The Vaccinium corymbosum FLOWERING LOCUS T-like gene (VcFT): a flowering activator reverses photoperiodic and chilling requirements in blueberry

Time-Course Transcriptome Analysis Unveils the CoFKF1-CoMYB4-CoFT1 Regulatory Module in Flowering Control of Camellia oleifera Abel

Camellia oleifera Abel. (C. oleifera) represents a significant woody edible oil species predominantly distributed in southern China. Timely flowering is essential for the growth, development and tea oil production of C. oleifera. However, the mechanisms underpinning this process remain insufficiently understood. In this study, it was demonstrated through time-course transcriptome analysis that we revealed that CoFKF1-like1 (CoFKF1) serves as a central regulatory gene in the flowering process of C. oleifera. The ectopic expression of CoFKF1 resulted in the induction of early flowering. Furthermore, it was observed that CoFKF1 interacts with the transcription factor CoMYB4 in a blue-light-dependent manner, facilitating its ubiquitination and subsequent degradation. Genetically, CoMYB4 was identified as functioning downstream of CoFKF1 by directly binding to the promoter of CoFT1 and repressing its promoter activity. In conclusion, these findings elucidate that CoFKF1 promotes flowering by reducing the stability of the CoMYB4 protein, thereby enhancing CoFT1 promoter activity. Collectively, the results provide critical insights into the flowering mechanisms of C. oleifera and present a promising avenue to optimise its flowering period via the CoFKF1-CoMYB4-CoFT1 module.

LoniComp: a platform for gene function comparison and analysis between Lonicera japonica and Lonicera macranthoides

Lonicera japonica and L. macranthoides are popular medicinal plants used for treating various diseases. Recently, new chromosome level genomes of Lonicera have provided a huge resource for understanding gene function. Although LjaFGD was created for analyzing L. japonica gene functions, it is now outdated due to updated genomes and more transcriptome data. Utilizing new chromosome-level genomic and transcriptomic data, we developed co-expression networks of L. japonica and L. macranthoides. Gene annotations were performed by comparing sequences with NR, TAIR, Swissprot, and trEMBL databases. GO and KEGG annotations were predicted using InterProScan and GhostKOALA software, while gene families were identified with iTAK, HMMER, and InParanoid. To fully leverage the utilization value of public resources and data, we developed LoniComp ( www.gzybioinformatics.cn/LoniComp ) as a newer and information-rich alternative, a platform for gene function comparison and analysis by integrating genomic, transcriptomic data and processed functional annotations. It features tools like BLAST, Extract Sequence, Enrichment, Heatmap, DEG, and JBrowse2. We demonstrated its use with examples like LjFT and LjMYB12. It offers superior genomic data, transcriptomic resources, and analysis tools compared to LjaFGD, aiding researchers in gene function studies and comparison.

Regulatory frameworks involved in the floral induction, formation and developmental programming of woody horticultural plants: a case study on blueberries

Flowering represents a crucial stage in the life cycles of plants. Ensuring strong and consistent flowering is vital for maintaining crop production amidst the challenges presented by climate change. In this review, we summarized key recent efforts aimed at unraveling the complexities of plant flowering through genetic, genomic, physiological, and biochemical studies in woody species, with a special focus on the genetic control of floral initiation and activation in woody horticultural species. Key topics covered in the review include major flowering pathway genes in deciduous woody plants, regulation of the phase transition from juvenile to adult stage, the roles of CONSTANS (CO) and CO-like gene and FLOWERING LOCUS T genes in flower induction, the floral regulatory role of GA-DELLA pathway, and the multifunctional roles of MADS-box genes in flowering and dormancy release triggered by chilling. Based on our own research work in blueberries, we highlighted the central roles played by two key flowering pathway genes, FLOWERING LOCUS T and SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1, which regulate floral initiation and activation (dormancy release), respectively. Collectively, our survey shows both the conserved and diverse aspects of the flowering pathway in annual and woody plants, providing insights into the potential molecular mechanisms governing woody plants. This paves the way for enhancing the resilience and productivity of fruit-bearing crops in the face of changing climatic conditions, all through the perspective of genetic interventions.

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.

A MADS-box gene-induced early flowering pear (Pyrus communis L.) for accelerated pear breeding

There have been a considerable number of studies that have successfully sped up the flowering cycle in woody perennial horticultural species. One particularly successful study in apple (Malus domestica) accelerated flowering using a silver birch (Betula pendula) APETALA1/FRUITFULL MADS-box gene BpMADS4, which yielded a good balance of vegetative growth to support subsequent flower and fruit development. In this study, BpMADS4 was constitutively expressed in European pear (Pyrus communis) to establish whether this could be used as a tool in a rapid pear breeding program. Transformed pear lines flowered within 6-18 months after grafting onto a quince (Cydonia oblonga) rootstock. Unlike the spindly habit of early flowering apples, the early flowering pear lines displayed a normal tree-like habit. Like apple, the flower appearance was normal, and the flowers were fertile, producing fruit and seed upon pollination. Seed from these transformed lines were germinated and 50% of the progeny flowered within 3 months of sowing, demonstrating a use for these in a fast breeding program.

