Flowering and fruit-set in cassava under extended red-light photoperiod supplemented with plant-growth regulators and pruning

BACKGROUND

Cassava (Manihot esculenta Crantz) is staple food and major source of calories for over 500 million people in sub-Saharan Africa. The crop is also a source of income for smallholder farmers, and has increasing potential for industrial utilization. However, breeding efforts to match the increasing demand of cassava are impeded by its inability to flower, delayed or unsynchronized flowering, low proportion of female flowers and high fruit abortions. To overcome these sexual reproductive bottlenecks, this study investigated the effectiveness of using red lights to extend the photoperiod (RLE), as a gateway to enhancing flowering and fruit set under field conditions.

MATERIALS AND METHODS

Panels of cassava genotypes, with non- or late and early flowering response, 10 in each case, were subjected to RLE from dusk to dawn. RLE was further evaluated at low (LL), medium (ML) and high (HL) red light intensities, at ~  ≤ 0.5; 1.0 and 1.5PFD (Photon Flux Density) in µmol m^-2 s^-1 respectively. Additionally, the effect of a cytokinin and anti-ethylene as plant growth regulators (PGR) and pruning under RLE treatment were examined.

RESULTS

RLE stimulated earlier flower initiation in all genotypes, by up to 2 months in the late-flowering genotypes. Height and number of nodes at first branching, particularly in the late-flowering genotypes were also reduced, by over 50%. Number and proportion of pistillate flowers more than doubled, while number of fruits and seeds also increased. Number of branching levels during the crop season also increased by about three. Earlier flowering in many genotypes was most elicited at LL to ML intensities. Additive effects on flower numbers were detected between RLE, PGR and pruning applications. PGR and pruning treatments further increased number and proportion of pistillate flowers and fruits. Plants subjected to PGR and pruning, developed bisexual flowers and exhibited feminization of staminate flowers. Pruning at first branching resulted in higher pistillate flower induction than at second branching.

CONCLUSIONS

These results indicate that RLE improves flowering in cassava, and its effectiveness is enhanced when PGR and pruning are applied. Thus, deployment of these technologies in breeding programs could significantly enhance cassava hybridizations and thus cassava breeding efficiency and impact.

'Yunnan' quince rootstock promoted flower bud formation of 'Abbé Fetel' pear by altering hormone levels and PbAGL9 expression

Flower busd formation is an important plant growth process. It has been reported that dwarfing rootstocks can significantly affect the flower bud formation of scions. In this study, we found the dwarfing rootstock 'Yunnan' quince could significantly increase the flowering rate of 'Abbé Fetel' pear scions. The RNA-sequencing data revealed significant changes in the expression of genes related to hormone pathways. Furthermore, hormone analyses indicated that 'Yunnan' quince significantly decreased the GA₃ content and increased the cytokinin/auxin ratio in 'Abbé Fetel' pear apical buds. The hormone contents were consistent with the RNA-sequencing data. Moreover, we found the flower development-related genes PbAGL9 and PbCAL-A1 were significantly upregulated and PbTFL1 was significantly downregulated in 'Abbé Fetel'/'Yunnan' quince apical buds. To further clarify the relationship between hormones and flowering-related genes, a hormone response assay was carried out. We found the expression levels of PbCAl-A1, PbTFL1 and PbAGL9 were regulated by hormones including GA₃, CPPU and NAA. Y1H and dual-luciferase assays indicated that PbAGL9 significantly decreased the promoter activity of PbTFL1. In summary, 'Yunnan' quince upregulated PbCAL-A1 and PbAGL9, and downregulated PbTFL1 expression by decreasing the GA₃ content and increasing the cytokinin/auxin ratio in 'Abbé Fetel' pear apical buds. Additionally, 'Yunnan' quince down-regulate PbTFL1 by upregulating the expression of PbAGL9, and eventually promoted floral induction in 'Abbé Fetel' pear.

