An RNA polymerase II- and AGO4-associated protein acts in RNA-directed DNA methylation. [鍏ㄦ枃]

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DNA Methylation Mediated by a MicroRNA Pathway [鍏ㄦ枃] [鎰熻阿绾㈠啗闀垮緛璁哄潧鐨刴hl]

In plants, the known microRNAs (miRNAs) are produced as 锝�21 nucleotide (nt) duplexes from their precursors by Dicer-like 1 (DCL1). They are incorporated into Argonaute 1 (AGO1) protein to regulate target gene expression primarily through mRNA cleavage. We report here the discovery of a class of miRNAs in the model monocot rice (Oryza sativa). These are 24 nt in length and require another member of the Dicer family, DCL3, for their biogenesis. The 24 nt long miRNAs (lmiRNAs) are loaded into AGO4 clade proteins according to hierarchical rules, depending on the upstream biogenesis machinery and the 5鈥�-terminal nucleotide. We demonstrated that lmiRNAs direct DNA methylation at loci from which they are produced as well as in trans at their target genes and play roles in gene regulation. Considered together, our findings define a miRNA pathway that mediates DNA methylation.

Haploid plants produced by centromere-mediated genome elimination 鍏ㄦ枃涓嬭浇

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锘縋roduction of haploid plants that inherit chromosomes from only 聽one parent can greatly accelerate plant breeding. Haploids 聽generated from a heterozygous individual and converted to diploid create instant homozygous lines, bypassing generations of inbreeding. Two methods are generally used to produce haploids. First, 聽cultured gametophyte cells may be regenerated into haploid plants , 聽but many species and genotypes are recalcitrant to this process. 聽Second, haploids can be induced from rare interspecific crosses, in 聽which one parental genome is eliminated after fertilization.The 聽molecular basis for genome elimination is not understood, but one theory posits that centromeres from the two parent species interact 聽unequally with the mitotic spindle, causing selective chromosome 聽loss. Here we show that haploid Arabidopsis thaliana plants can 聽be easily generated through seeds by manipulating a single centromere protein, the centromere-specific histone CENH3 (called 聽CENP-A in human). When cenh3 null mutants expressing altered 聽CENH3 proteins are crossed to wild type, chromosomes from the 聽mutant are eliminated, producing haploid progeny. Haploids are 聽spontaneously converted into fertile diploids through meiotic nonreduction, allowing their genotype to be perpetuated. Maternal and 聽paternal haploids can be generated through reciprocal crosses. We 聽have also exploited centromere-mediated genome elimination to 聽convert a natural tetraploid Arabidopsis into a diploid, reducing 聽its ploidy to simplify breeding. As CENH3 is universal in eukaryotes, 聽our method may be extended to produce haploids in any plant 聽species.

Arabidopsis thaliana life without phytochromes

(===璁哄潧閾炬帴===)

涓�绡囧緢鏈夋剰鎬濈殑鏂囩珷锛屾鐗╂病鏈変簡phytochromes浼氭�庢牱锛熻兘涓嶈兘娲伙紵绛旀鏄紝it depends锛侊紒

Plants use light as a source of energy for photosynthesis and as a source of environmental information perceived by photoreceptors. Testing whether plants can complete their cycle if light provides energy but no information about the environment requires a plant devoid of phytochromes because all photosynthetically active wavelengths activate phytochromes. Producing such a quintuple mutant of Arabidopsis thaliana has been challenging, but we were able to obtain it in the flowering locus T (ft) mutant background. The quintuple phytochrome mutant does not germinate in the FT background, but it germinates to some extent in the ft background. If germination problems are bypassed by the addition of gibberellins, the seedlings of the quintuple phytochrome mutant exposed to red light produce chlorophyll, indicating that phytochromes are not the sole red-light photoreceptors, but they become developmentally arrested shortly after the cotyledon stage. Blue light bypasses this blockage, rejecting the long-standing idea that the blue-light receptors cryptochromes cannot operate without phytochromes. After growth under white light, returning the quintuple phytochrome mutant to red light resulted in rapid senescence of already expanded leaves and severely impaired expansion of new leaves. We conclude that Arabidopsis development is stalled at several points in the presence of light suitable for photosynthesis but providing no photomorphogenic signal.

Transcriptional control of gene expression by microRNAs

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MicroRNAs (miRNAs) control gene expression in animals and plants. Like another class of small RNAs, siRNAs, they affect gene expression posttranscriptionally. While siRNAs in addition act in transcriptional gene silencing, a role of miRNAs in transcriptional regulation has been less clear. We show here that in moss Physcomitrella patens mutants without a DICER-LIKE1b gene, maturation of miRNAs is normal but cleavage of target RNAs is abolished and levels of these transcripts are drastically reduced. These mutants accumulate miRNA:target-RNA duplexes and show hypermethylation of the genes encoding target RNAs, leading to gene silencing. This pathway occurs also in the wild-type upon hormone treatment. We propose that initiation of epigenetic silencing by DNA methylation depends on the ratio of the miRNA and its target RNA.

Control of Arabidopsis apical鈥揵asal embryo polarity by antagonistic transcription factors

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apical fate, and show they are sufficient to drive the conversion of the embryonic root pole into a second shoot pole. Furthermore, genetic and misexpression studies show an antagonistic relationship between the PLT and HD-ZIP III genes in specifying the root and shoot poles.

Synthetic design of strong promoters

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H2A.Z-Containing Nucleosomes Mediate the Thermosensory Response in Arabidopsis

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Plants are highly sensitive to temperature and can perceive a difference of as little as 1掳C. How temperature is sensed and integrated in development is unknown. In a forward genetic screen in Arabidopsis, we have found that nucleosomes containing the alternative histone H2A.Z are essential to perceiving ambient temperature correctly. Genotypes deficient in incorporating H2A.Z into nucleosomes phenocopy warm grown plants, and show a striking constitutive warm temperature transcriptome. We show that nucleosomes containing H2A.Z display distinct responses to temperature in vivo, independently of transcription. Using purified nucleosomes, we are able to show that H2A.Z confers distinct DNA-unwrapping properties on nucleosomes, indicating a direct mechanism for the perception of temperature through DNA-nucleosome fluctuations. Our results show that H2A.Z-containing nucleosomes provide thermosensory information that is used to coordinate the ambient temperature transcriptome. We observe the same effect in budding yeast, indicating that this is an evolutionarily conserved mechanism.

Sugar signals and molecular networks controlling plant growth

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The Rate and Molecular Spectrum of Spontaneous Mutations in Arabidopsis thaliana

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瀹屾暣Abstract瑙佷笅锛�

To take complete advantage of information on within-species polymorphism and divergence from close relatives, one needs to know the rate and the molecular spectrum of spontaneous mutations. To this end, we have searched for de novo spontaneous mutations in the complete nuclear
genomes of five Arabidopsis thaliana mutation accumulation lines that had been maintained by single-seed descent for 30 generations. We identified and validated 99 base substitutions and 17 small and large insertions and deletions. Our results imply a spontaneous mutation rate of 7 脳 10鈭�9 base substitutions per site per generation, the majority of which are G:C鈫扐:T
transitions. We explain this very biased spectrum of base substitution mutations as a result of two main processes: deamination of methylated cytosines and ultraviolet light鈥搃nduced mutagenesis.