The cotyledons produce sufficient FT protein to induce flowering: evidence from cotyledon micrografting in Arabidopsis

Regulation of Flowering Time by Environmental Factors in Plants

The timing of floral transition is determined by both endogenous molecular pathways and external environmental conditions. Among these environmental conditions, photoperiod acts as a cue to regulate the timing of flowering in response to seasonal changes. Additionally, it has become clear that various environmental factors also control the timing of floral transition. Environmental factor acts as either a positive or negative signal to modulate the timing of flowering, thereby establishing the optimal flowering time to maximize the reproductive success of plants. This review aims to summarize the effects of environmental factors such as photoperiod, light intensity, temperature changes, vernalization, drought, and salinity on the regulation of flowering time in plants, as well as to further explain the molecular mechanisms that link environmental factors to the internal flowering time regulation pathway.

Study on the Applications and Regulatory Mechanisms of Grafting on Vegetables

Grafting can overcome problems with soil sensitivity, enhance plant stress tolerance, improve product quality, and increase crop yield and value. This paper reviews the various mechanisms of vegetable grafting, the graft survival process and its influencing factors, the practical applications of grafting, and the molecular regulation of grafting in vegetables. The importance of germplasm and rootstock interactions, the mechanization of vegetable grafting, and future aspects, including intelligence and digitalization, are discussed.

Establishment of a novel experimental system for studying the photoperiodic response of short-day dicots using soybean 'cotyledon-only plant' as material

Soybean is an important model crop for photoperiodic response studies in plants and contributes significantly to the study of plant development and physiology in the past century. Because soybean plant is much bigger in size and longer in life cycle than Arabidopsis, it needs much more space for growth and time for investigation, which significantly hamper the efficiency of research. In the current study, we tested the photoperiodic response of a distinctive artificially-made cotyledon-only plant (COP) using a photoperiod-sensitive soybean variety Zigongdongdou (ZGDD) and other varieties with diverse sensitivity to photoperiod. ZGDD COPs flowered 39.4 ± 2.5 d after emergence under short-day conditions but maintained vegetative growth under long-day and night break conditions, which is similar to the case in the intact ZGDD plants. The COPs of early-maturing and medium-maturing soybean varieties also grew and flowered normally under natural day-length conditions. At the molecular level, the key genes in the photoperiodic pathway such as E1, GmFT1a, GmFT2a, and GmFT5a in the COPs also showed the same photoperiod sensitivity as in the intact plants. In addition, a simpler material of COP with only one cotyledon and root was generated and found to be sensitive to photoperiod as well. Notably, the COPs are only one-fifth the height of intact plants and one-third the maximum diameter of the intact plants grown in chambers 30 d after emergence. Based on COPs, we established a novel experimental system characterized by an entire photoperiodic response and longer longevity of cotyledons in addition to small plant size, ensuring the consistency, reliability, and stability of plant materials. COPs have the potential to be a novel model material for studies of the developmental biology of soybean and other dicots.

Quantitative Analysis of Florigen for the Variability of Floral Induction in Cabbage/Radish Inter-generic Grafting

Grafting-induced flowering is a key phenomenon to understand systemic floral induction caused by florigen. It can also be used as a breeding technique enabling rapid seed production of crops with long generation times. However, the degree of floral induction in grafted plants is often variable. Moreover, it is difficult in some crop species. Here, we explored the factors promoting variability in the grafting-induced flowering of cabbage (Brassica oleracea L. var. capitata), an important vegetable crop with a long generation time, via the quantitative analysis of florigen accumulation. Significant variability in the flowering response of grafted cabbage was observed when rootstocks of different genotypes were used. As reported previously, B. oleracea rootstocks did not induce the flowering of grafted cabbage plants, but radish (Raphanus sativus L.) rootstocks unstably did, depending on the accessions used. Immunoblotting analysis of the FLOWERING LOCUS T (FT) protein, a main component of florigen, revealed that floral induction was quantitatively correlated with the level of accumulated FT protein in the grafted scion. To identify rootstock factors that cause variability in the floral induction of the grafted scion, we investigated FT protein accumulation and flowering response in grafted scions when the transcription levels of FT and the leaf area of rootstocks were altered by vernalization, daylength and leaf trimming treatments. We concluded that increasing the total amount of FT protein produced in the rootstock is important for the stable floral induction of the grafted cabbage, and this can be accomplished by increasing FT transcription and the leaf area of the rootstock.

