Expression of FLOWERING LOCUS T from Arabidopsis thaliana induces precocious flowering in soybean irrespective of maturity group and stem growth habit

Efficient virus-induced gene silencing system in pumpkin (Cucurbita maxima) using apple latent spherical virus vector

Crude-sap of apple latent spherical virus (ALSV)-infected Chenopodium quinoa leaves was rub-inoculated on the expanded cotyledons of various Cucurbitaceae plants. Most of the species were systemically infected with the virus without obvious symptoms, except pumpkin (Cucurbita maxima). In pumpkin, the ALSV infection was restricted to inoculated cotyledons; it did not spread to the upper true leaves. In situ hybridization showed that the ALSV was confined to part of the cotyledon tissues and it did not invade the phloem tissue, when inoculated at the expanded cotyledon stage. However, when total RNAs from ALSV-infected C. quinoa leaves were inoculated into the cotyledons immediately after germination (folded cotyledon stage) using particle bombardment, ALSV efficiently caused systemic infection. Systemic infection of pumpkin seedlings occurred only when the cotyledons were inoculated within a few days after germination. No systemic infection was observed in the seedlings 4 days after germination. In the grafting test, ALSV was not transmitted from the infected rootstocks to the healthy scions of pumpkins. An efficient virus-induced gene silencing system for pumpkins was established, in which infection with ALSV vectors harboring the phytoene desaturase or sulfur gene fragments resulted in a uniform phenotype in the true leaves of pumpkin seedlings.

Foxtail mosaic virus-induced flowering assays in monocot crops

Virus-induced flowering (VIF) exploits RNA or DNA viruses to express flowering time genes to induce flowering in plants. Such plant virus-based tools have recently attracted widespread attention for their fundamental and applied uses in flowering physiology and in accelerating breeding in dicotyledonous crops and woody fruit-trees. We now extend this technology to a monocot grass and a cereal crop. Using a Foxtail mosaic virus (FoMV)-based VIF system, dubbed FoMViF, we showed that expression of florigenic Flowering Locus T (FT) genes can promote early flowering and spikelet development in proso millet, a C4 grass species with potential as a nutritional food and biofuel resource, and in non-vernalized C3 wheat, a major food crop worldwide. Floral and spikelet/grain induction in the two monocot plants was caused by the virally expressed untagged or FLAG-tagged FT orthologs, and the florigenic activity of rice Hd3a was more pronounced than its dicotyledonous counterparts in proso millet. The FoMViF system is easy to use and its efficacy to induce flowering and early spikelet/grain production is high. In addition to proso millet and wheat, we envisage that FoMViF will be also applicable to many economically important monocotyledonous food and biofuel crops.

Virus-Induced Flowering by Apple Latent Spherical Virus Vector: Effective Use to Accelerate Breeding of Grapevine

Apple latent spherical virus (ALSV) was successfully used in promoting flowering (virus-induced flowering, VIF) in apple and pear seedlings. In this paper, we report the use of ALSV vectors for VIF in seedlings and in vitro cultures of grapevine. After adjusting experimental conditions for biolistic inoculation of virus RNA, ALSV efficiently infected not only progeny seedlings of Vitis spp. ‘Koshu,’ but also in vitro cultures of V. vinifera ‘Neo Muscat’ without inducing viral symptoms. The grapevine seedlings and in vitro cultures inoculated with an ALSV vector expressing the ‘florigen’ gene (Arabidopsis Flowering locus T, AtFT) started to set floral buds 20–30 days after inoculation. This VIF technology was successfully used to promote flowering and produce grapes with viable seeds in in vitro cultures of F₁ hybrids from crosses between V. ficifolia and V. vinifera and made it possible to analyze the quality of fruits within a year after germination. High-temperature (37 °C) treatment of ALSV-infected grapevine disabled virus movement to newly growing tissue to obtain ALSV-free shoots. Thus, the VIF using ALSV vectors can be used to shorten the generation time of grapevine seedlings and accelerate breeding of grapevines with desired traits.

