Chromatin-based processes are essential for cellular functions. Structural maintenance of chromosomes (SMCs) are evolutionarily conserved molecular machines that organize chromosomes throughout the ...cell cycle, mediate chromosome compaction, promote DNA repair, or control sister chromatid attachment. The SMC5/6 complex is known for its pivotal role during the maintenance of genome stability. However, a dozen recent plant studies expanded the repertoire of SMC5/6 complex functions to the entire plant sexual reproductive phase. The SMC5/6 complex is essential in meiosis, where its activity must be precisely regulated to allow for normal meiocyte development. Initially, it is attenuated by the recombinase RAD51 to allow for efficient strand invasion by the meiosis-specific recombinase DMC1. At later stages, it is essential for the normal ratio of interfering and non-interfering crossovers, detoxifying aberrant joint molecules, preventing chromosome fragmentation, and ensuring normal chromosome/sister chromatid segregation. The latter meiotic defects lead to the production of diploid male gametes in Arabidopsis SMC5/6 complex mutants, increased seed abortion, and production of triploid offspring. The SMC5/6 complex is directly involved in controlling normal embryo and endosperm cell divisions, and pioneer studies show that the SMC5/6 complex is also important for seed development and normal plant growth in cereals.
Epigenetic factors determine responses to internal and external stimuli in eukaryotic organisms. Whether and how environmental conditions feed back to the epigenetic landscape is more a matter of ...suggestion than of substantiation. Plants are suitable organisms with which to address this question due to their sessile lifestyle and diversification of epigenetic regulators. We show that several repetitive elements of Arabidopsis thaliana that are under epigenetic regulation by transcriptional gene silencing at ambient temperatures and upon short term heat exposure become activated by prolonged heat stress. Activation can occur without loss of DNA methylation and with only minor changes to histone modifications but is accompanied by loss of nucleosomes and by heterochromatin decondensation. Whereas decondensation persists, nucleosome loading and transcriptional silencing are restored upon recovery from heat stress but are delayed in mutants with impaired chromatin assembly functions. The results provide evidence that environmental conditions can override epigenetic regulation, at least transiently, which might open a window for more permanent epigenetic changes.
Gene flow between species in the genus Arabidopsis occurs in significant amounts, but how exactly gene flow is achieved is not well understood. Polyploidization may be one avenue to explain gene flow ...between species. One problem, however, with polyploidization as a satisfying explanation is the occurrence of lethal genomic instabilities in neopolyploids as a result of genomic exchange, erratic meiotic behavior, and genomic shock. We have created an autoallohexaploid by pollinating naturally co-occurring diploid Arabidopsis thaliana with allotetraploid Arabidopsis suecica (an allotetraploid composed of A. thaliana and Arabidopsis arenosa). Its triploid offspring underwent spontaneous genome duplication and was used to generate a multigenerational pedigree. Using genome resequencing, we show that 2 major mechanisms promote stable genomic exchange in this population. Legitimate meiotic recombination and chromosome segregation between the autopolyploid chromosomes of the 2 A. thaliana genomes occur without any obvious bias for the parental origin and combine the A. thaliana haplotypes from the A. thaliana parent with the A. thaliana haplotypes from A. suecica similar to purely autopolyploid plants. In addition, we repeatedly observed that occasional exchanges between regions of the homoeologous chromosomes are tolerated. The combination of these mechanisms may result in gene flow leading to stable introgression in natural populations. Unlike the previously reported resynthesized neoallotetraploid A. suecica, this population of autoallohexaploids contains mostly vigorous, and genetically, cytotypically, and phenotypically variable individuals. We propose that naturally formed autoallohexaploid populations might serve as an intermediate bridge between diploid and polyploid species, which can facilitate gene flow rapidly and efficiently.