Efficiency of graft-transmitted JcFT for floral induction in woody perennial species of the Jatropha genus depends on transport distance

FLOWERING LOCUS T (FT) promotes flowering by integrating six genetic pathways. In Arabidopsis, the FT protein is transported from leaves to shoot apices and induces flowering. However, contradictory conclusions about floral induction via graft-transmitted FT in trees were reported in previous studies. We obtained extremely early-flowering transgenic woody Jatropha curcas L. by overexpression of J. curcas FT using Arabidopsis thaliana SUCROSE TRANSPORTER 2 (SUC2) promoter (SUC2:JcFT) and non-flowering transgenic J. curcas by RNA interference (RNAi), which were used to investigate the function of graft-transmitted JcFT in floral induction in woody perennials. Scions from five wild-type species of the Jatropha genus and from JcFT-RNAi transgenic J. curcas were grafted onto SUC2:JcFT rootstocks. Most grafted plants produced flowers in 1-2 months, and the flowering percentage and frequency of various grafted plants decreased with increasing scion length. Consistently, FT protein abundance in scions also decreased with increasing distance from graft junctions to the buds. These findings suggest that FT proteins can be transmitted by grafting and can induce the floral transition in woody perennials, and the efficiency of graft-transmitted JcFT for floral induction depends on the scion length, which may help explain previous seemingly contradictory observations regarding floral induction via graft-transmitted FT in trees.

Use of Micrografting to Study the Role Played by Peptide Signals in ABA Biosynthesis

Plants survive by deploying appropriate stress responses under rapidly changing environmental conditions. Vascular plants transmit environmental information among distant organs. Long-distance signaling plays a crucial role in plant adaptation and subsequent survival under severe environmental conditions. In model plants, the micrografting method has recently emerged as an important method to elucidate tissue-to-tissue communication via hormones, RNAs, and peptides. In this chapter, I describe a micrografting method for Arabidopsis thaliana to graft shoots onto roots of plants with different genotypes. This grafting method may facilitate further research investigating how vascular plants integrate environmental information among distant organs via long-distance signaling.

Root-to-Shoot Long-Distance Mobile miRNAs Identified from Nicotiana Rootstocks

Root-derived mobile signals play critical roles in coordinating a shoot's response to underground conditions. However, the identification of root-to-shoot long-distance mobile signals has been scant. In this study, we aimed to characterize root-to-shoot endogenous mobile miRNAs by using an Arabidopsis/Nicotiana interfamilial heterograft in which these two taxonomically distant species with clear genetic backgrounds had sufficient diversity in differentiating miRNA sources. Small RNA deep sequencing analysis revealed that 82 miRNAs from the Arabidopsis scion could travel through the graft union to reach the rootstock, whereas only a very small subset of miRNA (6 miRNAs) preferred the root-to-shoot movement. We demonstrated in an ex vivo RNA imaging experiment that the root-to-shoot mobile Nb-miR164, Nb-miR395 and Nb-miR397 were targeted to plasmodesmata using the bacteriophage coat protein MS2 system. Furthermore, the Nb-miR164 was shown to move from the roots to the shoots to induce phenotypic changes when its overexpressing line was used as rootstock, strongly supporting that root-derived Nb-miR164 was able to modify the scion trait via its long-distance movement.

A Sequential Three-Phase Pathway Constitutes Tracheary Element Connection in the Arabidopsis/Nicotiana Interfamilial Grafts

Scion-rootstock union formation is a critical step toward the functional assemblage of heterogeneous plants. Interfamilial scion-rootstock interaction often results in graft incompatibility during the assemblage process, and the underlying mechanisms are largely unknown. In this study, we reported that tracheary element (TE) remodeling, including TE segmentation and deformation, rather than de novo formation from callus or adjacent tissues, took place at the early stage of grafting interface between Arabidopsis thaliana and Nicotiana benthamiana (At/Nb). Following cellular deposits, the short TEs from both partners were overlapping, dependent on the homogeneity of contacting TEs, with each other. Without overlapping, the TEs at the interface would grow laterally, and the TEs above and below the interface would undergo self-fusion to form insulating spiraling bundles. Finally, the overlapping TEs constituted a continuous network through alignment. Our results provide a definitive framework for the critical process of TE behavior in the At/Nb distant grafts, including (1) segmentation and/or deformation, (2) matching, overlapping, and cellular deposits, and (3) aligning or spiraling. These insights might guide us in the future into constructing more compatible distant grafts from the perspective of TE homogeneity.

