Genetic interactions reveal the antagonistic roles of FT/TSF and TFL1 in the determination of inflorescence meristem identity in Arabidopsis.

During the transition to the reproductive phase, the shoot apical meristem switches from the developmental program that generates vegetative organs to instead produce flowers. In this study, we examined the genetic interactions of FLOWERING LOCUS T (FT)/TWIN SISTER OF FT (TSF) and TERMINAL FLOWER 1 (TFL1) in the determination of inflorescence meristem identity in Arabidopsis thaliana. The ft-10 tsf-1 mutants produced a compact inflorescence surrounded by serrated leaves (hyper-vegetative shoot) at the early bolting stage, as did plants overexpressing TFL1. Plants overexpressing FT or TSF (or both FT and TFL1) generated a terminal flower, as did tfl1-20 mutants. The terminal flower formed in tfl1-20 mutants converted to a hyper-vegetative shoot in ft-10 tsf-1 mutants. Grafting ft-10 tsf-1 or ft-10 tsf-1 tfl1-20 mutant scions to 35S::FT rootstock plants produced a normal inflorescence and a terminal flower in the scion plants, respectively, although both scions showed similar early flowering. Misexpression of FT in the vasculature and in the shoot apex in wild-type plants generated a normal inflorescence and a terminal flower, respectively. By contrast, in ft-10 tsf-1 mutants the vasculature-specific misexpression of FT converted the hyper-vegetative shoot to a normal inflorescence, and in the ft-10 tsf-1 tfl1-20 mutants converted the shoot to a terminal flower. TFL1 levels did not affect the inflorescence morphology caused by FT/TSF overexpression at the early bolting stage. Taking these results together, we proposed that FT/TSF and TFL1 play antagonistic roles in the determination of inflorescence meristem identity, and that FT/TSF are more important than TFL1 in this process.

Chloroplast Signaling Gates Thermotolerance in Arabidopsis.

Temperature is a key environmental variable influencing plant growth and survival. Protection against high temperature stress in eukaryotes is coordinated by heat shock factors (HSFs), transcription factors that activate the expression of protective chaperones such as HEAT SHOCK PROTEIN 70 (HSP70); however, the pathway by which temperature is sensed and integrated with other environmental signals into adaptive responses is not well understood. Plants are exposed to considerable diurnal variation in temperature, and we have found that there is diurnal variation in thermotolerance in Arabidopsis thaliana, with maximal thermotolerance coinciding with higher HSP70 expression during the day. In a forward genetic screen, we identified a key role for the chloroplast in controlling this response, suggesting that light-induced chloroplast signaling plays a key role. Consistent with this, we are able to globally activate binding of HSFA1a to its targets by altering redox status in planta independently of a heat shock.

ELF3 controls thermoresponsive growth in Arabidopsis.

Plant development is highly responsive to ambient temperature, and this trait has been linked to the ability of plants to adapt to climate change. The mechanisms by which natural populations modulate their thermoresponsiveness are not known. To address this, we surveyed Arabidopsis accessions for variation in thermal responsiveness of elongation growth and mapped the corresponding loci. We find that the transcriptional regulator EARLY FLOWERING3 (ELF3) controls elongation growth in response to temperature. Through a combination of modeling and experiments, we show that high temperature relieves the gating of growth at night, highlighting the importance of temperature-dependent repressors of growth. ELF3 gating of transcriptional targets responds rapidly and reversibly to changes in temperature. We show that the binding of ELF3 to target promoters is temperature dependent, suggesting a mechanism where temperature directly controls ELF3 activity.

Generation and analysis of a complete mutant set for the Arabidopsis FT/TFL1 family shows specific effects on thermo-sensitive flowering regulation.

The FLOWERING LOCUS T (FT)/TERMINAL FLOWER 1 (TFL1) family proteins play an important role in the regulation of flowering time. In the Arabidopsis thaliana genome, there are six genes in the FT/TFL1 family. To determine how these FT/TFL1 family genes contribute to the regulation of flowering time, this study generated a comprehensive set of mutants (sixty-three multiple mutants in all combinations) of the FT/TFL1 family genes and analysed their flowering times at 23 and 16°C under long-day conditions. The analysis confirmed that FT and TFL1 are major determinants of flowering time under long-day conditions. At 23 °C, ft-10 tsf-1 mft-2 showed the latest flowering, whereas tfl1-20 atc-2 bft-2 showed the earliest flowering. Flowering occurred in the sextuple mutants. Introduction of tsf-1 led to reduced sensitivity to ambient temperature change. Introduction of tfl1-20 caused a stronger effect in accelerating flowering time at 16 °C than at 23 °C. Overexpression of miR156 did not block flowering of sextuple mutants, suggesting that there is a pathway to induce flowering independent of the FT/TFL1 pathway and miR156 pathway. This study proposes that this mutant population will be useful in further investigation of the functions of the FT/TFL1 family genes in plant development.

