Molecular Basis to Integrate Microgravity Signals into the Photoperiodic Flowering Pathway in Arabidopsis thaliana under Spaceflight Condition

Understanding the effects of spaceflight on plant flowering regulation is important to setup a life support system for long-term human space exploration. However, the way in which plant flowering is affected by spaceflight remains unclear. Here, we present results from our latest space experiments on the Chinese spacelab Tiangong-2, in which Arabidopsis wild-type and transgenic plants pFT::GFP germinated and grew as normally as their controls on the ground, but the floral initiation under the long-day condition in space was about 20 days later than their controls on the ground. Time-course series of digital images of pFT::GFP plants showed that the expression rhythm of FT in space did not change, but the peak appeared later in comparison with those of their controls on the ground. Whole-genome microarray analysis revealed that approximately 16% of Arabidopsis genes at the flowering stage changed their transcript levels under spaceflight conditions in comparison with their controls on the ground. The GO terms were enriched in DEGs with up-regulation of the response to temperature, wounding, and protein stabilization and down-regulation of the function in circadian rhythm, gibberellins, and mRNA processes. FT and SOC1 could act as hubs to integrate spaceflight stress signals into the photoperiodic flowering pathway in Arabidopsis in space.

A Daylength Recognition Model of Photoperiodic Flowering

The photoperiodic flowering pathway is crucial for plant development to synchronize internal signaling events and external seasons. One hundred years after photoperiodic flowering was discovered, the underlying core signaling network has been elucidated in model plants such as Arabidopsis (Arabidopsis thaliana), rice (Oryza sativa), and soybean (Glycine max). Here, we review the progress made in the photoperiodic flowering area and summarize previously accepted photoperiodic flowering models. We then introduce a new model based on daylength recognition by florigen. By determining the expression levels of the florigen gene, this model can assess the mechanism of daylength sensing and crop latitude adaptation. Future applications of this model under the constraints of global climate change are discussed.

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.

Discovery of UPSTREAM OF FLOWERING LOCUS C (UFC) and FLOWERING LOCUS C EXPRESSOR (FLX) in Gladiolus ×hybridus, G. dalenii

Gladiolus is a geophytic floricultural crop, cultivated for cut flower and garden ornamental uses. 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 environmental and hormonal factors leading to flowering are established in Arabidopsis. However, the lack of genetic regulation is poorly understood. 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 paramount. 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. FLX gene upregulates FRIGIDA (FRI) which upregulates FLC expression. The 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, have the UFC gene, consisting of four exons in two allelic forms. The UFC gene sequenced when translated into 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 species, reaching up to 57% in amino acid identity to Musa acuminata. The FLX gene in gladiolus has 3/5 (60%) exons in relative to Ananas comosus, i.e. lacking 2 exons and a partially complete gene sequence; the pair-alignment of the three exons shows up over all ~65% identity of FLX to Ananas comosus. The UFC protein consists of a conserved domain, DUF966, which is higher in identity and pair-alignment, with up to 86% identity in Elaeis guineensis. The discovered FLX gene in gladiolus has 3/5 (60%) exons, i.e. lacking 2 exons and a partially complete gene sequence; the pair-alignment of the 3 exons shows up to ~65% of identity of FLX to Ananas comosus. These discovered two genes in gladiolus provide insight to further our understanding of the flowering and vernalization response in ornamental geophytes.

Auxin-induced AUXIN RESPONSE FACTOR4 activates APETALA1 and FRUITFULL to promote flowering in woodland strawberry

Flowering time is known to be regulated by numerous pathways, such as the autonomous, gibberellin, aging, photoperiod-mediated, and vernalization pathways. These regulatory mechanisms involve both environmental triggers and endogenous hormonal cues. Additional flowering control mechanisms mediated by other phytohormones, such as auxin, are less well understood. We found that in cultivated strawberry (Fragaria × ananassa), the expression of auxin response factor4 (FaARF4) was higher in the flowering stage than in the vegetative stage. Overexpression of FaARF4 in Arabidopsis thaliana and woodland strawberry (Fragaria vesca) resulted in transgenic plants flowering earlier than control plants. In addition, FveARF4-silenced strawberry plants showed delayed flowering compared to control plants, indicating that FaARF4 and FveARF4 function similarly in regulating flowering. Further studies showed that ARF4 can bind to the promoters of the floral meristem identity genes APETALA1 (AP1) and FRUITFULL (FUL), inducing their expression and, consequently, flowering in woodland strawberry. Our studies reveal an auxin-mediated flowering pathway in strawberry involving the induction of ARF4 expression.

Gibberellin Promotes Bolting and Flowering via the Floral Integrators RsFT and RsSOC1-1 under Marginal Vernalization in Radish

Gibberellic acid (GA) is one of the factors that promotes flowering in radish (Raphanus Sativus L.), although the mechanism mediating GA activation of flowering has not been determined. To identify this mechanism in radish, we compared the effects of GA treatment on late-flowering (NH-JS1) and early-flowering (NH-JS2) radish lines. GA treatment promoted flowering in both lines, but not without vernalization. NH-JS2 plants displayed greater bolting and flowering pathway responses to GA treatment than NH-JS1. This variation was not due to differences in GA sensitivity in the two lines. We performed RNA-seq analysis to investigate GA-mediated changes in gene expression profiles in the two radish lines. We identified 313 upregulated, differentially expressed genes (DEGs) and 207 downregulated DEGs in NH-JS2 relative to NH-JS1 in response to GA. Of these, 21 and 8 genes were identified as flowering time and GA-responsive genes, respectively. The results of RNA-seq and quantitative PCR (qPCR) analyses indicated that RsFT and RsSOC1-1 expression levels increased after GA treatment in NH-JS2 plants but not in NH-JS1. These results identified the molecular mechanism underlying differences in the flowering-time genes of NH-JS1 and NH-JS2 after GA treatment under insufficient vernalization conditions.