- Open Access
The JatrophaFT ortholog is a systemic signal regulating growth and flowering time
Biotechnology for Biofuels volume 7, Article number: 91 (2014)
Jatropha curcas is being promoted as a new bioenergy crop in tropical and subtropical regions due to its high amount of seed oil and its potential capacity to grow on marginal land for biofuel production. However, the productivity of the plant is constrained by the unfavorable flowering time and inflorescence architecture, which render harvesting of seeds time-consuming and labor-intensive. These flowering-related traits have limited further widespread cultivation of Jatropha.
We identified a Jatropha curcas homolog of Flowering locus T (JcFT) and demonstrated its function by genetic complementation of the Arabidopsis ft mutant. The JcFT expression level was found to be remarkably correlated with leaf age. Overexpression of JcFT in Jatropha reduced flowering time and altered plant architecture by producing more branches. Grafting experiments suggested that the earlyflowering and alteration of plant architecture traits were graft-transmissible. We also showed that the FT-overexpressing transgenic Jatropha can be used as a root stock for grafting of scions derived from other Jatropha.
We generated early flowering transgenic Jatropha plants that accumulate higher levels of the florigen FT. Not only early flowering but also plant growth was affected in JcFT overexpression lines. More seeds can be produced in a shorter time frame by shortening the flowering time in Jatropha, suggesting the possibility to increase seed yield by manipulating the flowering time.
Biofuels have been recognized as a national priority for many countries as an alternative source to meet their energy security needs. The demand for biofuel has placed increasing pressure on food production, which further raises the 'food vs fuel' debate. For instance, to satisfy the biofuel need for Germany in 2017 as mandated by the German government, all agricultural land for food production has to also be used for rape seed derived biodiesel production . To ease the competition between food and fuel for arable land while satisfying the need for renewable fuels, it is well recognized that the first choice is to use marginal or degraded land for the production of biofuel crops .
Jatropha curcas, a small seed-propagated woody plant belonging to the family Euphorbiaceae, is a non-food plant used for biofuel production. This plant has the potential to solve in part the competition between food and fuel mainly because of its wide adaptability, drought tolerance and its ability to thrive on degraded soils [3, 4]. At present, the key agronomic trait targeted by Jatropha breeders is the productivity of the plant . Flowering is a prerequisite for crop production whenever fruits or seeds are harvested. The productivity of Jatropha is constrained by an unfavorable male-to-female flower ratio that has not been optimized by conventional breeding. Jatropha is monoecious and produces male and female flowers on the same inflorescence . Not surprisingly, the productivity of the Jatropha plant is also highly affected by geographical and environmental conditions [6, 7]. Physiological and genetic analysis of flowering has shown that multiple environmental and endogenous inputs influence the timing of the transition from vegetative to reproductive growth [7, 8]. Jatropha mainly grows in tropical and subtropical regions between 30°N and 35°S [6, 9]. It can produce limited amounts of seeds at high altitudes (>500 meters above sea level), where the majority of marginal land is located in the world. However, it is unable to flower and produce seeds in areas with low light intensity and poor light quality . Lastly, unsynchronized flowering and fruit setting renders harvesting of Jatropha seeds highly labor-intensive; in fact, high labor cost is the primary fixed cost in Jatropha fruit production [3, 4]. Therefore, it is essential to systematically study the Jatropha flowering biology so as to devise strategies to synchronize flowering and increase seed yield . With a better understanding of molecular and genetic mechanisms of flowering, it will be possible to generate next-generation Jatropha plants by traditional breeding and/or genetic engineering.
At a certain point in their life cycle, annual plants undergo a major developmental transition in which they switch from vegetative to reproductive growth . Although the draft genome sequence of Jatropha has been reported, little genetic and mechanistic research on regulation of flowering time or control of male/female flower ratio has been done . The male-biased ratio within an inflorescence limits Jatropha seed production; hence, more female flowers in a plant will produce more fruits and seeds. Meanwhile, to precisely control the time of flowering, plants have evolved mechanisms to integrate seasonally predictable environmental cues (such as changes in photoperiod and prolonged periods of cold temperatures) and developmental cues (such as maturity).
