Transposable elements (TEs) are the key players in generating genomic novelty by a combination of the chromosome rearrangements they cause and the genes that come under their regulatory sway. Genome ...size, gene content, gene order, centromere function, and numerous other aspects of nuclear biology are driven by TE activity. Although the origins and attitudes of TEs have the hallmarks of selfish DNA, there are numerous cases where TE components have been co-opted by the host to create new genes or modify gene regulation. In particular, epigenetic regulation has been transformed from a process to silence invading TEs and viruses into a key strategy for regulating plant genes. Most, perhaps all, of this epigenetic regulation is derived from TE insertions near genes or TE-encoded factors that act in trans. Enormous pools of genome data and new technologies for reverse genetics will lead to a powerful new era of TE analysis in plants.
By employing the nuclear DNA of the African rice Oryza glaberrima as a reference genome, the timing, natures, mechanisms, and specificities of recent sequence evolution in the indica and japonica ...subspecies of Oryza sativa were identified. The data indicate that the genome sizes of both indica and japonica have increased substantially, >2% and >6%, respectively, since their divergence from a common ancestor, mainly because of the amplification of LTR-retrotransposons. However, losses of all classes of DNA sequence through unequal homologous recombination and illegitimate recombination have attenuated the growth of the rice genome. Small deletions have been particularly frequent throughout the genome. In >1 Mb of orthologous regions that we analyzed, no cases of complete gene acquisition or loss from either indica or japonica were found, nor was any example of precise transposon excision detected. The sequences between genes were observed to have a very high rate of divergence, indicating a molecular clock for transposable elements that is at least 2-fold more rapid than synonymous base substitutions within genes. We found that regions prone to frequent insertions and deletions also exhibit higher levels of point mutation. These results indicate a highly dynamic rice genome with competing processes for the generation and removal of genetic variation.
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Low temperature restricts the planting range of all crops, but cold acclimation induces adaption to cold stress in many plants. Camellia sinensis, a perennial evergreen tree that is the source of ...tea, is mainly grown in warm areas. Camellia sinensis var. sinensis (CSS) has greater cold tolerance than Camellia sinensis var. assamica (CSA). To gain deep insight into the molecular mechanisms underlying cold adaptation, we investigated the physiological responses and transcriptome profiles by RNA-Seq in two tea varieties, cold resistant SCZ (classified as CSS) and cold susceptible YH9 (classified as CSA), during cold acclimation.
Under freezing stress, lower relative electrical conductivity and higher chlorophyll fluorescence (Fv/Fm) values were detected in SCZ than in YH9 when subjected to freezing acclimation. During cold treatment, 6072 and 7749 DEGs were observed for SCZ and YH9, respectively. A total of 978 DEGs were common for both SCZ and YH9 during the entire cold acclimation process. DEGs were enriched in pathways of photosynthesis, hormone signal transduction, and transcriptional regulation of plant-pathogen interactions. Further analyses indicated that decreased expression of Lhca2 and higher expression of SnRK2.8 are correlated with cold tolerance in SCZ.
Compared with CSA, CSS was significantly more resistant to freezing after cold acclimation, and this increased resistance was associated with an earlier expression of cold-induced genes. Because the greater transcriptional differentiation during cold acclimation in SCZ may contribute to its greater cold tolerance, our studies identify specific genes involved in photoinhibition, ABA signal conduction, and plant immunity that should be studied for understanding the processes involved in cold tolerance. Marker-assisted breeding focused on the allelic variation at these loci provides an avenue for the possible generation of CSA cultivars that have CSS-level cold tolerance.