Mutagenesis of GmFT2a and GmFT5a mediated by CRISPR/Cas9 contributes for expanding the regional adaptability of soybean

Flowering time is a key agronomic trait that directly influences the successful adaptation of soybean (Glycine max) to diverse latitudes and farming systems. GmFT2a and GmFT5a have been extensively identified as flowering activators and integrators in soybean. Here, we identified two quantitative trait loci (QTLs) regions harbouring GmFT2a and GmFT5a, respectively, associated with different genetic effects on flowering under different photoperiods. We analysed the flowering time of transgenic plants overexpressing GmFT2a or GmFT5a, ft2a mutants, ft5a mutants and ft2aft5a double mutants under long-day (LD) and short-day (SD) conditions. We confirmed that GmFT2a and GmFT5a are not redundant, they collectively regulate flowering time, and the effect of GmFT2a is more prominent than that of GmFT5a under SD conditions whereas GmFT5a has more significant effects than GmFT2a under LD conditions. GmFT5a, not GmFT2a, was essential for soybean to adapt to high latitude regions. The ft2aft5a double mutants showed late flowering by about 31.3 days under SD conditions and produced significantly increased numbers of pods and seeds per plant compared to the wild type. We speculate that these mutants may have enormous yield potential for the tropics. In addition, we examined the sequences of these two loci in 202 soybean accessions and investigated the flowering phenotypes, geographical distributions and maturity groups within major haplotypes. These results will contribute to soybean breeding and regional adaptability.

Functional divergence between soybean FLOWERING LOCUS T orthologues FT2a and FT5a in post-flowering stem growth

Genes in the FLOWERING LOCUS T (FT) family integrate external and internal signals to control various aspects of plant development. In soybean (Glycine max), FT2a and FT5a play a major role in floral induction, but their roles in post-flowering reproductive development remain undetermined. Ectopic overexpression analyses revealed that FT2a and FT5a similarly induced flowering, but FT5a was markedly more effective than FT2a for the post-flowering termination of stem growth. The down-regulation of Dt1, a soybean orthologue of Arabidopsis TERMINAL FLOWER1, in shoot apices in early growing stages of FT5a-overexpressing plants was concomitant with highly up-regulated expression of APETALA1 orthologues. The Dt2 gene, a repressor of Dt1, was up-regulated similarly by the overexpression of FT2a and FT5a, suggesting that it was not involved in the control of stem termination by FT5a. In addition to the previously reported interaction with FDL19, a homologue of the Arabidopsis bZIP protein FD, both FT2a and FT5a interacted with FDL12, but only FT5a interacted with FDL06. Our results suggest that FT2a and FT5a have different functions in the control of post-flowering stem growth. A specific interaction of FT5a with FDL06 may play a key role in determining post-flowering stem growth in soybean.

Virus-induced gene silencing and virus-induced flowering in strawberry (Fragaria × ananassa) using apple latent spherical virus vectors

Apple latent spherical virus (ALSV) vector is a convenient alternative to genetic transformation in horticultural plants, especially in species recalcitrant to genetic transformation. ALSV, an RNA virus, can infect a wide variety of plant species including major horticultural plants without inducing symptoms. Here, methodologies were developed for infection of ALSV vectors to strawberry seedlings and plantlets cultured in vitro. A seed-propagated F₁ hybrid strawberry cultivar 'Yotsuboshi' was aseptically grown on half-strength Murashige-Skoog medium for 1 month and true leaves were inoculated with an ALSV RNA preparation by particle bombardment. ALSV vector infection rates varied from 58 to 100% according to the insertion sequences, in 'Yotsuboshi' seedlings. Plantlets ('Dover') propagated in vitro could also be infected with ALSV vector at a similar infection rate. For virus-induced gene silencing (VIGS), we prepared an ALSV vector carrying a 201 nucleotide segment of the strawberry phytoene desaturase gene. 'Yotsuboshi' and 'Dover' plants infected by this vector generated completely white leaves at fifth or sixth true leaves and above. For virus-induced flowering (VIF), we used an ALSV vector expressing the Arabidopsis thaliana flowering locus T gene. Strawberry seedlings infected by this vector started to flower from about 2 months post inoculation and bore fruits with viable seeds. The ALSV vector was no longer detected in any of the seedlings from early-flowered strawberries. Thus, the ALSV vector may be beneficial for examination of gene functions by VIGS in strawberry, and VIF using ALSV vector constitutes an effective new plant breeding technique for the promotion of cross-breeding in strawberry.