Multiple transcriptome comparisons reveal the essential roles of FLOWERING LOCUS T in floral initiation and SOC1 and SVP in floral activation in blueberry

The flowering mechanisms, especially chilling requirement-regulated flowering, in deciduous woody crops remain to be elucidated. Flower buds of northern highbush blueberry cultivar Aurora require approximately 1,000 chilling hours to bloom. Overexpression of a blueberry FLOWERING LOCUS T (VcFT) enabled precocious flowering of transgenic “Aurora” mainly in non-terminated apical buds during flower bud formation, meanwhile, most of the mature flower buds could not break until they received enough chilling hours. In this study, we highlighted two groups of differentially expressed genes (DEGs) in flower buds caused by VcFT overexpression (VcFT-OX) and full chilling. We compared the two groups of DEGs with a focus on flowering pathway genes. We found: 1) In non-chilled flower buds, VcFT-OX drove a high VcFT expression and repressed expression of a major MADS-box gene, blueberry SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (VcSOC1) resulting an increased VcFT/VcSOC1 expression ratio; 2) In fully chilled flower buds that are ready to break, the chilling upregulated VcSOC1 expression in non-transgenic “Aurora” and repressed VcFT expression in VcFT-OX “Aurora”, and each resulted in a decreased ratio of VcFT to VcSOC1; additionally, expression of a blueberry SHORT VEGETATIVE PHASE (VcSVP) was upregulated in chilled flower buds of both transgenic and non-transgenic’ “Aurora”. Together with additional analysis of VcFT and VcSOC1 in the transcriptome data of other genotypes and tissues, we provide evidence to support that VcFT expression plays a significant role in promoting floral initiation and that VcSOC1 expression is a key floral activator. We thus propose a new hypothesis on blueberry flowering mechanism, of which the ratios of VcFT-to-VcSOC1 at transcript levels in the flowering pathways determine flower bud formation and bud breaking. Generally, an increased VcFT/VcSOC1 ratio or increased VcSOC1 in leaf promotes precocious flowering and flower bud formation, and a decreased VcFT/VcSOC1 ratio with increased VcSOC1 in fully chilled flower buds contributes to flower bud breaking.

Tobacco rattle virus-induced VcANS gene silencing in blueberry fruit

Blueberry (Vaccinium corymbosum L.), a woody perennial bush in the genus Vaccinium, is an economically important and popular fruit crop worldwide. Development the superior cultivars, which including excellent fruit traits, not only means higher yielding and economic efficiency, but also produce fruit that to meet the preferences of different consumers. Excavating fruit quality-related genes, studying their functions, and using transgenic or molecular-assisted breeding are beneficial to the development of excellent blueberry varieties. Genetic transformation is an excellent way to study the function of genes in plants, however, it is a labor-intensive and time-consuming process to genetically transform many woody plants, including blueberry. Virus-induced gene silencing (VIGS) provides an efficient approach to knock-down the expression of target genes for functional analysis. In this study, tobacco rattle virus induced genes silencing (TRV-VIGS) was established in blueberry fruits using the VcANS gene as a reporter. The silenced sector of the skin of blueberry fruits injected with pTRV2 (plasmid Tobacco Rattle Virus, TRV-RNA2)::VcANS remained green or white at 25 days after agroinfiltration. In agroinfiltrated materials, the VcANS transcript levels were much lower in fruits with phenotypic changes (delayed color change) than in those infiltrated with the pTRV2 empty vector. Silencing of VcANS also affected the expression of other genes involved in the anthocyanin synthesis pathway. The experimental results support that VcANS can be used as an effective marker gene for VIGS system. In addition, the TRV-VIGS system has been successfully established in blueberry fruits, which provided an effective verification method for functional identification of unknown genes in blueberry fruits.

Molecular mechanisms of flowering phenology in trees

Flower initiation is a phenological developmental process strictly regulated in all flowering plants. Studies in Arabidopsis thaliana, a model plant organism in plant biology and genetics, and major cereal crops have provided fundamental knowledge and understanding of the underlying molecular mechanisms and regulation in annuals. However, this flowering process and underly molecular mechanisms in perennials are much more complicated than those in annuals and remain poorly understood and documented. In recent years, the increasing availability of perennial plant genomes and advances in biotechnology have allowed the identification and characterization of flowering-associated gene orthologs in perennials. In this review, we compared and summarized the recent progress in regulation of flowering time in perennial trees, with an emphasis on the perennial-specific regulatory mechanisms. Pleiotropic effects on tree growth habits such as juvenility, seasonal activity-dormancy growth, and the applications of tree flowering phenology are discussed.

Transcriptome Analysis Reveals Putative Induction of Floral Initiation by Old Leaves in Tea-Oil Tree (Camellia oleifera ‘changlin53’)

Floral initiation is a major phase change in the spermatophyte, where developmental programs switch from vegetative growth to reproductive growth. It is a key phase of flowering in tea-oil trees that can affect flowering time and yield, but very little is known about the molecular mechanism of floral initiation in tea-oil trees. A 12-year-old Camellia oleifera (cultivar ‘changlin53’) was the source of experimental materials in the current study. Scanning electron microscopy was used to identify the key stage of floral initiation, and transcriptome analysis was used to reveal the transcriptional regulatory network in old leaves involved in floral initiation. We mined 5 DEGs related to energy and 55 DEGs related to plant hormone signal transduction, and we found floral initiation induction required a high level of energy metabolism, and the phytohormones signals in the old leaves regulate floral initiation, which occurred at stage I and II. Twenty-seven rhythm-related DEGs and 107 genes associated with flowering were also identified, and the circadian rhythm interacted with photoperiod pathways to induce floral initiation. Unigene0017292 (PSEUDO-RESPONSE REGULATOR), Unigene0046809 (LATE ELONGATED HYPOCOTYL), Unigene0009932 (GIGANTEA), Unigene0001842 (CONSTANS), and Unigene0084708 (FLOWER LOCUS T) were the key genes in the circadian rhythm-photoperiod regulatory network. In conjunction with morphological observations and transcriptomic analysis, we concluded that the induction of floral initiation by old leaves in C. oleifera ‘changlin53’ mainly occurred during stages I and II, floral initiation was completed during stage III, and rhythm–photoperiod interactions may be the source of the main signals in floral initiation induced by old leaves.

