The pseudogenes of barley

Four classic "de novo" genes all have plausible homologs and likely evolved from retro-duplicated or pseudogenic sequences

Despite being previously regarded as extremely unlikely, the idea that entirely novel protein-coding genes can emerge from non-coding sequences has gradually become accepted over the past two decades. Examples of "de novo origination", resulting in lineage-specific "orphan" genes, lacking coding orthologs, are now produced every year. However, many are likely cases of duplicates that are difficult to recognize. Here, I re-examine the claims and show that four very well-known examples of genes alleged to have emerged completely "from scratch"- FLJ33706 in humans, Goddard in fruit flies, BSC4 in baker's yeast and AFGP2 in codfish-may have plausible evolutionary ancestors in pre-existing genes. The first two are likely highly diverged retrogenes coding for regulatory proteins that have been misidentified as orphans. The antifreeze glycoprotein, moreover, may not have evolved from repetitive non-genic sequences but, as in several other related cases, from an apolipoprotein that could have become pseudogenized before later being reactivated. These findings detract from various claims made about de novo gene birth and show there has been a tendency not to invest the necessary effort in searching for homologs outside of a very limited syntenic or phylostratigraphic methodology. A robust approach is used for improving detection that draws upon similarities, not just in terms of statistical sequence analysis, but also relating to biochemistry and function, to obviate notable failures to identify homologs.

New Insights into Phylogenetic Relationship of Hydrocotyle (Araliaceae) Based on Plastid Genomes

Hydrocotyle, belonging to the Hydrocotyloideae of Araliaceae, consists of 95 perennial and 35 annual species. Due to the lack of stable diagnostic morphological characteristics and high-resolution molecular markers, the phylogenetic relationships of Hydrocotyle need to be further investigated. In this study, we newly sequenced and assembled 13 whole plastid genomes of Hydrocotyle and performed comparative plastid genomic analyses with four previously published Hydrocotyle plastomes and phylogenomic analyses within Araliaceae. The plastid genomes of Hydrocotyle exhibited typical quadripartite structures with lengths from 152,659 bp to 153,669 bp, comprising a large single-copy (LSC) region (83,958-84,792 bp), a small single-copy (SSC) region (18,585-18,768 bp), and a pair of inverted repeats (IRs) (25,058-25,145 bp). Each plastome encoded 113 unique genes, containing 79 protein-coding genes, 30 tRNA genes, and four rRNA genes. Comparative analyses showed that the IR boundaries of Hydrocotyle plastomes were highly similar, and the coding and IR regions exhibited more conserved than non-coding and single-copy (SC) regions. A total of 2932 simple sequence repeats and 520 long sequence repeats were identified, with specificity in the number and distribution of repeat sequences. Six hypervariable regions were screened from the SC region, including four intergenic spacers (IGS) (ycf3-trnS, trnS-rps4, petA-psbJ, and ndhF-rpl32) and two coding genes (rpl16 and ycf1). Three protein-coding genes (atpE, rpl16, and ycf2) were subjected to positive selection only in a few species, implying that most protein-coding genes were relatively conserved during the plastid evolutionary process. Plastid phylogenomic analyses supported the treatment of Hydrocotyle from Apiaceae to Araliaceae, and topologies with a high resolution indicated that plastome data can be further used in the comprehensive phylogenetic research of Hydrocotyle. The diagnostic characteristics currently used in Hydrocotyle may not accurately reflect the phylogenetic relationships of this genus, and new taxonomic characteristics may need to be evaluated and selected in combination with more comprehensive molecular phylogenetic results.

