高等植物开花时程的基因调控（Ⅰ）

高等植物从营养生长向生殖生长及发育转变的时程具有重要意义,但是了解得很少。近6年来利用分子遗传学方法详细地分析了拟南芥中的这一转变的时程变化,为高等植物开花时程的基因调控提供了一个很好的模式。有关早期或晚期开花表现型的大量突变体及遗传变异得到了阐述。这里谈到的表现型对影响开花转变的环境及内部因子的控制有重大作用。通过分子生物学、遗传学和生理学分析已经鉴定了参与此过程的不同组分,如光识别和昼夜节律（circadian rhythm）因子。另外,通过克隆某些花诱导基因及其相应的靶基因已经对参与开花信号转导途径（signal transduction pathway）的相关因子进行了系统的鉴定,这些开创性工作大大促进了高等植物开花时程的基因表达调控研究及其机理的阐明。本实验室在以黄瓜、新红宝西瓜、西葫芦为材料所获得的部分结果基础上,主要以近六年来在拟南芥方面获得的进展为依据,对高等植物开花时程的基因调控作一系统的总结,并对其开花时程基因调控的机理提出可能的作用理论模型。     

拟南芥开花时间调控的分子基础

在合适的时间开花对大多数植物的生存和成功繁衍极为重要。开花时间受错综复杂的环境因素和植物自身的遗传因子影响,由开花调控因子所构成的光周期、春化、温度、赤霉素、自主以及年龄等至少6条既相互独立又相互联系的遗传途径调控。该文综述了有关拟南芥(Arabidopsis thaliana)开花时间调控的分子机制的最新研究进展,并对今后的研究进行了展望。     

Floral transition mutants in Arabidopsis

An inventory of genetic differences in flowering time in Arabidopsis is presented and discussed. Many genes influence the transition to flowering in a quantitative way. Two groups of mutants and natural variants can be distinguished: those that are responsive to environmental factors and those that are less responsive or unresponsive. It is possible that all late/early-flowering mutants isolated to date carry a mutation with an effect, either promotive or repressive, on a floral repressor. The interaction between light perception and flowering has been studied by analysis of phytochrome- and cryptochrome-deficient mutants, which showed that phyA and probably also cryptochrome have a promotive role in flowering, whereas phyB and other stable phytochromes have an inhibitory role. A circadian rhythm is important in establishing daylength sensitivity, as was shown by the phenotype of the elf 3 mutants.     

Integration of photoperiod and cold temperature signals into flowering genetic pathways in Arabidopsis

Appropriate timing of flowering is critical for propagation and reproductive success in plants. Therefore, flowering time is coordinately regulated by endogenous developmental programs and external signals, such as changes in photoperiod and temperature. Flowering is delayed by a transient shift to cold temperatures that frequently occurs during early spring in the temperate zones. It is known that the delayed flowering by short-term cold stress is mediated primarily by the floral repressor FLOWERING LOCUS C (FLC). However, how the FLC-mediated cold signals are integrated into flowering genetic pathways is not fully understood. We have recently reported that the INDUCER OF CBF EXPRESSION 1 (ICE1), which is a master regulator of cold responses, FLC, and the floral integrator SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) constitute an elaborated feedforward-feedback loop that integrates photoperiod and cold temperature signals to regulate seasonal flowering in Arabidopsis. Cold temperatures promote the binding of ICE1 to FLC promoter to induce its expression, resulting in delayed flowering. However, under floral inductive conditions, SOC1 induces flowering by blocking the ICE1 activity. We propose that the ICE1-FLC-SOC1 signaling network fine-tunes the timing of photoperiodic flowering during changing seasons.     