In the model plant Arabidopsis thaliana, the FLOWERING LOCUS T (FT) protein serves as a flowering-inducing signal . FT is a member of a protein family that contains a plant-specific phosphatidylethanolamine-binding protein (PEBP) domain. In Arabidopsis, the PEBP family is divided into three subfamilies, namely FT-like, TERMINAL FLOWER1 (TFL1)-like, and MOTHER of FT and TFL1 (MFT)-like. FT and TFL1 are thought to be molecular switches regulating vegetative and reproductive growth, whereas MFT is phylogenetically ancestral to the two proteins. In other plants, such as tomato, the balance between the activity of tomato FT homolog SINGLE FLOWER TRUSS and that of the TFL1 homolog SELF-FRUNING affects a variety of developmental processes, such as flowering response, reiterative growth, termination cycles, leaf maturation and stem growth [12–14]. In the sugar beet, flowering time is controlled by the interplay between two paralogs of the Arabidopsis FT gene that have evolved antagonistic functions. BvFT2 is functionally conserved with FT and is essential for flowering. By contrast, BvFT1 functions as a flowering terminator (TFL1) to repress flowering, and its down-regulation is crucial for vernalization in beets . Therefore, all three subfamilies of PEBP genes can function as general developmental regulators rather than simple floral initiators or florigens .
In this study, we functionally identified an FT homolog JcFT gene from Jatropha by genetic complementation of the Arabidopsis thaliana ft-10 mutant. We showed how manipulation of Jatropha flowering time can lead to yield increase. A rapid breeding technology for Jatropha using earlyflowering transgenic plants and marker-assisted selection is also discussed.
Identification of FT genes from Jatrophaand other Euphorbiacious plants
By analysing the database of a sequenced cDNA library prepared from J. curcas seeds , several Flowering locus T-related genes and one Jatropha curcas Flowering locus T homolog (JcFT) were identified. At the amino acid level JcFT is 86% identical to Arabidopsis FT. Using the amino acid sequence of JcFT, we further identified FT-like genes from other Euphorbiacious plants such as castor bean (Ricinuscommunis) and cassava (Manihotesculenta) by mining data in Phytozome . Phylogenetic analysis of PEBP domain family proteins from various plants indicated that PEBP proteins fall into three groups: the FT group, which promotes flowering; the TFL1/CEN-like group, which prolongs vegetative identity; and the MFT group, which is ancestral to the FT and TFL1 groups. As the JcFT protein and FT-like proteins in Euphorbiacious plants are located in the FT-like group, we deduced that this JcFT is an FT-like protein (Figure 1A). Using the polymerase chain reaction (PCR) approach, we cloned an FT cDNA from a Jatropha leaf sample. Southern blot analysis, using the JcFT cDNA as a probe, revealed only one copy of the FT gene in the Jatropha genome (see Additional file 1). That there is only one FT homologous gene in the Jatropha genome was further confirmed through mining of both the deep sequencing data set of Jatropha performed at the Temasek Life Sciences Laboratory (Yan Hong, unpublished data) and the Jatropha genome database available online (Sato et al., ). Analysis of genomic structures of FT homologous genes from Euphorbiaceae showed that they are similar to that of the Arabidopsis FT gene (Figure 1B). The second intron of the JcFT and the Ricinuscommunis FT (RcFT) is very big in size (Figure 1B), and is similar to that of the sugar beet FT (BvFT2) .
We investigated the relationship between JcFT transcript levels in leaves of different ages and found that the JcFT transcript level increased from younger to older leaves (Figure 1C). These data suggest that FT expression is correlated with plant age and is important for Jatropha flowering regulation.