Unravelling the due importance of pseudogenes and their resurrection in plants

The complexities of a genome are underpinned to the vast expanses of the intergenic region, which constitutes ∼97-98% of the genome. This region is essentially composed of what is colloquially referred to as the "junk DNA" and is composed of various elements like transposons, repeats, pseudogenes, etc. The latter have long been considered as dead elements merely contributing to transcriptional noise in the genome. Many studies now describe the previously unknown regulatory functions of these genes. Recent advances in the Next-generation sequencing (NGS) technologies have allowed unprecedented access to these regions. With the availability of whole genome sequences of more than 788 different plant species in past 20 years, genome annotation has become feasible like never before. Different bioinformatic pipelines are available for the identification of pseudogenes. However, still little is known about their biological functions. The functional validation of these genes remains challenging and research in this area is still in infancy, particularly in plants. CRISPR/Cas-based genome editing could provide solutions to understand the biological roles of these genes by allowing creation of precise edits within these genes. The possibility of pseudogene reactivation or resurrection as has been demonstrated in a few studies might open new avenues of genetic manipulation to yield a desirable phenotype. This review aims at comprehensively summarizing the progress made with regards to the identification of pseudogenes and understanding their biological functions in plants.

Plant Transglutaminases: New Insights in Biochemistry, Genetics, and Physiology

Transglutaminases (TGases) are calcium-dependent enzymes that catalyse an acyl-transfer reaction between primary amino groups and protein-bound Gln residues. They are widely distributed in nature, being found in vertebrates, invertebrates, microorganisms, and plants. TGases and their functionality have been less studied in plants than humans and animals. TGases are distributed in all plant organs, such as leaves, tubers, roots, flowers, buds, pollen, and various cell compartments, including chloroplasts, the cytoplasm, and the cell wall. Recent molecular, physiological, and biochemical evidence pointing to the role of TGases in plant biology and the mechanisms in which they are involved allows us to consider their role in processes such as photosynthesis, plant fertilisation, responses to biotic and abiotic stresses, and leaf senescence. In the present paper, an in-depth description of the biochemical characteristics and a bioinformatics comparison of plant TGases is provided. We also present the phylogenetic relationship, gene structure, and sequence alignment of TGase proteins in various plant species, not described elsewhere. Currently, our knowledge of these proteins in plants is still insufficient. Further research with the aim of identifying and describing the regulatory components of these enzymes and the processes regulated by them is needed.

Gauging the trends of pseudogenes in plants

Pseudogenes, the debilitated parts of ancient genes, were previously scrapped off as junk or discarded genes with no functional significance. Pseudogenes have come under scrutiny for their functionality, since recent studies have unveiled their importance in the regulation of their corresponding parent genes and various biological mechanisms. Despite the enormous occurrence of pseudogenes in plants, the lack of experimental validation has contributed toward their unresolved roles in gene regulation. Contrarily, most of the studies associated with gene regulation have been mainly reported for humans, mice, and other mammalian genomes. Consequently, in order to present a cumulative report on plant-based pseudogenes research, an attempt has been made to assemble multiple studies presenting the pseudogene classification, the prediction and the determination of comparative accuracies of various computational pipelines, and recent trends in analyzing their biological functions, and regulatory mechanisms. This review represents the classical, as well as the recent advances on pseudogene identification and their potential roles in transcriptional regulation, which could possibly invigorate the quality of genome annotation, evolutionary analysis, and complexity surrounding the regulatory pathways in plants. Thus, when the ambiguous boundary girdling the pseudogenes eventually recedes on account of their explicit orchestration role, research in flora would no longer saunter compared to that on fauna.

Structural characterization and duplication modes of pseudogenes in plants

We identified and characterized the pseudogene complements of five plant species: four dicots (Arabidopsis thaliana, Vitis vinifera, Populus trichocarpa and Phaseolus vulgaris) and one monocot (Oryza sativa). Retroposition was considered of modest importance for pseudogene formation in all investigated species except V. vinifera, which showed an unusually high number of retro-pseudogenes in non coding genic regions. By using a pipeline for the classification of sequence duplicates in plant genomes, we compared the relative importance of whole genome, tandem, proximal, transposed and dispersed duplication modes in the pseudo and functional gene complements. Pseudogenes showed higher tendencies than functional genes to genomic dispersion. Dispersed pseudogenes were prevalently fragmented and showed high sequence divergence at flanking regions. On the contrary, those deriving from whole genome duplication were proportionally less than expected based on observations on functional loci and showed higher levels of flanking sequence conservation than dispersed pseudogenes. Pseudogenes deriving from tandem and proximal duplications were in excess compared to functional loci, probably reflecting the high evolutionary rate associated with these duplication modes in plant genomes. These data are compatible with high rates of sequence turnover at neutral sites and double strand break repairs mediated duplication mechanisms.

