Reduction of the geomagnetic field delays Arabidopsis thaliana flowering time through downregulation of flowering‐related genes

Variations in magnetic field (MF) intensity are known to induce plant morphological and gene expression changes. In Arabidopsis thaliana Col‐0, near‐null magnetic field (NNMF, i.e., <100 nT MF) causes a delay in the transition to flowering, but the expression of genes involved in this response has been poorly studied. Here, we showed a time‐course quantitative analysis of the expression of both leaf (including clock genes, photoperiod pathway, GA20ox, SVP, and vernalization pathway) and floral meristem (including GA2ox, SOC1, AGL24, LFY, AP1, FD, and FLC) genes involved in the transition to flowering in A. thaliana under NNMF. NNMF induced a delayed flowering time and a significant reduction of leaf area index and flowering stem length, with respect to controls under geomagnetic field. Generation experiments (F₁‐ and F₂‐NNMF) showed retention of flowering delay. The quantitative expression (qPCR) of some A. thaliana genes expressed in leaves and floral meristem was studied during transition to flowering. In leaves and flowering meristem, NNMF caused an early downregulation of clock, photoperiod, gibberellin, and vernalization pathways and a later downregulation of TSF, AP1, and FLC. In the floral meristem, the downregulation of AP1, AGL24, FT, and FLC in early phases of floral development was accompanied by a downregulation of the gibberellin pathway. The progressive upregulation of AGL24 and AP1 was also correlated to the delayed flowering by NNMF. The flowering delay is associated with the strong downregulation of FT, FLC, and GA20ox in the floral meristem and FT, TSF, FLC, and GA20ox in leaves. Bioelectromagnetics. 39:361–374, 2018. © 2018 The Authors. Bioelectromagnetics Published by Wiley Periodicals, Inc.     

Aldehyde dehydrogenase ALDH3F1 involvement in flowering time regulation through histone acetylation modulation on FLOWERING LOCUS C

Flowering time regulation is one of the most important processes in the whole life of flowering plants and FLOWERING LOCUS C (FLC) is a central repressor of flowering time. However, whether metabolic acetate level affects flowering time is unknown. Here we report that ALDEHYDE DEHYDROGENASE ALDH3F1 plays essential roles in floral transition via FLC‐dependent pathway. In the aldh3f1‐1 mutant, the flowering time was significant earlier than Col‐0 and the FLC expression level was reduced. ALDH3F1 had aldehyde dehydrogenase activity to affect the acetate level in plants, and the amino acids of E214 and C252 are essential for its catalytic activity. Moreover, aldh3f1 mutation reduced acetate level and the total acetylation on histone H3. The H3K9Ac level on FLC locus was decreased in aldh3f1‐1, which reduced FLC expression. Expression of ALDH3F1 could rescue the decreased H3K9Ac level on FLC, FLC expression and also the early‐flowering phenotype of aldh3f1‐1, however ALDH3F1^E214A or ALDH3F1^C252A could not. Our findings demonstrate that ALDH3F1 participates in flowering time regulation through modulating the supply of acetate for acetyl‐CoA, which functions as histone acetylation donor to modulate H3K9Ac on FLC locus.     

The BAF60 Subunit of the SWI/SNF Chromatin-Remodeling Complex Directly Controls the Formation of a Gene Loop at FLOWERING LOCUS C in Arabidopsis

SWI/SNF complexes mediate ATP-dependent chromatin remodeling to regulate gene expression. Many components of these complexes are evolutionarily conserved, and several subunits of Arabidopsis thaliana SWI/SNF complexes are involved in the control of flowering, a process that depends on the floral repressor FLOWERING LOCUS C (FLC). BAF60 is a SWI/SNF subunit, and in this work, we show that BAF60, via a direct targeting of the floral repressor FLC, induces a change at the high-order chromatin level and represses the photoperiod flowering pathway in Arabidopsis. BAF60 accumulates in the nucleus and controls the formation of the FLC gene loop by modulation of histone density, composition, and posttranslational modification. Physiological analysis of BAF60 RNA interference mutant lines allowed us to propose that this chromatin-remodeling protein creates a repressive chromatin configuration at the FLC locus.     

