AtAPOSTART1, an Arabidopsis thaliana PH-START domain protein involved in seed germination

The seed in the mature and dry state is metabolically inactive (quiescent) and is thus able to withstand extreme environmental conditions, such as drought and cold. Germination commences when the dry seed, shed from its parent plant, takes up water (imbibition) and ends when the root emerges through the seed coat. During seedling establishment, the reserves stored in the seed are metabolized, whereas the subsequent vegetative and reproductive growth is supported by photosynthesis. Here, we describe the functional characterization of the PH-START protein AtAPO1 (Arabidopsis thaliana APOSTART1), the putative homologue of PpAPO1 (Poa pratensis APOSTART1) in Arabidopsis thaliana. By using translational fusion of the AtAPO1 promoter to the uiaD gene and in situ hybridization analyses, we show that AtAPO1 is expressed in mature embryo sacs and developing embryos. The functional analysis of two at-apostart mutant alleles suggests that AtAPO1 is involved in the control of seed germination.     

Arabidopsis thaliana seed germination and early seedling growth are inhibited by monoethanolamine salts of para-halogenated benzoic acids

Abstract

In spite of the simplicity of its molecules, the complex effects of benzoic acids on the regulation of plant growth are an increasingly attractive field of research to chemists and biologists. Halide substituted benzoic acids offer an excellent opportunity to explore the effect of electron withdrawing substituents (fluoro-, chloro-, bromo- and iodo-) on the response of plant growth stage. Under normal physiological conditions, benzoic acids are ionized molecules that exhibit low solubility in water. Monoethanolamine, a natural alkanolamine, was used to generate salts of monoethanolamine of halogenated para-substituted benzoic acids, new compounds with biological activity. This study reports on the biological effects of these substances at different concentrations on Arabidopsis thaliana seed germination and early seedling growth. Seed germination at 22°C, in a vertical position, under a photoperiod of 16 h light and 8 h darkness, was variable depending on the concentration of the compounds applied. Final germination percentages were similar for all treatments and control at 0.05 mM and 0.1 mM (exception p-Br BA and p-I MEASPBA). No germination occurred when seeds were treated with more than 0.5 mM. The results also revealed that the primary root length and the number of secondary roots are reduced in a concentration-dependent manner and also in relation to increasing atomic size of the substituents (F < Cl < Br < I). It is concluded that uptake rates of benzoic acid anions by roots decrease with a decrease in hydrophilic character of the anion and with an increase in molecular size.     

Arabidopsis AtBECLIN 1/AtAtg6/AtVps30 is essential for pollen germination and plant development

Pollen germination on the surface of compatible stigmatic tissues is an essential step for plant fertilization. Here we report that the Arabidopsis mutant bcll is male sterile as a result of the failure ofpollen germination. We show that the bcll mutant allele cannot be transmitted by male gametophytes and no homozygous bcll mutants were obtained. Analysis of pollen developmental stages indicates that the bcll mutation affects pollen germination but not pollen maturation. Molecular analysis demonstrates that the failure of pollen germination was caused by the disruption of AtBECLIN 1. AtBECLIN 1 is expressed predominantly in mature pollen and encodes a protein with significant homology to Beclin1/Atg6/Vps30 required for the processes of autophagy and vacuolar protein sorting （VPS） in yeast. We also show that AtBECLIN 1 is required for normal plant development, and that genes related to autophagy, VPS and the glycosylphosphatidylinositol anchor system, were affected by the deficiency of AtBECLIN 1.     

VASCULAR PLANT ONE-ZINC FINGER1 and VOZ2 repress the FLOWERING LOCUS C clade members to control flowering time in Arabidopsis

Floral transition is regulated by environmental and endogenous signals. Previously, we identified VASCULAR PLANT ONE-ZINC FINGER1 (VOZ1) and VOZ2 as phytochrome B-interacting factors. VOZ1 and VOZ2 redundantly promote flowering and have pivotal roles in the downregulation of FLOWERING LOCUS C (FLC), a central repressor of flowering in Arabidopsis. Here, we showed that the late-flowering phenotypes of the voz1 voz2 mutant were suppressed by vernalization in the Columbia and FRIGIDA (FRI)-containing accessions, which indicates that the late-flowering phenotype of voz1 voz2 mutants was caused by upregulation of FLC. We also showed that the other FLC clade members, MADS AFFECTING FLOWERING (MAF) genes, were also a downstream target of VOZ1 and VOZ2 as their expression levels were also increased in the voz1 voz2 mutant. Our results suggest that the FLC clade genes integrate signals from VOZ1/VOZ2 and vernalization to regulate flowering.     