From Floral Induction to Blooming: The Molecular Mysteries of Flowering in Woody Plants

Flowering is a pivotal developmental process in response to the environment and determines the start of a new life cycle in plants. Woody plants usually possess a long juvenile nonflowering phase followed by an adult phase with repeated flowering cycles. The molecular mechanism underlying flowering regulation in woody plants is believed to be much more complex than that in annual herbs. In this review, we briefly describe the successive but distinct flowering processes in perennial trees, namely the vegetative phase change, the floral transition, floral organogenesis, and final blooming, and summarize in detail the most recent advances in understanding how woody plants regulate flowering through dynamic gene expression. Notably, the florigen gene FLOWERING LOCUS T(FT) and its antagonistic gene TERMINAL FLOWER 1 (TFL1) seem to play a central role in various flowering transition events. Flower development in different taxa requires interactions between floral homeotic genes together with AGL6 conferring floral organ identity. Finally, we illustrate the issues and corresponding measures of flowering regulation investigation. It is of great benefit to the future study of flowering in perennial trees.

Chromosome-level genome assembly of the diploid blueberry Vaccinium darrowii provides insights into its subtropical adaptation and cuticle synthesis

Vaccinium darrowii is a subtropical wild blueberry species that has been used to breed economically important southern highbush cultivars. The adaptive traits of V. darrowii to subtropical climates can provide valuable information for breeding blueberry and perhaps other plants, especially against the background of global warming. Here, we assembled the V. darrowii genome into 12 pseudochromosomes using Oxford Nanopore long reads complemented with Hi-C scaffolding technologies, and we predicted 41 815 genes using RNA-sequencing evidence. Syntenic analysis across three Vaccinium species revealed a highly conserved genome structure, with the highest collinearity between V. darrowii and Vaccinium corymbosum. This conserved genome structure may explain the high fertility observed during crossbreeding of V. darrowii with other blueberry cultivars. Analysis of gene expansion and tandem duplication indicated possible roles for defense- and flowering-associated genes in the adaptation of V. darrowii to the subtropics. Putative SOC1 genes in V. darrowii were identified based on phylogeny and expression analysis. Blueberries are covered in a thick cuticle layer and contain anthocyanins, which confer their powdery blue color. Using RNA sequencing, we delineated the cuticle biosynthesis pathways of Vaccinium species in V. darrowii. This result can serve as a reference for breeding berries whose colors are appealing to customers. The V. darrowii reference genome, together with the unique traits of this species, including its diploid genome, short vegetative phase, and high compatibility in hybridization with other blueberries, make V. darrowii a potential research model for blueberry species.

Synchronized flowering in pomegranate, following pruning, is associated with expression of the FLOWERING LOCUS T homolog, PgFT1

Flowering in angiosperms is a crucial event that marks the transition from the vegetative to the reproductive phase. In many perennials, pruning is an important horticultural practice that induces synchronized and profuse flowering. In pomegranate, vegetative growth immediately after pruning is associated with activation of PgCENa, a flowering suppressor of the phosphatidyl ethanolamine binding protein (PEBP) family, while a reduction is associated with synchronous flowering. We show that flowering in pomegranate is activated by expression of another PEBP family member, PgFT1, a homolog of the FLOWERING LOCUS T (FT) gene that promotes flowering. PgFT1 shows a rapid reduction in expression during the extensive vegetative growth immediately after pruning but shows robust expression during synchronous flowering post-pruning, in flower-bearing shoots but not in branches that do not bear flowers. A continuous low-level flowering in the absence of pruning is associated with continuous but reduced expression of PgFT1. Flowering by heterologous expression of PgFT1 in Arabidopsis is affected by a single amino acid change in the C-terminal region of PgFT1, which upon correction, promotes flowering in Arabidopsis. Our study provides insights into the molecular mechanisms by which pruning affects flowering pathways in tropical perennial fruit plants such as pomegranate.

Effect of Transgenic Rootstock Grafting on the Omics Profiles in Tomato

Grafting of non-transgenic scion onto genetically modified (GM) rootstocks provides superior agronomic traits in the GM rootstock, and excellent fruits can be produced for consumption. In such grafted plants, the scion does not contain any foreign genes, but the fruit itself is likely to be influenced directly or indirectly by the foreign genes in the rootstock. Before market release of such fruit products, the effects of grafting onto GM rootstocks should be determined from the perspective of safety use. Here, we evaluated the effects of a transgene encoding β-glucuronidase (GUS) on the grafted tomato fruits as a model case. An edible tomato cultivar, Stella Mini Tomato, was grafted onto GM Micro-Tom tomato plants that had been transformed with the GUS gene. The grafted plants showed no difference in their fruit development rate and fresh weight regardless of the presence or absence of the GUS gene in the rootstock. The fruit samples were subjected to transcriptome (NGS-illumina), proteome (shotgun LC-MS/MS), metabolome (LC-ESI-MS and GC-EI-MS), and general food ingredient analyses. In addition, differentially detected items were identified between the grafted plants onto rootstocks with or without transgenes (more than two-fold). The transcriptome analysis detected approximately 18,500 expressed genes on average, and only 6 genes were identified as differentially expressed. Principal component analysis of 2,442 peaks for peptides in proteome profiles showed no significant differences. In the LC-ESI-MS and GC-EI-MS analyses, a total of 93 peak groups and 114 peak groups were identified, respectively, and only 2 peak groups showed more than two-fold differences. The general food ingredient analysis showed no significant differences in the fruits of Stella scions between GM and non-GM Micro-Tom rootstocks. These multiple omics data showed that grafting on the rootstock harboring the GUS transgene did not induce any genetic or metabolic variation in the scion.