Model systems for regeneration: Arabidopsis

Plants encompass unparalleled multi-scale regenerative potential. Despite lacking specialized cells that are recruited to injured sites, and despite their cells being encased in rigid cell walls, plants exhibit a variety of regenerative responses ranging from the regeneration of specific cell types, tissues and organs, to the rebuilding of an entire organism. Over the years, extensive studies on embryo, shoot and root development in the model plant species Arabidopsis thaliana have provided insights into the mechanisms underlying plant regeneration. These studies highlight how Arabidopsis, with its wide array of refined molecular, genetic and cell biological tools, provides a perfect model to interrogate the cellular and molecular mechanisms of reprogramming during regeneration.

Insights Into Plant Surgery: An Overview of the Multiple Grafting Techniques for Arabidopsis thaliana

Plant grafting, the ancient practice of cutting and joining different plants, is gaining popularity as an elegant way to generate chimeras that combine desirable traits. Grafting was originally developed in woody species, but the technique has evolved over the past century to now encompass a large number of herbaceous species. The use of plant grafting in science is accelerating in part due to the innovative techniques developed for the model plant Arabidopsis thaliana. Here, we review these developments and discuss the advantages and limitations associated with grafting various Arabidopsis tissues at diverse developmental stages.

Transfer of endogenous small RNAs between branches of scions and rootstocks in grafted sweet cherry trees

Grafting is a well-established agricultural practice in cherry production for clonal propagation, altered plant vigor and architecture, increased tolerance to biotic and abiotic stresses, precocity, and higher yield. Mobile molecules, such as water, hormones, nutrients, DNAs, RNAs, and proteins play essential roles in rootstock-scion interactions. Small RNAs (sRNAs) are 19 to 30-nucleotides (nt) RNA molecules that are a group of mobile signals in plants. Rootstock-to-scion transfer of transgene-derived small interfering RNAs enabled virus resistance in nontransgenic sweet cherry scion. To determine whether there was long-distance scion-to-rootstock transfer of endogenous sRNAs, we compared sRNAs profiles in bud tissues of an ungrafted 'Gisela 6' rootstock, two sweet cherry 'Emperor Francis' scions as well as their 'Gisela 6' rootstocks. Over two million sRNAs were detected in each sweet cherry scion, where 21-nt sRNA (56.1% and 55.8%) being the most abundant, followed by 24-nt sRNAs (13.1% and 12.5%). Furthermore, we identified over three thousand sRNAs that were potentially transferred from the sweet cherry scions to their corresponding rootstocks. In contrast to the sRNAs in scions, among the transferred sRNAs in rootstocks, the most abundant were 24-nt sRNAs (46.3% and 34.8%) followed by 21-nt sRNAs (14.6% and 19.3%). In other words, 21-nt sRNAs had the least transferred proportion out of the total sRNAs in sources (scions) while 24-nt had the largest proportion. The transferred sRNAs were from 574 cherry transcripts, of which 350 had a match from the Arabidopsis thaliana standard protein set. The finding that "DNA or RNA binding activity" was enriched in the transcripts producing transferred sRNAs indicated that they may affect the biological processes of the rootstocks at different regulatory levels. Overall, the profiles of the transported sRNAs and their annotations revealed in this study facilitate a better understanding of the role of the long-distance transported sRNAs in sweet cherry rootstock-scion interactions as well as in branch-to-branch interactions in a tree.

Evolution and functional diversification of FLOWERING LOCUS T/TERMINAL FLOWER 1 family genes in plants

Plant growth and development, particularly the induction of flowering, are tightly controlled by key regulators in response to endogenous and environmental cues. The FLOWERING LOCUS T (FT)/TERMINAL FLOWER 1 (TFL1) family of phosphatidylethanolamine-binding protein (PEBP) genes is central to plant development, especially the regulation of flowering time and plant architecture. FT, the long-sought florigen, promotes flowering and TFL1 represses flowering. The balance between FT and TFL1 modulates plant architecture by switching the meristem from indeterminate to determinate growth, or vice versa. Recent studies in a broad range of plant species demonstrated that, in addition to their roles in flowering time and plant architecture, FT/TFL1 family genes participate in diverse aspects of plant development, such as bamboo seed germination and potato tuber formation. In this review, we briefly summarize the evolution of the FT/TFL1 family and highlight recent findings on their conserved and divergent functions in different species.