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Animal heterotrimeric G proteins are activated by guanine nucleotide exchange factors (GEF), typically seven transmembrane receptors that trigger GDP release and subsequent GTP binding. In contrast, ...the Arabidopsis thaliana G protein (AtGPA1) rapidly activates itself without a GEF and is instead regulated by a seven transmembrane Regulator of G protein Signaling (7TM-RGS) protein that promotes GTP hydrolysis to reset the inactive (GDP-bound) state. It is not known if this unusual activation is a major and constraining part of the evolutionary history of G signaling in eukaryotes. In particular, it is not known if this is an ancestral form or if this mechanism is maintained, and therefore constrained, within the plant kingdom. To determine if this mode of signal regulation is conserved throughout the plant kingdom, we analyzed available plant genomes for G protein signaling components, and we purified individually the plant components encoded in an informative set of plant genomes in order to determine their activation properties in vitro. While the subunits of the heterotrimeric G protein complex are encoded in vascular plant genomes, the 7TM-RGS genes were lost in all investigated grasses. Despite the absence of a Gα-inactivating protein in grasses, all vascular plant Gα proteins examined rapidly released GDP without a receptor and slowly hydrolyzed GTP, indicating that these Gα are self-activating. We showed further that a single amino acid substitution found naturally in grass Gα proteins reduced the Gα-RGS interaction, and this amino acid substitution occurred before the loss of the RGS gene in the grass lineage. Like grasses, non-vascular plants also appear to lack RGS proteins. However, unlike grasses, one representative non-vascular plant Gα showed rapid GTP hydrolysis, likely compensating for the loss of the RGS gene. Our findings, the loss of a regulatory gene and the retention of the "self-activating" trait, indicate the existence of divergent Gα regulatory mechanisms in the plant kingdom. In the grasses, purifying selection on the regulatory gene was lost after the physical decoupling of the RGS protein and its cognate Gα partner. More broadly these findings show extreme divergence in Gα activation and regulation that played a critical role in the evolution of G protein signaling pathways.
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Recent comprehensive sequence analysis of the maize genome now permits detailed discovery and description of all transposable elements (TEs) in this complex nuclear environment. Reiteratively ...optimized structural and homology criteria were used in the computer-assisted search for retroelements, TEs that transpose by reverse transcription of an RNA intermediate, with the final results verified by manual inspection. Retroelements were found to occupy the majority (>75%) of the nuclear genome in maize inbred B73. Unprecedented genetic diversity was discovered in the long terminal repeat (LTR) retrotransposon class of retroelements, with >400 families (>350 newly discovered) contributing >31,000 intact elements. The two other classes of retroelements, SINEs (four families) and LINEs (at least 30 families), were observed to contribute 1,991 and approximately 35,000 copies, respectively, or a combined approximately 1% of the B73 nuclear genome. With regard to fully intact elements, median copy numbers for all retroelement families in maize was 2 because >250 LTR retrotransposon families contained only one or two intact members that could be detected in the B73 draft sequence. The majority, perhaps all, of the investigated retroelement families exhibited non-random dispersal across the maize genome, with LINEs, SINEs, and many low-copy-number LTR retrotransposons exhibiting a bias for accumulation in gene-rich regions. In contrast, most (but not all) medium- and high-copy-number LTR retrotransposons were found to preferentially accumulate in gene-poor regions like pericentromeric heterochromatin, while a few high-copy-number families exhibited the opposite bias. Regions of the genome with the highest LTR retrotransposon density contained the lowest LTR retrotransposon diversity. These results indicate that the maize genome provides a great number of different niches for the survival and procreation of a great variety of retroelements that have evolved to differentially occupy and exploit this genomic diversity.
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5Gs for crop genetic improvement Varshney, Rajeev K; Sinha, Pallavi; Singh, Vikas K ...
Current opinion in plant biology,
08/2020, Volume:
56
Journal Article
Peer reviewed
Open access
•5G breeding approach brings precision and enhances efficiency in breeding programs.•Genome, germplasm, gene function, genomic breeding and genome editing are the 5Gs.•NGS platforms, speed breeding ...and express edit facility are the key drivers for 5G breeding.•Haplotype-based breeding, genomic selection and gene editing are the key genomic breeding approaches of the future.•Multi-disciplinary team of scientists need to be trained to deploy 5G breeding in developing countries.