Herbicide resistance is broadly recognized as the adaptive evolution of weed populations to the intense selection pressure imposed by the herbicide applications. Here, we tested whether ...transcriptional gene silencing (TGS) and RNA-directed DNA Methylation (RdDM) pathways modulate resistance to commonly applied herbicides. Using
wild-type plants exposed to sublethal doses of glyphosate, imazethapyr, and 2,4-D, we found a partial loss of TGS and increased susceptibility to herbicides in six out of 11 tested TGS/RdDM mutants. Mutation in
(
), that plays an important role in DNA demethylation, leading to strongly increased susceptibility to all applied herbicides, and imazethapyr in particular. Transcriptomic analysis of the imazethapyr-treated wild type and
plants revealed a relation of the herbicide upregulated genes to chemical stimulus, secondary metabolism, stress condition, flavonoid biosynthesis, and epigenetic processes. Hypersensitivity to imazethapyr of the flavonoid biosynthesis component
(
) mutant plants strongly suggests that ROS1-dependent accumulation of flavonoids is an important mechanism for herbicide stress response in
. In summary, our study shows that herbicide treatment affects transcriptional gene silencing pathways and that misregulation of these pathways makes Arabidopsis plants more sensitive to herbicide treatment.
The mobilization of transposable elements (TEs) is suppressed by host genome defense mechanisms. Recent studies showed that the cis-regulatory region of Arabidopsis thaliana COPIA78/ONSEN ...retrotransposons contains heat-responsive elements (HREs), which cause their activation during heat stress. However, it remains unknown whether this is a common and potentially conserved trait and how it has evolved.
We show that ONSEN, COPIA37, TERESTRA, and ROMANIAT5 are the major families of heat-responsive TEs in A. lyrata and A. thaliana. Heat-responsiveness of COPIA families is correlated with the presence of putative high affinity heat shock factor binding HREs within their long terminal repeats in seven Brassicaceae species. The strong HRE of ONSEN is conserved over millions of years and has evolved by duplication of a proto-HRE sequence, which was already present early in the evolution of the Brassicaceae. However, HREs of most families are species-specific, and in Boechera stricta, the ONSEN HRE accumulated mutations and lost heat-responsiveness.
Gain of HREs does not always provide an ultimate selective advantage for TEs, but may increase the probability of their long-term survival during the co-evolution of hosts and genomic parasites.
Chromosome organization, dynamics and stability are required for successful passage through cellular generations and transmission of genetic information to offspring. The key components involved are ...Structural maintenance of chromosomes (SMC) complexes. Cohesin complex ensures proper chromatid alignment, condensin complex chromosome condensation and the SMC5/6 complex is specialized in the maintenance of genome stability. Here we summarize recent knowledge on the composition and molecular functions of SMC5/6 complex. SMC5/6 complex was originally identified based on the sensitivity of its mutants to genotoxic stress but there is increasing number of studies demonstrating its roles in the control of DNA replication, sister chromatid resolution and genomic location-dependent promotion or suppression of homologous recombination. Some of these functions appear to be due to a very dynamic interaction with cohesin or other repair complexes. Studies in Arabidopsis indicate that, besides its canonical function in repair of damaged DNA, the SMC5/6 complex plays important roles in regulating plant development, abiotic stress responses, suppression of autoimmune responses and sexual reproduction.
We have used an approximately 8.7-Mb BAC contig of Arabidopsis thaliana Chromosome 4 to trace homeologous chromosome regions in 21 species of the family Brassicaceae. Homeologs of this segment could ...be identified in all tested species. Painting of pachytene chromosomes of Calepina, Conringia, and Sisymbrium species (2n = 14, 16), traditionally placed in tribe Brassiceae, showed one homeologous copy of the Arabidopsis contig, while the remaining taxa of the tribe (2n = 14-30) revealed three, and three Brassica species (2n = 34, 36, and 38) and Erucastrum gallicum (2n = 30) had six copies corresponding to the 8.7-Mb segment. The multiple homeologous copies corresponded structurally to the Arabidopsis segment or were rearranged by inversions and translocations within the diploidized genomes. These chromosome rearrangements accompanied by chromosome fusions/fissions led to the present-day chromosome number variation within the Brassiceae. Phylogenetic relationships based on the chloroplast 5'-trnL (UAA)-trnF(GAA) region and estimated divergence times based on sequence data of the chalcone synthase gene are congruent with comparative painting data and place Calepina, Conringia, and Sisymbrium outside the clade of Brassiceae species with triplicated genomes. Most likely, species containing three or six copy pairs descended from a common hexaploid ancestor with basic genomes similar to that of Arabidopsis. The presumed hexaploidization event occurred after the Arabidopsis-Brassiceae split, between 7.9 and 14.6 Mya.