Cut and paste: temperature-enhanced cotyledon micrografting for Arabidopsis thaliana seedlings

BACKGROUND

Cotyledon micrografting represents a useful tool for studying the central role of cotyledons during early plant development, especially their interplay with other plant organs with regard to long distance transport. While hypocotyl micrografting methods are well-established, cotyledon micrografting is still inefficient. By optimizing cotyledon micrografting, we aim for higher success rates and increased throughput in the model species Arabidopsis thaliana.

RESULTS

We established a cut and paste cotyledon surgery procedure on a flat and solid but moist surface which improved handling of small seedlings. By applying a specific cutting and joining pattern, throughput was increased up to 40 seedlings per hour. The combination of short-day photoperiods and low light intensities for germination and long days plus high light intensities, elevated temperature and vertical plate positioning after grafting significantly increased 'ligation' efficiency. In particular high temperatures affected success rates favorably. Altogether, we achieved up to 92% grafting success in A. thaliana. Reconnection of vasculature was demonstrated by transport of a vasculature-specific dye across the grafting site. Phloem and xylem reconnection were completed 3-4 and 4-6 days after grafting, respectively, in a temperature-dependent manner. We observed that plants with grafted cotyledons match plants with intact cotyledons in biomass production and rosette development.

CONCLUSIONS

This cut and paste cotyledon-to-petiole micrografting protocol simplifies the handling of plant seedlings in surgery, increases the number of grafted plants per hour and greatly improves success rates for A. thaliana seedlings. The developed cotyledon micrografting method is also suitable for other plant species of comparable size.

VcFT-induced mobile florigenic signals in transgenic and transgrafted blueberries

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

Methods for grafting Arabidopsis thaliana and Eutrema salsugineum

BACKGROUND

Grafting, an ancient agronomic technique, is an artificial mode of asexual reproduction in plants. Recently, grafting research has gradually shifted from modifying agronomic traits to the study of molecular mechanism. Grafting is an excellent tool to study long-range signaling processes in plants. And the grafting between species will help elucidate the molecular mechanisms underlying contrasting differences between different species. Arabidopsis thaliana is a salt-sensitive model glycophyte and Eutrema salsugineum (previously Thellungiella salsuginea, salt cress) is a salt-tolerant model halophyte. Successful grafting of these two model plants will help further study the physiological and molecular mechanisms underlying salt tolerance in plants. The aim of this study was to demonstrate two sterile micro-grafting methods for Arabidopsis and salt cress.

RESULTS

We developed the methods for sterile grafting between A. thaliana and E. salsugineum; this is the first report on inter-generic grafting between Arabidopsis and Eutrema. The method involves cut-in grafting under sterile conditions. The grafted plant part was placed in half strength Murashige and Skoog medium with 1% agar and 1% sugar, and then cultured vertically with 22 °C/18 °C short-day/night cycles. The plants were then transferred to half strength Hoagland nutrient solution for hydroponics. The reported method is simple and easy to operate. Self-grafted Arabidopsis-Arabidopsis and Eutrema-Eutrema plants were used as controls, which were obtained with an improved hypocotyl-cutting grafting method. Ion contents in grafted plants were detected by inductively coupled plasma optical emission spectroscopy. The results showed that the ion content in salt cress and Arabidopsis changed to different degrees after grafting.

CONCLUSIONS

The inter-species grafting technique described here makes it possible to study hybrid plants between Arabidopsis and Eutrema and will contribute to further understanding of long-distance communications in plants. This technique also provides a reference for improving plant varieties using grafting, such as gardening plants, as well as fruit and vegetable crops.

