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Rob Martienssen

Publications and source records attributed to Rob Martienssen.

12 recordsLinked to original sources
PubMed2006

ramosa2 encodes a LATERAL ORGAN BOUNDARY domain protein that determines the fate of stem cells in branch meristems of maize.

Genetic control of grass inflorescence architecture is critical given that cereal seeds provide most of the world's food. Seeds are borne on axillary branches, which arise from groups of stem cells in axils of leaves and whose branching patterns dictate most of the variation in plant form. Normal maize (Zea mays) ears are unbranched, and tassels have long branches only at their base. The ramosa2 (ra2) mutant of maize has increased branching with short branches replaced by long, indeterminate ones. ra2 was cloned by chromosome walking and shown to encode a LATERAL ORGAN BOUNDARY domain transcription factor. ra2 is transiently expressed in a group of cells that predicts the position of axillary meristem formation in inflorescences. Expression in different mutant backgrounds places ra2 upstream of other genes that regulate branch formation. The early expression of ra2 suggests that it functions in the patterning of stem cells in axillary meristems. Alignment of ra2-like sequences reveals a grass-specific domain in the C terminus that is not found in Arabidopsis thaliana. The ra2-dm allele suggests this domain is required for transcriptional activation of ra1. The ra2 expression pattern is conserved in rice (Oryza sativa), barley (Hordeum vulgare), sorghum (Sorghum bicolor), and maize, suggesting that ra2 is critical for shaping the initial steps of grass inflorescence architecture.

Amino Acid Sequence↗
PubMed2005

It's a small RNA world, after all.

Small RNAs (sRNAs) can regulate transcript and protein abundance. Previously, they have been identified using traditional cloning approaches, which has limited how many could be characterized. Now, the Meyers and Green laboratories have used massively parallel signature sequencing technology to find over 1.5 million sRNAs in Arabidopsis thaliana. These new sRNAs reveal a greater-than-expected potential role for sRNAs in gene regulation, preferential expression or usage of sRNAs in flowers, and the prospect of targeted sRNA-mediated regulation of pseudogenes. In addition, new plant microRNAs have been identified, some of which may be unique to Arabidopsis.

Arabidopsis↗
PubMed2005

Two novel proteins, dos1 and dos2, interact with rik1 to regulate heterochromatic RNA interference and histone modification.

BACKGROUND: Chromosomal behavior during mitosis and meiosis depends in part on heterochromatic modifications such as histone H3 lysine-9 methylation (H3K9me). In fission yeast, the Heterochromatin Protein 1 homolog Swi6 recognizes H3K9me, silences transcription, and retains cohesin at pericentromeric repeats. Heterochromatin formation also depends on processing of transcripts derived from centromeric repeats by the RNAi machinery. The DDB1 homolog, Rik1, and histone methyltransferase, Clr4, act in a complex to promote H3K9me. However, the mechanism underlying this interaction is poorly understood. RESULTS: Using a cytological screen, we have identified two novel genes, dos1(+) and dos2(+), which are required for localization of Swi6. Deletion of either of these genes results in mitotic and meiotic chromosome missegregation, defects in mitotic centromeric cohesion and meiotic telomere clustering, and loss of heterochromatic silencing. Dos1 is predominantly located in the nucleus in a Dos2-dependent manner and directly interacts with Rik1. Each of these genes is required for the association of H3K9me with centromeric repeats, as well as for the production of small interfering RNAs. CONCLUSIONS: Dos1 and Dos2 are required for the formation of heterochromatin in fission yeast. We hypothesize that the physical interaction between Dos1 and Rik1 represents a role in regulating activity of the Rik1/Clr4 complex. Dos2 contributes to this role by regulating Dos1 localization. Our findings suggest a mechanism for recruitment of Clr4 in the RNAi-dependent heterochromatin pathway, in which Dos1 and Dos2 are essential.

Carrier Proteins↗
PubMed2005

Replication, repair, and reactivation.

In a recent issue of Current Biology, Kapoor et al. (2005) and Elmayan et al. (2005) illuminate the linkage between DNA replication and repair and transcriptional gene silencing in plants by showing that mutants in RPA2, a homolog of yeast and mammalian replication protein A, exhibit loss of silencing at transgene loci as well as some transposable elements. This is accompanied by a shift in histone H3 methylation modifications at these loci from a heterochromatic to a euchromatic pattern. Intriguingly, cytosine methylation is unaffected at the reactivated loci, indicating that transmission of DNA methylation and histone modification status can be uncoupled.

Animals↗
PubMed2004

The role of RNA interference in heterochromatic silencing.

Soon after its discovery 75 years ago, heterochromatin, a dense chromosomal material, was found to silence genes. But its importance in regulating gene expression was controversial. Long thought to be inert, heterochromatin is now known to give rise to small RNAs, which, by means of RNA interference, direct the modification of proteins and DNA in heterochromatic repeats and transposable elements. Heterochromatin has thus emerged as a key factor in epigenetic regulation of gene expression, chromosome behaviour and evolution.