Circular RNAs-The Road Less Traveled

Circular RNAs are the most recent addition in the non-coding RNA family, which has started to gain recognition after a decade of obscurity. The first couple of reports that emerged at the beginning of this decade and the amount of evidence that has accumulated thereafter has, however, encouraged RNA researchers to navigate further in the quest for the exploration of circular RNAs. The joining of 5′ and 3′ ends of RNA molecules through backsplicing forms circular RNAs during co-transcriptional or post-transcriptional processes. These molecules are capable of effectively sponging microRNAs, thereby regulating the cellular processes, as evidenced by numerous animal and plant systems. Preliminary studies have shown that circular RNA has an imperative role in transcriptional regulation and protein translation, and it also has significant therapeutic potential. The high stability of circular RNA is rendered by its closed ends; they are nevertheless prone to degradation by circulating endonucleases in serum or exosomes or by microRNA-mediated cleavage due to their high complementarity. However, the identification of circular RNAs involves diverse methodologies and the delineation of its possible role and mechanism in the regulation of cellular and molecular architecture has provided a new direction for the continuous research into circular RNA. In this review, we discuss the possible mechanism of circular RNA biogenesis, its structure, properties, degradation, and the growing amount of evidence regarding the detection methods and its role in animal and plant systems.

Evolutionary analysis of six chloroplast genomes from three Persea americana ecological races: Insights into sequence divergences and phylogenetic relationships

Chloroplasts significantly influence species phylogenies because of their maternal inheritance and the moderate evolutionary rate of their genomes. Avocado, which is a member of the family Lauraceae, has received considerable attention from botanists, likely because of its position as a basal angiosperm. However, there is relatively little avocado genomic information currently available. In this study, six complete avocado chloroplast genomes from three ecological races were assembled to examine the sequence diversity among the three avocado ecological races. A comparative genomic analysis revealed that 515 simple sequence repeat loci and 176 repeats belonging to four other types were polymorphic across the six chloroplast genomes. Three highly variable regions (trnC-GCA-petN, petN-psbM, and petA-psbJ) were identified as highly informative markers. A phylogenetic analysis based on 79 common protein-coding genes indicated that the six examined avocado accessions from three ecological races form a monophyletic clade. The other three genera belonging to the Persea group clustered to form a sister clade with a high bootstrap value. These chloroplast genomes provide important genetic information for future attempts at identifying avocado races and for the related biological research.

Diversification of cytokinin phosphotransfer signaling genes in Medicago truncatula and other legume genomes

BACKGROUND

Legumes can establish on nitrogen-deprived soils a symbiotic interaction with Rhizobia bacteria, leading to the formation of nitrogen-fixing root nodules. Cytokinin phytohormones are critical for triggering root cortical cell divisions at the onset of nodule initiation. Cytokinin signaling is based on a Two-Component System (TCS) phosphorelay cascade, involving successively Cytokinin-binding Histidine Kinase receptors, phosphorelay proteins shuttling between the cytoplasm and the nucleus, and Type-B Response Regulator (RRB) transcription factors activating the expression of cytokinin primary response genes. Among those, Type-A Response Regulators (RRA) exert a negative feedback on the TCS signaling. To determine whether the legume plant nodulation capacity is linked to specific features of TCS proteins, a genome-wide identification was performed in six legume genomes (Cajanus cajan, pigeonpea; Cicer arietinum, chickpea; Glycine max, soybean; Phaseolus vulgaris, common bean; Lotus japonicus; Medicago truncatula). The diversity of legume TCS proteins was compared to the one found in two non-nodulating species, Arabidopsis thaliana and Vitis vinifera, which are references for functional analyses of TCS components and phylogenetic analyses, respectively.

RESULTS

A striking expansion of non-canonical RRBs was identified, notably leading to the emergence of proteins where the conserved phosphor-accepting aspartate residue is replaced by a glutamate or an asparagine. M. truncatula genome-wide expression datasets additionally revealed that only a limited subset of cytokinin-related TCS genes is highly expressed in different organs, namely MtCHK1/MtCRE1, MtHPT1, and MtRRB3, suggesting that this "core" module potentially acts in most plant organs including nodules.