Ovule Development and in Planta Transformation of Paphiopedilum Maudiae by Agrobacterium-Mediated Ovary-Injection

In this paper, the development of the Paphiopedilum Maudiae embryo sac at different developmental stages after pollination was assessed by confocal laser scanning microscopy. The mature seeds of P. Maudiae consisted of an exopleura and a spherical embryo, but without an endosperm, while the inner integument cells were absorbed by the developing embryo. The P. Maudiae embryo sac exhibited an Allium type of development. The time taken for the embryo to develop to a mature sac was 45-50 days after pollination (DAP) and most mature embryo sacs had completed fertilization and formed zygotes by about 50–54 DAP. In planta transformation was achieved by injection of the ovaries by Agrobacterium, resulting in 38 protocorms or seedlings after several rounds of hygromycin selection, corresponding to 2, 7, 5, 1, 3, 4, 9, and 7 plantlets from Agrobacterium-mediated ovary-injection at 30, 35, 42, 43, 45, 48, 50, and 53 DAP, respectively. Transformation efficiency was highest at 50 DAP (2.54%), followed by 2.48% at 53 DAP and 2.45% at 48 DAP. Four randomly selected hygromycin-resistant plants were GUS-positive after PCR analysis. Semi-quantitative PCR and quantitative real-time PCR analysis revealed the expression of the hpt gene in the leaves of eight hygromycin-resistant seedlings following Agrobacterium-mediated ovary-injection at 30, 35, 42, 43, 45, 48, 50, and 53 DAP, while hpt expression was not detected in the control. The best time to inject P. Maudiae ovaries in planta with Agrobacterium is 48-53 DAP, which corresponds to the period of fertilization. This protocol represents the first genetic transformation protocol for any Paphiopedilum species and will allow for expanded molecular breeding programs to introduce useful and interesting genes that can expand its ornamental and horticulturally important characteristics.

Isolation and Functional Characterization of MsFTa, a FLOWERING LOCUS T Homolog from Alfalfa (Medicago sativa)

The production of hay and seeds of alfalfa, an important legume forage for the diary industry worldwide, is highly related to flowering time, which has been widely reported to be integrated by FLOWERING LOCUS T (FT). However, the function of FT(s) in alfalfa is largely unknown. Here, we identified MsFTa, an FT ortholog in alfalfa, and characterized its role in flowering regulation. MsFTa shares the conserved exon/intron structure of FTs, and the deduced MsFTa is 98% identical to MtFTa1 in Medicago trucatula. MsFTa was diurnally regulated with a peak before the dark period, and was preferentially expressed in leaves and floral buds. Transient expression of MsFTa-GFP fusion protein demonstrated its localization in the nucleus and cytoplasm. When ectopically expressed, MsFTa rescued the late-flowering phenotype of ft mutants from Arabidopsis and M. trucatula. MsFTa over-expression plants of both Arabidopsis and M. truncatula flowered significantly earlier than the non-transgenic controls under long day conditions, indicating that exogenous MsFTa strongly accelerated flowering. Hence, MsFTa functions positively in flowering promotion, suggesting that MsFTa may encode a florigen that acts as a key regulator in the flowering pathway. This study provides an effective candidate gene for optimizing alfalfa flowering time by genetically manipulating the expression of MsFTa.

VcFT-induced mobile florigenic signals in transgenic and transgrafted blueberries

FLOWERING LOCUS T (FT) can promote early flowering in annual species, but such role has not been well demonstrated in woody species. We produced self and reciprocal grafts involving non-transgenic blueberry (NT) and transgenic blueberry (T) carrying a 35S-driven blueberry FT (VcFT-OX). We demonstrated that the transgenic VcFT-OX rootstock promoted flowering of non-transgenic blueberry scions in the NT (scion):T (rootstock) grafts. We further analyzed RNA-Seq profiles and six groups of phytohormones in both NT:T and NT:NT plants. We observed content changes of several hormone metabolites, in a descending order, in the transgenic NT:T, non-transgenic NT:T, and non-transgenic NT:NT leaves. By comparing differential expression transcripts (DETs) of these tissues in relative to their control, we found that the non-transgenic NT:T leaves had many DETs shared with the transgenic NT:T leaves, but very few with the transgenic NT:T roots. Interestingly, a number of these shared DETs belong to hormone pathway genes, concurring with the content changes of hormone metabolites in both transgenic and non-transgenic leaves of the NT:T plants. These results suggest that phytohormones induced by VcFT-OX in the transgenic leaves might serve as part of the signals that resulted in early flowering in both transgenic plants and the non-transgenic NT:T scions.