Using flowering times and leaf numbers to model the phases of photoperiod sensitivity in Antirrhinum majus L

A model has been developed that can be used to determine the phases of sensitivity to photoperiod for seedlings subjected to reciprocal transfers at regular intervals between long (LD) and short day (SD) conditions. The novel feature of this approach is that it enables the simultaneous analysis of the time to flower and number of leaves below the inflorescence. A range of antirrhinum cultivars were grown, all of which were shown to be quantitative long-day plants. Seedlings were effectively insensitive to photoperiod when very young (juvenile). However, after the end of the juvenile phase, SD delayed flowering and increased the number of leaves below the inflorescence. Plants transferred from LD to SD showed a sudden hastening of flowering and a decrease in leaf number once sufficient LD had been received for flower commitment. Photoperiod had little effect on the rate of flower development. The analysis clearly identified major cultivar differences in the length of the juvenile phase and the photoperiod-sensitive inductive phase in both LD and SD.     

Flowering and morphogenic responses of new Aster hybrids to photoperiod

Photoperiod treatments of 13, 14.5, 16 and 17.5 h were used to determine the photoperiodic response of the interspecific Aster hybrids 'Painted Lady', 'Snowflake' and 'Blue Butterfly' belonging to 'Butterfly' series, under glasshouse conditions. Rate of flowering was higher under 13-h photoperiods decreasing up to 16-h photoperiods. The rate of flowering for 13- and 17.5-h photoperiods was nearly similar but under the longest photoperiod flowering was erratic and sometimes abortion of the apical bud was observed. Pholoperiod affected the morphology of the plant. Increasing photo-periods up to 16 h induced an increase of internode length of the main axis, of total length of lateral shoots, the number of ray florets. In a 13-h photoperiod the plants produced a paniculate-racemose shaped inflorescence while in longer photoperiods the inflorescence was paniculate-corymbose shaped.     

Effects of temperature and photoperiod on flowering in Hyptis brevipes

Few tropical species have been tested for their flowering response under controlled conditions. Hyptis brevipes Poit, is an annual herb, commonly found in wet margins of streams and ponds, being considered a weed for some perennial plantations in Brazil. Under experimental glasshouse conditions, this species proved to be an obligate short-day plant. Flowering was delayed when photoperiods longer than 8 h were given, the critical photoperiod being between 12 and 13 h. When both temperature and photoperiod were controlled, at 20°C a longer protoperiod (by almost 1 h) is still inductive compared to 25 and 30°C. The number of short-day cycles required for full induction is relatively high and dependent upon temperature; at 20°C or above, 10 cycles are adequate, but at 15°C, more short-day cycles are needed. The number of inflorescences formed as well as the floral index vary according to daylength × temperature × inductive cycle number, allowing flowering to be assessed quantitatively. Long days are inhibitory to flowering, either suppressing it completely (when symmetrically intercalated among 24 inductive cycles) or preventing the floral index from increasing.     

SHB1 plays dual roles in photoperiodic and autonomous flowering

Flowering was initiated by the integration of environmental signals such as day-length with the internal development status in Arabidopsis, a facultative long-day plant. The photoperiodic flowering involves two key components, CONSTANS and FT, whereas the autonomous flowering is operated through a central quantitative floral repressor, FLC, and several other genes that act upstream of FLC. SOC1 acts downstream to integrate the flowering signals from the two pathways. Here, we report that SHB1 plays dual roles in both photoperiodic and autonomous flowering. shb1-D, a gain-of-function mutant, flowered early and shb1, a loss-of-function allele, flowered late under both long days and short days. The shb1-D mutation activated the expression of CO, FT, and SOC1 under both long and short days, and however, the co-2 mutation attenuated the shb1-D activated expression of FT and SOC1 only under long days but not short days. The shb1-D or shb1 mutations also reduced and increased, respectively, the expression of FLC under both long and short days. Transgenic remedy of FLC to wide-type level in shb1-D background also reverted shb1-D flowering and FT or SOC1 expression to wild type mostly under short days. Furthermore, the shb1-D suppression on FLC expression is likely operated through LD as ld-3 blocked this suppression and SHB1 appears to act upstream of LD. In summary, SHB1 represents signaling steps that regulate CO expression in leaves and LD or FLC expression in either leaves or shoot apical meristem, contributing to a threshold expression of SOC1 in shoot apical meristem for floral initiation.     