Transgenic Arabidopsis plants expressing JcFTwith early flowering phenotype
To determine the roles of JcFT in planta, we tested the function of JcFT in the model plant Arabidopsis thaliana. First, we constructed a binary vector pCAMBIA 2 × 35S:JcFT, containing the coding sequence of JcFT under the control of a double CaMV35S promoter. After transformation of Arabidopsis mutant ft-10 with pCAMBIA 2 × 35S: JcFT, the late flowering phenotype was rescued as indicated by the number of plant leaves at bolting and the final plant size of the complemented mutant (Figure 2A, B and C); by contrast, the ft-10 mutant remained in vegetative growth (Figure 2D). We further generated JcFT-overexpression lines in WT Arabidopsis Columbia (Col-0) background. All T1 plants showed expression of JcFT transcripts (see Additional file 2). Transgenic plants overexpressing JcFT displayed significantly earlyflowering phenotypes under either long day or short day conditions (Table 1). WT (Col-0) plants started bolting on average 16 days after sowing on soil, and their first flowers bloomed 21.5 days after sowing under long day conditions. By contrast, the JcFT overexpression #15 line required only 7.5 days for bolting and 13.8 days for blooming (Table 1, Figure 2E, F and G). Under long day conditions, JcFT overexpression significantly shortened the bolting time (only 47% of WT in #15) but had no effect on the time from bolting to blooming. The effect of JcFT overexpression on reducing the bolting time was even stronger under short day conditions (only 27% of WT in #15), whereas JcFT overexpression still did not have any effect on the time from bolting to blooming (Table 1). Overexpression of JcFT in Arabidopsis not only shortened plant flowering time, but also altered plant morphology such as upward curling of leaf and increased number of inflorescence branches (Figure 2G).
Early flowering by over expression of JcFT in Jatropha
We further tested the functional roles of JcFT in Jatropha. Transgenic shoots overexpressing JcFT from a 35S promoter were found to flower as early as the shoot regeneration stage in tissue culture (Figure 3A and B). These shoots with early flowering trait failed to generate roots. Moreover, it was difficult to graft these shoots onto WT rootstock. These observations prompted us to replace the 35S promoter with a weaker, synthetic G10-90 promoter to express JcFT (see Additional file 3). We successfully generated 10 transgenic Jatropha lines carrying G10-90::JcFT transgene. Both Southernblot and reverse transcriptase (RT)-PCR analysis verified the presence of the transgene and JcFT overexpression in transgenic Jatropha plants (see Additional file 1 and Additional file 4). Figures 4A and 5A show that ectopic expression of JcFT strongly accelerated Jatropha flowering and fruit setting. Under normal growth conditions in a greenhouse, WT Jatropha required around 8 months to produce the first inflorescence. By contrast, only 3.5 months were needed to obtain the first inflorescence for 10 primary independent JcFT overexpression lines (Figure 5A). Seeds of two early flowering T0 lines JcFT overexpression lines (#33 and #43) were germinated on plates and the seedlings transferred into soil at the two-true-leafstage together with the WT control. The flowering times of the T1 plants of #33 and #43 were dramatically shortened to 1 to 2 months, from 8 months (Figures 4E, 5B and Additional file 5). The great reduction of time to flowering considerably shortened the time for seed set, which is one of the key determinant factors for Jatropha yield. We further found that the JcFT expression level was correlated with flowering time in Jatropha (Figure 5B). However, there was no significant role of JcFT on the time from in florescene emergence to blooming, and the time from seed set to seed maturation (data not shown).
The time for flowering dormancy, a period between the initiation of inflorescence and the next, was also drastically reduced by JcFT overexpression as indicated by the number of leaves developed between the first to the second inflorescence (Compare Figure 4A with B, and Figure 5C). Besides early flowering, JcFT-overexpressing plants produced more leaves but with smaller leaf size (Figure 4A, E and Additional file 5).