Animals↗
PubMed2004

Role of transposable elements in heterochromatin and epigenetic control.

Heterochromatin has been defined as deeply staining chromosomal material that remains condensed in interphase, whereas euchromatin undergoes de-condensation. Heterochromatin is found near centromeres and telomeres, but interstitial sites of heterochromatin (knobs) are common in plant genomes and were first described in maize. These regions are repetitive and late-replicating. In Drosophila, heterochromatin influences gene expression, a heterochromatin phenomenon called position effect variegation. Similarities between position effect variegation in Drosophila and gene silencing in maize mediated by "controlling elements" (that is, transposable elements) led in part to the proposal that heterochromatin is composed of transposable elements, and that such elements scattered throughout the genome might regulate development. Using microarray analysis, we show that heterochromatin in Arabidopsis is determined by transposable elements and related tandem repeats, under the control of the chromatin remodelling ATPase DDM1 (Decrease in DNA Methylation 1). Small interfering RNAs (siRNAs) correspond to these sequences, suggesting a role in guiding DDM1. We also show that transposable elements can regulate genes epigenetically, but only when inserted within or very close to them. This probably accounts for the regulation by DDM1 and the DNA methyltransferase MET1 of the euchromatic, imprinted gene FWA, as its promoter is provided by transposable-element-derived tandem repeats that are associated with siRNAs.

Arabidopsis↗
PubMed2003

Distinct mechanisms determine transposon inheritance and methylation via small interfering RNA and histone modification.

Heritable, but reversible, changes in transposable element activity were first observed in maize by Barbara McClintock in the 1950s. More recently, transposon silencing has been associated with DNA methylation, histone H3 lysine-9 methylation (H3mK9), and RNA interference (RNAi). Using a genetic approach, we have investigated the role of these modifications in the epigenetic regulation and inheritance of six Arabidopsis transposons. Silencing of most of the transposons is relieved in DNA methyltransferase (met1), chromatin remodeling ATPase (ddm1), and histone modification (sil1) mutants. In contrast, only a small subset of the transposons require the H3mK9 methyltransferase KRYPTONITE, the RNAi gene ARGONAUTE1, and the CXG methyltransferase CHROMOMETHYLASE3. In crosses to wild-type plants, epigenetic inheritance of active transposons varied from mutant to mutant, indicating these genes differ in their ability to silence transposons. According to their pattern of transposon regulation, the mutants can be divided into two groups, which suggests that there are distinct, but interacting, complexes or pathways involved in transposon silencing. Furthermore, different transposons tend to be susceptible to different forms of epigenetic regulation.

Adenosine Triphosphatases↗
PubMed2003

Maize-targeted mutagenesis: A knockout resource for maize.

We describe an efficient system for site-selected transposon mutagenesis in maize. A total of 43,776 F1 plants were generated by using Robertson's Mutator (Mu) pollen parents and self-pollinated to establish a library of transposon-mutagenized seed. The frequency of new seed mutants was between 10-4 and 10-5 per F1 plant. As a service to the maize community, maize-targeted mutagenesis selects insertions in genes of interest from this library by using the PCR. Pedigree, knockout, sequence, phenotype, and other information is stored in a powerful interactive database (maize-targeted mutagenesis database) that enables analysis of the entire population and the handling of knockout requests. By inhibiting Mu activity in most F1 plants, we sought to reduce somatic insertions that may cause false positives selected from pooled tissue. By monitoring the remaining Mu activity in the F2, however, we demonstrate that seed phenotypes depend on it, and false positives occur in lines that appear to lack it. We conclude that more than half of all mutations arising in this population are suppressed on losing Mu activity. These results have implications for epigenetic models of inbreeding and for functional genomics.

Base Sequence↗
PubMed2002

SCHIZORIZA controls an asymmetric cell division and restricts epidermal identity in the Arabidopsis root.

The primary root of Arabidopsis has a simple cellular organisation. The fixed radial cell pattern results from stereotypical cell divisions that occur in the meristem. Here we describe the characterisation of schizoriza (scz), a mutant with defective radial patterning. In scz mutants, the subepidermal layer (ground tissue) develops root hairs. Root hairs normally only form on epidermal cells of wild-type plants. Moreover, extra periclinal divisions (new wall parallel to surface of the root) occur in the scz root resulting in the formation of supernumerary layers in the ground tissue. Both scarecrow (scr) and short root (shr) suppress the extra periclinal divisions characteristic of scz mutant roots. This results in the formation of a single layered ground tissue in the double mutants. Cells of this layer develop root hairs, indicating that mis-specification of the ground tissue in scz mutants is uncoupled to the cell division defect. This suggests that during the development of the ground tissue SCZ has two distinct roles: (1) it acts as a suppressor of epidermal fate in the ground tissue, and (2) it is required to repress periclinal divisions in the meristem. It may act in the same pathway as SCR and SHR.

Arabidopsis↗