VcRR2 regulates chilling-mediated flowering through expression of hormone genes in a transgenic blueberry mutant

The molecular mechanism underlying dormancy release and the induction of flowering remains poorly understood in woody plants. Mu-legacy is a valuable blueberry mutant, in which a transgene insertion caused increased expression of a RESPONSE REGULATOR 2-like gene (VcRR2). Mu-legacy plants, compared with nontransgenic 'Legacy' plants, show dwarfing, promotion of flower bud formation, and can flower under nonchilling conditions. We conducted transcriptomic comparisons in leaves, chilled and nonchilled flowering buds, and late-pink buds, and analyzed a total of 41 metabolites of six groups of hormones in leaf tissues of both Mu-legacy and 'Legacy' plants. These analyses uncovered that increased VcRR2 expression promotes the expression of a homolog of Arabidopsis thaliana ENT-COPALYL DIPHOSPHATE SYNTHETASE 1 (VcGA1), which induces new homeostasis of hormones, including increased gibberellin 4 (GA4) levels in Mu-legacy leaves. Consequently, increased expression of VcRR2 and VcGA1, which function in cytokinin responses and gibberellin synthesis, respectively, initiated the reduction in plant height and the enhancement of flower bud formation of the Mu-legacy plants through interactions of multiple approaches. In nonchilled flower buds, 29 differentially expressed transcripts of 17 genes of five groups of hormones were identified in transcriptome comparisons between Mu-legacy and 'Legacy' plants, of which 22 were chilling responsive. Thus, these analyses suggest that increased expression of VcRR2 was collectively responsible for promoting flower bud formation in highbush blueberry under nonchilling conditions. We report here for the first time the importance of VcRR2 to induce a suite of downstream hormones that promote flowering in woody plants.

Overexpression of the MADS-box gene K-domain increases the yield potential of blueberry

MADS-box genes play a significant role for plant flowering. Keratin-like (K) domains are involved in protein-to-protein interactions in the formation of the MIKC-type MADS-box domain proteins. In this study, the potential of utilizing the K domain of a Vaccinium corymbosum SOC1-like gene (VcSOC1K) was investigated to modulate expression of other blueberry MADS-box genes for increasing blueberry productivity. Chilled transgenic blueberry plants overexpressing the VcSOC1K showed a significant increase in the number of canes, floral buds, and flower and fruit clusters compared to chilled non-transgenic plants. Additionally, nonchilled transgenic plants flowered whereas nonchilled non-transgenic plants did not. Transgenic plants showed an increase in tolerance to high soil pH. Comparative transcriptome analysis of transgenic and non-transgenic leaves showed differential expression of 17% of the MADS-box genes identified in blueberry. These differentially expressed (DE) MADS-box genes were associated with genes related to plant flowering, phytohormones, and response to various biotic and abiotic stimuli. The phenotypic changes and the DE MADS-box genes caused by the overexpression of VcSOC1K not only reveal that the MADS-box genes are involved in chilling/vernalization-mediated flowering in blueberry but also demonstrated that the overexpression of the K domain can effectively modulate plant reproductive processes.

An invaluable transgenic blueberry for studying chilling-induced flowering in woody plants

BACKGROUND

Many deciduous woody crops require a minimum level of chilling to break dormancy and allow the seasonal growth of vegetative and floral buds. In this study, we report the discovery of an invaluable transgenic event of the blueberry cultivar 'Legacy' (hereafter, Mu-Legacy) for studying chilling-induced flowering in woody plants. Mu-legacy and its progeny provide a unique material to study the unknown mechanism of chilling-mediated flowering in woody plants.

RESULTS

Unlike nontransgenic 'Legacy' and plants of 48 other transgenic events, Mu-Legacy plants were able to flower under nonchilling conditions and had early flower bud formation, reduced plant size, and reduced chilling requirement for normal flowering. These characteristics were heritable and also observed in self-pollinated, transgenic T1 progenies of Mu-Legacy. A 47-Kbp genomic sequence surrounding the transgene insertion position was identified. RNA-sequencing data showed increased expression of a RESPONSE REGULATOR 2-like gene (VcRR2), located adjacent to the insertion position in Mu-Legacy and likely driven by the CaMV 35S promoter of the transgene. The Mu-Legacy showed 209 differentially expressed genes (DEGs) in nonchilled flower buds (compared to nontransgenic 'Legacy'), of which only four DEGs were in the flowering pathway. This suggests altered expression of these few genes, VcRR2 and four flowering DEGs, is sufficient to significantly change flowering behavior in Mu-Legacy.

CONCLUSIONS

The significance of VcRR2 in Mu-Legacy suggests that the VcRR2-involved cytokinin pathway likely contributes to the major differences in chilling-mediated flowering between woody and herbaceous plants. More importantly, Mu-Legacy shows increased yield potential, a decreased chilling requirement, and better winter hardiness than many low-chilling cultivars growing in southern warm winter conditions.

Genetic diversity assessed by genotyping by sequencing (GBS) and for phenological traits in blueberry cultivars

Blueberry is a small fruit crop which includes a complex group of different Vaccinium species of various ploidy levels. Commercial blueberries have been grown in Europe most recently, so there is not much information available about their adaptation into new regions. In this work we investigated adaptation to the environmental conditions of northern Spain, in terms of flowering and ripening seasons, of a set of 70 blueberry cultivars including several of the most important cultivated American species (V. corymbosum, V. virgatum, V. macrocarpon and V. uliginosum) in order to identify which types are best-suited in this geographical area of Europe. Most materials showed high chilling requirements for flowering under local conditions, while materials with low-chilling requirements showed problems in the maturation process of the flowers. Most cultivars were early or mid-season while a relative lack of late-season cultivars was observed. GBS was used for the analysis of genetic diversity in this sample of 70 cultivars. A total of 5255 SNP markers were obtained and a cluster analysis revealed three main groups associated with the ploidy level of the species. A Principal Component Analysis revealed a grouping of the V. corymbosum cultivars according to their chilling requirements. A total of 29 SNPs were identified as being highly informative for diversity analysis and potentially useful for cultivar identification and for breeding purposes. The results obtained from this research should contribute to the expansion of this crop, as well as providing data about genetic diversity useful for the preservation of genetic resources or for future breeding programs.