Overexpression of JcFT was found to also change the Jatropha architecture. Figure 4D and F show a typical WT Jatropha sympodial unit structure including two inflorescences and two lateral shoots. The sympodial unit structure in JcFT-overexpressing plants included only one inflorescence and two shoots (Figure 4D, G and H). Due to the short dormancy time, more branches were also found in JcFT-overexpressing Jatropha plants (Figures 4A, I and 5D). Importantly, early flowering JcFT-overexpressing Jatropha consistently produced three times more seeds without compromising seed weight (Figures 4A, K, 5E, F and G), indicating the potential agronomic value of JcFT overexpression.
In addition to genetics, environmental conditions highly affect plant growth and flowering time, especially for Jatropha, a tropical plant. Our data showed that the early flowering time trait obtained by JcFT ectopic expression was not affected by lower temperature (22°C vs 28°C as optimal temperature, Figure 5H).
Graft-transmissible action of JcFT
In general, it takes 8 months for transgenic Jatropha to develop the first inflorescence under greenhouse conditions. Therefore, a method to shorten flowering time will be beneficial for the development of genetically modified Jatropha. As a mobile, long-distance signal, it is generally believed that FT is produced in the leaf and is transported to the shoot apex, where it triggers floral morphogenesis. We asked whether Jatropha FT protein can act as a mobile and graft-transmissible signal to promote flowering. To this end, we used JcFT-overexpressing plant (#43) as a root stock and a high oleic acid line of transgenic Jatropha (X8-FAD2 RNAi #34) as scion . Figure 6 shows that FT overexpression in the root stock was capable of promoting flowering in the recipient scion. A parallel control experiment showed that a WT Jatropha root stock had no effect on flowering time of the grafted scion. The use of transgenic JcFT-overexpressing rootstock enabled us to save 4 months in seed production of the T0 generation of transgenic Jatropha. More interestingly, the role of JcFT on sympodial unit structure was also found to be graft-transmissible (Figure 6F).
Regulation of flowering time of biodiesel plants has the potential to increase yield. Using Miscanthussacchariflorus, Jensen et al. (2013) found that delayed flowering results in a greater than 50% increase in biomass . Here, we demonstrate that direct manipulation of the florigen FT could yield a larger number of seeds. The great reduction of time to flowering considerably shortened the time for seed set, which is one of the key determinant factors for Jatropha yield. Overexpression of JcFT was found to also change the Jatropha architecture to semidwarf stature (Figure 4D), which is ideal for mechanical harvesting of seeds to reduce labor costs. Not only flowering time, but also the inflorescence structure was affected by overexpression of JcFT in plants with distinct growth habits, monopodial Arabidopsis and sympodial Jatropha (shown in Figures 2 and 4). Overexpression of JcFT resulted in one instead of two inflorescences in one sympodial unit in transgenic Jatropha. No branching inflorescence phenotype was found in the loss-of-function Arabidopsis TFL1 mutant. TFL1 and FT have highly conserved amino acid sequences but opposing functions. Previous studies in Arabidopsis have suggested that an antagonistic interaction between the TFL1 and floral meristem identity genes, such as LEAFY (LFY) and APETALA1 (AP1), regulates the inflorescence branching pattern . There are two TFL1-like proteins in the Jatropha genome [10, 20]. The roles of Jatropha TFL1 orthologs in the determination of flowering traits are being investigated by our group . Nevertheless, the observed impact on plant architecture, that of increasing the number of branches in Jatropha by manipulation of the florigen FT, is likely an indirect consequence of the early and rapid initiation of flowering, rather than a direct effect on branch initiation, since each flowering event in Jatropha is accompanied by a subsequent branching event, each of which terminates in a second flowering event and subsequent branching.
Other than flowering, FT-like proteins in plants have also been recognized as major regulatory factors in a number of developmental processes including stomatal control and tuberization. These multifunctional roles of FT-like proteins are derived from extensive gene duplication that occurred during evolution. As the gene evolutionary process occurred independently in nearly all modern angiosperm lineages, it is essential to determine the spatiotemporal pattern of ectopic FT expression to minimize its negative effect on normal vegetative growth in plants . An alternative way is to induce expression of FT-like proteins by using, for example, an ethanol-inducible promoter [21, 22]. Controllable flowering allows the synchronization of fruit set and collection, thus reducing labor cost, although more research is needed on the feasibility of chemical induction on an industrial scale.
