Changes in DNA methylation and transgenerational mobilization of a transposable element (mPing) by the topoisomerase II inhibitor, etoposide, in rice

Exploring the Effect of High-Energy Heavy Ion Beam on Rice Genome: Transposon Activation

High-energy heavy ion beams are a new type of physical mutagen that can produce a wide range of phenotypic variations. In order to understand the mechanism of high-energy heavy ion beams, we resequenced the whole genome of individual plants with obvious phenotypic variations in rice. The sequence alignment results revealed a large number of SNPs and InDels, as well as genetic variations related to grain type and heading date. The distribution of SNP and InDel on chromosomes is random, but they often occur in the up/downstream regions and the intergenic region. Mutagenesis can cause changes in transposons such as Dasheng, mPing, Osr13 and RIRE2, affecting the stability of the genome. This study obtained the major gene mutation types, discovered differentially active transposons, screened out gene variants related to phenotype, and explored the mechanism of high-energy heavy ion beam radiation on rice genes.

Oak stands along an elevation gradient have different molecular strategies for regulating bud phenology

BACKGROUND

Global warming raises serious concerns about the persistence of species and populations locally adapted to their environment, simply because of the shift it produces in their adaptive landscape. For instance, the phenological cycle of tree species may be strongly affected by higher winter temperatures and late frost in spring. Given the variety of ecosystem services they provide, the question of forest tree adaptation has received increasing attention in the scientific community and catalyzed research efforts in ecology, evolutionary biology and functional genomics to study their adaptive capacity to respond to such perturbations.

RESULTS

In the present study, we used an elevation gradient in the Pyrenees Mountains to explore the gene expression network underlying dormancy regulation in natural populations of sessile oak stands sampled along an elevation cline and potentially adapted to different climatic conditions mainly driven by temperature. By performing analyses of gene expression in terminal buds we identified genes displaying significant dormancy, elevation or dormancy-by-elevation interaction effects. Our Results highlighted that low- and high-altitude populations have evolved different molecular strategies for minimizing late frost damage and maximizing the growth period, thereby increasing potentially their respective fitness in these contrasting environmental conditions. More particularly, population from high elevation overexpressed genes involved in the inhibition of cell elongation and delaying flowering time while genes involved in cell division and flowering, enabling buds to flush earlier were identified in population from low elevation.

CONCLUSION

Our study made it possible to identify key dormancy-by-elevation responsive genes revealing that the stands analyzed in this study have evolved distinct molecular strategies to adapt their bud phenology in response to temperature.

Roles of Topoisomerases in Heterochromatin, Aging, and Diseases

Heterochromatin is a transcriptionally repressive chromatin architecture that has a low abundance of genes but an enrichment of transposons. Defects in heterochromatin can cause the de-repression of genes and transposons, leading to deleterious physiological changes such as aging, cancer, and neurological disorders. While the roles of topoisomerases in many DNA-based processes have been investigated and reviewed, their roles in heterochromatin formation and function are only beginning to be understood. In this review, we discuss recent findings on how topoisomerases can promote heterochromatin organization and impact the transcription of genes and transposons. We will focus on two topoisomerases: Top2α, which catenates and decatenates double-stranded DNA, and Top3β, which can change the topology of not only DNA, but also RNA. Both enzymes are required for normal heterochromatin formation and function, as the inactivation of either protein by genetic mutations or chemical inhibitors can result in defective heterochromatin formation and the de-silencing of transposons. These defects may contribute to the shortened lifespan and neurological disorders observed in individuals carrying mutations of Top3β. We propose that topological stress may be generated in both DNA and RNA during heterochromatin formation and function, which depend on multiple topoisomerases to resolve.

Early embryogenesis-specific expression of the rice transposon Ping enhances amplification of the MITE mPing

Miniature inverted-repeat transposable elements (MITEs) are numerically predominant transposable elements in the rice genome, and their activities have influenced the evolution of genes. Very little is known about how MITEs can rapidly amplify to thousands in the genome. The rice MITE mPing is quiescent in most cultivars under natural growth conditions, although it is activated by various stresses, such as tissue culture, gamma-ray irradiation, and high hydrostatic pressure. Exceptionally in the temperate japonica rice strain EG4 (cultivar Gimbozu), mPing has reached over 1000 copies in the genome, and is amplifying owing to its active transposition even under natural growth conditions. Being the only active MITE, mPing in EG4 is an appropriate material to study how MITEs amplify in the genome. Here, we provide important findings regarding the transposition and amplification of mPing in EG4. Transposon display of mPing using various tissues of a single EG4 plant revealed that most de novo mPing insertions arise in embryogenesis during the period from 3 to 5 days after pollination (DAP), and a large majority of these insertions are transmissible to the next generation. Locus-specific PCR showed that mPing excisions and insertions arose at the same time (3 to 5 DAP). Moreover, expression analysis and in situ hybridization analysis revealed that Ping, an autonomous partner for mPing, was markedly up-regulated in the 3 DAP embryo of EG4, whereas such up-regulation of Ping was not observed in the mPing-inactive cultivar Nipponbare. These results demonstrate that the early embryogenesis-specific expression of Ping is responsible for the successful amplification of mPing in EG4. This study helps not only to elucidate the whole mechanism of mPing amplification but also to further understand the contribution of MITEs to genome evolution.

Transposable elements are major components of eukaryotic genomes, comprising a large portion of the genome in some species. Miniature inverted-repeat transposable elements (MITEs), which belong to the class II DNA transposable elements, are abundant in gene-rich regions, and their copy numbers are very high; therefore, they have been considered to contribute to genome evolution. Because MITEs are short and have no coding capacity, they cannot transpose their positions without the aid of transposase, provided in trans by their autonomous element(s). It has been unknown how MITEs amplify themselves to high copy numbers in the genome. Our results demonstrate that the rice active MITE mPing is mobilized in the embryo by the developmental stage-specific up-regulation of an autonomous element, Ping, and thereby successfully amplifies itself to a high copy number in the genome. The short-term expression of Ping is thought to be a strategy of the mPing family for amplifying mPing by escaping the silencing mechanism of the host genome.

Epigenetic control of mobile DNA as an interface between experience and genome change

Mobile DNA in the genome is subject to RNA-targeted epigenetic control. This control regulates the activity of transposons, retrotransposons and genomic proviruses. Many different life history experiences alter the activities of mobile DNA and the expression of genetic loci regulated by nearby insertions. The same experiences induce alterations in epigenetic formatting and lead to trans-generational modifications of genome expression and stability. These observations lead to the hypothesis that epigenetic formatting directed by non-coding RNA provides a molecular interface between life history events and genome alteration.

Transgenerational changes in plant physiology and in transposon expression in response to UV-C stress in Arabidopsis thaliana

Stress has a negative impact on crop yield by altering a gain in biomass and affecting seed set. Recent reports suggest that exposure to stress also influences the response of the progeny. In this paper, we analyzed seed size, leaf size, bolting time and transposon expression in 2 consecutive generations of Arabidopsis thaliana plants exposed to moderate UV-C stress. Since previous reports suggested a potential role of Dicer-like (DCL) proteins in the establishment of transgenerational response, we used dcl2, dcl3 and dcl4 mutants in parallel with wild-type plants. These studies revealed that leaf number decreased in the progeny of UV-C stressed plants, and bolting occurred later. Transposons were also re-activated in the progeny of stressed plants. Changes in the dcl mutants were less prominent than in wild-type plants. DCL2 and DCL3 appeared to be more important in the transgenerational stress memory than DCL4 because transgenerational changes were less profound in the dcl2 and dcl3 mutants.