Plant MITEs: useful tools for plant genetics and genomics

Temporal changes in transcripts of miniature inverted-repeat transposable elements during rice endosperm development

The repression of transcription from transposable elements (TEs) by DNA methylation is necessary to maintain genome integrity and prevent harmful mutations. However, under certain circumstances, TEs may escape from the host defense system and reactivate their transcription. In Arabidopsis (Arabidopsis thaliana) and rice (Oryza sativa), DNA demethylases target the sequences derived from TEs in the central cell, the progenitor cell for the endosperm in the female gametophyte. Genome-wide DNA demethylation is also observed in the endosperm after fertilization. In the present study, we used a custom microarray to survey the transcripts generated from TEs during rice endosperm development and at selected time points in the embryo as a control. The expression patterns of TE transcripts are dynamically up- and downregulated during endosperm development, especially those of miniature inverted-repeat TEs (MITEs). Some TE transcripts were directionally controlled, whereas the other DNA transposons and retrotransposons were not. We also discovered the NUCLEAR FACTOR Y binding motif, CCAAT, in the region near the 5' terminal inverted repeat of Youren, one of the transcribed MITEs in the endosperm. Our results uncover dynamic changes in TE activity during endosperm development in rice.

The transposable element-rich genome of the cereal pest Sitophilus oryzae

BACKGROUND

The rice weevil Sitophilus oryzae is one of the most important agricultural pests, causing extensive damage to cereal in fields and to stored grains. S. oryzae has an intracellular symbiotic relationship (endosymbiosis) with the Gram-negative bacterium Sodalis pierantonius and is a valuable model to decipher host-symbiont molecular interactions.

RESULTS

We sequenced the Sitophilus oryzae genome using a combination of short and long reads to produce the best assembly for a Curculionidae species to date. We show that S. oryzae has undergone successive bursts of transposable element (TE) amplification, representing 72% of the genome. In addition, we show that many TE families are transcriptionally active, and changes in their expression are associated with insect endosymbiotic state. S. oryzae has undergone a high gene expansion rate, when compared to other beetles. Reconstruction of host-symbiont metabolic networks revealed that, despite its recent association with cereal weevils (30 kyear), S. pierantonius relies on the host for several amino acids and nucleotides to survive and to produce vitamins and essential amino acids required for insect development and cuticle biosynthesis.

CONCLUSIONS

Here we present the genome of an agricultural pest beetle, which may act as a foundation for pest control. In addition, S. oryzae may be a useful model for endosymbiosis, and studying TE evolution and regulation, along with the impact of TEs on eukaryotic genomes.

Characterization and chromosomal mapping of the DgmarMITE transposon in populations of Dichotomius (Luederwaldtinia) sericeus species complex (Coleoptera: Scarabaeidae)

Transposable elements are dispersed repetitive DNA sequences that can move within the genome and are related to genome and chromosome evolution, adaptation, and speciation. The aim of this study was to characterize and determine the chromosomal location and accumulation of a Mariner-like element in populations of four phylogenetically related species of the Dichotomius (Luederwaldtinia) sericeus complex. Mapping of the isolated element was performed by fluorescent in situ hybridization in different populations of analyzed species. Characterization of the isolated element revealed a degenerated transposon, named DgmarMITE. This transposon is 496-bp-long, AT rich (57%), and contains 24 bp terminal inverted repeats. In situ mapping revealed presence of this element only in two out of four species analyzed. DgmarMITE sites were located in heterochromatic and euchromatic regions and varied in location and number on the karyotypes of Dichotomius (L.) gilletti and D. (L.) guaribensis across different populations. These results demonstrate differential accumulation of the DgmarMITE in genomes of these species, which is probably due to the occurrence of ectopic recombination and cross-mobilization of the element mediated by the transposase of closely related or unrelated transposable elements.