Molecular characterization of SPL gene family during flower morphogenesis and regulation in blueberry

The SPL gene is a plant-specific transcription factor involved in the regulation of plant growth and development, which have been identified in woody plants. The process of floral bud differentiation affects the timing of flowering and fruit set and regulates plant growth, however, the mechanism of regulation of flower development by SPL genes is less studied. In this study, 56 VcSPL genes were identified in the tetraploid blueberry. The VcSPL gene family was classified into six subfamilies, and analysis of cis-elements showed that VcSPL genes were regulated by light, phytohormones (abscisic acid, MeJA), and low temperature. In the evolutionary analysis, segmental replication may play an important role in VcSPL gene amplification. Interestingly, we also studied diploid blueberry (Bilberry), in which 24 SPL genes were identified, and 36 homologous pairs were found, suggesting a high degree of convergence in the syntenic relationship between blueberry (Vaccinium corymbosum L) and bilberry (Vaccinium darrowii). Based on the expression profile, VcSPL genes were expressed at high levels in flowers, shoots, and roots, indicating a diversity of gene functions. Then we selected 20 differentially-expressed SPL genes to further investigate the role of VcSPL in floral induction and initiation. It showed that the genes VcSPL40, VcSPL35, VcSPL45, and VcSPL53 may play a crucial role in the blueberry floral transition phase (from vegetative growth to flower initiation). These results provided important information for understanding and exploring the role of VcSPLs in flower morphogenesis and plant growth.

Widely targeted metabolite profiling of mango stem apex during floral induction by compond of mepiquat chloride, prohexadione-calcium and uniconazole

Background

Insufficient low temperatures in winter and soil residues caused by paclobutrazol (PBZ) application pose a considerable challenge for mango floral induction (FI). Gibberellin inhibitors SPD (compound of mepiquat chloride, prohexadione-calcium and uniconazole) had a significant influence on enhancing the flowering rate and yield of mango for two consecutive years (2020-2021). Researchers have indicated that FI is regulated at the metabolic level; however, little is known about the metabolic changes during FI in response to SPD treatment.

Methods

Here, ultra-performance liquid chromatography-electrospray ionization-tandem mass spectrometry (UPLC-ESI-MS/MS)-based widely targeted metabolomic analysis was carried out to assess the metabolic differences in the mango stem apex during different stage of mango FI (30, 80, 100 days after SPD/water treatment).

Results

A total of 582 compounds were annotated and 372 metabolites showed two-fold differences in abundance (variable importance in projection, VIP ≥ 1 and fold change, FC≥ 2 or≤ 0.5) between buds at 30, 80, 100 days after SPD/water treatment or between buds under different treatment. Lipids, phenolic acids, amino acids, carbohydrates, and vitamins were among metabolites showing significant differences over time after SPD treatment. Here, 18 out of 20 lipids, including the lysophosphatidylethanolamine (12, LPE), lysophosphatidylcholine (7, LPC), and free fatty acids (1, FA), were significantly upregulated from 80 to 100 days after SPD treatment comared to water treatment. Meanwhile, the dormancy release of mango buds from 80 to 100 days after SPD treatment was accompanied by the accumulation of proline, ascorbic acid, carbohydrates, and tannins. In addition, metabolites, such as L-homocysteine, L-histidine, and L-homomethionine, showed more than a ten-fold difference in relative abundance from 30 to 100 days after SPD treatment, however, there were no significant changes after water treatment. The present study reveals novel metabolites involved in mango FI in response to SPD, which would provide a theoretical basis for utilizing SPD to induce mango flowering.

Assessing Flowering Time Under Different Photoperiods

Flowering time is one of the most important developmental transitions in plants, especially in annuals such as Arabidopsis thaliana. However, flowering is also a critical agronomic trait, as it impacts the level of vegetative biomass produced (e.g., leaves) or the amount of seed (grain) generated. Therefore, uncovering flowering phenotypes would help understand the impact of any regulatory network on the overall plant life cycle, since flowering integrates multiple cues, both environmental (e.g., photoperiod, temperature) and internal (e.g., induction/repression of specific genes, phytohormone accumulation, plant age). Although the photoperiod flowering pathway has been extensively studied, and its gene circuitry characterized in great detail, specific flowering time protocols are mostly accessible to specialized laboratories in this field. In this report, we address this knowledge gap by generating a reproducible, non-expensive, and step-by-step protocol to assess flowering time under different photoperiods. We provide a comprehensive description and highlight the major pitfalls in the process. Moreover, this protocol could be expanded to include temperature changes and thus contribute to assess the impact of both environmental conditions in the plant's decision to flower.