CONCLUSIONS

Further functional analyses are required to determine the relevance of these numerous non-canonical TCS RRBs in symbiotic nodulation, as well as of canonical MtHPT1 and MtRRB3 core signaling elements.

Evolutionary Analysis of Plastid Genomes of Seven Lonicera L. Species: Implications for Sequence Divergence and Phylogenetic Relationships

Plant plastomes play crucial roles in species evolution and phylogenetic reconstruction studies due to being maternally inherited and due to the moderate evolutionary rate of genomes. However, patterns of sequence divergence and molecular evolution of the plastid genomes in the horticulturally- and economically-important Lonicera L. species are poorly understood. In this study, we collected the complete plastomes of seven Lonicera species and determined the various repeat sequence variations and protein sequence evolution by comparative genomic analysis. A total of 498 repeats were identified in plastid genomes, which included tandem (130), dispersed (277), and palindromic (91) types of repeat variations. Simple sequence repeat (SSR) elements analysis indicated the enriched SSRs in seven genomes to be mononucleotides, followed by tetra-nucleotides, dinucleotides, tri-nucleotides, hex-nucleotides, and penta-nucleotides. We identified 18 divergence hotspot regions (rps15, rps16, rps18, rpl23, psaJ, infA, ycf1, trnN-GUU-ndhF, rpoC2-rpoC1, rbcL-psaI, trnI-CAU-ycf2, psbZ-trnG-UCC, trnK-UUU-rps16, infA-rps8, rpl14-rpl16, trnV-GAC-rrn16, trnL-UAA intron, and rps12-clpP) that could be used as the potential molecular genetic markers for the further study of population genetics and phylogenetic evolution of Lonicera species. We found that a large number of repeat sequences were distributed in the divergence hotspots of plastid genomes. Interestingly, 16 genes were determined under positive selection, which included four genes for the subunits of ribosome proteins (rps7, rpl2, rpl16, and rpl22), three genes for the subunits of photosystem proteins (psaJ, psbC, and ycf4), three NADH oxidoreductase genes (ndhB, ndhH, and ndhK), two subunits of ATP genes (atpA and atpB), and four other genes (infA, rbcL, ycf1, and ycf2). Phylogenetic analysis based on the whole plastome demonstrated that the seven Lonicera species form a highly-supported monophyletic clade. The availability of these plastid genomes provides important genetic information for further species identification and biological research on Lonicera.

Systematic analysis of DEMETER-like DNA glycosylase genes shows lineage-specific Smi-miR7972 involved in SmDML1 regulation in Salvia miltiorrhiza

DEMETER-like DNA glycosylases (DMLs) initiate the base excision repair-dependent DNA demethylation to regulate a wide range of biological processes in plants. Six putative SmDML genes, termed SmDML1-SmDML6, were identified from the genome of S. miltiorrhiza, an emerging model plant for Traditional Chinese Medicine (TCM) studies. Integrated analysis of gene structures, sequence features, conserved domains and motifs, phylogenetic analysis and differential expression showed the conservation and divergence of SmDMLs. SmDML1, SmDML2 and SmDML4 were significantly down-regulated by the treatment of 5Aza-dC, a general DNA methylation inhibitor, suggesting involvement of SmDMLs in genome DNA methylation change. SmDML1 was predicted and experimentally validated to be target of Smi-miR7972. Computational analysis of forty whole genome sequences and almost all of RNA-seq data from Lamiids revealed that MIR7972s were only distributed in some plants of the three orders, including Lamiales, Solanales and Boraginales, and the number of MIR7972 genes varied among species. It suggests that MIR7972 genes underwent expansion and loss during the evolution of some Lamiids species. Phylogenetic analysis of MIR7972s showed closer evolutionary relationships between MIR7972s in Boraginales and Solanales in comparison with Lamiales. These results provide a valuable resource for elucidating DNA demethylation mechanism in S. miltiorrhiza.