SummaryTransposable elements (TEs) are considered to be important components of the maintenance and diversification of genomes. The recent increase in genome sequence data has created an opportunity to evaluate the impact of these active mobile elements on the evolution of plant genomes. Analysis of the sugarcane transcriptome identified 267 clones with significant similarity to previously described plant TEs. After full cDNA sequencing, 68 sugarcane TE clones were assigned to 11 families according to their best sequence alignment against a fully characterized element. Expression was further investigated through a combined study utilizing electronic Northerns, macroarray, transient and stable sugarcane transformation. Newly synthesized cDNA probes from flower, leaf roll, apical meristem and callus tissues confirm previous results. Callus was identified as the tissue with the highest number of TEs being expressed, revealing that tissue culture drastically induced the expression of different elements. No tissue-specific family was identified. Different representatives within a TE family displayed differential expression patterns, showing that each family presented expression in almost every tissue. Transformation experiments demonstrated that most Hopscotch clone-derived U3 regions are, indeed, active promoters, although under a strong transcriptional regulation. This is a large-scale study about the expression pattern of TEs and indicates that mobile genetic elements are transcriptionally active in the highly polyploid and complex sugarcane genome.

The maize Mutator ( Mu) system has been described as the most active and mutagenic plant transposon so far discovered. Mu -like elements (MULEs) are widespread among plants, and many and diverse variants can coexist in a particular genome. The autonomous regulatory element MuDR contains two genes: mudrA encodes the transposase, while the function of the mudrB gene product remains unknown. Although mudrA -like sequences are ubiquitous in plants, mudrB seems to be restricted to the genus Zea. In the SUCEST (the Brazilian Sugarcane EST Sequencing Project) database, several mudrA -like cDNAs have been identified, suggesting the presence of a transcriptionally active Mu system in sugarcane. Phylogenetic studies have revealed the presence in plants of four classes of mudrA -like sequences, which arose prior to the monocot/eudicot split. At least three of the four classes are also found in the progenitors of the sugarcane hybrid (Saccharum spp.), Saccharum officinarum and S. spontaneum. The frequency of putatively functional transposase ORFs varies among the classes, as revealed at both cDNA and genomic levels. The predicted products of some sugarcane mudrA -like transcripts contain both a DNA-binding domain and a transposase catalytic-site motif, supporting the idea that an active Mu system exists in this hybrid genome.

Transposons are abundant components of eukaryotic genomes, and play important role in genome evolution. The knowledge about these elements should contribute to the understanding of their impact on the host genomes. The hAT transposon superfamily is one of the best characterized superfamilies in diverse organisms, nevertheless, a detailed study of these elements was never carried in sugarcane. To address this question we analyzed 32 cDNAs similar to that of hAT superfamily of transposons previously identified in the sugarcane transcriptome. Our results revealed that these hAT-like transposases cluster in one highly homogeneous and other more heterogeneous lineage. We present evidences that support the hypothesis that the highly homogeneous group is a domesticated transposase while the remainder of the lineages are composed of transposon units. The first is common to grasses, clusters significantly with domesticated transposases from Arabidopsis, rice and sorghum and is expressed in different tissues of two sugarcane cultivars analyzed. In contrast, the more heterogeneous group represents at least two transposon lineages. We recovered five genomic versions of one lineage, characterizing a novel transposon family with conserved DDE motif, named SChAT. These results indicate the presence of at least three distinct lineages of hAT-like transposase paralogues in sugarcane genome, including a novel transposon family described in Saccharum and a domesticated transposase. Taken together, these findings permit to follow the diversification of some hAT transposase paralogues in sugarcane, aggregating knowledge about the co-evolution of transposons and their host genomes.Electronic supplementary materialThe online version of this article (doi:10.1007/s00438-011-0670-8) contains supplementary material, which is available to authorized users.

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Background: Transposable elements (TEs) are major components of plant genomes. Despite being regarded as junk DNA at first, TEs play important roles for the organisms they are found in. The most obvious and easily recognizable effects caused by TEs result from their mobility, which can disrupt coding sequences or promoter regions. However, with the recent advances in transcriptomics, it is becoming increasingly evident that TEs can act as an additional layer of gene expression regulation through a number of processes, which can involve production of non-coding RNAs. Here, we describe how Tnt1, a stress-responsive LTR-retrotransposon, interferes with gene expression and modulate a number of developmental aspects in tobacco. Results: Through an RNAi approach, we generated tobacco (HP) lines knocked-down for Tnt1 expression. Quantitative RT-PCR experiments confirm that Tnt1 is downregulated in HP lines after ethylene exposure. A RNA-seq experiment was performed and through two independent bioinformatic approaches (with different stringencies) we found 932 and 97 differentially expressed genes in HP lines. A number of phenotypes were observed in such lines, namely lesion mimicry in leaves, underdevelopment of the root system, overproduction of root hairs and early loss of seed viability. Folding prediction of part of the Tnt1 mRNA reveals putative stem-loop secondary structures containing transcriptional regulation sequences, suggesting it could be a source of small RNAs. We also propose a model to explain the Tnt1 expression in both homeostatic and stress conditions, and how it could interact with stress-responsive genes. Conclusions: Our results are consistent that interferences with Tnt1 transcript levels correlate with transcriptomic and phenotypic changes, suggesting a functional role for this element during plant development and stress response.

De novo synthesis of thiamine (vitamin B1) in plants depends on the action of thiamine thiazole synthase, which synthesizes the thiazole ring, and is encoded by the THI1 gene. Here, we investigated the evolution and diversity of THI1 in Poaceae, where C4 and C3 photosynthetic plants co-evolved. An ancestral duplication of THI1 is observed in Panicoideae that remains in many modern monocots, including sugarcane. In addition to the two sugarcane copies (ScTHI1-1 and ScTHI1-2), we identified ScTHI1-2 alleles showing differences in their sequence, indicating divergence between ScTHI1-2a and ScTHI1-2b. Such variations are observed only in the Saccharum complex, corroborating the phylogeny. At least five THI1 genomic environments were found in Poaceae, two in sugarcane, M. sinensis, and S. bicolor. The THI1 promoter in Poaceae is highly conserved at 300 bp upstream of the start codon ATG and has cis-regulatory elements that putatively bind to transcription factors associated with development, growth, development and biological rhythms. An experiment set to compare gene expression levels in different tissues across the sugarcane R570 life cycle showed that ScTHI1-1 was expressed mainly in leaves regardless of age. Furthermore, ScTHI1 displayed relatively high expression levels in meristem and culm, which varied with the plant age. Finally, yeast complementation studies with THI4-defective strain demonstrate that only ScTHI1-1 and ScTHI1-2b isoforms can partially restore thiamine auxotrophy, albeit at a low frequency. Taken together, the present work supports the existence of multiple origins of THI1 harboring genomic regions in Poaceae with predicted functional redundancy. In addition, it questions the contribution of the levels of the thiazole ring in C4 photosynthetic plant tissues or potentially the relevance of the THI1 protein activity.