Molecular epigenetic mechanisms for the memory of temperature stresses in plants

The sessile plants encounter various stresses; some are prolonged, whereas some others are recurrent. Temperature is crucial for plant growth and development, and plants often encounter adverse high temperature fluctuations (heat stresses) as well as prolonged cold exposure such as seasonal temperature drops in winter when grown in temperate regions. Many plants can remember past temperature stresses to get adapted to adverse local temperature changes to ensure survival and/or reproductive success. Here, we summarize chromatin-based mechanisms underlying acquired thermotolerance or thermomemory in plants and review recent progresses on molecular epigenetic understanding of ‘remembering of prolonged cold in winter’ or vernalization, a process critical for various over-wintering plants to acquire competence to flower in the coming spring. In addition, perspectives on future study in temperature stress memories of economically-important crops are discussed.     

Mutations causing defects in the biosynthesis and response to gibberellins, abscisic acid and phytochrome B do not inhibit vernalization in Arabidopsis fca-1

 The roles of gibberellins, abscisic acid and phytochrome B in the vernalization response were investigated by combining mutations causing defects in their biosynthesis and response with the Arabidopsis thaliana (L.) Heynh. fca-1 mutation. The fca-1 mutation confers a very late-flowering phenotype which can be reversed to wild-type flowering if the seedlings are vernalized. Vernalization was unaffected in ga1-3, gai, abi1-1, abi2-1, abi3-1 and phyB-1 backgrounds, suggesting that gibberellin action mediated via GA1 and GAI, abscisic acid action mediated through ABI1 and ABI2, and phytochrome B, function independently of vernalization. However, the mutations did interact with fca-1 to change flowering time in the absence of vernalization. The abi1 fca-1 and abi2 fca-1 double mutants flowered earlier than fca-1 implying a role for abscisic acid in floral repression. Combination of ga1-3 or gai with fca-1 unexpectedly resulted in opposite interactions, with gai partially suppressing the late flowering of fca-1. Received: 17 July 1999 / Accepted: 11 October 1999     

FLC基因表达在植物春化过程中的作用

在对以往有关不同开花途径研究简要总结的基础上综述了FLC基因在春化过程中的作用。近期以拟南芥不同生态型和突变体为模式的研究结果表明基因FLC可能是春化反应的关键基因。研究发现 ,FLC的表达水平与植株低温处理的时间呈数量关系 ,低温处理时间越长 ,FLC的表达越弱 ,去甲基化也可能对FLC起负调控的作用。同时FLC也存在于自主开花途径中 ,与其他基因共同作用以调节植株开花时间。而FLC的表达对开花起抑制作用。一系列研究表明 ,春化的低温作用可能在于相关基因的去甲基化 ,消除了FLC对开花的抑制作用 ,从而解除赤霉素合成途径的封锁最终导致植株在一定时期开花。     

Changes of Soluble Protein Components in Winter Wheat (Triticum aestivum L.) Shoots During Vernalization and its Relation to Morphogenesis

Winter wheat “Nong Da 139” and spring wheat “Zhong 8022” were used for this experiment. The effect of low temperature treatment at different periods on protein content and composition of the seedling shoots as well as subsequent development was studied. The main results are as follows: 1. More than 40 days of vernalizing treatment were required for a rapid and uniform earing by winter wheat “Nong Da 139”. After 14–21 days vernalization, the winter wheat is possible to ear in summer, but in an irregular manner. The results indicate that during the whole vernalization process, the effect of low temperature on earing development varies with the duration of treatment. At the earlier stage, it is likely that the Iow temperature induces a change in physiological process, and in the later stage, it only accelerates the development. It is there- fore, suggested that there are two distinct processes existing during vernalization. The transformation from the former to the latter state appears to occur at the middle period of vernalization process. 2. The protein metabolic inhibitors, such as ethionine and p-fluorophenylalanine, interfere with the vernalizing process of winter wheat also at its middle period. 3. With low temperature treatment for different periods, the soluble protein content and composition are found to be changed in winter wheat shoots. At the middle-stage of vernalization (after 14 days low temperature treatment), not only is the content of protein increased twice as compared with the control, but new proteins (on electrophorestic gel) are also produced. On the contrary, there was no difference in protein bands for spring wheat. Spring wheat not to be treated by low temperature, has already possessed the proteins that appeared in vernalized winter wheat shoots. These results demonstrated that the days of 14–21 are the critical time for the vernalization of the winter wheat. The new proteins synthesized at this stage might be the factor of the determination whether or not the plants will transform from vegetative stage into reproductive phase.     

