating that OsPEBPs may be involved in the control of the response to the environmental signals. These results lay sound foundation to further explore their functions in development of rice and crops.

FLOWERING LOCUS T paralogs control the annual growth cycle in Populus trees

In temperate and boreal regions, perennials adapt their annual growth cycle to the change of seasons. These adaptations ensure survival in harsh environmental conditions, allowing growth at different latitudes and altitudes, and are therefore tightly regulated. Populus tree species cease growth and form terminal buds in autumn when photoperiod falls below a certain threshold.¹ This is followed by establishment of dormancy and cold hardiness over the winter. At the center of the photoperiodic pathway in Populus is the gene FLOWERING LOCUS T2 (FT2), which is expressed during summer and harbors significant SNPs in its locus associated with timing of bud set.^1-4 The paralogous gene FT1, on the other hand, is hyper-induced in chilling buds during winter.^3,5 Even though its function is so far unknown, it has been suggested to be involved in the regulation of flowering and the release of winter dormancy.^3,5 In this study, we employ CRISPR-Cas9-mediated gene editing to individually study the function of the FT-like genes in Populus trees. We show that while FT2 is required for vegetative growth during spring and summer and regulates the entry into dormancy, expression of FT1 is absolutely required for bud flush in spring. Gene expression profiling suggests that this function of FT1 is linked to the release of winter dormancy rather than to the regulation of bud flush per se. These data show how FT duplication and sub-functionalization have allowed Populus trees to regulate two completely different and major developmental control points during the yearly growth cycle.

A single nucleotide deletion in the third exon of FT-D1 increases the spikelet number and delays heading date in wheat (Triticum aestivum L.)

The spikelet number and heading date are two crucial and correlated traits for yield in wheat. Here, a quantitative trait locus (QTL) analysis was conducted in F₈ recombinant inbred lines (RILs) derived from crossing two common wheats with different spikelet numbers. A total of 15 stable QTL influencing total spikelet number (TSN) and heading date (HD) were detected. Notably, FT-D1, a well-known flowering time gene in wheat, was located within the finely mapped interval of a major QTL on 7DS (QTsn/Hd.cau-7D). A causal indel of one G in the third exon of FT-D1 was significantly associated with total spikelet number and heading date. Consistently, CRISPR/Cas9 mutant lines with homozygous mutations in FT-D1 displayed an increase in total spikelet number and heading date when compared with wild type. Moreover, one simple and robust marker developed according to the polymorphic site of FT-D1 revealed that this one G indel had been preferentially selected to adapt to different environments. Collectively, these data provide further insights into the genetic basis of spikelet number and heading date, and the diagnostic marker of FT-D1 will be useful for marker-assisted pyramiding in wheat breeding.

Analysis of the PEBP gene family and identification of a novel FLOWERING LOCUS T orthologue in sugarcane

Sugarcane (Saccharum spp.) is an important economic crop for both sugar and biomass, the yields of which are negatively affected by flowering. The molecular mechanisms controlling flowering in sugarcane are nevertheless poorly understood. RNA-seq data analysis and database searches have enabled a comprehensive description of the PEBP gene family in sugarcane. It is shown to consist of at least 13 FLOWERING LOCUS T (FT)-like genes, two MOTHER OF FT AND TFL (MFT)-like genes, and four TERMINAL FLOWER (TFL)-like genes. As expected, these genes all show very high homology to their corresponding genes in Sorghum, and also to FT-like, MFT-like, and TFL-like genes in maize, rice, and Arabidopsis. Functional analysis in Arabidopsis showed that the sugarcane ScFT3 gene can rescue the late flowering phenotype of the Arabidopsis ft-10 mutant, whereas ScFT5 cannot. High expression levels of ScFT3 in leaves of short day-induced sugarcane plants coincided with initial stages of floral induction in the shoot apical meristem as shown by histological analysis of meristem dissections. This suggests that ScFT3 is likely to play a role in floral induction in sugarcane; however, other sugarcane FT-like genes may also be involved in the flowering process.

Functional Redundancy of FLOWERING LOCUS T 3b in Soybean Flowering Time Regulation

Photoperiodic flowering is an important agronomic trait that determines adaptability and yield in soybean and is strongly influenced by FLOWERING LOCUS T (FT) genes. Due to the presence of multiple FT homologs in the genome, their functions in soybean are not fully understood. Here, we show that GmFT3b exhibits functional redundancy in regulating soybean photoperiodic flowering. Bioinformatic analysis revealed that GmFT3b is a typical floral inducer FT homolog and that the protein is localized to the nucleus. Moreover, GmFT3b expression was induced by photoperiod and circadian rhythm and was more responsive to long-day (LD) conditions. We generated a homozygous ft3b knockout and three GmFT3b-overexpressing soybean lines for evaluation under different photoperiods. There were no significant differences in flowering time between the wild-type, the GmFT3b overexpressors, and the ft3b knockouts under natural long-day, short-day, or LD conditions. Although the downstream flowering-related genes GmFUL1 (a, b), GmAP1d, and GmLFY1 were slightly down-regulated in ft3b plants, the floral inducers GmFT5a and GmFT5b were highly expressed, indicating potential compensation for the loss of GmFT3b. We suggest that GmFT3b acts redundantly in flowering time regulation and may be compensated by other FT homologs in soybean.