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.

Developmental stage-specific metabolite signatures in Arabidopsis thaliana under optimal and mild nitrogen limitation

Metabolites influence flowering time, and thus are among the major determinants of yield. Despite the reported role of trehalose 6-phosphate and nitrate signaling on the transition from the vegetative to the reproductive phase, little is known about other metabolites contributing and responding to developmental phase changes. To increase our understanding which metabolic traits change throughout development in Arabidopsis thaliana and to identify metabolic markers for the vegetative and reproductive phases, especially among individual amino acids (AA), we profiled metabolites of plants grown in optimal (ON) and limited nitrogen (N) (LN) conditions, the latter providing a mild but consistent limitation of N. We found that although LN plants adapt their growth to a decreased level of N, their metabolite profiles are strongly distinct from ON plant profiles, with N as the driving factor for the observed differences. We demonstrate that the vegetative and the reproductive phase are not only marked by growth parameters such as biomass and rosette area, but also by specific metabolite signatures including specific single AA. In summary, we identified N-dependent and -independent indicators manifesting developmental stages, indicating that the plant's metabolic status also reports on the developmental phases.

A potential endogenous gibberellin-mediated signaling cascade regulated floral transition in Magnolia × soulangeana 'Changchun'

The floral transition is a critical developmental switch in plants, and has profound effects on the flower production and yield. Magnolia × soulangeana 'Changchun' is known as a woody ornamental plant, which can bloom in spring and summer, respectively. In this study, anatomical observation, physiological measurement, transcriptome, and small RNA sequencing were performed to investigate potential endogenous regulatory mechanisms underlying floral transition in 'Changchun'. Transition of the shoot apical meristem from vegetative to reproductive growth occurred between late April and early May. During this specific developmental process, a total of 161,645 unigenes were identified, of which 73,257 were significantly differentially expressed, while a number of these two categories of miRNAs were 299 and 148, respectively. Further analysis of differentially expressed genes (DEGs) revealed that gibberellin signaling could regulate floral transition in 'Changchun' in a DELLA-dependent manner. In addition, prediction and analysis of miRNA targeted genes suggested that another potential molecular regulatory module was mediated by the miR172 family and other several novel miRNAs (Ms-novel_miR139, Ms-novel_miR229, and Ms-novel_miR232), with the participation of up- or down-regulating genes, including MsSVP, MsAP2, MsTOE3, MsAP1, MsGATA6, MsE2FA, and MsMDS6. Through the integrated analysis of mRNA and miRNA, our research results will facilitate the understanding of the potential molecular mechanism underlying floral transition in 'Changchun', and also provide basic experimental data for the plant germplasm resources innovation in Magnolia.

Genome-wide identification and expression analysis of flowering-related genes reveal putative floral induction and differentiation mechanisms in tea plant (Camellia sinensis)

The tea leaf is economically important, while reproductive growth reduce tea output. However, little is known about flowering mechanisms in tea plants. Here, we determined the approximate times of floral induction, floral transition and floral organ differentiation by morphological observation. We identified 401 and 356 flowering-related genes from the genomes of Camellia sinensis var. sinensis and Camellia sinensis var. assamica, respectively. Then, we compared the expression profiles of flowering-related genes in floriferous and oliganthous cultivars, the result showed that PRR7, GI, GID1B and GID1C expression is correlated with the floral induction; LFY, PNF and PNY expression was correlated with floral bud formation. Transcriptome analysis also showed that GI, PRR7 and GID1 were correlated with stress-induced flowering. Thus, we proposed putative mechanisms of flowering in tea plants. This study provides new insights into flowering and a theoretical basis for balancing vegetative and reproductive growth in tea plants and other economical plants.

