Comparative genetics of flowering time

Analysis of genes controlling flowering time (heading date) contributes to our understanding of fundamental principles of plant development and is of practical importance because of the effects of flowering time on plant adaptation and crop yield. This review discusses the extent to which plants may share common genetic mechanisms for the control of flowering time and the implications of such conservation for gene isolation from the major cereal crops. Gene isolation may exploit the small genome of rice in map-based approaches, utilizing the conservation of gene order that is revealed when common DNA markers are mapped in different species. Alternatively, mechanisms may be conserved within plants as a whole, in which case genes cloned from the model dicot Arabidopsis thaliana provide an alternative route.     

Reduced peroxisomal import triggers peroxisomal retrograde signaling

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植物开花时间的遗传调控通路研究进展

开花时间对植物的繁殖成功至关重要。广泛分布的物种经常发生开花时间的分化, 从而能够更好地适应不同的环境条件。为了探索植物开花行为发生适应性分化的分子机制, 首先要明确调控开花行为的遗传通路。本文梳理了植物各类群调控开花时间的遗传通路, 以期为开花时间适应性分化的分子机制研究提供依据。 植物从营养生长向繁殖转变时, 其开花行为主要受到光照、温度、水分等外界环境因子和赤霉素等内在因素的影响。通过对模式植物拟南芥(Arabidopsis thaliana)和其他类群的研究, 总结出了调控植物开花时间的6条通路, 包括日照长度和光质影响开花的光依赖通路, 长时间冷暴露后促进植物开花的春化通路, 高温或低温环境影响开花的温度通路, 以及赤霉素通路、年龄通路和自主通路3条内部调节过程。植物开花时间调控的6条上游通路信号传递到下游的开花整合基因FT(FLOWERING LOCUS T)和SOC1(SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1), 整合基因将这些复杂的调节因子整合后进一步传递到下游花分生组织, 从而启动开花。此外, 非编码RNA、转座子对开花时间的调控也具有重要作用。部分遗传通路被证实在植物适应环境的过程中起到了重要作用。目前对植物开花调控的研究已经有一百多年历史, 理论相对成熟。然而, 仍然存在许多具有争议和未解决的问题, 如开花基因的表达方式、开花行为的特殊调控机制、开花时间变异的适应性意义等等, 需要更进一步的研究。     

Ambient temperature signaling in plants: An emerging field in the regulation of flowering time

Plants show remarkable developmental plasticity to survive in a continually changing environment. One example is their capability to adjust flowering time in response to environmental changes. Ambient growth temperature, which is strongly affected by global temperature changes, has a profound effect on flowering time. However, those effects have been largely ignored in research. Recent molecular genetic studies ofArabidopsis as a model system have implicated several genes, and have identified a molecular mechanism underlying the responses of plants to changes in ambient temperature. Here, we describe recent discoveries related to ambient temperature signaling and the control of flowering time inArabidopsis. We also discuss current perspectives on how plants sense and respond to such changes.     

Salicylic acid regulates flowering time and links defence responses and reproductive development

Flowering relies on signaling networks that integrate endogenous and external cues. Normally, plants flower at a particular season, reflecting day length and/or temperature cues. However, plants can surpass this seasonal regulation and show precocious flowering under stress environmental conditions. Here, we show that UV-C light stress activates the transition to flowering in Arabidopsis thaliana through salicylic acid (SA). Moreover, SA also regulates flowering time in non-stressed plants, as SA-deficient plants are late flowering. The regulation of flowering time by SA seems to involve the photoperiod and autonomous pathways, but it does not require the function of the flowering time genes CONSTANS (CO), FCA, or FLOWERING LOCUS C (FLC).     

Effect of planting date on flowering time in wheat

The time from seed germination to anthesis varied for spring wheat in experiments in climate chambers with plants grown hydroponically at different nitrogen regimes. Time to anthesis was related to the time of seed germination during the calendar year. Seed germinating earlier in the calendar year required a shorter time to anthesis compared to seed germinating later in the year, a pattern found for all the spring wheat cultivars investigated. Time to anthesis was also found to be independent of factors such as year in which the seed was produced, nitrogen regime used, or year or site of cultivation. We suggest the existence of an annual rhythm for flowering in spring wheat. This variation in time to flowering can be due either to external factors or more likely to an endogenous rhythm in the plant. When investigating plant processes, it is of importance to be aware of such a rhythm, since it may influence the results depending on when during the year the experiments are performed.     