INDUCER OF CBF EXPRESSION 1 integrates cold signals into FLOWERING LOCUS C‐mediated flowering pathways in Arabidopsis

Plants constantly monitor changes in photoperiod and temperature throughout the year to synchronize flowering with optimal environmental conditions. In the temperate zones, both photoperiod and temperature fluctuate in a somewhat predictable manner through the seasons, although a transient shift to low temperature is also encountered during changing seasons, such as early spring. Although low temperatures are known to delay flowering by inducing the floral repressor FLOWERING LOCUS C (FLC), it is not fully understood how temperature signals are coordinated with photoperiodic signals in the timing of seasonal flowering. Here, we show that the cold signaling activator INDUCER OF CBF EXPRESSION 1 (ICE1), FLC and the floral promoter SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) constitute an elaborate signaling network that integrates cold signals into flowering pathways. The cold‐activated ICE1 directly induces the gene encoding FLC, which represses SOC1 expression, resulting in delayed flowering. In contrast, under floral promotive conditions, SOC1 inhibits the binding of ICE1 to the promoters of the FLC gene, inducing flowering with a reduction of freezing tolerance. These observations indicate that the ICE1‐FLC‐SOC1 signaling network contributes to the fine‐tuning of flowering during changing seasons.     

FLC: A key regulator of flowering time in Arabidopsis

The timing of floral transition has significant consequences for reproductive success in plants. The molecular genetic dissection of flowering time control in Arabidopsis identified an integrated network of pathways that quantitatively control this developmental switch. A central player in this process is the FLOWERING LOCUS C gene (FLC), which blocks flowering by inhibiting the genes required to switch the meristem from vegetative to floral development. Three systems (the FRIGIDA gene, vernalization, and the autonomous pathway) all influence the state of FLC. Last years many new genes have been identified that regulate FLC expression, and most of them are involved in the modification of FLC chromatin. This review focuses on recent insights in FLC regulation.     

Polycomb-Group Proteins and FLOWERING LOCUS T Maintain Commitment to Flowering in Arabidopsis thaliana

The switch from vegetative to reproductive growth is extremely stable even if plants are only transiently exposed to environmental stimuli that trigger flowering. In the photoperiodic pathway, a mobile signal, florigen, encoded by FLOWERING LOCUS T (FT) in Arabidopsis thaliana, induces flowering. Because FT activity in leaves is not maintained after transient photoperiodic induction, the molecular basis for stable floral commitment is unclear. Here, we show that Polycomb-group (Pc-G) proteins, which mediate epigenetic gene regulation, maintain the identity of inflorescence and floral meristems after floral induction. Thus, plants with reduced Pc-G activity show a remarkable increase of cauline leaves under noninductive conditions and floral reversion when shifted from inductive to noninductive conditions. These phenotypes are almost completely suppressed by loss of FLOWERING LOCUS C (FLC) and SHORT VEGETATIVE PHASE, which both delay flowering and promote vegetative shoot identity. Upregulation of FLC in Pc-G mutants leads to a strong decrease of FT expression in inflorescences. We find that this activity of FT is needed to prevent floral reversion. Collectively, our results reveal that floral meristem identity is at least partially maintained by a daylength-independent role of FT whose expression is indirectly sustained by Pc-G activity.     

A photo‐responsive F‐box protein FOF2 regulates floral initiation by promoting FLC expression in Arabidopsis

Floral initiation is regulated by various genetic pathways in response to light, temperature, hormones and developmental status; however, the molecular mechanisms underlying the interactions between different genetic pathways are not fully understood. Here, we show that the photoresponsive gene FOF2 (F‐box of flowering 2) negatively regulates flowering. FOF2 encodes a putative F‐box protein that interacts specifically with ASK14, and its overexpression results in later flowering under both long‐day and short‐day photoperiods. Conversely, transgenic plants expressing the F‐box domain deletion mutant of FOF2 (FOF2ΔF), or double loss of function mutant of FOF2 and FOL1 (FOF2‐LIKE 1) present early flowering phenotypes. The late flowering phenotype of the FOF2 overexpression lines is suppressed by the flc‐3 loss‐of‐function mutation. Furthermore, FOF2 mRNA expression is regulated by autonomous pathway gene FCA, and the repressive effect of FOF2 in flowering can be overcome by vernalization. Interestingly, FOF2 expression is regulated by light. The protein level of FOF2 accumulates in response to light, whereas it is degraded under dark conditions via the 26S proteasome pathway. Our findings suggest a possible mechanistic link between light conditions and the autonomous floral promotion pathway in Arabidopsis.     