Chenopodium ficifolium flowers under long days without upregulation of FLOWERING LOCUS T (FT) homologs

Chenopodium ficifoliumflowered under long days despite much lower expression ofFLOWERING LOCUS Thomolog than under short days. Frequent duplications of the FLOWERING LOCUS T (FT) gene across various taxonomic lineages resulted in FT paralogs with floral repressor function, whereas others duplicates maintained their floral-promoting role. The FT gene has been confirmed as the inducer of photoperiodic flowering in most angiosperms analyzed to date. We identified all FT homologs in the transcriptome of Chenopodium ficifolium and in the genome of Chenopodium suecicum, which are closely related to diploid progenitors of the tetraploid crop Chenopodium quinoa, and estimated their expression during photoperiodic floral induction. We found that expression of FLOWERING LOCUS T like 1 (FTL1), the ortholog of the sugar beet floral activator BvFT2, correlated with floral induction in C. suecicum and short-day C. ficifolium, but not with floral induction in C. ficifolium with accelerated flowering under long days. This C. ficifolium accession was induced to flowering without the concomitant upregulation of any FT homolog.

The transcriptomic response to a short day to long day shift in leaves of the reference legume Medicago truncatula

Photoperiodic flowering aligns plant reproduction to favourable seasons of the year to maximise successful production of seeds and grains. However understanding of this process in the temperate legumes of the Fabaceae family, which are important both agriculturally and ecologically, is incomplete. Previous work in the reference legume Medicago truncatula has shown that the FT-like gene MtFTa1 is a potent floral activator. While MtFTa1 is upregulated by long-day photoperiods (LD) and vernalisation, the molecular basis of this is unknown as functional homologues of key regulatory genes present in other species, notably CONSTANS in A. thaliana, have not been identified. In LD MtFTa1 maintains a near constant diurnal pattern of expression unlike its homologue FT in A. thaliana, which has a notable peak in expression at dusk. This suggests a different manner of regulation. Furthermore, M. truncatula possesses other FT-like genes such as two LD induced MtFTb genes which may also act in the regulation of flowering time. MtFTb genes have a diurnal pattern of expression with peaks at both four and sixteen hours after dawn. This study utilises RNA-Seq to analyse the transcriptome of M. truncatula leaves to identify genes which may regulate or be co-expressed with these FT-like genes following a shift from short-day photoperiods to inductive long-days. Specifically this study focuses on the first four hours of the day in the young leaves, which coincides with the first diurnal peak of the FTb genes. Following differential expression analysis at each timepoint, genes which alter their pattern of expression are distinguished from those which just alter their magnitude of expression (and those that do neither). It goes on to categorise these genes into groups with similar patterns of expression using c-means clustering and identifies a number of potential candidate photoperiod flowering time genes for future studies to consider.

2' apple varieties reveals novel insights into the regulatory mechanisms underlying floral induction

BACKGROUND

Floral induction is an important stage in the apple tree life cycle. In 'Nagafu No. 2', which was derived from a 'Fuji' bud sport, flower bud formation is associated with serious problems, such as fewer and inferior flower buds, a long juvenile phase, and an alternate bearing phenotype. Moreover, the molecular regulatory mechanisms underlying apple floral induction remain unknown. To characterize these mechanisms, we compared the RNA-sequencing-based transcriptome profiles of buds during floral induction in profusely flowering 'Qinguan' and weakly flowering 'Nagafu No. 2' apple varieties.

RESULTS

Genes differentially expressed between the buds of the two varieties were mainly related to carbohydrate, fatty acid, and lipid pathways. Additionally, the steady up-regulated expression of genes related to the fatty acid and lipid pathways and the down-regulated expression of starch synthesis-related genes in the carbon metabolic pathway of 'Qinguan' relative to 'Nagafu No. 2' were observed to contribute to the higher flowering rate of 'Qinguan'. Additionally, global gene expression profiling revealed that genes related to cytokinin, indole-3-acetic acid, and gibberellin synthesis, signalling, and responses (i.e., factors contributing to cell division and differentiation and bud growth) were significantly differentially expressed between the two varieties. The up-regulated expression of genes involved in abscisic acid and salicylic acid biosynthesis via shikimate pathways as well as jasmonic acid production through fatty acid pathways in 'Qinguan' buds were also revealed to contribute to the floral induction and relatively high flowering rate of this variety. The differential expression of transcription factor genes (i.e., SPL, bZIP, IDD, and MYB genes) involved in multiple biological processes was also observed to play key roles in floral induction. Finally, important flowering genes (i.e., FT, FD, and AFL) were significantly more highly expressed in 'Qinguan' buds than in 'Nagafu No. 2' buds during floral induction.