AcFT promotes kiwifruit in vitro flowering when overexpressed and Arabidopsis flowering when expressed in the vasculature under its own promoter

Kiwifruit (Actinidia chinensis) has three FLOWERING LOCUS T (FT) genes, AcFT, AcFT1, and AcFT2, with differential expression and potentially divergent roles. AcFT was previously shown to be expressed in source leaves and induced in dormant buds by winter chilling. Here, we show that AcFT promotes flowering in A. chinensis, despite a short sequence insertion not present in other FT-like genes. A 3.5-kb AcFT promoter region contained all the regulatory elements required to mediate vascular expression in transgenic Arabidopsis thaliana (Arabidopsis). The promoter activation was initially confined to the veins in the distal end of the leaf, before extending to the veins in the base of the leaf, and was detected in inductive and noninductive photoperiods. The 3-kb and 2.7-kb promoter regions of AcFT1 and AcFT2, respectively, demonstrated different activation patterns in Arabidopsis, corresponding to differential expression in kiwifruit. Expression of AcFT cDNA from the AcFT promoter was capable to induce early flowering in transgenic Arabidopsis in noninductive photoperiods. Further, expression of AcFT cDNA fused to the green fluorescent protein was detected in the vasculature and was also capable to advance flowering in noninductive photoperiods. Taken together, these studies implicate AcFT in regulation of kiwifruit flowering time and as a candidate for kiwifruit florigen.

Comparative transcriptome analysis of nonchilled, chilled, and late-pink bud reveals flowering pathway genes involved in chilling-mediated flowering in blueberry

BACKGROUND

Blueberry cultivars require a fixed quantity of chilling hours during winter endo-dormancy for vernalization. In this study, transcriptome analysis using RNA sequencing data from nonchilled, chilled, and late pink buds of southern highbush blueberry 'Legacy' was performed to reveal genes associated with chilling accumulation and bud break.

RESULTS

Fully chilled 'Legacy' plants flowered normally whereas nonchilled plants could not flower. Compared to nonchilled flower buds, chilled flower buds showed differential expression of 89% of flowering pathway genes, 86% of MADS-box genes, and 84% of cold-regulated genes. Blueberry orthologues of FLOWERING LOCUS T (FT) did not show a differential expression in chilled flower buds (compared to nonchilled flower bud) but were up-regulated in late-pink buds (compared to chilled flower bud). Orthologoues of major MADS-box genes were significantly up-regulated in chilled flower buds and down-regulated in late-pink buds. Functional orthologues of FLOWERING LOCUS C (FLC) were not found in blueberry. Orthologues of Protein FD (FD), TERMINAL FLOWER 1 (TFL1), and LEAFY (LFY) were down-regulated in chilled flower buds and in late-pink buds compared to nonchilled flower bud.

CONCLUSIONS

The changes from nonchilled to chilled and chilled to late-pink buds are associated with transcriptional changes in a large number of differentially expressed (DE) phytohormone-related genes and DE flowering pathway genes. The profile of DE genes suggests that orthologues of FT, FD, TFL1, LFY, and MADS-box genes are the major genes involved in chilling-mediated blueberry bud-break. The results contribute to the comprehensive investigation of the vernalization-mediated flowering mechanism in woody plants.

Overexpression of Arabidopsis FLOWERING LOCUS T (FT) gene improves floral development in cassava (Manihot esculenta, Crantz)

Cassava is a tropical storage-root crop that serves as a worldwide source of staple food for over 800 million people. Flowering is one of the most important breeding challenges in cassava because in most lines flowering is late and non-synchronized, and flower production is sparse. The FLOWERING LOCUS T (FT) gene is pivotal for floral induction in all examined angiosperms. The objective of the current work was to determine the potential roles of the FT signaling system in cassava. The Arabidopsis thaliana FT gene (atFT) was transformed into the cassava cultivar 60444 through Agrobacterium-mediated transformation and was found to be overexpressed constitutively. FT overexpression hastened flower initiation and associated fork-type branching, indicating that cassava has the necessary signaling factors to interact with and respond to the atFT gene product. In addition, overexpression stimulated lateral branching, increased the prolificacy of flower production and extended the longevity of flower development. While FT homologs in some plant species stimulate development of vegetative storage organs, atFT inhibited storage-root development and decreased root harvest index in cassava. These findings collectively contribute to our understanding of flower development in cassava and have the potential for applications in breeding.

Transcriptomic changes reveal gene networks responding to the overexpression of a blueberry DWARF AND DELAYED FLOWERING 1 gene in transgenic blueberry plants

BACKGROUND

Constitutive expression of the CBF/DREB1 for increasing freezing tolerance in woody plants is often associated with other phenotypic changes including dwarf plant and delayed flowering. These phenotypic changes have been observed when Arabidopsis DWARF AND DELAYED FLOWERING 1 (DDF1) was overexpressed in A. thaliana plants. To date, the DDF1 orthologues have not been studied in woody plants. The aim of this study is to investigate transcriptomic responses to the overexpression of blueberry (Vaccinium corymbosum) DDF1 (herein, VcDDF1-OX).