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

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

Re-Evaluation of Florigen Transport Kinetics with Separation of Functions by Mutations That Uncouple Flowering Initiation and Long-Distance Transport

In many plants, timing of flowering is regulated by day length. In Arabidopsis, florigen, FLOWERING LOCUS T (FT) protein, is synthesized in leaf phloem companion cells in response to long days and is transported to the shoot apical meristem (SAM) through the phloem. The temporal aspects of florigen transportation have been studied in various plants by physiological experiments. Nevertheless, little is known about how FT protein transportation is regulated in Arabidopsis. In this study, we performed heat shock-based transient FT induction in a single leaf blade and detected the FT protein in the shoot apex by 2D-PAGE. We demonstrated that detectable amounts of FT were transported from the leaf to the shoot apex within 8 h, and subsequent FT-induced target gene expression was detected within 8-12 h. Furthermore, we identified three amino acid residues (V70, S76 and R83) where missense mutations led to reduced mobility. Interestingly, these FT variants lost only their transportation ability, but retained their flowering promotion capacity, suggesting that discrete amino acids are involved in flowering regulation and transport regulation. Since the interaction with FT-INTERACTING PROTEIN 1 (FTIP1) was not affected in these FT variants, we hypothesize that the three amino acid residues are not involved in the FTIP1-mediated pathway of uploading, but rather in the subsequent step(s) of FT transport.

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

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

Grafting with Arabidopsis thaliana

Generating chimeric organisms is an invaluable way to study cell-to-cell movement and non-cell-autonomous actions of molecules. Plant grafting is an ancient method of generating chimeric organisms and recently has been used to study the movement of hormones, proteins, and RNAs. Here, I describe a simple and efficient way to graft Arabidopsis thaliana at the seedling stage to generate plants with roots and shoots of different genotypes. Using this protocol, success rates of over 80 % with up to 80 grafts assembled per hour can be achieved.

The Use of Grafting to Study Systemic Signaling in Plants

Grafting has long been an important technique in agriculture. Nowadays, grafting is a widely used technique also to study systemic long-distance signaling in plants. Plants respond to their surrounding environment, and at that time many aspects of their physiology are regulated systemically; these start from local input signals and are followed by the transmission of information to the rest of the plant. For example, soil nutrient conditions, light/photoperiod, and biotic and abiotic stresses affect plants heterogeneously, and plants perceive such information in specific plant tissues or organs. Such environmental cues are crucial determinants of plant growth and development, and plants drastically change their morphology and physiology to adapt to various events in their life. Hitherto, intensive studies have been conducted to understand systemic signaling in plants, and grafting techniques have permitted advances in this field. The breakthrough technique of micrografting in Arabidopsis thaliana was established in 2002 and led to the development of molecular genetic tools in this field. Thereafter, various phenomena of systemic signaling have been identified at the molecular level, including nutrient fixation, flowering, circadian clock and defense against pathogens. The significance of grafting is that it can clarify the transmission of the stimulus and molecules. At present, many micro- and macromolecules have been identified as mobile signals, which are transported through plant vascular tissues to co-ordinate their physiology and development. In this review, we introduce the various grafting techniques that have been developed, we report on the recent advances in the field of plant systemic signaling where grafting techniques have been applied and provide insights for the future.

Monitoring Vascular Regeneration and Xylem Connectivity in Arabidopsis thaliana

Plants have a remarkable ability to regenerate vascular tissue after damage or wounding. A striking example of this phenomenon is the cutting and rejoining of plants during the process of grafting, which humans have used for millennia. Here, I describe how to graft Arabidopsis seedlings and how to monitor the vascular reconnection process during wound healing using fluorescent dyes and fluorescent proteins. Furthermore, I describe how to visualize xylem formation during graft healing. These techniques are useful for studying how the vascular system regenerates and for better understanding the process of plant grafting.

Plant grafting: insights into tissue regeneration

For millennia, people have cut and joined different plants together through a process known as grafting. The severed tissues adhere, the cells divide and the vasculature differentiates through a remarkable process of regeneration between two genetically distinct organisms as they become one. Grafting is becoming increasingly important in horticulture where it provides an efficient means for asexual propagation. Grafting also combines desirable roots and shoots to generate chimeras that are more vigorous, more pathogen resistant and more abiotic stress resistant. Thus, it presents an elegant and efficient way to improve plant productivity in vegetables and trees using traditional techniques. Despite this horticultural importance, we are only beginning to understand how plants regenerate tissues at the graft junction. By understanding grafting better, we can shed light on fundamental regeneration pathways and the basis for self/non‐self recognition. We can also better understand why many plants efficiently graft whereas others cannot, with the goal of improving grafting so as to broaden the range of grafted plants to create even more desirable chimeras. Here, I review the latest findings describing how plants graft and provide insight into future directions in this emerging field.