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

In Arabidopsis, long-distance movement of FLOWERING LOCUS T (FT) protein from the leaf to the shoot apex triggers flower development. In wild-type Arabidopsis plants under long-day conditions, FT is mainly expressed in the cotyledon but is weakly expressed in the first true leaf prior to floral induction. To test the importance of the cotyledon in floral induction, we developed a cotyledon micrografting (Cot-grafting) method that, unlike other grafting methods, allows the FT protein from the graft to be transported via its native route from leaves to the shoot apex. By using Cot-grafting, we found that grafting a single wild-type cotyledon onto an ft-10 mutant strongly suppressed the ft-10 late flowering phenotype. Neither Y-grafting wild-type shoots nor butt-grafting wild-type roots to ft-10 plants resulted in comparably accelerated flowering in the ft-10 recipient plants. ft-10 mutants grafted with a 35S::FT cotyledon flowered as early as wild-type plants. When phloem-specific tracers were applied to a donor cotyledon, the tracers were detected in the vein of the true leaf of recipient plants 6 d after Cot-grafting. Also, macromolecule trafficking of an FT:yellow fluorescent protein:hemagglutinin fusion occurred across the graft junction 6 d after Cot-grafting. These results suggest that Cot-grafting, which allows protein movement in a manner consistent with the natural flow of FT protein from the leaf to the shoot apex, can efficiently suppress the late flowering of ft-10 mutants. Our results further suggest that in Arabidopsis, the cotyledon is an important organ for producing FT protein to induce flowering.

BROTHER OF FT AND TFL1 (BFT), a member of the FT/TFL1 family, shows distinct pattern of expression during the vegetative growth of Arabidopsis.

Transition to the flowering stage is precisely controlled by a few classes of regulatory molecules. BROTHER OF FT AND TFL1 (BFT) is a member of FLOWERING LOCUS T (FT)/TERMINAL FLOWER 1 (TFL1) family, an important class of flower development regulators with unidentified biochemical function. BFT has a TFL1-like activity and plays a role in axillary inflorescence development. To elucidate the expression pattern of BFT, we analyzed the subcellular localization and conditional expression of BFT in this study. We generated 35S::BFT:GFP plants to investigate the subcellular localization of BFT protein. 35S::BFT:GFP plants showed late flowering, similarly as did 35S::BFT plants. BFT:GFP fusion protein was localized in the nucleus and the plasma membrane, which was different from the localization pattern of FT and TFL1. BFT expression was induced by abiotic stress conditions. ABA, drought, and osmotic stress treatments induced BFT expression, whereas cold, salt, and heat stress conditions did not, suggesting that BFT plays a role in regulating flowering time and inflorescence structure under drought conditions. The induction pattern of BFT was different from those of other FT/TFL1 family genes. Our studies indicated that BFT showed a distinct expression pattern from its homologous genes during the vegetative growth in Arabidopsis.

BROTHER OF FT AND TFL1 (BFT) has TFL1-like activity and functions redundantly with TFL1 in inflorescence meristem development in Arabidopsis.

The FLOWERING LOCUS T (FT)/TERMINAL FLOWER 1 (TFL1) family is a small gene family that encodes important regulators that control flower development in Arabidopsis. Here, we investigated the biological role of the product of BROTHER OF FT AND TFL1 (BFT), a member of this family, whose function remains unknown. Comparison of the critical residues that play a role in distinguishing FT- or TFL1-like activity revealed that BFT is more similar to FT. Similar to FT expression, BFT expression showed a diurnal oscillation pattern, peaking in the evening. In situ hybridization revealed BFT expression in the shoot apical meristem, young leaf and axillary inflorescence meristem. Transgenic plants over-expressing BFT exhibited delayed flowering and severe floral defects (floral indeterminacy and compact inflorescences surrounded by serrate leaves), similar to 35S::TFL1 plants. LEAFY (LFY) and APETALA1 (AP1) expression was significantly reduced in 35S::BFT plants. BFT over-expression failed to rescue the terminal flower phenotype of tfl1 mutants; however, it delayed both terminal flower formation in the primary inflorescence and axillary inflorescence development in the tfl1 mutant background. Consistent with this, the loss-of-function BFT alleles, bft-2 and an BFT RNAi line, accelerated termination of the primary inflorescence and formation of axillary inflorescences in the tfl1 mutant background. Taken together, our results suggest that, despite similarities in the critical residues of BFT and FT, BFT possesses a TFL1-like activity and functions redundantly with TFL1 in inflorescence meristem development, and possibly contributes to the regulation of plant architecture.

Genetic framework for flowering-time regulation by ambient temperature-responsive miRNAs in Arabidopsis.

Flowering is the primary trait affected by ambient temperature changes. Plant microRNAs (miRNAs) are small non-coding RNAs playing an important regulatory role in plant development. In this study, to elucidate the mechanism of flowering-time regulation by small RNAs, we identified six ambient temperature-responsive miRNAs (miR156, miR163, miR169, miR172, miR398 and miR399) in Arabidopsis via miRNA microarray and northern hybridization analyses. We also determined the expression profile of 120 unique miRNA loci in response to ambient temperature changes by miRNA northern hybridization analysis. The expression of the ambient temperature-responsive miRNAs and their target genes was largely anticorrelated at two different temperatures (16 and 23 degrees C). Interestingly, a lesion in short vegetative phase (SVP), a key regulator within the thermosensory pathway, caused alteration in the expression of miR172 and a subset of its target genes, providing a link between a thermosensory pathway gene and miR172. The miR172-overexpressing plants showed a temperature-independent early flowering phenotype, suggesting that modulation of miR172 expression leads to temperature insensitivity. Taken together, our results suggest a genetic framework for flowering-time regulation by ambient temperature-responsive miRNAs under non-stress temperature conditions.