RNA Silencing-Mediated Apple Latent Spherical Virus Vaccine in Plants

The apple latent spherical virus (ALSV), originally isolated from an apple tree in Japan, is a small spherical virus with a diameter of 25 nm and comprises a bisegmented, single-stranded RNA genome (RNA1 and RNA2) and three different capsid proteins (Vp25, Vp20, and Vp24). The virus can experimentally infect a broad range of plants including, not only model plants (Arabidopsis thaliana and Nicotiana species) but also economically important crops such as cucumber, soybean, tomato, fruit trees, and flowers. ALSV has been used as an effective plant virus vector for virus-induced gene silencing (VIGS) to assess gene functions because the virus infects most of the host plants without showing any symptoms and induces a uniform knockout phenotype in infected plants. Moreover, the VIGS persists throughout plant growth in infected plants. Here, we show that genetically engineered ALSV vectors (ALSV vaccines) containing a partial genome sequence of pathogenic viruses display a high degree of cross-protection against the challenge inoculation of the corresponding pathogenic viruses. Treatment effects can also be expected in virus-infected plants by subsequent inoculation with ALSV vaccine.

Gentian (Gentiana triflora) prevents transmission of apple latent spherical virus (ALSV) vector to progeny seeds

MAIN CONCLUSION

Gentian plants ( Gentiana triflora ) severely restrict apple latent spherical virus (ALSV) invasion to the gametes (pollens and ovules) and block seed transmission to progeny plants. Early flowering of horticultural plants can be induced by infection of ALSV vector expressing Flowering Locus T (FT) gene. In the present study, flowering of gentian plants was induced by infection with an ALSV vector expressing a gentian FT gene and the patterns of seed transmission of ALSV in gentian were compared with those in apple and Nicotiana benthamiana. Infection of gentian progeny plants with ALSV was examined by quantitative reverse transcription-polymerase chain reaction (qRT-PCR), reverse transcription-loop-mediated isothermal amplification (RT-LAMP), and enzyme-linked immunosorbent assay (ELISA). ALSV was not transmitted to the progeny gentian plants, whereas small proportions of apple and N. benthamiana progeny plants are infected with ALSV. The in situ hybridization analyses indicated that ALSVs are not present in gentian pollen and ovules, but detected in most of gametes in apple and N. benthamiana. Collectively, these results suggest that seed transmission of ALSV is blocked in gentian plants through the unknown barriers present in their gametes. On the other hand, apple and N. benthamiana seem to minimize ALSV seed transmission by inhibiting viral propagation in embryos.

Expression patterns of flowering genes in leaves of 'Pineapple' sweet orange [Citrus sinensis (L.) Osbeck] and pummelo (Citrus grandis Osbeck)

BACKGROUND

In citrus the transition from juvenility to mature phase is marked by the capability of a tree to flower and fruit consistently. The long period of juvenility in citrus severely impedes the use of genetic based strategies to improve fruit quality, disease resistance, and responses to abiotic environmental factors. One of the genes whose expression signals flower development in many plant species is FLOWERING LOCUS T (FT).

RESULTS

In this study, gene expression levels of flowering genes CiFT1, CiFT2 and CiFT3 were determined using reverse-transcription quantitative real-time PCR in citrus trees over a 1 year period in Florida. Distinct genotypes of citrus trees of different ages were used. In mature trees of pummelo (Citrus grandis Osbeck) and 'Pineapple' sweet orange (Citrus sinensis (L.) Osbeck) the expression of all three CiFT genes was coordinated and significantly higher in April, after flowering was over, regardless of whether they were in the greenhouse or in the field. Interestingly, immature 'Pineapple' seedlings showed significantly high levels of CiFT3 expression in April and June, while CiFT1 and CiFT2 were highest in June, and hence their expression induction was not simultaneous as in mature plants.

CONCLUSIONS

In mature citrus trees the induction of CiFTs expression in leaves occurs at the end of spring and after flowering has taken place suggesting it is not associated with dormancy interruption and further flower bud development but is probably involved with shoot apex differentiation and flower bud determination. CiFTs were also seasonally induced in immature seedlings, indicating that additional factors must be suppressing flowering induction and their expression has other functions.