Genome-Wide Comparative Analysis of Miniature Inverted Repeat Transposable Elements in 19 Arabidopsis thaliana Ecotype Accessions

Miniature inverted repeat transposable elements (MITEs) are prevalent in eukaryotic genomes. They are known to critically influence the process of genome evolution and play a role in gene regulation. As the first study concentrated in the transposition activities of MITEs among different ecotype accessions within a species, we conducted a genome-wide comparative analysis by characterizing and comparing MITEs in 19 Arabidopsis thaliana accessions. A total of 343485 MITE putative sequences, including canonical, diverse and partial ones, were delineated from all 19 accessions. Within the entire population of MITEs sequences, 80.7% of them were previously unclassified MITEs, demonstrating a different genomic distribution and functionality compared to the classified MITEs. The interactions between MITEs and homologous genes across 19 accessions provided a fine source for analyzing MITE transposition activities and their impacts on genome evolution. Moreover, a significant proportion of MITEs were found located in the last exon of genes besides the ordinary intron locality, thus potentially modifying the end of genes. Finally, analysis of the impact of MITEs on gene expression suggests that migrations of MITEs have no detectable effect on the expression level for host genes across accessions.

Programmed translational bypassing elements in mitochondria: structure, mobility, and evolutionary origin

Programmed translational bypassing enables ribosomes to 'ignore' a precise mRNA interval of several dozen nucleotides. Well-characterized bypassed sequences include hop and byp elements, present in bacteriophage T4 and mitochondria of the yeast Magnusiomyces capitatus, respectively. The bypassing mechanism of byps is probably similar to that of hop, yet the former appears more effective and less constrained as to sequence context. Furthermore, both elements are mobile but hop moves as part of a cassette including a homing endonuclease, whereas byps seem to spread like miniature DNA transposable elements known as GC clusters. Here, we argue that hop and byps arose independently by convergent evolution, and that byps evolved in magnusiomycete mitochondria due to (as yet unknown) alterations of the mitochondrial translation machinery.

Functional analysis of the HS185 regulatory element in the rice HSP70 promoter

A series of HSP70 promoter deletion constructs was established. Analysis of beta-glucuronidase activities from the promoter deletion constructs in transient expression assays identified a cis-element, located from -493 to -308 bp upstream of the ATG start site. This element was designated as HS185 and has a crucial role in HSP70 promoter activity. HS185 has some characteristics of a miniature inverted-repeat transposable element (MITE), such as terminal inverted repeats (TIRs) (GGTCCCACA) and a putative target site duplication. There are 362 copies of homologous sequences of HS185 in the rice genome, which are preferentially distributed to non-coding regions. Based on these sequence features, we propose that HS185 is an uncharacterized rice MITE, possibly derived from the rice transposon Mutator-like element VIII family. Further transient expression assays showed that HS185 inhibited the enhancer activity of the cauliflower mosaic virus 35S promoter. These results demonstrate that not only is HS185 necessary for HSP70 promoter activity, but it also has a functional role as an insulator. This study explored new regulatory functions of non-coding repeat sequences in rice.

Identification of a novel gene (Hsdr4) involved in water-stress tolerance in wild barley

Drought is one of the most severe stresses limiting plant growth and yield. Genes involved in water stress tolerance of wild barley (Hordeum spontaneoum), the progenitor of cultivated barley, were investigated using genotypes contrasting in their response to water stress. Gene expression profiles of water-stress tolerant vs. water-stress sensitive wild barley genotypes, under severe dehydration stress applied at the seedling stage, were compared using cDNA-AFLP analysis. Of the 1100 transcript-derived fragments (TDFs) amplified about 70 displayed differential expression between control and stress conditions. Eleven of them showed clear difference (up- or down-regulation) between tolerant and susceptible genotypes. These TDFs were isolated, sequenced and tested by RT-PCR. The differential expression of seven TDFs was confirmed by RT-PCR, and TDF-4 was selected as a promising candidate gene for water-stress tolerance. The corresponding gene, designated Hsdr4 (Hordeum spontaneum dehydration-responsive), was sequenced and the transcribed and flanking regions were determined. The deduced amino acid sequence has similarity to the rice Rho-GTPase-activating protein-like with a Sec14 p-like lipid-binding domain. Analysis of Hsdr4 promoter region that was isolated by screening a barley BAC library, revealed a new putative miniature inverted-repeat transposable element (MITE), and several potential stress-related binding sites for transcription factors (MYC, MYB, LTRE, and GT-1), suggesting a role of the Hsdr4 gene in plant tolerance to dehydration stress. Furthermore, the Hsdr4 gene was mapped using wild barley mapping population to the long arm of chromosome 3H between markers EBmac541 and EBmag705, within a region that previously was shown to affect osmotic adaptation in barley.