Phytochrome A, phytochrome B and other phytochrome(s) regulate ATHB-2 gene expression in etiolated and green Arabidopsis plants

The expression of the Arabidopsis ATHB-2 gene is light-regulated both in seedlings and in adult plants. The gene is expressed at high levels in rapidly elongating etiolated seedlings and is down-regulated by a pulse of red light (R) through the action of a phytochrome other than phytochrome A or B, or by a pulse of far-red light (FR) through the action of phytochrome A. In green plants, the expression of the ATHB-2 gene is rapidly and strongly enhanced by lowering the R:FR ratio perceived by a phytochrome other than A or B. Returning the plant to a high R:FR ratio results in an equally rapid decrease of the ATHB-2 mRNA. Consistently, plants overproducing ATHB-2 show developmental phenotypes characteristic of plants grown in low R:FR: elongated petioles, reduced leaf area, early flowering, and reduced number of rosette leaves. Taken together, the data strongly suggest a direct involvement of ATHB-2 in light-regulated growth phenomena throughout Arabidopsis development.     

Distinct cell-specific expression patterns of early and late gibberellin biosynthetic genes during Arabidopsis seed germination

Gibberellins (GAs) are biosynthesized through a complex pathway that involves several classes of enzymes. To predict sites of individual GA biosynthetic steps, we studied cell type-specific expression of genes encoding early and late GA biosynthetic enzymes in germinating Arabidopsis seeds. We showed that expression of two genes, AtGA3ox1 and AtGA3ox2, encoding GA 3-oxidase, which catalyzes the terminal biosynthetic step, was mainly localized in the cortex and endodermis of embryo axes in germinating seeds. Because another GA biosynthetic gene, AtKO1, coding for ent-kaurene oxidase, exhibited a similar cell-specific expression pattern, we predicted that the synthesis of bioactive GAs from ent-kaurene oxidation occurs in the same cell types during seed germination. We also showed that the cortical cells expand during germination, suggesting a spatial correlation between GA production and response. However, promoter activity of the AtCPS1 gene, responsible for the first committed step in GA biosynthesis, was detected exclusively in the embryo provasculature in germinating seeds. When the AtCPS1 cDNA was expressed only in the cortex and endodermis of non-germinating ga1-3 seeds (deficient in AtCPS1) using the AtGA3ox2 promoter, germination was not as resistant to a GA biosynthesis inhibitor as expression in the provasculature. These results suggest that the biosynthesis of GAs during seed germination takes place in two separate locations with the early step occurring in the provasculature and the later steps in the cortex and endodermis. This implies that intercellular transport of an intermediate of the GA biosynthetic pathway is required to produce bioactive GAs.     

Constitutive expression of CIR1 (RVE2) affects several circadian-regulated processes and seed germination in Arabidopsis

Circadian clocks are endogenous auto-regulatory mechanisms that allow organisms, from bacteria to humans, to advantageously time a wide range of activities within 24 h environmental cycles. Here we report the identification and characterization of an MYB-related gene, designated Circadian 1 (CIR1), that is involved in circadian regulation in Arabidopsis. Expression of CIR1 is transiently induced by light and oscillates with a circadian rhythm. The rhythmic expression of CIR1 is controlled by the central oscillator. Constitutive expression of CIR1 resulted in a shorter period length for the rhythms of four central oscillator components, and much lower amplitude for the rhythms of central oscillator components CCA1 and LHY. Furthermore, CIR1 over-expression severely affected the circadian rhythms of its own RNA and those of the slave oscillator EPR1 and effector genes Lhcb and CAT3. Plants that constitutively expressed CIR1 displayed delayed flowering, longer hypocotyls and reduced seed germination in the dark. These results suggest that CIR1 is possibly part of a regulatory feedback loop that controls a subset of the circadian outputs and modulates the central oscillator.     