Identification and functional analysis of SVP ortholog in herbaceous perennial plant Gentiana triflora: Implication for its multifunctional roles

Information concerning to regulation of vegetative phase and floral initiation in herbaceous perennial plants has been limited to a few plant species. To know and compare flowering regulation in a wider range of plant species, we identified and characterized SHORT VEGETATIVE PHASE (SVP)-like genes (GtSVP-L1 and GtSVP-L2) from herbaceous perennial gentian (Gentiana triflora). Apple latent spherical virus (ALSV)-mediated silencing of the GtSVP-L1 in G. triflora seedlings resulted in early flowering and shortened vegetative phase by about one-third period of time, without vernalization. This indicated that GtSVP-L1 acts as a negative regulator of flowering and vegetative phase. Seasonal change in the expression of GtSVP was monitored in the overwinter buds (OWBs) of G. triflora. It was found that the levels of GtSVP-L1 mRNA in OWBs increased concomitantly with induction and/or maintenance of dormancy, then decreased toward release from dormancy, while that of GtSVP-L2 mRNA remained low and unchanged. These results implied that, in herbaceous perennial plants, SVP ortholog might concern to activity-dormancy control, as well as negative regulation in flowering. Practically, these results can be applicable to non-time-consuming technologies for breeding.

Apple latent spherical virus vector-induced flowering for shortening the juvenile phase in Japanese gentian and lisianthus plants

Infection by apple latent spherical virus (ALSV) vectors that promote the expression of Arabidopsis thaliana FLOWERING LOCUS T ( AtFT ) or Gentiana triflora GtFT s accelerates flowering in gentian and lisianthus plants. Apple latent spherical virus (ALSV) has isometric virus particles (25 nm in diameter) that contain two ssRNA species (RNA1 and RNA2) and three capsid proteins (Vp25, Vp20, and Vp24). ALSV vectors are used for foreign gene expression and virus-induced gene silencing in a broad range of plant species. Here, we report the infection by ALSV vectors that express FLOWERING LOCUS T (AtFT) from Arabidopsis thaliana or its homolog GtFT1 from Gentiana triflora in three gentian cultivars ('Iwate Yume Aoi' [early flowering], 'Iwate' [medium flowering], and 'Alta' [late flowering]), and two lisianthus cultivars ('Newlination Pink ver. 2' and 'Torukogikyou daburu mikkusu') promotes flowering within 90 days post-inoculation using particle bombardment. Additionally, seedlings from the progeny of the early-flowering plants were tested by tissue blot hybridization, and the results showed that ALSV was not transmitted to the next generation. The promotion of flowering in the family Gentianaceae by ALSV vectors shortened the juvenile phase from 1-3 years to 3-5 months, and thus, it could be considered as a new plant breeding technique in ornamental gentian and lisianthus plants.

Promotion of Flowering by Apple Latent Spherical Virus Vector and Virus Elimination at High Temperature Allow Accelerated Breeding of Apple and Pear

Plant viral vectors are superior tools for genetic manipulation, allowing rapid induction or suppression of expression of a target gene in plants. This is a particularly effective technology for use in breeding fruit trees, which are difficult to manipulate using recombinant DNA technologies. We reported previously that if apple seed embryos (cotyledons) are infected with an Apple latent spherical virus (ALSV) vector (ALSV-AtFT/MdTFL1) concurrently expressing the Arabidopsis thaliana florigen (AtFT) gene and suppressing the expression of the apple MdTFL1-1 gene, the period prior to initial flowering (generally lasts 5-12 years) will be reduced to about 2 months. In this study, we examined whether or not ALSV vector technology can be used to promote flowering in pear, which undergoes a very long juvenile period (germination to flowering) similar to that of apple. The MdTFL1 sequence in ALSV-AtFT/MdTFL1 was replaced with a portion of the pear PcTFL1-1 gene. The resulting virus (ALSV-AtFT/PcTFL1) and ALSV-AtFT/MdTFL1 were used individually for inoculation to pear cotyledons immediately after germination in two inoculation groups. Those inoculated with ALSV-AtFT/MdTFL1 and ALSV-AtFT/PcTFL1 then initiated flower bud formation starting one to 3 months after inoculation, and subsequently exhibited continuous flowering and fruition by pollination. Conversely, Japanese pear exhibited extremely low systemic infection rates when inoculated with ALSV-AtFT/MdTFL1, and failed to exhibit any induction of flowering. We also developed a simple method for eliminating ALSV vectors from infected plants. An evaluation of the method for eliminating the ALSV vectors from infected apple and pear seedlings revealed that a 4-week high-temperature (37°C) incubation of ALSV-infected apples and pears disabled the movement of ALSV to new growing tissues. This demonstrates that only high-temperature treatment can easily eliminate ALSV from infected fruit trees. A method combining the promotion of flowering in apple and pear by ALSV vector with an ALSV elimination technique is expected to see future application as a new plant breeding technique that can significantly shorten the breeding periods of apple and pear.