CONCLUSIONS

A complex genetic network of regulatory mechanisms involving carbohydrate, fatty acid, lipid, and hormone pathways may mediate the induction of apple tree flowering.

Expression of genes in the potential regulatory pathways controlling alternate bearing in 'Fuji' (Malus domestica Borkh.) apple trees during flower induction

Most perennial fruit trees have an alternate bearing problem where a heavy fruit load is produced one year (ON year) but few flowers and fruits produced the next year (OFF year), resulting in a significant fluctuation in production. In the present study, comparative transcriptome analysis of terminal buds of apple (Malus domestica Borkh., cv. Nagafu No. 2) trees was conducted during the floral induction period in the ON and OFF years to identify the potential regulatory pathways controlling alternate bearing. A total of 1027 differentially expressed genes (DEGs), most of which were involved in secondary metabolism, sugar metabolism, plant hormone pathways, were identified. The analysis focused on differences in sugar content and hormone levels between the ON and OFF trees. Sucrose content, zeatin-riboside (ZR), and abscisic acid (ABA) levels were lower in ON-year buds than in OFF-year buds. ON buds also had elevated levels of gibberellins (GAs), with a higher expression of GA20 oxidase (GA20ox) and a significant lower level of RGA-like2 (RGL2). Expression analyses also revealed a significantly higher level of SQUAMOSA-PROMOTER BINDING PROTEIN-LIKE genes (MdSPL1, MdSPL6 and MdSPL12) transcripts levels in buds of OFF trees at 45 days after full bloom (DAFB). LEAFY (LFY) expression increased significantly prior to flower induction in OFF buds. These findings provide new information of the role of hormones in alternate bearing, as well as other processes, and provide new insights into the molecular mechanisms regulating alternate bearing in perennial fruit trees.

Functional analysis of a homologue of the FLORICAULA/LEAFY gene in litchi (Litchi chinensis Sonn.) revealing its significance in early flowering process

Litchi (Litchi chinensis Sonn.) is an important subtropical fruit crop with high commercial value due to its high nutritional values and favorable tastes. However, irregular bearing attributed to unstable flowering is a major ongoing problem for litchi producers. Previous studies indicate that low-temperature is a key factor in litchi floral induction. In order to reveal the genetic and molecular mechanisms underlying the reproductive process in litchi, we had analyzed the transcriptome of buds before and after low-temperature induction using RNA-seq technology. A key flower bud differentiation associated gene, a homologue of FLORICAULA/LEAFY, was identified and named LcLFY (GenBank Accession No. KF008435). The cDNA sequence of LcLFY encodes a putative protein of 388 amino acids. To gain insight into the role of LcLFY, the temporal expression level of this gene was measured by real-time RT-PCR. LcLFY was highly expressed in flower buds and its expression correlated with the floral developmental stage. Heterologous expression of LcLFY in transgenic tobacco plants induced precocious flowering. Meantime, we investigated the sub-cellular localization of LcLFY. The LcLFY-Green fluorescent protein (GFP) fusion protein was found in the nucleus. The results suggest that LcLFY plays a pivotal role as a transcription factor in controlling the transition to flowering and in the development of floral organs in litchi.

Identification of TPS family members in apple (Malus x domestica Borkh.) and the effect of sucrose sprays on TPS expression and floral induction