NaKR1 regulates long-distance movement of FLOWERING LOCUS T in Arabidopsis

Flowering plants perceive photoperiodic signals in leaves to generate mobile stimuli required for the induction of flower formation at shoot apices. Although FLOWERING LOCUS T (FT) has been identified as part of the mobile floral stimuli in Arabidopsis thaliana, the mechanisms underlying long-distance movement of FT from leaves to shoot apices remain largely unclear. Here we show that a heavy-metal-associated (HMA) domain-containing protein, SODIUM POTASSIUM ROOT DEFECTIVE 1 (NaKR1), is activated by CONSTANS (CO) under long-day conditions and regulates long-distance movement of FT in Arabidopsis. Loss of function of NaKR1 compromises FT transport to shoot apices through sieve elements, causing late flowering under long-day conditions. NaKR1 and FT share similar expression patterns and subcellular localization, and interact with each other in vivo. Grafting experiments demonstrate that NaKR1 promotes flowering through mediating FT translocation from leaves to shoot apices. Thus, photoperiodic control of floral induction requires NaKR1-mediated long-distance delivery of florigenic signals.

FLOWERING LOCUS T has higher protein mobility than TWIN SISTER OF FT

In plants, successful reproduction requires the proper timing of flowering under changing environmental conditions. Arabidopsis FLOWERING LOCUS T (FT), which encodes a proposed phloem-mobile florigen, has a close homologue, TWIN SISTER OF FT (TSF). During the vegetative phase, TSF shows high levels of expression in the hypocotyl before FT induction, but the tsf mutation does not have an apparent flowering-time phenotype on its own under long-day conditions. This study compared the protein mobility of FT and TSF. With TSF-overexpressing plants as the rootstock, the flowering time of ft tsf scion plants was only slightly accelerated. Previous work has shown that FT is graft-transmissible; by contrast, this study did not detect movement of TSF from the roots into the shoot of the scion plants. This study used plants overexpressing FT/TSF chimeric proteins to map a region responsible for FT movement. A chimeric TSF with region II of FT (L28 to G98) expressed in the rootstock caused early flowering in ft tsf scion plants; movement of the chimeric protein from the rootstocks into the shoot apical region of the ft tsf scion plants was also detected. Misexpression of TSF in the leaf under the control of the FT promoter or grafting of 35S::TSF cotyledons accelerated flowering of ft-10 plants. FT was more stable than TSF. Taking these results together, we propose that protein mobility of FT is higher than that of TSF, possibly due to a protein domain that confers mobility and/or protein stability.

Extensive Analysis of GmFTL and GmCOL Expression in Northern Soybean Cultivars in Field Conditions

The FLOWERING LOCUS T (FT) gene is a highly conserved florigen gene among flowering plants. Soybean genome encodes six homologs of FT, which display flowering activity in Arabidopsis thaliana. However, their contributions to flowering time in different soybean cultivars, especially in field conditions, are unclear. We employed six soybean cultivars with different maturities to extensively investigate expression patterns of GmFTLs (Glycine max FT-like) and GmCOLs (Glycine max CO-like) in the field conditions. The results show that GmFTL3 is an FT homolog with the highest transcript abundance in soybean, but other GmFTLs may also contribute to flower induction with different extents, because they have more or less similar expression patterns in developmental-, leaf-, and circadian-specific modes. And four GmCOL genes (GmCOL1/2/5/13) may confer to the expression of GmFTL genes. Artificial manipulation of GmFTL expression by transgenic strategy (overexpression and RNAi) results in a distinct change in soybean flowering time, indicating that GmFTLs not only impact on the control of flowering time, but have potential applications in the manipulation of photoperiodic adaptation in soybean. Additionally, transgenic plants show that GmFTLs play a role in formation of the first flowers and in vegetative growth.

Photoperiodic Regulation of Florigen Function in Arabidopsis thaliana

One mechanism through which flowering in response to seasonal change is brought about is by sensing the fluctuation in day-length; the photoperiod. Flowering induction occurs through the production of the florigenic protein FLOWERING LOCUS T (FT) and its movement from the phloem companion cells in the leaf vasculature into the shoot apex, where meristematic reprogramming occurs. FT activation in response to photoperiod condition is accomplished largely through the activity of the transcription factor CONSTANS (CO). Regulation of CO expression and protein stability, as well as the timing of other components via the circadian clock, is a critical mechanism by which plants are able to respond to photoperiod to initiate the floral transition. Modulation of FT expression in response to external and internal stimuli via components of the flowering network is crucial to mediate a fluid flowering response to a variety of environmental parameters. In addition, the regulated movement of FT protein from the phloem to the shoot apex, and interactions that determine floral meristem cell fate, constitute novel mechanisms through which photoperiodic information is translated into flowering time.