RESULTS

The VcDDF1-OX resulted in enhanced freezing tolerance in tetraploid blueberry plants and did not result in significant changes in plant size, chilling requirement, and flowering time. Comparative transcriptome analysis of transgenic 'Legacy-VcDDF1-OX' plants containing an overexpressed VcDDF1 with non-transgenic highbush blueberry 'Legacy' plants revealed the VcDDF1-OX derived differentially expressed (DE) genes and transcripts in the pathways of cold-response, plant flowering, DELLA proteins, and plant phytohormones. The increase in freezing tolerance was associated to the expression of cold-regulated genes (CORs) and the ethylene pathway genes. The unchanged plant size, dormancy and flowering were due to the minimal effect of the VcDDF1-OX on the expression of DELLA proteins, flowering pathway genes, and the other phytohormone genes related to plant growth and development. The DE genes in auxin and cytokinin pathways suggest that the VcDDF1-OX has also altered plant tolerance to drought and high salinity.

CONCLUSION

A DDF1 orthologue in blueberry functioned differently from the DDF1 reported in Arabidopsis. The overexpression of VcDDF1 or its orthologues is a new approach to increase freezing tolerance of deciduous woody plant species with no obvious effect on plant size and plant flowering time.

Three FT and multiple CEN and BFT genes regulate maturity, flowering, and vegetative phenology in kiwifruit

Kiwifruit is a woody perennial horticultural crop, characterized by excessive vegetative vigor, prolonged juvenility, and low productivity. To understand the molecular factors controlling flowering and winter dormancy, here we identify and characterize the kiwifruit PEBP (phosphatidylethanolamine-binding protein) gene family. Five CEN-like and three BFT-like genes are differentially expressed and act as functionally conserved floral repressors, while two MFT-like genes have no impact on flowering time. FT-like genes are differentially expressed, with AcFT1 confined to shoot tip and AcFT2 to mature leaves. Both act as potent activators of flowering, but expression of AcFT2 in Arabidopsis resulted in a greater impact on plant morphology than that of AcFT1. Constitutive expression of either construct in kiwifruit promoted in vitro flowering, but AcFT2 displayed a greater flowering activation efficiency than AcFT1, leading to immediate floral transition and restriction of leaf development. Both leaf and flower differentiation were observed in AcFT1 kiwifruit lines. Sequential activation of specific PEBP genes in axillary shoot buds during growth and dormancy cycles indicated specific roles in regulation of kiwifruit vegetative and reproductive phenologies. AcCEN and AcCEN4 marked active growth, AcBFT2 was associated with suppression of latent bud growth during winter, and only AcFT was activated after cold accumulation and dormancy release.

Overexpression of blueberry FLOWERING LOCUS T is associated with changes in the expression of phytohormone-related genes in blueberry plants

Flowering locus T (FT) is a primary integrator in the regulation of plant flowering. Overexpressing a blueberry (Vaccinium corymbosum L.) FT gene (VcFT) (herein VcFT-OX) resulted in early flowering and dwarfing in 'Aurora' plants (herein 'VcFT-Aurora'). In this study, we found that VcFT-OX reduced shoot regeneration from leaf explants. To investigate the potential roles of the phytohormone pathway genes associated with VcFT-OX, differentially expressed (DE) genes in leaf tissues of 'VcFT-Aurora' plants were annotated and analyzed using non-transgenic 'Aurora' plants as a control. Three DE floral genes, including the blueberry SUPPRESSOR of Overexpression of constans 1 (VcSOC1) (gibberellin related), Abscisic acid responsive elements-binding factor 2 (VcABF2) and protein related to ABI3/VP1 (VcABI3/VP1) (ethylene-related), are present under both the phytohormone-responsive and the dwarfing-related Gene Ontology terms. The gene networks of the DE genes overall showed the molecular basis of the multifunctional aspects of VcFT overexpression beyond flowering promotion and suggested that phytohormone changes could be signaling molecules with important roles in the phenotypic changes driven by VcFT-OX.

Transcript Profile of Flowering Regulatory Genes in VcFT-Overexpressing Blueberry Plants

In order to identify genetic components in flowering pathways of highbush blueberry (Vaccinium corymbosum L.), a transcriptome reference composed of 254,396 transcripts and 179,853 gene contigs was developed by assembly of 72.7 million reads using Trinity. Using this transcriptome reference and a query of flowering pathway genes of herbaceous plants, we identified potential flowering pathway genes/transcripts of blueberry. Transcriptome analysis of flowering pathway genes was then conducted on leaf tissue samples of transgenic blueberry cv. Aurora (‘VcFT-Aurora’), which overexpresses a blueberry FLOWERING LOCUS T-like gene (VcFT). Sixty-one blueberry transcripts of 40 genes showed high similarities to 33 known flowering-related genes of herbaceous plants, of which 17 down-regulated and 16 up-regulated genes were identified in ‘VcFT-Aurora’. All down-regulated genes encoded transcription factors/enzymes upstream in the signaling pathway containing VcFT. A blueberry CONSTANS-LIKE 5-like (VcCOL5) gene was down-regulated and associated with five other differentially expressed (DE) genes in the photoperiod-mediated flowering pathway. Three down-regulated genes, i.e., a MADS-AFFECTING FLOWERING 2-like gene (VcMAF2), a MADS-AFFECTING FLOWERING 5-like gene (VcMAF5), and a VERNALIZATION1-like gene (VcVRN1), may function as integrators in place of FLOWERING LOCUS C (FLC) in the vernalization pathway. Because no CONSTAN1-like or FLOWERING LOCUS C-like genes were found in blueberry, VcCOL5 and VcMAF2/VcMAF5 or VRN1 might be the major integrator(s) in the photoperiod- and vernalization-mediated flowering pathway, respectively. The major down-stream genes of VcFT, i.e., SUPPRESSOR of Overexpression of Constans 1-like (VcSOC1), LEAFY-like (VcLFY), APETALA1-like (VcAP1), CAULIFLOWER 1-like (VcCAL1), and FRUITFULL-like (VcFUL) genes were present and showed high similarity to their orthologues in herbaceous plants. Moreover, overexpression of VcFT promoted expression of all of these VcFT downstream genes. These results suggest that VcFT’s down-stream genes appear conserved in blueberry.