The Venus flytrap (
Dionaea muscipula
J. Ellis) is a carnivorous plant able to synthesize large amounts of phenolic compounds, such as phenylpropanoids, flavonoids, phenolic acids, and ...1,4-naphtoquinones. In this study, the first genetic transformation of
D. muscipula
tissues is presented. Two wild-type
Rhizobium rhizogenes
strains (LBA 9402 and ATCC 15834) were suitable vector organisms in the transformation process. Transformation led to the formation of teratoma (transformed shoot) cultures with the bacterial
rol
B gene incorporated into the plant genome in a single copy. Using high-pressure liquid chromatography, we demonstrated that transgenic plants were characterized by an increased quantity of phenolic compounds, including 1,4-naphtoquinone derivative, plumbagin (up to 106.63 mg × g
−1
DW), and phenolic acids (including salicylic, caffeic, and ellagic acid), in comparison to non-transformed plants. Moreover,
Rhizobium
-mediated transformation highly increased the bactericidal properties of teratoma-derived extracts. The antibacterial properties of transformed plants were increased up to 33% against
Staphylococcus aureus
,
Enterococcus faecalis
, and
Escherichia coli
and up to 7% against
Pseudomonas aeruginosa
. For the first time, we prove the possibility of
D. muscipula
transformation. Moreover, we propose that transformation may be a valuable tool for enhancing secondary metabolite production in
D. muscipula
tissue and to increase bactericidal properties against human antibiotic-resistant bacteria.
Key points
•
Rhizobium-mediated transformation created Dionaea muscipula teratomas
.
•
Transformed plants had highly increased synthesis of phenolic compounds
.
•
The MBC value was connected with plumbagin and phenolic acid concentrations
.
In plants, transposable elements (TEs) are kept inactive by transcriptional gene silencing (TGS). TGS is established and perpetuated by RNA‐directed DNA methylation (RdDM) and maintenance methylation ...pathways, respectively. Here, we describe a novel RdDM function specific for shoot apical meristems that reinforces silencing of TEs during early vegetative growth. In meristems, RdDM counteracts drug‐induced interference with TGS maintenance and consequently prevents TE activation. Simultaneous disturbance of both TGS pathways leads to transcriptionally active states of repetitive sequences that are inherited by somatic tissues and partially by the progeny. This apical meristem‐specific mechanism is mediated by increased levels of TGS factors and provides a checkpoint for correct epigenetic inheritance during the transition from vegetative to reproductive phase and to the next generation.
Synopsis
Repetitive DNA sequences are prone to be activated in plant embryonic tissues. Reinforced silencing in the shoot apical meristem is shown here to prevent their expression in post‐embryonic tissues.
The DNA methylation inhibitor zebularine induces transcriptional activation of repetitive sequences in embryonic, but not post‐embryonic, plant tissues.
RNA‐dependent DNA methylation (RdDM) is required for re‐silencing of these sequences in the shoot apical meristem (SAM).
Abundance of de novo and maintenance methylation factors in the SAM secure correct transmission of epigenetic states during vegetative growth.
Repetitive DNA sequences are prone to be activated in plant embryonic tissues. Reinforced silencing in the shoot apical meristem is shown here to prevent their expression in post‐embryonic tissues.
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