Allelic variants of the esterase gene W14/15 differentially regulate overwinter survival in perennial gentian (Gentiana L.)

Overwinter survival has to be under critical regulation in the lifecycle of herbaceous perennial plants. Gentians (Gentiana L.) maintain their perennial life style through producing dormant and freezing-tolerant overwinter buds (OWBs) to overcome cold winter. However, the mechanism acting on such an overwinter survival and the genes/proteins contributing to it have been poorly understood. Previously, we identified an OWB-enriched protein W14/15, a member of a group of α/β hydrolase fold superfamily that is implicated in regulation of hormonal action in plants. The W14/15 gene has more than ten variant types in Gentiana species. However, roles of the W14/15 gene in OWB survival and functional difference among those variants have been unclear. In the present study, we examined whether the W14/15 gene variants are involved in the mechanism acting on overwinter survival, by crossing experiments using cultivars carrying different W14/15 variant alleles and virus-induced gene silencing experiments. We found that particular types of the W14/15 variants (W15a types) contributed toward obtaining high ability of overwinter survival, while other types (W14b types) did not, or even interfered with the former type gene. This study demonstrates two findings; first, contribution of esterase genes to winter hardiness, and second, paired set or paired partner among the allelic variants determines the ability of overwinter survival.

Genetic analysis of bolting after winter in sugar beet (Beta vulgaris L.)

This study reveals for the first time a major QTL for post-winter bolting resistance in sugar beet ( Beta vulgaris L.). The knowledge of this QTL is a major contribution towards the development of a winter sugar beet with controlled bolting behavior. In cool temperate climates, sugar beets are currently grown as a spring crop. They are sown in spring and harvested in autumn. Growing sugar beet as a winter crop with an extended vegetation period fails due to bolting after winter. Bolting after winter might be controlled by accumulating genes for post-winter bolting resistance. Previously, we had observed in field experiments a low post-winter bolting rate of 0.5 for sugar beet accession BETA 1773. This accession was crossed with a biennial sugar beet with regular bolting behavior to develop a F3 mapping population. The population was grown in the greenhouse, exposed to artificial cold treatment for 16 weeks and transplanted to the field. Bolting was recorded twice a week from May until October. Post-winter bolting behavior was assessed by two different factors, bolting delay (determined as days to bolt after cold treatment) and post-winter bolting resistance (bolting rate after winter). For days to bolt, means of F3 families ranged from 25 to 164 days while for bolting rate F3 families ranged from 0 to 1. For each factor one QTL explaining about 65% of the phenotypic variation was mapped to the same region on linkage group 9 with a partially recessive allele increasing bolting delay and post-winter bolting resistance. The results are discussed in relation to the potential use of marker-assisted breeding of winter sugar beets with controlled bolting.

Transcription factor-mediated cell-to-cell signalling in plants

Plant cells utilize mobile transcription factors to transmit intercellular signals when they perceive environmental stimuli or initiate developmental programmes. Studies on these novel cell-to-cell signals have accumulated multiple pieces of evidence showing that non-cell-autonomous transcription factors play pivotal roles in most processes related to the formation and development of plant organs. Recent studies have explored the evolution of mobile transcription factors and proposed mechanisms for their trafficking through plasmodesmata, where a selective system exists to facilitate this process. Mobile transcription factors contribute to the diversity of the intercellular signalling network, which is also established by peptides, hormones, and RNAs. Crosstalk between mobile transcription factors and other intercellular molecules leads to the development of complex biological signalling networks in plants. The regulation of plasmodesmata appears to have been another major step in controlling the intercellular trafficking of transcription factors based on studies of many plasmodesmal components. Furthermore, diverse omics approaches are being successfully applied to explore a large number of candidate transcription factors as mobile signals in plants. Here, we review these fascinating discoveries to integrate current knowledge of non-cell-autonomous transcription factors.

