Mobile Flowering Locus T RNA – Biological Relevance and Biotechnological Potential

Many systemically mobile mRNAs have been revealed in phloem. However, very few of them have been found to be of clear signaling functions. One of such rare examples is the mobile Flowering locus T (FT) mRNA despite the continuous debate about its mobility and biological relevance to the control of flowering time in plants. Nevertheless, accumulating evidence supports the notion of the long-distance movement of FT mRNA from leaf to shoot apex meristem and its role in flowering. In this review, we discuss the discovery of florigenic FT, the initial debate on long-distance movement of FT mRNA, emerging evidence to prove its mobility, and the use of mobile FT mRNA to generate heritable transgenerational gene editing in plants. We elaborate on evidence from virus-based RNA mobility assay, plant grafting, RNA with fluorescent protein labeling, and CRISPR/Cas9 gene-editing technology, to demonstrate that the FT mRNA besides the FT protein can move systemically and function as an integral component of the florigenic signal in flowering. We also propose a model to prompt further research on the molecular mechanism underlying the long-distance movement of this important mobile signaling RNA in plants.

PRMT6 physically associates with nuclear factor Y to regulate photoperiodic flowering in Arabidopsis

The timing of floral transition is critical for reproductive success in flowering plants. In long-day (LD) plant Arabidopsis, the floral regulator gene FLOWERING LOCUS T (FT) is a major component of the mobile florigen. FT expression is rhythmically activated by CONSTANS (CO), and specifically accumulated at dusk of LDs. However, the underlying mechanism of adequate regulation of FT transcription in response to day-length cues to warrant flowering time still remains to be investigated. Here, we identify a homolog of human protein arginine methyltransferases 6 (HsPRMT6) in Arabidopsis, and confirm AtPRMT6 physically interacts with three positive regulators of flowering Nuclear Factors YC3 (NF-YC3), NF-YC9, and NF-YB3. Further investigations find that AtPRMT6 and its encoding protein accumulate at dusk of LDs. PRMT6-mediated H3R2me2a modification enhances the promotion of NF-YCs on FT transcription in response to inductive LD signals. Moreover, AtPRMT6 and its homologues proteins AtPRMT4a and AtPRMT4b coordinately inhibit the expression of FLOWERING LOCUS C, a suppressor of FT. Taken together, our study reveals the role of arginine methylation in photoperiodic pathway and how the PRMT6-mediating H3R2me2a system interacts with NF-CO module to dynamically control FT expression and facilitate flowering time.

Discovery of UPSTREAM of FLOWERING LOCUS C (UFC) and FLOWERING LOCUS C EXPRESSOR (FLX) in Gladiolus ×Hybridus, G. Dalenii

Background. Gladiolus is a geophytic floricultural crop cultivated for cut flower and garden ornamental uses. Monocotyledonous flower crops have few, if any, flowering genes identified. Ornamental geophytes such as gladiolus, lily, tulip and daffodil are examples of floral crops that are currently being investigated to understand the flowering pathway. While the flower genes and environmental / hormonal factors leading to flowering are established in Arabidopsis, the lack of identified flowering genes in economically important ornamental geophytic crops, such as gladiolus, is critical to further genetic research. Thus, the importance of such an ornamental crop that relies on flowers (flowering) for economic purposes encourages researchers to discover the flowering genes to breed vigorous, flowering cultivars. The understanding of the flowering mechanisms in the flowering pathway is also of paramount importance. Results. Herein we show the discovery of UPSTREAM OF FLOWERING LOCUS C (UFC) and FLOWERING LOCUS C EXPRESSOR (FLX) genes in Gladiolus ×hybridus and G. dalenii. The UFC gene is adjacent to FLOWERING LOCUS C (FLC) which is a floral repressor in many temperate species. The FLX gene upregulates FRIGIDA (FRI) which upregulates FLC expression. Discovery of both genes is a step forward in finding the FLC gene in gladiolus, provided they are linked. Seventeen gladiolus genotypes, consisting of early flowering and commercial cultivars, were discovered to possess the UFC gene, consisting of four exons in two allelic forms. The sequenced UFC gene, when translated into its amino acid sequence and set in pair-alignment to other species, has up to 57% in amino acid identity to Musa acuminata. The UFC protein ranges in identity with pair-alignment to other monocot species, also with 57% amino acid identity to M. acuminata. The FLX gene in gladiolus has 3/5 (60%) exons in common relative to Ananas comosus, i.e. lacking 2 exons and a partially complete gene sequence; the pair-alignment of the three exons shows an overall ~65% identity of FLX to A. comosus. The UFC protein consists of a conserved domain, DUF966, which is higher in identity (86%) and pair-alignment with Elaeis guineensis. Conclusions. The two newly-discovered genes in gladiolus, UFC and FLX, provide insight to further our understanding of the flowering mechanism, flowering pathway genes, and vernalization response in ornamental geophytes. This knowledge will be valuable for gladiolus breeders and geneticists to finding the FLC gene, identify segregating seedlings for both UFC and FLX, and aid in marker assisted selection for flowering gene expression.

