MSI1 belongs to a family of histone binding WD40-repeat proteins. Arabidopsis thaliana contains five genes encoding MSI1-like proteins, but their functions in diverse chromatin-associated complexes are poorly understood. Here, we show that MSI1 is part of a histone deacetylase complex. We copurified HISTONE DEACETYLASE19 (HDA19) with MSI1 and transcriptional regulatory SIN3-like proteins and provide evidence that MSI1 and HDA19 associate into the same complex in vivo. These data suggest that MSI1, HDA19, and HISTONE DEACETYLATION COMPLEX1 protein form a core complex that can integrate various SIN3-like proteins. We found that reduction of MSI1 or HDA19 causes upregulation of abscisic acid (ABA) receptor genes and hypersensitivity of ABA-responsive genes. The MSI1-HDA19 complex fine-tunes ABA signaling by binding to the chromatin of ABA receptor genes and by maintaining low levels of acetylation of histone H3 at lysine 9, thereby affecting the expression levels of ABA receptor genes. Reduced MSI1 or HDA19 levels led to increased tolerance to salt stress corresponding to the increased ABA sensitivity of gene expression. Together, our results reveal the presence of an MSI1-HDA19 complex that fine-tunes ABA signaling in Arabidopsis.

Background Stable gene repression is essential for normal growth and development. Polycomb repressive complexes 1 and 2 (PRC1&2) are involved in this process by establishing monoubiquitination of histone 2A (H2Aub1) and subsequent trimethylation of lysine 27 of histone 3 (H3K27me3). Previous work proposed that H2Aub1 removal by the ubiquitin-specific proteases 12 and 13 (UBP12 and UBP13) is part of the repressive PRC1&2 system, but its functional role remains elusive. Results We show that UBP12 and UBP13 work together with PRC1, PRC2, and EMF1 to repress genes involved in stimulus response. We find that PRC1-mediated H2Aub1 is associated with gene responsiveness, and its repressive function requires PRC2 recruitment. We further show that the requirement of PRC1 for PRC2 recruitment depends on the initial expression status of genes. Lastly, we demonstrate that removal of H2Aub1 by UBP12/13 prevents loss of H3K27me3, consistent with our finding that the H3K27me3 demethylase REF6 is positively associated with H2Aub1. Conclusions Our data allow us to propose a model in which deposition of H2Aub1 permits genes to switch between repression and activation by H3K27me3 deposition and removal. Removal of H2Aub1 by UBP12/13 is required to achieve stable PRC2-mediated repression.

Agrobacterium tumefaciens, a pathogenic bacterium capable of transforming plants through horizontal gene transfer, is nowadays the preferred vector for plant genetic engineering. The vehicle for transfer is the T-strand, a single-stranded DNA molecule bound by the bacterial protein VirD2, which guides T-DNA into the plants nucleus where it integrates. How VirD2 is removed from T-DNA, and which mechanism acts to attach the liberated end to the plant genome is currently unknown. Here, using newly developed technology that yields hundreds of T-DNA integrations in somatic tissue of Arabidopsis thaliana, we uncover two redundant mechanisms for the genomic capture of the T-DNA's 5' end. Different from capture of the 3' end of the T-DNA, which is the exclusive action of polymerase theta-mediated end joining (TMEJ), 5' attachment is accomplished either by TMEJ or by canonical non-homologous end joining (cNHEJ). We further nd that TMEJ needs MRE11, whereas cNHEJ requires TDP2 to remove the 5'-end blocking protein VirD2. As a consequence, T-DNA integration is severely impaired in plants de cient for both MRE11 and TDP2 (or other cNHEJ factors). In support of MRE11 and cNHEJ speci cally acting on the 5' end, we demonstrate rescue of the integration defect of double-de cient plants by using T-DNAs that are capable of forming telomeres upon 3' capture. Our study provides a mechanistic model for how Agrobacterium exploits the plant's own DNA repair machineries to transform them. Main TextAgrobacterium tumefaciens-mediated transformation (AMT) is the most widely used method for generating transgenic plants. In nature, this soil bacterium transforms dicotyledonous plants by translocating part of its DNA, the transferred (T)-DNA, into plant cells, where it integrates into the plant's genome 1 . Subsequent expression of the Agrobacterium genes causes crown gall disease. Within Agrobacterium the T-DNA is located on a Tumor-inducing (Ti) plasmid anked by a repeated sequence of 25 bp, the Left and Right Border repeat (LB and RB). These sequences are recognition sites for the virulence proteins VirD1 and VirD2, which generate ssDNA breaks required to liberate T-DNA as a singlestranded DNA molecule, the T-strand 2 . The VirD2 protein remains covalently bound to the 5' end of the Tstrand 3,4 and pilots it into the plant cell through the type 4 secretion system 5 that is created by the Agrobacterium virulence program upon detection of wounded plant cells 6 . The T-DNA is subsequently imported into the nucleus 7 where it integrates at a random position in the genome 8 . The molecular mechanism by which the T-DNA is integrated into the plant genome has remained enigmatic until recently when it was found for Arabidopsis thaliana that this process critically depends on polymerase theta (Pol θ) 9 , a host protein that acts in the repair of DNA double-strand breaks (DSBs) via end joining. Abundant genetic and biochemical research performed over the last few years has established that Pol θ facilitates repair of DSBs in a multitude of species ...

North European invasion by common ragweed is associated with earlier flowering, dominant changes in FT/TFL1 expression, and reduced seed production.

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Agrobacterium tumefaciens, a pathogenic bacterium capable of transforming plants through horizontal gene transfer, is nowadays the preferred vector for plant genetic engineering. The vehicle for transfer is the T-strand, a single-stranded DNA molecule bound by the bacterial protein VirD2, which guides T-DNA into the plants nucleus where it integrates. How VirD2 is removed from T-DNA, and which mechanism acts to attach the liberated end to the plant genome is currently unknown. Here, using newly developed technology that yields hundreds of T-DNA integrations in somatic tissue of Arabidopsis thaliana, we uncover two redundant mechanisms for the genomic capture of the T-DNA’s 5’ end. Different from capture of the 3’ end of the T-DNA, which is the exclusive action of polymerase theta-mediated end joining (TMEJ), 5’ attachment is accomplished either by TMEJ or by canonical non-homologous end joining (cNHEJ). We further find that TMEJ needs MRE11, whereas cNHEJ requires TDP2 to remove the 5’-end blocking protein VirD2. As a consequence, T-DNA integration is severely impaired in plants deficient for both MRE11 and TDP2 (or other cNHEJ factors). In support of MRE11 and cNHEJ specifically acting on the 5’ end, we demonstrate rescue of the integration defect of double-deficient plants by using T-DNAs that are capable of forming telomeres upon 3’ capture. Our study provides a mechanistic model for how Agrobacterium exploits the plant’s own DNA repair machineries to transform them.

No abstract