Timing of Photoperiodic Flowering:Light Perception and Circadian Clock

Flowering symbolizes the transition of a plant from vegetative phase to reproductive phase and is controlled by fairly complex and highly coordinated regulatory pathways. Over the last decade, genetic studies in Arabidopsis have aided the discovery of many signaling components involved in these pathways. In this review, we discuss how the timing of flowering is regulated by photoperiod and the involvement of light perception and the circadian clock in this process. The specific regulatory mechanisms on CONSTANS expression and CONSTANS stability by the circadian clock and photoreceptors are described in detail. In addition, the roles of CONSTANS, FLOWERING LOCUS T, and several other light signaling and circadiandependent components in photoperiodic flowering are also highlighted.     

Rapid reset of the corticosterone rhythm by food presentation in rats under acircadian restricted feeding schedule

Corticosterone levels were determined in the 7-week-old male rat maintained under different feeding and lighting schedules. At 4 weeks of age, the animals were kept either under a natural photoperiod (LD) or were subjected to continuous illumination (LL). Access to food was either ad libitum or restricted to an 8 hr span per 24 hr (circadian) or 32 hr (acircadian).

The food signal seemed able to synchronize the corticosterone rhythm to its own circadian periodicity, irrespective of the lighting regimen. No synchronization was observed in serially sampled LL or LD rats under an acircadian feeding schedule. Instead, the group acrophase appeared 24 hr subsequent to food presentation. Regarding individual patterns, many rats showed an acrophase or a peak also at that time. We speculate that an endogenous circadian mechanism was reset by the food signal, whenever it appeared.     

Florigen goes molecular: Seventy years of the hormonal theory of flowering regulation

As early as in 1936, the comprehensive studies of flowering led M.Kh. Chailakhyan to the concept of florigen, a hormonal floral stimulus, and let him establish several characteristics of this stimulus. These studies set up for many years the main avenues for research into the processes that control plant flowering, and the notion of florigen became universally accepted by scientists worldwide. The present-day evidence of genetic control of plant flowering supports the idea that florigen participates in floral signal transduction. The recent study of arabidopsis plants led the authors to conclusion that the immediate products of the gene FLOWERING LOCUS I, its mRNA and/or protein, move from an induced leaf into the shoot apex and evoke flowering therein.     

Timing of Photoperiodic Flowering：Light Perception and Circadian Clock

Flowering symbolizes the transition of s plant from vegetative phase to reproductive phase and is controlled by fairly complex and highly coordinated regulatory pathways. Over the last decade, genetic studies in Arabidopsis have aided the discovery of many signaling components involved in these pathways. In this review, we discuss how the timing of flowering is regulated by photoperiod and the involvement of light perception and the circadian clock in this process. The specific regulatory mechanisms on CONSTANS expression and CONSTANS stability by the circadian clock and photoreceptors are described in detail. In addition, the roles of CONSTANS, FLOWERING LOCUS T, and several other light signaling and circadian-dependent components in photoperiodic flowering are also highlighted.     

Photorefractoriness and Its Termination in the Subtropical House Sparrow, Passer Domesticus: Involvement of Circadian Rhythm

Groups of photorefractory female subtropical house sparrows, Passer domestkus, when treated with 6 weeks of a short photocycle (8L : 16D) showed significant ovarian growth on their return to a long photocycle (15L :9D). A 6-hr photophase coupled with scotophase of varying durations does not terminate the refractory period under photoperiod cycles of 12 (6L : 6D), 36 (6L :30D) and 60 (6L : S4D) hr but the refractory period is terminated by light-dark cycles of 24 (6L: 18D), 48 (6L :42D) and 72 (6L : 66D) hr. These results are consistent with the Biinning hypothesis of coincidence between endogenous photosensitive rhythmicity and environmental photoperiod timing that an endogenous circadian rhythm is involved in the maintenance and termination of photorefractoriness.