The Genetic and Hormonal Inducers of Continuous Flowering in Orchids: An Emerging View

Orchids are the flowers of magnetic beauty. Vivid and attractive flowers with magnificent shapes make them the king of the floriculture industry. However, the long-awaited flowering is a drawback to their market success, and therefore, flowering time regulation is the key to studies about orchid flower development. Although there are some rare orchids with a continuous flowering pattern, the molecular regulatory mechanisms are yet to be elucidated to find applicable solutions to other orchid species. Multiple regulatory pathways, such as photoperiod, vernalization, circadian clock, temperature and hormonal pathways are thought to signalize flower timing using a group of floral integrators. This mini review, thus, organizes the current knowledge of floral time regulators to suggest future perspectives on the continuous flowering mechanism that may help to plan functional studies to induce flowering revolution in precious orchid species.

Identification and Functional Characterization of FLOWERING LOCUS T in Platycodon grandiflorus

Platycodon grandiflorus roots have been used as a foodstuff and traditional medicine for thousands of years in East Asia. In order to increase the root development of P. grandiflorus, cultivators removed the inflorescences, suggesting the possible negative effect of flowering on root development. This indicates that the genetic improvement of P. grandiflorus by late flowering is a potential approach to increase productivity. However, nothing is known about key genes integrating multiple flowering pathways in P. grandiflorus. In order to fill this gap, we identified potential homologs of the FLOWERING LOCUS T (FT) gene in P. grandiflorus. The alignment with other FT members and phylogenetic analysis revealed that the P. grandiflorus FT (PlgFT) protein contains highly conserved functional domains and belongs to the FT-like clade. The expression analysis revealed spatial variations in the transcription of PlgFT in different organs. In addition, the expression level of PlgFT was increased by high temperature but not by photoperiodic light input signals, presumably due to lacking the CONSTANS binding motif in its promoter region. Furthermore, PlgFT induced early flowering upon its overexpression in P. grandiflorus, suggesting the functional role of PlgFT in flowering. Taken together, we functionally characterized PlgFT as a master regulator of P. grandiflorus flowering under inductive high temperature, which will serve as an important target gene for improving the root productivity.

FT-like paralogs are repressed by an SVP protein during the floral transition in Phalaenopsis orchid

An SVP protein, PhSVP, bound to the CArG-boxes in the promoter regions of FT-like paralogs and repressed their expression, thus affecting the floral transition in Phalaenopsis orchid. Phalaenopsis is an important ornamental flower native to tropical rain forests. It usually reaches vegetative maturity after 4-5 leaves and, after a juvenile stage, forms a flower spike (inflorescence) from the axillary buds. The PEBP gene family encodes a phosphatidyl-ethanolamine-binding protein (PEBP) domain involved in regulating flowering and other aspects of plant development. Here, we identified eight PEBP family genes in Phalaenopsis and detected the expression patterns of seven of them in various organs. Among them, PhFT1 (Phalaenopsis hybrid FLOWERING LOCUS T1), PhFT3, PhFT5, and PhMFT (Phalaenopsis hybrid MOTHER OF FT AND TFL1) promoted flowering in transgenic Arabidopsis, while PhFT6 inhibited flowering. PhSVP (Phalaenopsis hybrid SHORT VEGETATIVE PHASE), an SVP protein that repressed flowering in Arabidopsis, bound to the CArG-boxes in the promoter regions of PhFT3, PhFT6, and PhMFT in a yeast one-hybrid assay. Additionally, dual-luciferase and transient expression assays showed that PhSVP significantly inhibits the expression of both PhFT3 and PhFT6. Together, our work provides a comprehensive understanding of the PhFT-like genes that can promote or repress flowering, and it suggests strategies for regulating the floral transition in Phalaenopsis that exploit the evolutionary versatility of PhFTs to respond to various signals stimuli.