GmFT4, a homolog of FLOWERING LOCUS T, is positively regulated by E1 and functions as a flowering repressor in soybean

The major maturity gene E1 has the most prominent effect on flowering time and photoperiod sensitivity of soybean, but the pathway mediated by E1 is largely unknown. Here, we found the expression of GmFT4, a homolog of Flowering Locus T, was strongly up-regulated in transgenic soybean overexpressing E1, whereas expression of flowering activators, GmFT2a and GmFT5a, was suppressed. GmFT4 expression was strongly up-regulated by long days exhibiting a diurnal rhythm, but down-regulated by short days. Notably, the basal expression level of GmFT4 was elevated when transferred to continous light, whereas repressed when transferred to continuous dark. GmFT4 was primarily expressed in fully expanded leaves. Transcript abundance of GmFT4 was significantly correlated with that of functional E1, as well as flowering time phenotype in different cultivars. Overexpression of GmFT4 delayed the flowering time in transgenic Arabidopsis. Taken together, we propose that GmFT4 acts downstream of E1 and functions as a flowering repressor, and the balance of two antagonistic factors (GmFT4 vs GmFT2a/5a) determines the flowering time of soybean.

Phloem-mobile signals affecting flowers: applications for crop breeding

Transport of endogenous macromolecules within and between tissues serves as a signaling pathway to regulate numerous aspects of plant growth. The florigenic FT gene product moves via the phloem from leaves to apical tissues and induces the flowering program in meristems. Similarly, short interfering RNA (siRNA) signals produced in source or sink tissues move cell-to-cell and long distance via the phloem to apical tissues. Recent advances in identifying these mobile signals regulating flowering or the epigenetic status of targeted tissues can be applicable to crop-breeding programs. In this review, we address the identity of florigen, the mechanism of allocation, and how virus-induced flowering and grafting of transgenes producing siRNA signals affecting meiosis can produce transgene-free progenies useful for agriculture.

GmGBP1, a homolog of human ski interacting protein in soybean, regulates flowering and stress tolerance in Arabidopsis

BACKGROUND

SKIP is a transcription cofactor in many eukaryotes. It can regulate plant stress tolerance in rice and Arabidopsis. But the homolog of SKIP protein in soybean has been not reported up to now.

RESULTS

In this study, the expression patterns of soybean GAMYB binding protein gene (GmGBP1) encoding a homolog of SKIP protein were analyzed in soybean under abiotic stresses and different day lengths. The expression of GmGBP1 was induced by polyethyleneglycol 6000, NaCl, gibberellin, abscisic acid and heat stress. GmGBP1 had transcriptional activity in C-terminal. GmGBP1 could interact with R2R3 domain of GmGAMYB1 in SKIP domain to take part in gibberellin flowering pathway. In long-day (16 h-light) condition, transgenic Arabidopsis with the ectopic overexpression of GmGBP1 exhibited earlier flowering and less number of rosette leaves; Suppression of AtSKIP in Arabidopsis resulted in growth arrest, flowering delay and down-regulation of many flowering-related genes (CONSTANS, FLOWERING LOCUS T, LEAFY); Arabidopsis myb33 mutant plants with ectopic overexpression of GmGBP1 showed the same flowering phenotype with wild type. In short-day (8 h-light) condition, transgenic Arabidopsis plants with GmGBP1 flowered later and showed a higher level of FLOWERING LOCUS C compared with wild type. When treated with abiotic stresses, transgenic Arabidopsis with the ectopic overexpression of GmGBP1 enhanced the tolerances to heat and drought stresses but reduced the tolerance to high salinity, and affected the expressions of several stress-related genes.

CONCLUSIONS

In Arabidopsis, GmGBP1 might positively regulate the flowering time by affecting CONSTANS, FLOWERING LOCUS T, LEAFY and GAMYB directly or indirectly in photoperiodic and gibberellin pathways in LDs, but GmGBP1 might represse flowering by affecting FLOWERING LOCUS C and SHORT VEGETATIVE PHASE in autonomous pathway in SDs. GmGBP1 might regulate the activity of ROS-eliminating to improve the resistance to heat and drought but reduce the high-salinity tolerance.

Manipulating plant architecture with members of the CETS gene family

The shape or architecture of a plant is specified through the activities of indeterminate and determinate meristems, and the sum of these events sharply impacts plant growth habit, productivity, and crop management. The CENTRORADIALIS/TERMINAL FLOWER 1/SELF-PRUNING (CETS) gene family shares homology to phosphatidylethanolamine binding protein (PEBP) genes and is prominent in controlling the timing and location of the developmental transition from indeterminate to determinate growth, with different family members balancing the activities of others through antagonistic functions. The CETS members FLOWERING LOCUS T (FT) of Arabidopsis and related genes (e.g. SINGLE FLOWER TRUSS, SFT, in tomato) are important in promoting the transition to determinate growth while TERMINAL FLOWER 1 (TFL1) and its homologs (e.g. tomato SELF PRUNING, SP) oppose this activity by maintaining meristems in an indeterminate state. FT orthologs, and perhaps other CETS family members, act as mobile proteinaceous hormones, and can amplify their impact by accumulating in recipient organs. A universal model is emerging for the timing and placement of determinate and indeterminate growth through a balance of FT-like and TFL1-like gene activities, and it is now clear that the domestication of many wild exotics into crops with desired growth habits resulted from selection of altered FT/TFL1 balances. Manipulating this ratio further, through transgenic or viral-based technologies, holds promise for improved agricultural sustainability.

