Flowering and expression of flowering-related genes under long-day conditions with light-emitting diodes

The effects of light quality on flowering time were investigated in Gypsophila paniculata, which is a long-day cut flower, and with Arabidopsis under long-day conditions with light-emitting diodes (LEDs). Gypsophila paniculata plants were grown under natural daylight and flowering was controlled by long-day treatment with a weak LED light of a single color in the night. Flowering was promoted not by blue light, but by far-red light in G. paniculata, while flowering was promoted by both light colors in Arabidopsis. FT homologs of G. paniculata GpFT1 and GpFT2 were differentially expressed under long-day conditions with white light, suggesting that they play roles in flowering at different stages of reproductive development. GpFTs and FT gene expression was not induced by far-red light in G. paniculata or Arabidopsis. Instead, the expression of the SOC1 homolog of G. paniculata GpSOC1 and SOC1 was induced by far-red light in G. paniculata and Arabidopsis. Flowering was promoted by induction of FT and SOC1 expression with blue light in Arabidopsis, whereas GpFTs and GpSOC1 expression was low with blue light induction in G. paniculata. The relationship between flowering and the expression of FT and SOC1 in Arabidopsis was confirmed with ft and soc1 mutants. These results suggest that long-day conditions with far-red light promote flowering through SOC1 and its homologs, while the conditions with blue light do not promote flowering in G. paniculata, because of low expression of GpFTs and GpSOC1 in contrast to that in Arabidopsis.     

Functional conservation of clock-related genes in flowering plants: overexpression and RNA interference analyses of the circadian rhythm in the monocotyledon Lemna gibba

Circadian rhythms are found in organisms from cyanobacteria to plants and animals. In flowering plants, the circadian clock is involved in the regulation of various physiological phenomena, including growth, leaf movement, stomata opening, and floral transitions. Molecular mechanisms underlying the circadian clock have been identified using Arabidopsis (Arabidopsis thaliana); the functions and genetic networks of a number of clock-related genes, including CIRCADIAN CLOCK ASSOCIATED1, LATE ELONGATED HYPOCOTYL (LHY), TIMING OF CAB EXPRESSION1, GIGANTEA (GI), and EARLY FLOWERING3 (ELF3), have been analyzed. The degree to which clock systems are conserved among flowering plants, however, is still unclear. We previously isolated homologs for Arabidopsis clock-related genes from monocotyledon Lemna plants. Here, we report the physiological roles of these Lemna gibba genes (LgLHYH1, LgLHYH2, LgGIH1, and LgELF3H1) in the circadian system. We studied the effects of overexpression and RNA interference (RNAi) of these genes on the rhythmic expression of morning- and evening-specific reporters. Overexpression of each gene disrupted the rhythmicity of either or both reporters, suggesting that these four homologs can be involved in the circadian system. RNAi of each of the genes except LgLHYH2 affected the bioluminescence rhythms of both reporters. These results indicated that these homologs are involved in the circadian system of Lemna plants and that the structure of the circadian clock is likely to be conserved between monocotyledons and dicotyledons. Interestingly, RNAi of LgGIH1 almost completely abolished the circadian rhythm; because this effect appeared to be much stronger than the phenotype observed in an Arabidopsis gi loss-of-function mutant, the precise role of each clock gene may have diverged in the clock systems of Lemna and Arabidopsis.     

Repression of FLOWERING LOCUS C and FLOWERING LOCUS T by the Arabidopsis Polycomb repressive complex 2 components

Polycomb group (PcG) proteins are evolutionarily conserved in animals and plants, and play critical roles in the regulation of developmental gene expression. Here we show that the Arabidopsis Polycomb repressive complex 2 (PRC2) subunits CURLY LEAF (CLF), EMBRYONIC FLOWER 2 (EMF2) and FERTILIZATION INDEPENDENT ENDOSPERM (FIE) repress the expression of FLOWERING LOCUS C (FLC), a central repressor of the floral transition in Arabidopsis and FLC relatives. In addition, CLF directly interacts with and mediates the deposition of repressive histone H3 lysine 27 trimethylation (H3K27me3) into FLC and FLC relatives, which suppresses active histone H3 lysine 4 trimethylation (H3K4me3) in these loci. Furthermore, we show that during vegetative development CLF and FIE strongly repress the expression of FLOWERING LOCUS T (FT), a key flowering-time integrator, and that CLF also directly interacts with and mediates the deposition of H3K27me3 into FT chromatin. Our results suggest that PRC2-like complexes containing CLF, EMF2 and FIE, directly interact with and deposit into FT, FLC and FLC relatives repressive trimethyl H3K27 leading to the suppression of active H3K4me3 in these loci, and thus repress the expression of these flowering genes. Given the central roles of FLC and FT in flowering-time regulation in Arabidopsis, these findings suggest that the CLF-containing PRC2-like complexes play a significant role in control of flowering in Arabidopsis.     