Precise control of flowering time is a critical developmental process that determines the reproductive success of flowering plants. Our data demonstrated that JcFT is a mobile signal to control flowering time and a major flowering integrator in Jatropha. The earlyflowering lines reported here provide valuable germplasm, especially for marginal land and mountainous regions with conditions of poor light and high altitude (>500 meters above sea level), for example, the Sichuan, Yunnan and Guizhou provinces in China, to produce seeds in a cost-effective way. These lines can also be used to shorten the development time for other GM traits by grafting as shown in Figure 6. Transgenic seeds obtained from grafting can be used to integrate with traditional breeding processes to accelerate advancement of transgenic traits.
In comparison to herbaceous plants, the breeding of trees is more time-consuming owing to their long generation time. Shortened juvenility and precocious flowering are therefore important breeding goals. Flower initiation has been intensively studied in Arabidopsis, and orthologs/homologs of genes for LFY, AP1, and TFL1 have been cloned from apple trees among others [23–25]. There are a few successful reports on a rapid breeding program in apple by genetic modification of the flowering pathway [26–28]. A similar strategy can be used in breeding programs of Jatropha. Viral-based systems such as virus- induced gene silencing (VIGS) or transient expression of flowering time genes can also be good alternatives to shorten flowering time to accelerate the breeding process [29–31].
We generated early flowering transgenic Jatropha plants that accumulate higher levels of the florigen FT. Not only early flowering but also plant growth was affected in JcFT- overexpressing lines. More seeds can be produced in a shorter time frame by reducing flowering time in Jatropha, suggesting the possibility to increase seed yield by flowering time manipulation.
Plant materials and growth condition
Seeds were obtained from Jatropha curcas (Jc-MD) elite plants preselected by Drs. Yan Hong and Chengxin Yi . All control or transgenic plants were grown in a biosafety level 2 greenhouse . Plant management, such fertilization, pesticide spraying, watering and artificial fertilization, was carried out according to normal practice.
JcFTcDNA isolation and plasmid construction
The Jatropha curcas Flowering locus T (JcFT) gene was first identified from the database of a sequenced cDNA library prepared from Jatropha seeds . A full-length cDNA fragment was PCR-amplified with forward primer 5'-ATAAGTCGACATGAGGGATCAATTTAGAGA-3' and reverse primer 5'-TTATTTCTAGATCACCGTCTCCGTCCTCCGGT-3'. The PCR fragment was inserted in the sense orientation into the Sal I/XabI sites of the pCABMIA1300-3HA vector.
To generate β-estradiol-inducible JcFT overexpression lines, we used a cDNA fragment encoding JcFT protein. This fragment was PCR-amplified with forward primer 5'-ATAACTCGAGATGAGGGATCAATTTAGAGA-3' and reverse primer 5'-TTATTACTAGTTCACCGTCTCCGTCCTCCGGT-3'. The PCR fragment was inserted in the sense orientation into the XhoI/SpeI sites of the pX7-GFP vector as described previously by Qu et al. . The construct was named pX7-JcFT.
We used the transformation protocol described by Qu et al. .
Nuclear acid extraction and analysis
DNA and RNA were isolated and analyzed according to previously described methods . For quantitative PCR analysis, two primers were used for the JcFT gene (F: GTTACTTATAATCACAGAGAGGT, R: TCTCATAGCACACTATCTCTTGC). The Jatropha UBQ transcript served as an internal control for RNA samples with primers (F: GAGGTGGAAAGCTCAGATACAATT, R: AAAGTGATGGTCTTTCCGGTCAATG). For Southern blot analysis, nylon Hybond-N+ membranes were hybridized with DNA probes encoding the HPT or JcFT open reading frame .