Metabolic profiling of the methylerythritol phosphate pathway reveals the source of post‐illumination isoprene burst from leaves

The methylerythritol phosphate (MEP) pathway in plants produces the prenyl precursors for all plastidic isoprenoids, including carotenoids and quinones. The MEP pathway is also responsible for synthesis of approximately 600 Tg of isoprene per year, the largest non‐methane hydrocarbon flux into the atmosphere. There have been few studies of the regulation of the MEP pathway in plants under physiological conditions. In this study, we combined gas exchange techniques and high‐performance liquid chromatography–tandem mass spectrometry (HPLC‐MS‐MS) and measured the profile of MEP pathway metabolites under different conditions. We report that in the MEP pathway, metabolites immediately preceding steps requiring reducing power were in high concentration. Inhibition of the MEP pathway by fosmidomycin caused deoxyxylulose phosphate accumulation in leaves as expected. Evidence is presented that accumulation of MEP pathway intermediates, primarily methylerythritol cyclodiphosphate, is responsible for the post‐illumination isoprene burst phenomenon. Pools of intermediate metabolites stayed at approximately the same level 10 min after light was turned off, but declined eventually under prolonged darkness. In contrast, a strong inhibition of the second‐to‐last step of the MEP pathway caused suppression of isoprene emission in pure N₂. Our study suggests that reducing equivalents may be a key regulator of the MEP pathway and therefore isoprene emission from leaves.     

Divergent selection on flowering time contributes to local adaptation in Mimulus guttatus populations

The timing of when to initiate reproduction is an important transition in any organism's life cycle. There is much variation in flowering time among populations, but we do not know to what degree this variation contributes to local adaptation. Here we use a reciprocal transplant experiment to examine the presence of divergent natural selection for flowering time and local adaptation between two distinct populations of Mimulus guttatus. We plant both parents and hybrids (to tease apart differences in suites of associated parental traits) between these two populations into each of the two native environments and measure floral, vegetative, life-history, and fitness characters to assess which traits are under selection at each site. Analysis of fitness components indicates that each of these plant populations is locally adapted. We obtain striking evidence for divergent natural selection on date of first flower production at these two sites. Early flowering is favored at the montane site, which is inhabited by annual plants and characterized by dry soils in midsummer, whereas intermediate (though later) flowering dates are selectively favored at the temperate coastal site, which is inhabited by perennial plants and is almost continually moist. Divergent selection on flowering time contributes to local adaptation between these two populations of M. guttatus, suggesting that genetic differentiation in the timing of reproduction may also serve as a partial reproductive isolating barrier to gene flow among populations.     

The rhythmic expression of genes controlling flowering time in southern and northern populations of invasive Ambrosia artemisiifolia

Aims Flowering time has been suggested to be an important adaptive trait during the dispersal of invasive species, and identifying the molecular mechanisms underlying flowering time may provide insight into the local adaptation during the process of invasion. Here, we conducted a preliminary exploration on the genetic basis of the differentiation of flowering time in Ambrosia artemisiifolia .Methods Using relative real-time fluorescent quantitative polymerase chain reaction, we investigated the expression levels of eight flowering-related genes, including AP1, FT, SOC1, CRY2, FKF1, GI, CO2 and SPY, in leaves and flowers at different time points in individuals from northern Beijing and southern Wuhan populations that exhibit significant differences in flowering times to identify any rhythmic changes in gene expression and their association with differential flowering times.Important findings The differentiation of flowering time in the A. artemisiifolia populations was closely associated with five genes involved in flowering pathways. The floral pathway integrators FT and SOC1 and floral meristem identity gene AP1 exhibited increased expression during flowering. The photoreceptor CRY2 in the light-dependent pathway and the SPY gene in the gibberellin pathway displayed specific expression patterns over time. In earlier-flowering Beijing plants, CRY2 expression was lower and SPY expression was higher than in Wuhan plants. The expression patterns of these five genes suggest a molecular basis for the differentiation of flowering time in A. artemisiifolia .     