Trehalose (α-D-glucopyranosyl α-D-glucopyranoside) is a non-reducing disaccharide that serves as a carbon source and stress protectant in apple trees. Trehalose-6-phosphate (T6P) is the biosynthetic precursor of trehalose. It functions as a crucial signaling molecule involved in the regulation of floral induction, and is closely related to sucrose. Trehalose-6-phosphate synthase (TPS) family members are pivotal components of the T6P biosynthetic pathway. The present study identified 13 apple TPS family members and characterized their expression patterns in different tissues and in response to exogenous application of sucrose during floral induction. 'Fuji' apple trees were sprayed with sucrose prior to the onset of floral induction. Bud growth, flowering rate, and endogenous sugar levels were then monitored. The expression of genes associated with sucrose metabolism and flowering were also characterized by RT-quantitative PCR. Results revealed that sucrose applications significantly improved flower production and increased bud size and fresh weight, as well as the sucrose content in buds and leaves. Furthermore, the expression of MdTPS1, 2, 4, 10, and 11 was rapidly and significantly up-regulated in response to the sucrose treatments. In addition, the expression levels of flowering-related genes (e.g., SPL genes, FT1, and AP1) also increased in response to the sucrose sprays. In summary, apple TPS family members were identified that may influence the regulation of floral induction and other responses to sucrose. The relationship between sucrose and T6P or TPS during the regulation of floral induction in apple trees is discussed.

Repression of TERMINAL FLOWER1 primarily mediates floral induction in pear (Pyrus pyrifolia Nakai) concomitant with change in gene expression of plant hormone-related genes and transcription factors

Floral induction is an important event in the annual growth cycle of perennial fruit trees. For pear, this event directly affects fruit production in the following year. The flower buds in many species are induced by FLOWERING LOCUS T (FT), whose effect is repressed by the meristem-expressed gene TERMINAL FLOWER1 (TFL1). In this study, we investigated the functions of pear FT and TFL1 genes during floral development. Expression of pear FTs (PpFT1a and PpFT2a) in reproductive meristems was not obviously induced prior to floral initiation, while expression of TFL1s (PpTFL1-1a and PpTFL1-2a) rapidly decreased. The induction of the productive meristem identity MADS-box gene AP1 after repression of PpTFL1s suggested a primary role for PpTFL1 in floral induction. RNA-seq analysis suggested that plant hormone-related genes and several transcription factors that were coexpressed with PpTFL1 were potentially involved in the PpTFL1-mediated floral induction. Our data indicate the essential function of TFL1 in pear floral induction and add another species in the family Rosaceae in addition to strawberry and rose that shows a role for TFL1 in floral induction.

Transcriptome profiling of litchi leaves in response to low temperature reveals candidate regulatory genes and key metabolic events during floral induction

Background

Litchi (Litchi chinensis Sonn.) is an economically important evergreen fruit tree widely cultivated in subtropical areas. Low temperature is absolutely required for floral induction of litchi, but its molecular mechanism is not fully understood. Leaves of litchi played a key role during floral induction and could be the site of low temperature perception. Therefore, leaves were treated under different temperature (15 °C/25 °C), and high-throughput RNA sequencing (RNA-Seq) performed with leaf samples for the de novo assembly and digital gene expression (DGE) profiling analyses to investigate low temperature-induced gene expression changes.

Results

83,107 RNA-Seq unigenes were de novo assembled with a mean length of 1221 bp and approximately 61% of these unigenes (50,345) were annotated against public protein databases. Differentially-expressed genes (DEGs) under low temperature treatment in comparison with the control group were the main focus of our study. Hierarchical clustering analysis arranged 2755 DEGs into eight groups with three significant expression clusters (p-value ≤ 0.05) during floral induction. With the increasing contents of sugars and starch, the expression of genes involved in metabolism of sugars increased dramatically after low temperature induction. One FT gene (Unigene0025396, LcFT1) which produces a protein called ‘florigen’ was also detected among DEGs of litchi. LcFT1 exhibited an apparent specific tissue and its expression was highly increased after low temperature induction, GUS staining results also showed GUS activity driven by LcFT1 gene promoter can be induced by low temperature, which indicated LcFT1 probably played a pivotal role in the floral induction of litchi under low temperature.

Conclusions

Our study provides a global survey of transcriptomes to better understand the molecular mechanisms underlying changes of leaves in response to low temperature induction in litchi. The analyses of transcriptome profiles and physiological indicators will help us study the complicated metabolism of floral induction in the subtropic evergreen plants.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3747-x) contains supplementary material, which is available to authorized users.

