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

An explanatory model of temperature influence on flowering through whole-plant accumulation of FLOWERING LOCUS T in Arabidopsis thaliana

We assessed mechanistic temperature influence on flowering by incorporating temperature-responsive flowering mechanisms across developmental age into an existing model. Temperature influences the leaf production rate as well as expression of FLOWERING LOCUS T (FT), a photoperiodic flowering regulator that is expressed in leaves. The Arabidopsis Framework Model incorporated temperature influence on leaf growth but ignored the consequences of leaf growth on and direct temperature influence of FT expression. We measured FT production in differently aged leaves and modified the model, adding mechanistic temperature influence on FT transcription, and causing whole-plant FT to accumulate with leaf growth. Our simulations suggest that in long days, the developmental stage (leaf number) at which the reproductive transition occurs is influenced by day length and temperature through FT, while temperature influences the rate of leaf production and the time (in days) the transition occurs. Further, we demonstrate that FT is mainly produced in the first 10 leaves in the Columbia (Col-0) accession, and that FT accumulation alone cannot explain flowering in conditions in which flowering is delayed. Our simulations supported our hypotheses that: (i) temperature regulation of FT, accumulated with leaf growth, is a component of thermal time, and (ii) incorporating mechanistic temperature regulation of FT can improve model predictions when temperatures change over time

An explanatory model of temperature influence on flowering through whole-plant accumulation of FLOWERING LOCUS T in Arabidopsis thaliana

We assessed mechanistic temperature influence on flowering by incorporating temperature-responsive flowering mechanisms across developmental age into an existing model. Temperature influences the leaf production rate as well as expression of FLOWERING LOCUS T (FT), a photoperiodic flowering regulator that is expressed in leaves. The Arabidopsis Framework Model incorporated temperature influence on leaf growth but ignored the consequences of leaf growth on and direct temperature influence of FT expression. We measured FT production in differently aged leaves and modified the model, adding mechanistic temperature influence on FT transcription, and causing whole-plant FT to accumulate with leaf growth. Our simulations suggest that in long days, the developmental stage (leaf number) at which the reproductive transition occurs is influenced by day length and temperature through FT, while temperature influences the rate of leaf production and the time (in days) the transition occurs. Further, we demonstrate that FT is mainly produced in the first 10 leaves in the Columbia (Col-0) accession, and that FT accumulation alone cannot explain flowering in conditions in which flowering is delayed. Our simulations supported our hypotheses that: (i) temperature regulation of FT, accumulated with leaf growth, is a component of thermal time, and (ii) incorporating mechanistic temperature regulation of FT can improve model predictions when temperatures change over time

Clock-Controlled and Cold-Induced CYCLING DOF FACTOR6 Alters Growth and Development in Arabidopsis

The circadian clock represents a critical regulatory network, which allows plants to anticipate environmental changes as inputs and promote plant survival by regulating various physiological outputs. Here, we examine the function of the clock-regulated transcription factor, CYCLING DOF FACTOR 6 (CDF6), during cold stress in Arabidopsis thaliana. We found that the clock gates CDF6 transcript accumulation in the vasculature during cold stress. CDF6 mis-expression results in an altered flowering phenotype during both ambient and cold stress. A genome-wide transcriptome analysis links CDF6 to genes associated with flowering and seed germination during cold and ambient temperatures, respectively. Analysis of key floral regulators indicates that CDF6 alters flowering during cold stress by repressing photoperiodic flowering components, FLOWERING LOCUS T (FT), CONSTANS (CO), and BROTHER OF FT (BFT). Gene ontology enrichment further suggests that CDF6 regulates circadian and developmental-associated genes. These results provide insights into how the clock-controlled CDF6 modulates plant development during moderate cold stress

Role of the Arabidopsis Glucose Sensor Hexokinase1 (HXK1) in seedling establishment

Light energy serve as primary substrates for photosynthetic sugar production in plant metabolic maintenance. In addition, light also functions as a crucial environmental cue initiating a series of signal transduction cascades downstream to the photoreceptors. Limited knowledge exists on the molecular connections integrating the metabolic and photoreceptor signalling pathways. The primary focus of this thesis is the plant glucose receptor Hexokinase1 (HXK1). Arabidopsis HXK1 performs a dual function 1) an enzymatic role in glycolysis catalysing glucose phosphorylation to generate glucose-6-phosphate (G6P) and 2) a transcriptional repressor role in response to exogenous sugar. While the enzymatic role required for ATP production during respiration is conserved across biological systems, the sugar induced nuclear signalling role has only been identified in yeast and Arabidopsis. In this thesis, I show that HXK1 operates during nutrient limiting conditions such as extended periods of darkness or suboptimal light in seedlings. A first-ever RNAseq enabled us to gain insights into the mutant transcriptome. Energy demanding pathways were downregulated and carbon starvation induced Branched Chain Amino Acid (BCAA) degradation pathway is upregulated as an alternate energy source. Supplying Glucose-6-Phosphate (G6P – HXK1 enzymatic endproduct) restored the mutant phenotype and the C starvation response during nutrient limited conditions. This highlights the requirement of HXK1 enzyme rather than the signalling component during seedling establishment. Nuclear HXK1 operates as a transcriptional repressor in response to exogenous sugar. It is postulated to function during feedback inhibition of photosynthetic genes. However, our data indicates that this does not appear to be the case when endogenous sugars are naturally elevated in seedlings. Further, I present preliminary data on the potential feedback regulation by HXK1Overpression (HXK1OX) in blue and red light signalling pathway. HXK1 exerts a negative control on blue light mediated photomorphogenesis. Red light negates this effect in a PHYB dependent manner. Although the transcriptomes are reflective of the phenotype, the molecular mechanism behind this response is unknown. Taken together, my thesis discovers novel facets of HXK1 during seedling establishment

Circadian rhythms in floral scent emission

To successfully recruit pollinators, plants often release attractive floral scents at specific times of day to coincide with pollinator foraging. This timing of scent emission is thought to be evolutionarily beneficial to maximize resource efficiency while attracting only useful pollinators. Temporal regulation of scent emission is tied to the activity of the specific metabolic pathways responsible for scent production. Although floral volatile profiling in various plants indicated a contribution by the circadian clock, the mechanisms by which the circadian clock regulates timing of floral scent emission remained elusive. Recent studies using two species in the Solanaceae family provided initial insight into molecular clock regulation of scent emission timing. In Petunia hybrida, the floral volatile benzenoid/phenylpropanoid (FVBP) pathway is the major metabolic pathway that produces floral volatiles. Three MYB-type transcription factors, ODORANT 1 (ODO1), EMISSION OF BENZENOIDS I (EOBI), and EOBII, all of which show diurnal rhythms in mRNA expression, act as positive regulators for several enzyme genes in the FVBP pathway. Recently, in P. hybrida and Nicotiana attenuata, homologs of the Arabidopsis clock gene LATE ELONGATED HYPOCOTYL (LHY) have been shown to have a similar role in the circadian clock in these plants, and to also determine the timing of scent emission. In addition, in P. hybrida, PhLHY directly represses ODO1 and several enzyme genes in the FVBP pathway during the morning as an important negative regulator of scent emission. These findings facilitate our understanding of the relationship between a molecular timekeeper and the timing of scent emission, which may influence reproductive success.ISSN:1664-462