Characterization of Aquilegia Polycomb Repressive Complex 2 homologs reveals absence of imprinting

Epigenetic regulation is important for maintaining gene expression patterns in multicellular organisms. The Polycomb Group (PcG) proteins form several complexes with important and deeply conserved epigenetic functions in both the plant and animal kingdoms. The plant Polycomb Repressive Complex 2 (PRC2) contains four core proteins, Enhancer of Zeste (E(z)), Suppressor of Zeste 12 (Su(z)12), Extra Sex Combs (ESC), and Multicopy Suppressor of IRA 1 (MSI1), and functions in many developmental transitions. In some plant species, including rice and Arabidopsis, duplications in the core PRC2 proteins allow the formation of PRC2s with distinct developmental functions. In addition, members of the plant specific VEL PHD family have been shown to associate with the PRC2 complex in Arabidopsis and may play a role in targeting the PRC2 to specific loci. Here we examine the evolution and expression of the PRC2 and VEL PHD families in Aquilegia, a member of the lower eudicot order Ranunculales and an emerging model for the investigation of plant ecology, evolution and developmental genetics. We find that Aquilegia has a relatively simple PRC2 with only one homolog of Su(z)12, ESC and MSI1 and two ancient copies of E(z), AqSWN and AqCLF. Aquilegia has four members of the VEL PHD family, three of which appear to be closely related to Arabidopsis proteins known to associate with the PRC2. The PRC2 and VEL PHD family proteins are expressed at a relatively constant level throughout Aquilegia vulgaris development, with the VEL PHD family and MSI1 expressed at higher levels during and after vernalization and in the inflorescence. Both AqSWN and AqCLF are expressed in Aquilegia endosperm but neither copy is imprinted.     

Genetic analysis of <Emphasis Type="Italic">Vrn-B1</Emphasis> for vernalization requirement by using linked dCAPS markers in bread wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.)

To identify a molecular marker closely linked to Vrn-B1, the Vrn-1 ortholog on chromosome 5B, sequence polymorphism at four orthologous RFLP loci closely linked to the Vrn-1 gene family was analyzed by using near-isogenic lines of ”Triple Dirk.” At Xwg644, a RFLP locus, three types of nucleotide sequence differing by the number of (TG) repeats, two or three times, and base changes were detected. A (TG)₃-type sequence proved to be specific to chromosome 5B by nulli-tetrasomic analysis, and substitution of single nucleotide (C/T) was detected between TD(B) carrying the former Vrn2 allele and TD(C) carrying the vrn2 allele. A mismatch primer was designed for dCAPS analysis of this single nucleotide polymorphism (SNP). Polymorphism was successfully detected between two NILs, through nested PCR by using a (TG)₃-specific primer (1st) and a dCAPS primer (2nd) followed by a NsiI digest. The analysis of a BF₂ population [(TD(B)//TD(C)] revealed the close linkage (1.7 cM) between WG644–5B and Vrn2. It was therefore concluded that the former Vrn2 locus is located on chromosome 5B and equivalent to Vrn-B1. Received: 3 May 2001 / Accepted: 19 July 2001     

Phenotypic plasticity in Cardamine flexuosa: variation among populations in plastic response to chilling treatments and photoperiods

Variability in the plastic responses of seven life history traits to different chilling and photoperiod regimes was studied in four wild populations of Cardamine flexuosa. This species, a winter-green or year-long annual, showed a facultative long-day and chilling requirement for flowering. Considerable variation among populations was noted in plasticity of all traits. Differences in plasticity were greater among three paddy field populations from different climatic areas than between adjacent populations under different disturbance regimes. A paddy field population (OP) and an adjacent orchard population (OG) exhibited similar plasticity, in both amounts and patterns of response. TP, a Japan Sea coast population, was distinct from three other populations, especially in the small amounts of plasticity. Differences in amount of response were much more common than differences in pattern of response. Character expressions of five traits were significantly correlated with the number of days to flowering. Days to flowering and the numbers of inflorescences and siliques showed high negative correlations because the branching ability of meristems decreased with delay of flowering.     

The regulatory role of vernalization in the expression of low-temperature-induced genes in wheat and rye

Low temperature is one of the primary stresses limiting the growth and productivity of wheat (Triticum aestivum L.) and rye (Secale cereale L.). Winter cereals low-temperature-acclimate when exposed to temperatures colder than 10°C. However, they gradually lose their ability to tolerate below-freezing temperatures when they are maintained for long periods of time in the optimum range for low-temperature acclimation. The overwinter decline in low-temperature response has been attributed to an inability of cereals to maintain low-temperature-tolerance genes in an up-regulated state once vernalization saturation has been achieved. In the present study, the low-temperature-induced Wcs120 gene family was used to investigate the relationship between low-temperature gene expression and vernalization response at the molecular level in wheat and rye. The level and duration of gene expression determined the degree of low-temperature tolerance, and the vernalization genes were identified as the key factor responsible for the duration of expression of low-temperature-induced genes. Spring-habit cultivars that did not have a vernalization response were unable to maintain low-temperature-induced genes in an up-regulated condition when exposed to 4°C. Consequently, they were unable to achieve the same levels of low-temperature tolerance as winter-habit cultivars. A close association between the point of vernalization saturation and the start of a decline in the Wcs120 gene-family mRNA level and protein accumulation in plants maintained at 4°C indicated that vernalization genes have a regulatory influence over low-temperature gene expression in winter cereals.