PIE1, an ISWI family gene,is required for FLC activation and floral repression in Arabidopsis

Proper control of the floral transition is critical for reproductive success in flowering plants. In Arabidopsis, FLOWERING LOCUS C (FLC) is a floral repressor upon which multiple floral regulatory pathways converge. Mutations in PHOTOPERIOD-INDEPENDENT EARLY FLOWERING1 (PIE1) suppress the FLC-mediated delay of flowering as a result of the presence of FRIGIDA or of mutations in autonomous pathway genes. PIE1 is required for high levels of FLC expression in the shoot apex, but it is not required for FLC expression in roots. PIE1 is similar to ATP-dependent, chromatin-remodeling proteins of the ISWI and SWI2/SNF2 family. The role of PIE1 as an activator of FLC is consistent with the general role of ISWI and SWI2/SNF2 family genes as activators of gene expression. The pie1 mutation also causes early flowering in noninductive photoperiods independently of FLC; thus, PIE1 appears to be involved in multiple flowering pathways. PIE1 also plays a role in petal development, as revealed by the suppression of petal defects of the curly leaf mutant by the pie1 mutation.     

FRIGIDA-independent variation in flowering time of natural Arabidopsis thaliana accessions

FRIGIDA (FRI) and FLOWERING LOCUS C (FLC) are two genes that, unless plants are vernalized, greatly delay flowering time in Arabidopsis thaliana. Natural loss-of-function mutations in FRI cause the early flowering growth habits of many A. thaliana accessions. To quantify the variation among wild accessions due to FRI, and to identify additional genetic loci in wild accessions that influence flowering time, we surveyed the flowering times of 145 accessions in long-day photoperiods, with and without a 30-day vernalization treatment, and genotyped them for two common natural lesions in FRI. FRI is disrupted in at least 84 of the accessions, accounting for only approximately 40% of the flowering-time variation in long days. During efforts to dissect the causes for variation that are independent of known dysfunctional FRI alleles, we found new loss-of-function alleles in FLC, as well as late-flowering alleles that do not map to FRI or FLC. An FLC nonsense mutation was found in the early flowering Van-0 accession, which has otherwise functional FRI. In contrast, Lz-0 flowers late because of high levels of FLC expression, even though it has a deletion in FRI. Finally, eXtreme array mapping identified genomic regions linked to the vernalization-independent, late-flowering habit of Bur-0, which has an alternatively spliced FLC allele that behaves as a null allele.     

Differential expression of genes important for adaptation in Capsella bursa-pastoris (Brassicaceae)

Understanding the genetic basis of natural variation is of primary interest for evolutionary studies of adaptation. In Capsella bursa-pastoris, a close relative of Arabidopsis (Arabidopsis thaliana), variation in flowering time is correlated with latitude, suggestive of an adaptation to photoperiod. To identify pathways regulating natural flowering time variation in C. bursa-pastoris, we have studied gene expression differences between two pairs of early- and late-flowering C. bursa-pastoris accessions and compared their response to vernalization. Using Arabidopsis microarrays, we found a large number of significant differences in gene expression between flowering ecotypes. The key flowering time gene FLOWERING LOCUS C (FLC) was not differentially expressed prior to vernalization. This result is in contrast to those in Arabidopsis, where most natural flowering time variation acts through FLC. However, the gibberellin and photoperiodic flowering pathways were significantly enriched for gene expression differences between early- and late-flowering C. bursa-pastoris. Gibberellin biosynthesis genes were down-regulated in late-flowering accessions, whereas circadian core genes in the photoperiodic pathway were differentially expressed between early- and late-flowering accessions. Detailed time-series experiments clearly demonstrated that the diurnal rhythm of CIRCADIAN CLOCK-ASSOCIATED1 (CCA1) and TIMING OF CAB EXPRESSION1 (TOC1) expression differed between flowering ecotypes, both under constant light and long-day conditions. Differential expression of flowering time genes was biologically validated in an independent pair of flowering ecotypes, suggesting a shared genetic basis or parallel evolution of similar regulatory differences. We conclude that genes involved in regulation of the circadian clock, such as CCA1 and TOC1, are strong candidates for the evolution of adaptive flowering time variation in C. bursa-pastoris.     