Geminivirus-mediated delivery of florigen promotes determinate growth in aerial organs and uncouples flowering from photoperiod in cotton

Background

Plant architecture and the timing and distribution of reproductive structures are fundamental agronomic traits shaped by patterns of determinate and indeterminate growth. Florigen, encoded by FLOWERING LOCUS T (FT) in Arabidopsis and SINGLE FLOWER TRUSS (SFT) in tomato, acts as a general growth hormone, advancing determinate growth. Domestication of upland cotton (Gossypium hirsutum) converted it from a lanky photoperiodic perennial to a highly inbred, compact day-neutral plant that is managed as an annual row-crop. This dramatic change in plant architecture provides a unique opportunity to analyze the transition from perennial to annual growth.

Methodology/Principal Findings

To explore these architectural changes, we addressed the role of day-length upon flowering in an ancestral, perennial accession and in a domesticated variety of cotton. Using a disarmed Cotton leaf crumple virus (CLCrV) as a transient expression system, we delivered FT to both cotton accessions. Ectopic expression of FT in ancestral cotton mimicked the effects of day-length, promoting photoperiod-independent flowering, precocious determinate architecture, and lanceolate leaf shape. Domesticated cotton infected with FT demonstrated more synchronized fruiting and enhanced “annualization”. Transient expression of FT also facilitated simple crosses between wild photoperiodic and domesticated day-neutral accessions, effectively demonstrating a mechanism to increase genetic diversity among cultivated lines of cotton. Virus was not detected in the F₁ progeny, indicating that crosses made by this approach do not harbor recombinant DNA molecules.

Conclusions

These findings extend our understanding of FT as a general growth hormone that regulates shoot architecture by advancing organ-specific and age-related determinate growth. Judicious manipulation of FT could benefit cotton architecture to improve crop management.

New dimensions for VIGS in plant functional genomics

Virus-induced gene silencing (VIGS) is an efficient tool for gene function studies. It has been used to perform both forward and reverse genetics to identify plant genes involved in several plant processes. However, this technology has not yet been used to its full potential. This can be attributed to several of its limitations such as inability to silence genes during seed germination and the non-stable nature of silencing. However, several recent studies have shown that these limitations can now be overcome. In this review, we will discuss the limitations of VIGS and suitable solutions. In addition, we also describe the recent improvements and future prospects of using VIGS in plant biology.

The alleles at the E1 locus impact the expression pattern of two soybean FT-like genes shown to induce flowering in Arabidopsis

A small gene family of phosphatidyl ethanolamine-binding proteins (PEBP) has been shown to function as key regulators in flowering; in Arabidopsis thaliana the FT protein promotes flowering whilst the closely related TFL1 protein represses flowering. Control of flowering time in soybean [Glycine max (L.) Merrill] is important for geographic adaptation and maximizing yield. Soybean breeders have identified a series of loci, the E-genes, that control photoperiod-mediated flowering time, yet how these loci control flowering is poorly understood. The objectives of this study were to evaluate the expression of GmFT-like genes in the E1 near-isogenic line (NIL) background. Of the 20 closely related PEBP proteins in the soybean genome, ten are similar to the Arabidopsis FT protein. Expression analysis of these ten GmFT-like genes confirmed that only two are detectable in the conditions tested. Further analysis of these two genes in the E1 NILs grown under short-day (SD) and long-day (LD) conditions showed a diurnal expression and tissue specificity expression commensurate with soybean flowering time under SD and LD conditions, suggesting that these were good candidates for flowering induction in soybean. Arabidopsis ft mutant lines flowered early when transformed with the two soybean genes, suggesting that the soybean genes can complement the Arabidopsis FT function. Flowering time in E1 NILs is consistent with the differential expression of the two GmFT-like genes under SD and LD conditions, suggesting that the E1 locus, at least in part, impacts time to flowering through the regulation of soybean FT expression.