A pin-fasten grafting method provides a non-sterile and highly efficient method for grafting Arabidopsis at diverse developmental stages

BACKGROUND

Higher plants have evolved sophisticated communication systems to integrate environmental stimuli into their developmental programs. Grafting provides a powerful technique to examine transportation and systemic effects of mobile molecules. In Arabidopsis, many grafting approaches have been developed to investigate systemic molecules. However, these methods are usually limited to specific developmental stages or require sterilized conditions. To broaden the application of grafting for examining systemic signals at diverse developmental stages, we developed an Arabidopsis pin-fasten grafting method with insect pins used to assemble stocks and scions.

RESULTS

We report the step-by-step protocol of Arabidopsis pin-fasten grafting. Arabidopsis wild-type or gl1-1 plants were grown under long- or short-day conditions. Insect pins were inserted into gl1-1 scions at different developmental stages for grafting onto epicotyls or hypocotyls of stocks. Successfully grafted scions with newly developed glabrous leaves were observed at 14 days after grafting. Further longitudinal sections of the graft union showed well-connected vascular tissues between grafted plants. Use of fluorescent phloem-limited dye carboxyfluorescein diacetate in grafted plants demonstrated a symplastic connection established at 6 days after grafting and almost fully developed at 8 days.

CONCLUSIONS

Our method provides a simple and robust approach to grafting Arabidopsis at different developmental stages. Sterilized conditions are not required, which greatly improves the success of grafting and plant growth.

Conserved CO-FT regulons contribute to the photoperiod flowering control in soybean

BACKGROUND

CO and FT orthologs, belonging to the BBX and PEBP family, respectively, have important and conserved roles in the photoperiod regulation of flowering time in plants. Soybean genome experienced at least three rounds of whole genome duplications (WGDs), which resulted in multiple copies of about 75% of genes. Subsequent subfunctionalization is the main fate for paralogous gene pairs during the evolutionary process.

RESULTS

The phylogenic relationships revealed that CO orthologs were widespread in the plant kingdom while FT orthologs were present only in angiosperms. Twenty-eight CO homologous genes and twenty-four FT homologous genes were gained in the soybean genome. Based on the collinear relationship, the soybean ancestral CO ortholog experienced three WGD events, but only two paralogous gene pairs (GmCOL1/2 and GmCOL5/13) survived in the modern soybean. The paralogous gene pairs, GmCOL1/2 or GmCOL5/13, showed similar expression patterns in pair but different between pairs, indicating that they functionally diverged. GmFTL1 to 7 were derived from the same ancestor prior to the whole genome triplication (WGT) event, and after the Legume WGD event the ancestor diverged into two branches, GmFTL3/5/7 and GmFTL1/2/4/6. GmFTL7 were truncated in the N-terminus compared to other FT-lineage genes, but ubiquitously expressed. Expressions of GmFTL1 to 6 were higher in leaves at the flowering stage than that at the seedling stage. GmFTL3 was expressed at the highest level in all tissues except roots at the seedling stage, and its circadian pattern was different from the other five ones. The transcript of GmFTL6 was highly accumulated in seedling roots. The circadian rhythms of GmCOL5/13 and GmFT1/2/4/5/6 were synchronized in a day, demonstrating the complicate relationship of CO-FT regulons in soybean leaves. Over-expression of GmCOL2 did not rescue the flowering phenotype of the Arabidopsis co mutant. However, ectopic expression of GmCOL5 did rescue the co mutant phenotype. All GmFTL1 to 6 showed flower-promoting activities in Arabidopsis.

CONCLUSIONS

After three recent rounds of whole genome duplications in the soybean, the paralogous genes of CO-FT regulons showed subfunctionalization through expression divergence. Then, only GmCOL5/13 kept flowering-promoting activities, while GmFTL1 to 6 contributed to flowering control. Additionally, GmCOL5/13 and GmFT1/2/3/4/5/6 showed similar circadian expression profiles. Therefore, our results suggested that GmCOL5/13 and GmFT1/2/3/4/5/6 formed the complicate CO-FT regulons in the photoperiod regulation of flowering time in soybean.