HDA6 is required for jasmonate response, senescence and flowering in Arabidopsis

Post-translational modifications of histones, including acetylation, play a key role in modulating dynamic changes in chromatin structure and gene activity. Histone acetylation is modulated through the action of histone acetyltransferases and deacetylases. HDA6 is a RPD3-type histone deacetylase in Arabidopsis. The Arabidopsis HDA6 mutant, axe1-5, and HDA6 RNA-interfering (HDA6-RNAi) plants displayed higher levels of acetylated H3 compared with wild-type, suggesting that HDA6 affects histone acetylation levels globally. The expression of the jasmonate responsive genes, PDF1.2, VSP2, JIN1, and ERF1, was down-regulated in axe1-5 and HDA6-RNAi plants. Furthermore, axe1-5 and HDA6-RNAi plants displayed increased leaf longevity compared with the wild type. The expression of the senescence-associated genes, SAG12 and SEN4, was down-regulated in the axe1-5 and HDA6-RNAi plants. In addition, axe1-5 and HDA6-RNAi plants displayed late-flowering. The expression of FLC was up-regulated and hyperacetylated in axe1-5 and HDA6-RNAi plants, suggesting that HDA6 is required to deacetylate FLC chromatin and thereby repress its expression. Our results suggest that HDA6 is involved in jasmonate response, senescence, and flowering in Arabidopsis.     

Flowering of strict photoperiodic <Emphasis Type="Italic">Nicotiana</Emphasis> varieties in non-inductive conditions by transgenic approaches

The genus Nicotiana contains species and varieties that respond differently to photoperiod for flowering time control as day-neutral, short-day and long-day plants. In classical photoperiodism studies, these varieties have been widely used to analyse the physiological nature for floral induction by day length. Since key regulators for flowering time control by day length have been identified in Arabidopsis thaliana by molecular genetic studies, it was intriguing to analyse how closely related plants in the Nicotiana genus with opposite photoperiodic requirements respond to certain flowering time regulators. SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) and FRUITFULL (FUL) are two MADS box genes that are involved in the regulation of flowering time in Arabidopsis. SOC1 is a central flowering time pathway integrator, whereas the exact role of FUL for floral induction has not been established yet. The putative Nicotiana orthologs of SOC1 and FUL, NtSOC1 and NtFUL, were studied in day-neutral tobacco Nicotiana tabacum cv Hicks, in short-day tobacco N. tabacum cv Hicks Maryland Mammoth (MM) and long-day N. sylvestris plants. Both genes were similarly expressed under short- and long-day conditions in day-neutral and short-day tobaccos, but showed a different expression pattern in N. sylvestris. Overexpression of NtSOC1 and NtFUL caused flowering either in strict short-day (NtSOC1) or long-day (NtFUL) Nicotiana varieties under non-inductive photoperiods, indicating that these genes might be limiting for floral induction under non-inductive conditions in different Nicotiana varieties.     

利用RNA干扰技术获得晚抽薹开花转基因大白菜

LEAFY基因在植物的成花转变中起重要作用。在拟南芥中,降低LEAFY基因的表达使开花延迟,并且增加分枝。利用RNAi技术降低大白菜LEAFY基因表达,获得晚抽薹开花的转基因大白菜。开花时转基因大白菜的株高明显低于对照,而叶片数、叶面积和侧枝数则高于对照;转基因植株的LEAFY基因表达明显降低,导致大白菜抽薹开花延迟。因此可以通过降低LEAFY基因表达来延迟需低温春化作用长日植物开花。也为利用生物技术获得晚抽薹转基因大白菜提供了理论依据。