光照和温度对滇丁香种子萌发的影响

研究了不同光照和温度条件对滇丁香(Luculia pinciana)种子萌发的影响。结果表明,滇丁香种子是需光种子,有明显的光休眠现象;种子在光下萌发的最适温度范围为20~25℃,8~10d开始萌发,2~3周萌发完全,萌发率可达97%,温度的升高或降低均会降低种子萌发率。在15~30℃,用250mg/L GA₃处理24h能代替光照解除光休眠。     

生长素调控种子的休眠与萌发

植物种子的休眠与萌发,是植物生长发育过程中的关键阶段,也是生命科学领域的研究热点。种子从休眠向萌发的转换是极为复杂的生物学过程,由外界环境因子、体内激素含量及信号传导和若干关键基因协同调控。大量研究表明,植物激素脱落酸(Abscisic acid, ABA)和赤霉素(Gibberellin acid, GA)是调控种子休眠水平,决定种子从休眠转向萌发的主要内源因子。ABA与GA在含量和信号传导两个层次上的精确平衡,确保了植物种子能以休眠状态在逆境中存活,并在适宜的时间启动萌发程序。生长素(Auxin)是经典植物激素之一,其对向性生长和组织分化等生物学过程的调控已有大量研究。但最近有研究证实,生长素对种子休眠有正向调控作用,这表明生长素是继ABA之后的第二个促进种子休眠的植物激素。本文在回顾生长素的发现历程、阐释生长素体内合成途径及信号传导通路的基础上,重点综述了生长素通过与ABA的协同作用调控种子休眠的分子机制,并对未来的研究热点进行了讨论和展望。     

Light activates the degradation of PIL5 protein to promote seed germination through gibberellin in Arabidopsis

Angiosperm seeds integrate various environmental signals, such as water availability and light conditions, to make a proper decision to germinate. Once the optimal conditions are sensed, gibberellin (GA) is synthesized, triggering germination. Among environmental signals, light conditions are perceived by phytochromes. However, it is not well understood how phytochromes regulate GA biosynthesis. Here we investigated whether phytochromes regulate GA biosynthesis through PIL5, a phytochrome-interacting bHLH protein, in Arabidopsis. We found that pil5 seed germination was inhibited by paclobutrazol, the ga1 mutation was epistatic to the pil5 mutation, and the inhibitory effect of PIL5 overexpression on seed germination could be rescued by exogenous GA, collectively indicating that PIL5 regulates seed germination negatively through GA. Expression analysis revealed that PIL5 repressed the expression of GA biosynthetic genes (GA3ox1 and GA3ox2), and activated the expression of a GA catabolic gene (GA2ox) in both PHYA- and PHYB-dependent germination assays. Consistent with these gene-expression patterns, the amount of bioactive GA was higher in the pil5 mutant and lower in the PIL5 overexpression line. Lastly, we showed that red and far-red light signals trigger PIL5 protein degradation through the 26S proteasome, thus releasing the inhibition of bioactive GA biosynthesis by PIL5. Taken together, our data indicate that phytochromes promote seed germination by degrading PIL5, which leads to increased GA biosynthesis and decreased GA degradation.     

拟南芥SDIR1基因转录的选择性剪接

采用PCR及RT-PCR法分别克隆了拟南芥SDIR1基因的DNA和cDNA序列。根据序列比对分析结果,发现了3种不同的转录本,提示SDIR1基因的转录中存在选择性剪接。3种转录本的长度分别为822bp、691bp和666bp,依次命名为：SDIR1-822、SDIR1-691、SDIR1-666。与SDIR1基因的DNA序列及已报道的SDIR1cDNA序列比较,除转录本SDIR1-822包含了完整的编码序列外,其余2种转录本的编码序列都存在不同长度的缺失。其中,SDIR1-691缺失了131bp的片段：第2外显子3′端缺失33bp,第3外显子53bp全部缺失,第4外显子5′端缺失45bp;转录本SDIR1-666缺失了156bp的片段：第3外显子3′端缺失18bp,第4外显子5′端缺失138bp。进而随机挑取101个克隆子对三种转录本的表达比例进行初步分析,结果表明3种分子的比值为SDIR1-822：SDIR1-691：SDIR1-666=26.00：1.33：1.00,反映出SDIR1基因不同转录本在拟南芥中的相对表达量。