Photoperiodic flowering regulation in Arabidopsis thaliana

Photoperiod, or the duration of light in a given day, is a critical cue that flowering plants utilize to effectively assess seasonal information and coordinate their reproductive development in synchrony with the external environment. The use of the model plant, Arabidopsis thaliana, has greatly improved our understanding of the molecular mechanisms that determine how plants process and utilize photoperiodic information to coordinate a flowering response. This mechanism is typified by the transcriptional activation of FLOWERING LOCUS T (FT) gene by the transcription factor CONSTANS (CO) under inductive long-day conditions in Arabidopsis. FT protein then moves from the leaves to the shoot apex, where floral meristem development can be initiated. As a point of integration from a variety of environmental factors in the context of a larger system of regulatory pathways that affect flowering, the importance of photoreceptors and the circadian clock in CO regulation throughout the day has been a key feature of the photoperiodic flowering pathway. In addition to these established mechanisms, the recent discovery of a photosynthate derivative trehalose-6-phosphate as an activator of FT in leaves has interesting implications for the involvement of photosynthesis in the photoperiodic flowering response that were suggested from previous physiological experiments in flowering induction.

An efficient flat-surface collar-free grafting method for Arabidopsis thaliana seedlings

BACKGROUND

Grafting procedures are an excellent tool to study long range signalling processes within a plant. In the last decade, suitable flat-surface grafting procedures for young Arabidopsis seedlings using a collar to support the graft have been developed, allowing the study of long-range signals from a molecular perspective.

RESULTS

In the modification presented here, scion and stock are put together on the medium without supporting elements, while cotyledons are removed from the scion, resulting in increased grafting success that can reach up to 100%. At the same time, the protocol enables to process as many as 36 seedlings per hour, which combined with the high success percentage represents increased efficiency per time unit.

CONCLUSIONS

Growing cotyledons usually push the scion and the rootstock away in the absence of a supporting element. Removing them at the grafting step greatly improved success rate and reduced post-grafting manipulations.

Florigen in rice: complex gene network for florigen transcription, florigen activation complex, and multiple functions

Regulation of flowering time directly influences successful rice grain production; thus, the long history of domestication and breeding has improved the genetic network of flowering. Recent advances using molecular genomic approaches have revealed the targets of these modifications and the underlying molecular mechanism for flowering. These efforts contributed to identifying the molecular nature of the systemic floral signal 'florigen' and have shown how florigen functions, how florigen expression is controlled, and how regulatory pathways are diversified. In this review, we summarize the advances in our understanding of the detailed molecular and genetic mechanisms that allow rice plants to produce flowers at the proper time to ensure grain production.

Functional diversification of FD transcription factors in rice, components of florigen activation complexes

Florigen, a protein encoded by the FLOWERING LOCUS T (FT) in Arabidopsis and Heading date 3a (Hd3a) in rice, is the universal flowering hormone in plants. Florigen is transported from leaves to the shoot apical meristem and initiates floral evocation. In shoot apical cells, conserved cytoplasmic 14-3-3 proteins act as florigen receptors. A hexameric florigen activation complex (FAC) composed of Hd3a, 14-3-3 proteins, and OsFD1, a transcription factor, activates OsMADS15, a rice homolog of Arabidopsis APETALA1, leading to flowering. Because FD is a key component of the FAC, we characterized the FD gene family and their functions. Phylogenetic analysis of FD genes indicated that this family is divided into two groups: (i) canonical FD genes that are conserved among eudicots and non-Poaceae monocots; and (ii) Poaceae-specific FD genes that are organized into three subgroups: Poaceae FD1, FD2 and FD3. The Poaceae FD1 group shares a small sequence motif, T(A/V)LSLNS, with FDs of eudicots and non-Poaceae monocots. Overexpression of OsFD2, a member of the Poaceae FD2 group, produced smaller leaves with shorter plastochrons, suggesting that OsFD2 controls leaf development. In vivo subcellular localization of Hd3a, 14-3-3 and OsFD2 suggested that in contrast to OsFD1, OsFD2 is restricted to the cytoplasm through its interaction with the cytoplasmic 14-3-3 proteins, and interaction of Hd3a with 14-3-3 facilitates nuclear translocation of the FAC containing OsFD2. These results suggest that FD function has diverged between OsFD1 and OsFD2, but formation of a FAC is essential for their function.