Components and Functional Diversification of Florigen Activation Complexes in Cotton

In shoot apex cells of rice, a hexameric florigen activation complex (FAC), comprising flowering locus T (FT), 14-3-3 and the basic leucine zipper transcription factor FD, activates downstream target genes and regulates several developmental transitions, including flowering. The allotetraploid cotton (Gossypium hirsutum L.) contains only one FT locus in both of the A- and D-subgenomes. However, there is limited information regarding cotton FACs. Here, we identified a 14-3-3 protein that interacts strongly with GhFT in the cytoplasm and the nuclei, and five FD homoeologous gene pairs were characterized. In vivo, all five GhFD proteins interacted with Gh14-3-3 and GhFT in the nucleus. GhFT, 14-3-3 and all the GhFDs interacted in the nucleus as well, suggesting that they formed a ternary complex. Virus-induced silencing of GhFD1, -2 and -4 in cotton delayed flowering and inhibited the expression of floral meristem identity genes. Silencing GhFD3 strongly decreased lateral root formation, suggesting a function in lateral root development. GhFD overexpression in Arabidopsis and transcriptional activation assays suggested that FACs containing GhFD1 and GhFD2 function mainly in promoting flowering with partial functional redundancy. Moreover, GhFD3 was specifically expressed in lateral root meristems and dominantly activated the transcription of auxin response factor genes, such as ARF19. Thus, the diverse functions of FACs may depend on the recruited GhFD. Creating targeted genetic mutations in the florigen system using Clustered regularly interspaced short palindromic repeats (CRISPR) and their associated proteins (Cas) genome editing may fine-tune flowering and improve plant architecture.

The FveFT2 florigen/FveTFL1 antiflorigen balance is critical for the control of seasonal flowering in strawberry while FveFT3 modulates axillary meristem fate and yield

Plant architecture is central in determining crop yield. In the short-day species strawberry, a crop vegetatively propagated by daughter-plants produced by stolons, fruit yield is further dependent on the trade-off between sexual reproduction (fruits) and asexual reproduction (daughter-plants). Both are largely dependent on meristem identity, which establishes the development of branches, stolons and inflorescences. Floral initiation and plant architecture are modulated by the balance between two related proteins, FLOWERING LOCUS T (FT) and TERMINAL FLOWER 1 (TFL1). We explored in woodland strawberry the role of the uncharacterised FveFT2 and FveFT3 genes and of the floral repressor FveTFL1 through gene expression analyses, grafting and genetic transformation (overexpression and gene editing). We demonstrate the unusual properties of these genes. FveFT2 is a nonphotoperiodic florigen permitting short-day (SD) flowering and FveTFL1 is the long-hypothesised long-day systemic antiflorigen that contributes, together with FveFT2, to the photoperiodic regulation of flowering. We additionally show that FveFT3 is not a florigen but promotes plant branching when overexpressed, that is likely to be through changing axillary meristem fate, therefore resulting in a 3.5-fold increase in fruit yield at the expense of stolons. We show that our findings can be translated into improvement of cultivated strawberry in which FveFT2 overexpression significantly accelerates flowering.

Asymmetric expansions of FT and TFL1 lineages characterize differential evolution of the EuPEBP family in the major angiosperm lineages

Background

In flowering plants, precise timing of the floral transition is crucial to maximize chances of reproductive success, and as such, this process has been intensively studied. FLOWERING LOCUS T (FT) and TERMINAL FLOWER1 (TFL1) have been identified as closely related eukaryotic phosphatidylethanolamine-binding proteins (‘EuPEBPs’) that integrate multiple environmental stimuli, and act antagonistically to determine the optimal timing of the floral transition. Extensive research has demonstrated that FT acts similar to hormonal signals, being transported in the phloem from its primary site of expression in leaves to its primary site of action in the shoot meristem; TFL1 also appears to act as a mobile signal. Recent work implicates FT, TFL1, and the other members of the EuPEBP family, in the control of other important processes, suggesting that the EuPEBP family may be key general regulators of developmental transitions in flowering plants. In eudicots, there are a small number of EuPEBP proteins, but in monocots, and particularly grasses, there has been a large, but uncharacterized expansion of EuPEBP copy number, with unknown consequences for the EuPEBP function.

Results

To systematically characterize the evolution of EuPEBP proteins in flowering plants, and in land plants more generally, we performed a high-resolution phylogenetic analysis of 701 PEBP sequences from 208 species. We refine previous models of EuPEBP evolution in early land plants, demonstrating the algal origin of the family, and pin-pointing the origin of the FT/TFL1 clade at the base of monilophytes. We demonstrate how a core set of genes (MFT1, MFT2, FT, and TCB) at the base of flowering plants has undergone differential evolution in the major angiosperm lineages. This includes the radical expansion of the FT family in monocots into 5 core lineages, further re-duplicated in the grass family to 12 conserved clades.

Conclusions

We show that many grass FT proteins are strongly divergent from other FTs and are likely neo-functional regulators of development. Our analysis shows that monocots and eudicots have strongly divergent patterns of EuPEBP evolution.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01128-8.