The enzymatic activity of Arabidopsis protein arginine methyltransferase 10 is essential for flowering time regulation

Arabidopsis AtPRMT10 is a plant-specific type I protein arginine methyltransferase that can asymmetrically dimethylate arginine 3 of histone H4 with auto-methylation activity. Mutations of AtPRMT10 derepress FLOWERING LOCUS C (FLC) expression resulting in a late-flowering phenotype. Here, to further investigate the biochemical characteristics of AtPRMT10, we analyzed a series of mutated forms of the AtPRMT10 protein. We demonstrate that the conserved “VLD” residues and “double-E loop” are essential for enzymatic activity of AtPRMT10. In addition, we show that Arg54 and Cys259 of AtPRMT10, two residues unreported in animals, are also important for its enzymatic activity. We find that Arg13 of AtPRMT10 is the auto-methylation site. However, substitution of Arg13 to Lys13 does not affect its enzymatic activity. In vivo complementation assays reveal that plants expressing AtPRMT10 with VLD-AAA, E143Q or E152Q mutations retain high levels of FLC expression and fail to rescue the late-flowering phenotype of atprmt10 plants. Taken together, we conclude that the methyltransferase activity of AtPRMT10 is essential for repressing FLC expression and promoting flowering in Arabidopsis.     

Accelerated flowering time reduces lifetime water use without penalizing reproductive performance in Arabidopsis

Natural selection driven by water availability has resulted in considerable variation for traits associated with drought tolerance and leaf‐level water‐use efficiency (WUE). In Arabidopsis, little is known about the variation of whole‐plant water use (PWU) and whole‐plant WUE (transpiration efficiency). To investigate the genetic basis of PWU, we developed a novel proxy trait by combining flowering time and rosette water use to estimate lifetime PWU. We validated its usefulness for large‐scale screening of mapping populations in a subset of ecotypes. This parameter subsequently facilitated the screening of water use and drought tolerance traits in a recombinant inbred line population derived from two Arabidopsis accessions with distinct water‐use strategies, namely, C24 (low PWU) and Col‐0 (high PWU). Subsequent quantitative trait loci mapping and validation through near‐isogenic lines identified two causal quantitative trait loci, which showed that a combination of weak and nonfunctional alleles of the FRIGIDA (FRI) and FLOWERING LOCUS C (FLC) genes substantially reduced plant water use due to their control of flowering time. Crucially, we observed that reducing flowering time and consequently water use did not penalize reproductive performance, as such water productivity (seed produced per unit of water transpired) improved. Natural polymorphisms of FRI and FLC have previously been elucidated as key determinants of natural variation in intrinsic WUE (δ¹³C). However, in the genetic backgrounds tested here, drought tolerance traits, stomatal conductance, δ¹³C. and rosette water use were independent of allelic variation at FRI and FLC, suggesting that flowering is critical in determining lifetime PWU but not always leaf‐level traits.     

A physiological overview of the genetics of flowering time control

Physiological studies on flowering time control have shown that plants integrate several environmental signals. Predictable factors, such as day length and vernalization, are regarded as 'primary', but clearly interfere with, or can even be substituted by, less predictable factors. All plant parts participate in the sensing of these interacting factors. In the case of floral induction by photoperiod, long-distance signalling is known to occur between the leaves and the shoot apical meristem (SAM) via the phloem. In the long-day plant, Sinapis alba, this long-distance signalling has also been shown to involve the root system and to include sucrose, nitrate, glutamine and cytokinins, but not gibberellins. In Arabidopsis thaliana, a number of genetic pathways controlling flowering time have been identified. Models now extend beyond 'primary' controlling factors and show an ever-increasing number of cross-talks between pathways triggered or influenced by various environmental factors and hormones (mainly gibberellins). Most of the genes involved are preferentially expressed in meristems (the SAM and the root tip), but, surprisingly, only a few are expressed preferentially or exclusively in leaves. However, long-distance signalling from leaves to SAM has been shown to occur in Arabidopsis during the induction of flowering by long days. In this review, we propose a model integrating physiological data and genes activated by the photoperiodic pathway controlling flowering time in early-flowering accessions of Arabidopsis. This model involves metabolites, hormones and gene products interacting as long- or short-distance signalling molecules.