Multiple alignments were generated using Clustal W. The Neighbor-Joining method in MEGA4 was used to reconstruct a phylogeny tree. Bootstrap analysis was performed to estimate nodal support on the basis of 1,000 resamplings. The phylogenetic tree was constructed with proteins including sugar beet (Beta vulgaris – BvFT1, No. HM448910; BvFT2, No.HM448912; BvBFT1, No.HM448916; BvCEN1, No. HM448914; and BvMFT1, No. HM448918), apple (Malus x domestica– MdFT1, No.AB161112; MdFT2, No.AB458504; MdCENa, No.AB366641; MdCENb, No.AB366642; MdTFL1, No.AB052994; BFTL1, No.EB138045; and MFTL1, No. EB134193), black cottonwood (Populustrichocarpa– PtFT1, No.XM_002311228; PtFT2, No.XM_002316137; PtCENL1, No.XM_002328224; PtCENL2, No. XM_002312775; and PtMFT, No. XM_002321471), morning glory (Ipomoea nil – PnFT1, No. EU178859; and PnFT2, No. EU178860), orange (Citrus sinensis– CiFT, No. CK939149; and CsTFL, No. AY344244), rice (Oryza sativa – Hd3a, No. NM_001063395; RFT1, No.NM_001063394; RCN1, Os11g05470), snapdragon (Antirrhinum majus– CEN, No.AJ251994), arabidopsis (Arabidopsis thaliana– FT, No.NM_105222; TFL1, No.NM_120465; MFT, No.NM_101672; ATC, No.NM_128315; TSF, No. NM_118156; and BFT, No. NM_125597), and tomato (Solanumlycopersicum– SP, No.U84140; SP3D, No.AY186735; SP2G, No. AY186734; and SP9D, No. AY186738). The amino acid sequences encoded by JcFT (Jatropha curcas FT), RcFT (Ricinuscommunis FT) and Me FT (Manihotesculenta) are listed in Additional file 6 and .
- AP1 :
- BFT :
Brother of FT
- FT :
Flowering locus T
- LFY :
- MFT :
Mother of FT
- TFL1 :
Jatropha curcas MD isolate
virus-induced gene silencing.
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We would like to thank Drs. Yan Hong, Chengxin Yi (JOil) and Yuehui He for the J. curcas Jc-MD and Arabidopsis ft-10 seeds, and Mr. Khar Meng Ng for his help in taking care of the plants. This work was supported by the Temasek Life Sciences Laboratory, J Oil and the Singapore Millennium Foundation.
Patents relating to the JcFT and its usage have been filed by the Temasek Life Sciences Laboratory.
JY and NHC designed the experiments, analyzed the data and drafted the manuscript. JY, YFG, BPZ, and JQ performed vector construction, genotyping, molecular analysis, flowering traits collection and analysis. HZM did the Jatropha transformation. All authors read and approved the final manuscript.
Electronic supplementary material
Additional file 1: Figure S1: Southern blot analysis of transgenic Jatropha lines overexpressing JcFT. Four independent transgenic lines were analyzed using wild-type (WT) Jatropha as a negative control. Southern blots were hybridized to a JcFT probe (left) or a hygromycinphosphotransferase (hgy) probe (right). Molecular size markers (kb) are given on the right. (TIFF 104 KB)
Additional file 2: Figure S2: RT-PCR analysis of JcFT and AtFT transcripts in transgenic Arabidopsis plants expressing 35S:JcFT. Total RNAs were extracted from leaves of 25 day-old plants. Nine independent transgenic lines were analyzed. WT Arabidopsis thaliana (Col-0) was used as a negative control. JcFT: Jatropha curcas Flowering locus T, AtFT: Arabidopsis thaliana Flowering locus T, AtUBQ: Arabidopsis thaliana ubiquitin 10.(TIFF 70 KB)
Authors’ original submitted files for images
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