Expression of coffee florigen CaFT1 reveals a sustained floral induction window associated with asynchronous flowering in tropical perennials

The behavior of florigen(s) and environment-influenced regulatory pathways that control flowering in tropical perennials with complex phenological cycles is poorly understood. Understanding the mechanisms underlying this process is important for food production in the face of climate change. To explore this, homologs of Arabidopsis florigen FLOWERING LOCUS T (CaFT1) and environment-related regulators CONSTANS (CO), PHYTOCHROME INTERACTING FACTOR 4 (PIF4) and FLOWERING LOCUS C (FLC) were isolated from Coffea sp. L. (Rubiaceae). Overexpression of CaFT1 in Arabidopsis showed typical early-flowering and yeast two hybrid studies indicated CaFT1 binding to bZIP floral regulator, FD, demonstrates that CaFT1 is a coffee orthologue of florigen. Expression of CaFT1 and floral regulators were evaluated over one year using three contrasting genotypes: two C. arabica and one C. canephora. All genotypes showed active CaFT1 transcription from February until October, indicating a potential window for floral induction. CaCO expression, as expected, varied over the day period and monthly with day length, whereas expression of temperature-responsive homologs, CaFLC and CaPIF4, did not correlate with temperature changes. Using coffee as a model, we suggest a continuum of floral induction that allows different starting points for floral activation, which explains developmental asynchronicity and prolonged anthesis events in tropical perennial species.

Flowering Times of Wild Arabidopsis Accessions From Across Norway Correlate With Expression Levels of FT, CO, and FLC Genes

Temperate species often require or flower most rapidly in the long daylengths, or photoperiods, experienced in summer or after prolonged periods of cold temperatures, referred to as vernalization. Yet, even within species, plants vary in the degree of responsiveness to these cues. In Arabidopsis thaliana, CONSTANS (CO) and FLOWERING LOCUS C (FLC) genes are key to photoperiod and vernalization perception and antagonistically regulate FLOWERING LOCUS T (FT) to influence the flowering time of the plants. However, it is still an open question as to how these genes vary in their interactions among wild accessions with different flowering behaviors and adapted to different microclimates, yet this knowledge could improve our ability to predict plant responses in variable natural conditions. To assess the relationships among these genes and to flowering time, we exposed 10 winter-annual Arabidopsis accessions from throughout Norway, ranging from early to late flowering, along with two summer-annual accessions to 14 weeks of vernalization and either 8- or 19-h photoperiods to mimic Norwegian climate conditions, then assessed gene expression levels 3-, 5-, and 8-days post vernalization. CO and FLC explained both FT levels and flowering time (days) but not rosette leaf number at flowering. The correlation between FT and flowering time increased over time. Although vernalization suppresses FLC, FLC was high in the late-flowering accessions. Across accessions, FT was expressed only at low FLC levels and did not respond to CO in the late-flowering accessions. We proposed that FT may only be expressed below a threshold value of FLC and demonstrated that these three genes correlated to flowering times across genetically distinct accessions of Arabidopsis.

VDAC1 Negatively Regulates Floral Transition in Arabidopsis thaliana

Voltage-dependent anion channels (VDACs) are the most important proteins in mitochondria. They localize to the outer mitochondrial membrane and contribute to the metabolite transport between the mitochondria and cytoplasm, which aids plant growth regulation. Here, we report that Arabidopsis thaliana VDAC1 is involved in the floral transition, with the loss of AtVDAC1 function, resulting in an early-flowering phenotype. AtVDAC1 is expressed ubiquitously in Arabidopsis. To identify the flowering pathway integrators that may be responsible for AtVDAC1′s function during the floral transition, an RNA-seq analysis was performed. In total, 106 differentially expressed genes (DEGs) were identified between wild-type and atvdac1-5 mutant seedlings. However, none were involved in flowering-related pathways. In contrast, AtVDAC1 physically associated with FLOWERING LOCUS T. Thus, in the floral transition, AtVDAC1 may function partly through the FLOWERING LOCUS T protein.

TEM1 combinatorially binds to FLOWERING LOCUS T and recruits a Polycomb factor to repress the floral transition in Arabidopsis

Arabidopsis TEMPRANILLO 1 (TEM1) is a transcriptional repressor that participates in multiple flowering pathways and negatively regulates the juvenile-to-adult transition and the flowering transition. To understand the molecular basis for the site-specific regulation of FLOWERING LOCUS T (FT) by TEM1, we determined the structures of the two plant-specific DNA-binding domains in TEM1, AP2 and B3, in complex with their target DNA sequences from the FT gene 5′-untranslated region (5′-UTR), revealing the molecular basis for TEM1 specificity for its DNA targets. In vitro binding assays revealed that the combination of the AP2 and B3 binding sites greatly enhanced the overall binding of TEM1 to the FT 5′-UTR, indicating TEM1 combinatorically recognizes the FT gene 5′-UTR. We further showed that TEM1 recruits the Polycomb repressive complex 2 (PRC2) to the FT 5′-UTR. The simultaneous binding of the TEM1 AP2 and B3 domains to FT is necessary for deposition of H3K27me3 at the FT 5′-UTR and for the flowering repressor function of TEM1. Overall, our data suggest that the combinatorial recognition of FT 5′-UTR by TEM1 ensures H3K27me3 deposition to precisely regulate the floral transition.