The FLOWERING LOCUS T/TERMINAL FLOWER 1 Gene Family: Functional Evolution and Molecular Mechanisms

In plant development, the flowering transition and inflorescence architecture are modulated by two homologous proteins, FLOWERING LOCUS T (FT) and TERMINAL FLOWER 1 (TFL1). The florigen FT promotes the transition to reproductive development and flowering, while TFL1 represses this transition. Despite their importance to plant adaptation and crop improvement and their extensive study by the plant community, the molecular mechanisms controlling the opposing actions of FT and TFL1 have remained mysterious. Recent studies in multiple species have unveiled diverse roles of the FT/TFL1 gene family in developmental processes other than flowering regulation. In addition, the striking evolution of FT homologs into flowering repressors has occurred independently in several species during the evolution of flowering plants. These reports indicate that the FT/TFL1 gene family is a major target of evolution in nature. Here, we comprehensively survey the conserved and diverse functions of the FT/TFL1 gene family throughout the plant kingdom, summarize new findings regarding the unique evolution of FT in multiple species, and highlight recent work elucidating the molecular mechanisms of these proteins.

Chilling requirement of Ribes cultivars

It is usually thought that adequate winter chill is required for the full flowering of many temperate woody species. This paper investigates the sensitivity of blackcurrant bud burst and flowering to natural weather fluctuations in a temperate maritime climate, and compares a range of chill models that have been proposed for assessing the accumulation of winter chill. Bud break for four contrasting cultivars are compared in an exceptionally cold and in a mild winter in Eastern Scotland. The results confirm the importance of chilling at temperatures lower than 0°C and demonstrate that no single chilling function applies equally to all blackcurrant cultivars. There is a pressing need for further model development to take into account the relationship between chilling temperatures and warming temperatures occurring both during and after the chill accumulation period.

Promoting flowering, lateral shoot outgrowth, leaf development, and flower abscission in tobacco plants overexpressing cotton FLOWERING LOCUS T (FT)-like gene GhFT1

FLOWERING LOCUS T (FT) encodes a mobile signal protein, recognized as major component of florigen, which has a central position in regulating flowering, and also plays important roles in various physiological aspects. A mode is recently emerging for the balance of indeterminate and determinate growth, which is controlled by the ratio of FT-like and TERMINAL FLOWER 1 (TFL1)-like gene activities, and has a strong influence on the floral transition and plant architecture. Orthologs of GhFT1 was previously isolated and characterized from Gossypium hirsutum. We demonstrated that ectopic overexpression of GhFT1 in tobacco, other than promoting flowering, promoted lateral shoot outgrowth at the base, induced more axillary bud at the axillae of rosette leaves, altered leaf morphology, increased chlorophyll content, had higher rate of photosynthesis and caused flowers abscission. Analysis of gene expression suggested that flower identity genes were significantly upregulated in transgenic plants. Further analysis of tobacco FT paralogs indicated that NtFT4, acting as flower inducer, was upregulated, whereas NtFT2 and NtFT3 as flower inhibitors were upregulated in transgenic plants under long-day conditions, but downregulated under short-day conditions. Our data suggests that sufficient level of transgenic cotton FT might disturb the balance of the endogenous tobacco FT paralogs of inducers and repressors and resulted in altered phenotype in transgenic tobacco, emphasizing the expanding roles of FT in regulating shoot architecture by advancing determine growth. Manipulating the ratio for indeterminate and determinate growth factors throughout FT-like and TFL1-like gene activity holds promise to improve plant architecture and enhance crop yield.

Blueberry (Vaccinium corymbosum L.)

Vaccinium consists of approximately 450 species, of which highbush blueberry (Vaccinium corymbosum) is one of the three major Vaccinium fruit crops (i.e., blueberry, cranberry, and lingonberry) domesticated in the twentieth century. In blueberry the adventitious shoot regeneration using leaf explants has been the most desirable regeneration system to date; Agrobacterium tumefaciens-mediated transformation is the major gene delivery method and effective selection has been reported using either the neomycin phosphotransferase II gene (nptII) or the bialaphos resistance (bar) gene as selectable markers. The A. tumefaciens-mediated transformation protocol described in this chapter is based on combining the optimal conditions for efficient plant regeneration, reliable gene delivery, and effective selection. The protocol has led to successful regeneration of transgenic plants from leaf explants of four commercially important highbush blueberry cultivars for multiple purposes, providing a powerful approach to supplement conventional breeding methods for blueberry by introducing genes of interest.