Thermoperiodic Control of Floral Induction Involves Modulation of the Diurnal FLOWERING LOCUS T Expression Pattern

Thermoperiodism is defined as the ability to discriminate between day temperature (DT) and night temperature (NT). Our aim was to shed light on the mechanistic basis of thermoperiodic floral induction with acceleration under lower DT than NT compared with other DT-NT combinations at the same average daily temperature (ADT), a response exploited in temperate area greenhouses. Arabidopsis thaliana floral pathway mutants and a lhy circadian clock mutant as well as the expression of floral integrators and LHY (LATE ELONGATED HYPOCOTYL) were studied under different DT-NT combinations, all at the same ADT. We show that acceleration of floral induction under lower DT than NT is linked to increased FT expression early during the day and generally increased LFY expression preceding visible flower buds, compared with higher DT than NT or equal DT and NT. Consistent with FLOWERING LOCUS T (FT) action through LEAFY (LFY), time to floral transition in ft-1 and lfy-1 was similar under all treatments, in contrast to the situation for soc1-1, which behaved like the wild type (WT). The lhy-21 mutants did not discriminate between opposite DT-NT combinations, whereas LHY expression in the WT differed in these temperature regimes. This might suggest that LHY plays a role in thermoperiodic control of floral induction. We conclude that thermoperiodic control of floral transition is associated with modulation of the diurnal expression patterns of FT, with timing of temperature alteration being important rather than ADT.

Effect of exogenous GA3 and its inhibitor paclobutrazol on floral formation, endogenous hormones, and flowering-associated genes in 'Fuji' apple (Malus domestica Borkh.)

Gibberellins (GAs) reduce apple (Malus domestica) flowering rates; however, the mechanism of their action is not fully understood. To gain a better insight into gibberellin-regulated flowering, here, 5 year-old 'Fuji' apple trees were used to explore the responses of hormones [GA1+3, GA4+7, indole-3-acetic acid (IAA), zeatin-riboside (ZR), and abscisic acid (ABA)], and gibberellin- and flowering-associated genes, to applications of gibberellin acid (GA3) and paclobutrazol (PAC). Results showed that GA3 relatively stimulated vegetative growth and delayed floral induction. Moreover, GA3 spraying significantly affected contents of all endogenous hormones and all the genes tested in at least one time points: the content of endogenous GAs was increased instantly and that of ZR was reduced at 44 days after fullbloom (DAF), which might constitute an unfavorable factor for flower formation; MdKO (ent-kaurene oxidase gene) and MdGA20ox (GA20 oxidase gene) were significantly repressed by a high level of GAs through the negative feedback regulation of GA; additionally, the MdSPLs (SQUAMOSA-PROMOTER BINDING PROTEIN-LIKE) in this study were all significantly repressed by GA3 but promoted by PAC; the expression of MdFT1/2 (FLOWERING LOCUS T), MdSOC1 (SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1) and MdAP1 (APETALA1) in GA3-treated buds changed in the same way, and they were repressed at 44 DAF. We suppose that GA3 spraying disrupts the balance between ZR and GAs, and inhibits floral induction, probably by suppressing MdSPLs and the floral integrators in flower induction, which ultimately contributed to inhibiting flower formation.

Flowering and trichome development share hormonal and transcription factor regulation

Gibberellins (GAs) and cytokinins (CKs) are plant hormones that act either synergistically or antagonistically during the regulation of different developmental processes. In Arabidopsis thaliana, GAs and CKs overlap in the positive regulation of processes such as the transition from the vegetative to the reproductive phase and the development of epidermal adaxial trichomes. Despite the fact that both developmental processes originate in the rosette leaves, they occur separately in time and space. Here we review how, as genetic and molecular mechanisms are being unraveled, both processes might be closely related. Additionally, this shared genetic network is not only dependent on GA and CK hormone signaling but is also strictly controlled by specific clades of transcription factor families. Some key flowering genes also control other rosette leaf developmental processes such as adaxial trichome formation. Conversely, most of the trichome activator genes, which belong to the MYB, bHLH and C2H2 families, were found to positively control the floral transition. Furthermore, three MADS floral organ identity genes, which are able to convert leaves into floral structures, are also able to induce trichome proliferation in the flower. These data lead us to propose that the spatio-temporal regulation and integration of diverse signals control different developmental processes, such as floral induction and trichome formation, which are intimately connected through similar genetic pathways.