Overexpression of a Brassica rapa MADS-box gene, BrAGL20, induces early flowering time phenotypes in Brassica napus

BrAGL20 (SOC1) containing MADS box, a floral integrator gene, was introduced into Brassica napus cv. “Youngsan” by Agrobacterium-mediated transformation. Constitutively overexpressed BrAGL20 under the CaMV 35S promoter induced early flowering time compared to the wild-type. These phenotypes were stably inherited through generations T₂ and T₃, regardless of planting season. The expression of the floral meristem identity genes LFY and AP1 seemed to appear rapidly in the shoot apex region of transgenic plants showing the early flowering time phenotype. These results suggest that overexpression of BrAGL20 can significantly affect the flowering time of B. napus, and regulation of floral integrator gene expression could be applied for adaptation of crops to local environments and climate changes.     

Requirement of histone acetyltransferases HAM1 and HAM2 for epigenetic modification of FLC in regulating flowering in Arabidopsis

Histone acetylation is an important posttranslational modification associated with gene activation. In Arabidopsis, two MYST histone acetyltransferases HAM1 and HAM2 work redundantly to acetylate histone H4 lysine 5 (H4K5ace) in vitro. The double mutant ham1/ham2 is lethal, which suggests the critical role of HAM1 and HAM2 in development. Here, we used an artificial microRNA (amiRNA) strategy in Arabidopsis to uncover a novel function of HAM1 and HAM2. The amiRNA-HAM1/2 transgenic plants showed early flowering and reduced fertility. In addition, they responded normally to photoperiod, gibberellic acid treatment, and vernalization. The expression of flowering-repressor FLOWERING LOCUS C (FLC) and its homologues, MADS-box Affecting Flowering genes 3/4 (MAF3/4), were decreased in amiRNA-HAM1/2 lines. HAM1 overexpression caused late flowering and elevated expression of FLC and MAF3/4. Mutation of FLC almost rescued the late flowering with HAM1 overexpression, which suggests that HAM1 regulation of flowering time depended on FLC. Global H4 acetylation was decreased in amiRNA-HAM1/2 lines, but increased in HAM1-OE lines, which further confirmed the acetyltransferase activity of HAM1 in vivo. Chromatin immunoprecipitation revealed that H4 hyperacetylation and H4K5ace at FLC and MAF3/4 were less abundant in amiRNA-HAM1/2 lines than the wild type, but were enriched in HAM1-OE lines. Thus, HAM1 and HAM2 may affect flowering time by epigenetic modification of FLC and MAF3/4 chromatins at H4K5 acetylation.     

Stress and salicylic acid induce the expression of PnFT2 in the regulation of the stress-induced flowering of Pharbitis nil

Poor nutrition and low temperature stress treatments induced flowering in the Japanese morning glory Pharbitis nil (synonym Ipomoea nil) cv. Violet. The expression of PnFT2, one of two homologs of the floral pathway integrator gene FLOWERING LOCUS T (FT), was induced by stress, whereas the expression of both PnFT1 and PnFT2 was induced by a short-day treatment. There was no positive correlation between the flowering response and the homolog expression of another floral pathway integrator gene SUPPRESSOR OF OVEREXPRESSION OF CO1 and genes upstream of PnFT, such as CONSTANS. In another cultivar, Tendan, flowering and PnFT2 expression were not induced by poor nutrition stress. Aminooxyacetic acid (AOA), a phenylalanine ammonia-lyase inhibitor, inhibited the flowering and PnFT2 expression induced by poor nutrition stress in Violet. Salicylic acid (SA) eliminated the inhibitory effects of AOA. SA enhanced PnFT2 expression under the poor nutrition stress but not under non-stress conditions. These results suggest that SA induces PnFT2 expression, which in turn induces flowering; SA on its own, however, may not be sufficient for induction.     

The molecular mechanism of vernalization in Arabidopsis and cereals: role of Flowering Locus C and its homologs

Winter varieties of plants can flower only after exposure to prolonged cold. This phenomenon is known as vernalization and has been widely studied in the model plant Arabidopsis thaliana as well as in monocots. Through the repression of floral activator genes, vernalization prevents flowering in winter. In Arabidopsis, FLOWERING LOCUS C or FLC is the key repressor during vernalization, while in monocots vernalization is regulated through VRN1, VRN2 and VRN3 (or FLOWERING LOCUS T). Interestingly, VRN genes are not homologous to FLC but FLC homologs are found to have a significant role in vernalization response in cereals. The presence of FLC homologs in monocots opens new dimensions to understand, compare and retrace the evolution of vernalization pathways between monocots and dicots. In this review, we discuss the molecular mechanism of vernalization-induced flowering along with epigenetic regulations in Arabidopsis and temperate cereals. A better understanding of cold-induced flowering will be helpful in crop breeding strategies to modify the vernalization requirement of economically important temperate cereals.