The circadian clock acts as the timekeeping mechanism in photoperiodism. In Arabidopsis thaliana, a circadian clockcontrolled flowering pathway comprising the genes GIGANTEA (GI), CONSTANS (CO), and FLOWERING LOCUS T (FT) promotes flowering specifically under long days. Within this pathway, GI regulates circadian rhythms and flowering and acts earlier in the hierarchy than CO and FT, suggesting that GI might regulate flowering indirectly by affecting the control of circadian rhythms. We studied the relationship between the roles of GI in flowering and the circadian clock using late elongated hypocotyl circadian clock associated1 double mutants, which are impaired in circadian clock function, plants overexpressing GI (35S:GI), and gi mutants. These experiments demonstrated that GI acts between the circadian oscillator and CO to promote flowering by increasing CO and FT mRNA abundance. In addition, circadian rhythms in expression of genes that do not control flowering are altered in 35S:GI and gi mutant plants under continuous light and continuous darkness, and the phase of expression of these genes is changed under diurnal cycles. Therefore, GI plays a general role in controlling circadian rhythms, and this is different from its effect on the amplitude of expression of CO and FT. Functional GI:green fluorescent protein is localized to the nucleus in transgenic Arabidopsis plants, supporting the idea that GI regulates flowering in the nucleus. We propose that the effect of GI on flowering is not an indirect effect of its role in circadian clock regulation, but rather that GI also acts in the nucleus to more directly promote the expression of flowering-time genes. INTRODUCTIONInduction of flowering in response to daylength synchronizes flowering to the changing seasons and is believed to be important in adaptation of plants to growth at different latitudes (Ray and Alexander, 1966). Physiological experiments implicated the circadian clock as the timekeeping mechanism that enables the measurement of daylength Yanovsky and Kay, 2003). Forward genetics in Arabidopsis thaliana identified a genetic pathway that promotes flowering specifically on exposure to long days (LDs) (Searle and Coupland, 2004), and the role of the circadian clock in photoperiodic time measurement was confirmed by demonstrating that transcription of the genes that act in this pathway is circadian clock controlled. Mutations in one of these genes, GIGANTEA (GI), both impair circadian rhythms and delay flowering. Here, we use moleculargenetic approaches to compare the role of GI in the circadian system with its function in controlling flowering. GI, CONSTANS (CO), and FLOWERING LOCUS T (FT)were placed in the Arabidopsis photoperiod pathway based on genetic analysis (Redei, 1962;Koornneef et al., 1991Koornneef et al., , 1998. Loss-offunction mutations in each of these genes delay flowering under LDs but have little or no effect under short days (SDs). Genetic epistasis and analysis of expression of these three genes in mutant and wild-type background...
Daily rhythms of gene expression provide a benefit to most organisms by ensuring that biological processes are activated at the optimal time of day. Although temporal patterns of expression control plant traits of agricultural importance, how natural genetic variation modifies these patterns during the day and how precisely these patterns influence phenotypes is poorly understood. The circadian clock regulates the timing of gene expression, and natural variation in circadian rhythms has been described, but circadian rhythms are measured in artificial continuous conditions that do not reflect the complexity of biologically relevant day/ night cycles. By studying transcriptional rhythms of the eveningexpressed gene GIGANTEA (GI) at high temporal resolution and during day/night cycles, we show that natural variation in the timing of GI expression occurs mostly under long days in 77 Arabidopsis accessions. This variation is explained by natural alleles that alter light sensitivity of GI, specifically in the evening, and that act at least partly independent of circadian rhythms. Natural alleles induce precise changes in the temporal waveform of GI expression, and these changes have detectable effects on PHYTOCHROME INTERACTING FACTOR 4 expression and growth. Our findings provide a paradigm for how natural alleles act within day/night cycles to precisely modify temporal gene expression waveforms and cause phenotypic diversity. Such alleles could confer an advantage by adjusting the activity of temporally regulated processes without severely disrupting the circadian system. diurnal | circadian | rhythms | Arabidopsis | GIGANTEA
Many plants flower in response to seasonal changes in daylength. This response often varies between accessions of a single species. We studied the variation in photoperiod response found in the model species Arabidopsis (Arabidopsis thaliana). Seventy-two accessions were grown under six daylengths varying in 2-h intervals from 6 to 16 h. The typical response was sigmoidal, so that plants flowered early under days longer than 14 h, late under days shorter than 10 h, and at intermediate times under 12-h days. However, many accessions diverged from this pattern and were clustered into groups showing related phenotypes. Thirty-one mutants and transgenic lines were also scored under the same conditions. Statistical comparisons demonstrated that some accessions show stronger responses to different daylengths than are found among the mutants. Genetic analysis of two such accessions demonstrated that different quantitative trait loci conferred an enhanced response to shortening the daylength from 16 to 14 h. Our data illustrate the spectrum of daylength response phenotypes present in accessions of Arabidopsis and demonstrate that similar phenotypic variation in photoperiodic response can be conferred by different combinations of loci.Growth and reproduction of many plant species are regulated by seasonal changes in daylength. Specific traits controlled by daylength include flowering, bud dormancy in trees, and tuberization of potato (Solanum tuberosum; Thomas and Vince-Prue, 1997). Within a species, there is often quantitative variation for the precise length of day that induces a response, and the distribution of accessions that respond to different daylengths suggests that this trait is associated with adaptation to growth at particular latitudes. Examples of such distributions include induction of flowering by daylength in cultivated populations of soybean (Glycine max; Borthwick and Parker, 1939) and natural populations of Xanthium strumarium (Ray and Alexander, 1966) or repression of bud growth in poplar (Populus spp.; Bohlenius et al., 2006). The mechanisms controlling photoperiodic flowering are best understood in Arabidopsis (Arabidopsis thaliana; Kobayashi and Weigel, 2007; Turck et al., 2008), but no comprehensive analysis of quantitative variation in photoperiod response within this species has been reported.Arabidopsis is a quantitative long-day plant that flowers earlier under long days (LDs) of spring and early summer than during short days (SD) of winter. Commonly used laboratory accessions such as Columbia (Col) and Landsberg erecta (Ler) show a marked flowering response to daylength and were used to screen for mutations that impair photoperiodic flowering (Redei, 1962;
The effect of different environmental conditions on flowering time and the number of leaves produced before the first flower is formed has been investigated in Antirrhinum majus L. The effect of light quality has been tested by decreasing the red/far‐red ratio, generally resulting in a reduced flowering time and leaf number. Furthermore, it could be shown that photoperiod, temperature and light intensity are inversely correlated with flowering time and leaf number. However, lowering the temperature from 15 to 12°C resulted in a reduction of flowering time. This observation shows that Antirrhinum can be vernalised. Using defined combinations of the four environmental factors we have been able to reduce flowering time to only 42 days or to delay flowering for at least 2 years. The results obtained allow an optimisation of the screening conditions for identifying flowering time mutants in Antirrhinum.
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