Here we propose a 5G breeding approach for bringing much-needed disruptive changes to crop improvement. These 5Gs are Genome assembly, Germplasm characterization, Gene function identification, Genomic breeding (GB), and Gene editing (GE). In our view, it is important to have genome assemblies available for each crop and a deep collection of germplasm characterized at sequencing and agronomic levels for identification of marker-trait associations and superior haplotypes. Systems biology and sequencing-based mapping approaches can be used to identify genes involved in pathways leading to the expression of a trait, thereby providing diagnostic markers for target traits. These genes, markers, haplotypes, and genome-wide sequencing data may be utilized in GB and GE methodologies in combination with a rapid cycle breeding strategy.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
A fundamental tenet of multicellular eukaryotic evolution is that vertical inheritance is paramount, with natural selection acting on genetic variants transferred from parents to offspring. This ...lineal process means that an organism’s adaptive potential can be restricted by its evolutionary history, the amount of standing genetic variation, and its mutation rate. Lateral gene transfer (LGT) theoretically provides a mechanism to bypass many of these limitations, but the evolutionary importance and frequency of this process in multicellular eukaryotes, such as plants, remains debated. We address this issue by assembling a chromosome-level genome for the grass Alloteropsis semialata, a species surmised to exhibit two LGTs, and screen it for other grass-to-grass LGTs using genomic data from 146 other grass species. Through stringent phylogenomic analyses, we discovered 57 additional LGTs in the A. semialata nuclear genome, involving at least nine different donor species. The LGTs are clustered in 23 laterally acquired genomic fragments that are up to 170 kb long and have accumulated during the diversification of Alloteropsis. The majority of the 59 LGTs in A. semialata are expressed, and we show that they have added functions to the recipient genome. Functional LGTs were further detected in the genomes of five other grass species, demonstrating that this process is likely widespread in this globally important group of plants. LGT therefore appears to represent a potent evolutionary force capable of spreading functional genes among distantly related grass species.
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Analysis of LTR retrotransposon structures in five diploid angiosperm genomes uncovered very different relative levels of different types of genomic diversity. All species exhibited recent LTR ...retrotransposon mobility and also high rates of DNA removal by unequal homologous recombination and illegitimate recombination. The larger plant genomes contained many LTR retrotransposon families with >10,000 copies per haploid genome, whereas the smaller genomes contained few or no LTR retrotransposon families with >1,000 copies, suggesting that this differential potential for retroelement amplification is a primary factor in angiosperm genome size variation. The average ratios of transition to transversion mutations (Ts/Tv) in diverging LTRs were >1.5 for each species studied, suggesting that these elements are mostly 5-methylated at cytosines in an epigenetically silenced state. However, the diploid wheat Triticum monococcum and barley have unusually low Ts/Tv values (respectively, 1.9 and 1.6) compared with maize (3.9), medicago (3.6), and lotus (2.5), suggesting that this silencing is less complete in the two Triticeae. Such characteristics as the ratios of point mutations to indels (insertions and deletions) and the relative efficiencies of DNA removal by unequal homologous recombination compared with illegitimate recombination were highly variable between species. These latter variations did not correlate with genome size or phylogenetic relatedness, indicating that they frequently change during the evolutionary descent of plant lineages. In sum, the results indicate that the different sizes, contents, and structures of angiosperm genomes are outcomes of the same suite of mechanistic processes, but acting with different relative efficiencies in different plant lineages.
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Our knowledge of the structure and composition of genomes is rapidly progressing in pace with their sequencing. The emerging data show that a significant portion of eukaryotic genomes is composed of ...transposable elements (TEs). Given the abundance and diversity of TEs and the speed at which large quantities of sequence data are emerging, identification and annotation of TEs presents a significant challenge. Here we propose the first unified hierarchical classification system, designed on the basis of the transposition mechanism, sequence similarities and structural relationships, that can be easily applied by non-experts. The system and nomenclature is kept up to date at the WikiPoson web site.
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