Fine-Tuning Florigen Increases Field Yield Through Improving Photosynthesis in Soybean

Crop yield has been maintaining its attraction for researchers because of the demand of global population growth. Mutation of flowering activators, such as florigen, increases plant biomass at the expense of later flowering, which prevents crop maturity in the field. As a result, it is difficult to apply flowering activators in agriculture production. Here, we developed a strategy to utilize florigen to significantly improve soybean yield in the field. Through the screening of transgenic lines of RNAi-silenced florigen homologs in soybean (Glycine-max-Flowering Locus T Like, GmFTL), we identified a line, GmFTL-RNAi#1, with minor changes in both GmFTL expression and flowering time but with notable increase in soybean yield. As expected, GmFTL-RNAi#1 matured normally in the field and exhibited markedly high yield over multiple locations and years, indicating that it is possible to reach a trade-off between flowering time and high yield through the fine-tuning expression of flowering activators. Further studies uncovered an unknown mechanism by which GmFTL negatively regulates photosynthesis, a substantial source of crop yield, demonstrating a novel function of florigen. Thus, because of the highly conserved functions of florigen in plants and the classical RNAi approach, the findings provide a promising strategy to harness early flowering genes to improve crop yield.

Tuber and Tuberous Root Development

Root and tuber crops have been an important part of human nutrition since the early days of humanity, providing us with essential carbohydrates, proteins, and vitamins. Today, they are especially important in tropical and subtropical regions of the world, where they help to feed an ever-growing population. Early induction and storage organ size are important agricultural traits, as they determine yield over time. During potato tuberization, environmental and metabolic status are sensed, ensuring proper timing of tuberization mediated by phloem-mobile signals. Coordinated cellular restructuring and expansion growth, as well as controlled storage metabolism in the tuber, are executed. This review summarizes our current understanding of potato tuber development and highlights similarities and differences to important tuberous root crop species like sweetpotato and cassava. Finally, we point out knowledge gaps that need to be filled before a complete picture of storage organ development can emerge.

Functional analysis and expression patterns of members of the FLOWERING LOCUS T (FT) gene family in Lilium

Lilium is an important commercial flowering species, and there are many varieties and more than 100 species of wild Lilium. Lilium × formolongi is usually propagated from seedlings, and the flowering of these plants is driven mainly by the photoperiodic pathway. Most of the other lily plants are propagated via bulblets and need to be vernalized; these plants can be simply divided into pretransplantation types and posttransplantation types according to the time at which the floral transition occurs. We identified three Lilium FLOWERING LOCUS T (LFT) family members in 7 Lilium varieties, and for each gene, the coding sequence of the different varieties was identical. Among these genes, the LFT1 gene of Lilium was most homologous to the AtFT gene, which promotes flowering in Arabidopsis. We analyzed the expression patterns of LFT genes in Lilium × formolongi seedlings and in different Lilium varieties, and the results showed that LFT1 and LFT3 may promote floral induction. Compared with LFT3, LFT1 may have a greater effect on floral induction in Lilium, which is photoperiod sensitive, while LFT3 may play a more important role in the floral transition of lily plants, which have a high requirement for vernalization. LFT2 may be involved in the differentiation of bulblets, which was verified by tissue culture experiments, and LFT1 may have other functions involved in promoting bulblet growth. The functions of LFT genes were verified by the use of transgenic Arabidopsis thaliana plants, which showed that both the LFT1 and LFT3 genes can promote early flowering in Arabidopsis. Compared with LFT3, LFT1 promoted flowering more obviously, and thus, this gene could be an important promoter of floral induction in Lilium.

Auxin signaling and vascular cambium formation enable storage metabolism in cassava tuberous roots

Cassava storage roots are among the most important root crops worldwide, and represent one of the most consumed staple foods in sub-Saharan Africa. The vegetatively propagated tropical shrub can form many starchy tuberous roots from its stem. These storage roots are formed through the activation of secondary root growth processes. However, the underlying genetic regulation of storage root development is largely unknown. Here we report distinct structural and transcriptional changes occurring during the early phases of storage root development. A pronounced increase in auxin-related transcripts and the transcriptional activation of secondary growth factors, as well as a decrease in gibberellin-related transcripts were observed during the early stages of secondary root growth. This was accompanied by increased cell wall biosynthesis, most notably increased during the initial xylem expansion within the root vasculature. Starch storage metabolism was activated only after the formation of the vascular cambium. The formation of non-lignified xylem parenchyma cells and the activation of starch storage metabolism coincided with increased expression of the KNOX/BEL genes KNAT1, PENNYWISE, and POUND-FOOLISH, indicating their importance for proper xylem parenchyma function.