AGO1-miR173 complex initiates phased siRNA formation in plants.

MicroRNA (miRNA)-guided cleavage initiates entry of primary transcripts into the transacting siRNA (tasiRNA) biogenesis pathway involving RNA-DEPENDENT RNA POLYMERASE6, DICER-LIKE4, and SUPPRESSOR OF GENE SILENCING3. Arabidopsis thaliana TAS1 and TAS2 families yield tasiRNA that form through miR173-guided initiation-cleavage of primary transcripts and target several transcripts encoding pentatricopeptide repeat proteins and proteins of unknown function. Here, the TAS1c locus was modified to produce synthetic (syn) tasiRNA to target an endogenous transcript encoding PHYTOENE DESATURASE and used to analyze the role of miR173 in routing of transcripts through the tasiRNA pathway. miR173 was unique from other miRNAs in its ability to initiate TAS1c-based syn-tasiRNA formation. A single miR173 target site was sufficient to route non-TAS transcripts into the pathway to yield phased siRNA. We also show that miR173 functions in association with ARGONAUTE 1 (AGO1) during TAS1 and TAS2 tasiRNA formation, and we provide data indicating that the miR173-AGO1 complex possesses unique functionality that many other miRNA-AGO1 complexes lack.

Role of SVP in the control of flowering time by ambient temperature in Arabidopsis.

Plants must perceive and rapidly respond to changes in ambient temperature for their successful reproduction. Here we demonstrate that Arabidopsis SHORT VEGETATIVE PHASE (SVP) plays an important role in the response of plants to ambient temperature changes. The loss of SVP function elicited insensitivity to ambient temperature changes. SVP mediates the temperature-dependent functions of FCA and FVE within the thermosensory pathway. SVP controls flowering time by negatively regulating the expression of a floral integrator, FLOWERING LOCUS T (FT), via direct binding to the CArG motifs in the FT sequence. We propose that this is one of the molecular mechanisms that modulate flowering time under fluctuating temperature conditions.

Isolation of 151 mutants that have developmental defects from T-DNA tagging.

In order to understand the mechanisms underlying plant development, a necessary first step involves the elucidation of the functions of the genes, via the analysis of mutants that exhibit developmental defects. In this study, an activation tagging mutant library harboring 80,650 independent Arabidopsis transformants was generated in order to screen for developmental mutants. A total of 129 mutants manifesting dominant developmental abnormalities were isolated, and their T-DNA insertion loci were mapped. The activation of one or more genes adjacent to a T-DNA insertion locus was confirmed in eight dominant mutants. A gene adjacent to the right border was usually activated by the 35S enhancers. Interestingly, the transcriptional activation of multiple genes within a broad range was observed in one of the mutants, which raises the possibility that activation by the 35S enhancers was not limited strictly to a single gene. In order to gain a better understanding of sexual reproduction in higher plants, we isolated 22 mutants exhibiting defects in female gametophyte development, and determined their T-DNA insertion loci. We propose that this mutant population may prove useful in the further determination of the functions of genes that play important roles in plant development.

CONSTANS activates SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 through FLOWERING LOCUS T to promote flowering in Arabidopsis.

CONSTANS (CO) regulates flowering time by positively regulating expression of two floral integrators, FLOWERING LOCUS T (FT) and SUPPRESSOR OF OVEREXPRESSION OF CO 1 (SOC1), in Arabidopsis (Arabidopsis thaliana). FT and SOC1 have been proposed to act in parallel pathways downstream of CO based on genetic analysis using weak ft alleles, since ft soc1 double mutants showed an additive effect in suppressing the early flowering of CO overexpressor plants. However, this genetic analysis was inconsistent with the sequential induction pattern of FT and SOC1 found in inducible CO overexpressor plants. Hence, to identify genetic interactions of CO, FT, and SOC1, we carried out genetic and expression analyses with a newly isolated T-DNA allele of FT, ft-10. We found that ft-10 almost completely suppressed the early flowering phenotype of CO overexpressor plants, whereas soc1-2 partially suppressed the phenotype, suggesting that FT is the major output of CO. Expression of SOC1 was altered in gain- or loss-of-function mutants of FT, whereas expression of FT remained unchanged in gain- or loss-of-function mutants of SOC1, suggesting that FT positively regulates SOC1 to promote flowering. In addition, inactivation of FT caused down-regulation of SOC1 even in plants overexpressing CO, indicating that FT is required for SOC1 induction by CO. Taken together, these data suggest that CO activates SOC1 through FT to promote flowering in Arabidopsis.