Gramene: A Resource for Comparative Analysis of Plants Genomes and Pathways

Heat stress at the bicellular stage inhibits sperm cell development and transport into pollen tubes

For successful double fertilization in flowering plants (angiosperms), pollen tubes deliver 2 nonmotile sperm cells toward female gametes (egg and central cell, respectively). Heatwaves, especially during the reproduction period, threaten male gametophyte (pollen) development, resulting in severe yield losses. Using maize (Zea mays) as a crop and grass model system, we found strong seed set reduction when moderate heat stress was applied for 2 d during the uni- and bicellular stages of pollen development. We show that heat stress accelerates pollen development and impairs pollen germination capabilities when applied at the unicellular stage. Heat stress at the bicellular stage impairs sperm cell development and transport into pollen tubes. To understand the course of the latter defects, we used marker lines and analyzed the transcriptomes of isolated sperm cells. Heat stress affected the expression of genes associated with transcription, RNA processing and translation, DNA replication, and the cell cycle. This included the genes encoding centromeric histone 3 (CENH3) and α-tubulin. Most genes that were misregulated encode proteins involved in the transition from metaphase to anaphase during pollen mitosis II. Heat stress also activated spindle assembly check point and meta- to anaphase transition genes in sperm cells. In summary, misregulation of the identified genes during heat stress at the bicellular stage results in sperm cell development and transport defects ultimately leading to sterility.

Haplotype-resolved genome assembly and implementation of VitExpress, an open interactive transcriptomic platform for grapevine

Haplotype-resolved genome assemblies were produced for Chasselas and Ugni Blanc, two heterozygous Vitis vinifera cultivars by combining high-fidelity long-read sequencing and high-throughput chromosome conformation capture (Hi-C). The telomere-to-telomere full coverage of the chromosomes allowed us to assemble separately the two haplo-genomes of both cultivars and revealed structural variations between the two haplotypes of a given cultivar. The deletions/insertions, inversions, translocations, and duplications provide insight into the evolutionary history and parental relationship among grape varieties. Integration of de novo single long-read sequencing of full-length transcript isoforms (Iso-Seq) yielded a highly improved genome annotation. Given its higher contiguity, and the robustness of the IsoSeq-based annotation, the Chasselas assembly meets the standard to become the annotated reference genome for V. vinifera. Building on these resources, we developed VitExpress, an open interactive transcriptomic platform, that provides a genome browser and integrated web tools for expression profiling, and a set of statistical tools (StatTools) for the identification of highly correlated genes. Implementation of the correlation finder tool for MybA1, a major regulator of the anthocyanin pathway, identified candidate genes associated with anthocyanin metabolism, whose expression patterns were experimentally validated as discriminating between black and white grapes. These resources and innovative tools for mining genome-related data are anticipated to foster advances in several areas of grapevine research.

Molecular Evolution of RAMOSA1 (RA1) in Land Plants

RAMOSA1 (RA1) is a Cys2-His2-type (C2H2) zinc finger transcription factor that controls plant meristem fate and identity and has played an important role in maize domestication. Despite its importance, the origin of RA1 is unknown, and the evolution in plants is only partially understood. In this paper, we present a well-resolved phylogeny based on 73 amino acid sequences from 48 embryophyte species. The recovered tree topology indicates that, during grass evolution, RA1 arose from two consecutive SUPERMAN duplications, resulting in three distinct grass sequence lineages: RA1-like A, RA1-like B, and RA1; however, most of these copies have unknown functions. Our findings indicate that RA1 and RA1-like play roles in the nucleus despite lacking a traditional nuclear localization signal. Here, we report that copies diversified their coding region and, with it, their protein structure, suggesting different patterns of DNA binding and protein-protein interaction. In addition, each of the retained copies diversified regulatory elements along their promoter regions, indicating differences in their upstream regulation. Taken together, the evidence indicates that the RA1 and RA1-like gene families in grasses underwent subfunctionalization and neofunctionalization enabled by gene duplication.

Four enzymes control natural variation in the steroid core of Erysimum cardenolides

Plants commonly produce families of structurally related metabolites with similar defensive functions. This apparent redundancy raises the question of underlying molecular mechanisms and adaptive benefits of such chemical variation. Cardenolides, a class defensive compounds found in the wallflower genus Erysimum (L., Brassicaceae) and scattered across other plant families, show substantial structural variation, with glycosylation and hydroxylation being common modifications of a steroid core, which itself may vary in terms of stereochemistry and saturation. Through a combination of chemical mutagenesis and analysis of gene coexpression networks, we identified four enzymes involved in cardenolide biosynthesis in Erysimum that work together to determine stereochemistry at carbon 5 of the steroid core: Ec3βHSD, a 3β-hydroxysteroid dehydrogenase, Ec3KSI, a ketosteroid isomerase, EcP5βR2, a progesterone 5β-reductase, and EcDET2, a steroid 5α-reductase. We biochemically characterized the activity of these enzymes in vitro and generated CRISPR/Cas9 knockout lines to confirm activity in vivo. Cardenolide biosynthesis was not eliminated in any of the knockouts. Instead, mutant plants accumulated cardenolides with altered saturation and stereochemistry of the steroid core. Furthermore, we found variation in carbon 5 configuration among the cardenolides of 44 species of Erysimum, where the occurrence of some 5β-cardenolides is associated with the expression and sequence of P5βR2. This may have allowed Erysimum species to fine-tune their defensive profiles to target specific herbivore populations over the course of evolution.

SIGNIFICANCE STATEMENT

Plants use an array of toxic compounds to defend themselves from attack against insects and other herbivores. One mechanism through which plants may evolve more toxic compounds is through modifications to the structure of compounds they already produce. In this study, we show how plants in the wallflower genus Erysimum use four enzymes to fine-tune the structure of toxic metabolites called cardenolides. Natural variation in the sequence and expression of a single enzyme called progesterone 5β-reductase 2 partly explains the variation in cardenolides observed across the Erysimum genus. These alterations to cardenolide structure over the course of evolution suggests that there may be context-dependent benefits to Erysimum to invest in one cardenolide variant over another.

Expression Atlas update: insights from sequencing data at both bulk and single cell level

Expression Atlas (www.ebi.ac.uk/gxa) and its newest counterpart the Single Cell Expression Atlas (www.ebi.ac.uk/gxa/sc) are EMBL-EBI's knowledgebases for gene and protein expression and localisation in bulk and at single cell level. These resources aim to allow users to investigate their expression in normal tissue (baseline) or in response to perturbations such as disease or changes to genotype (differential) across multiple species. Users are invited to search for genes or metadata terms across species or biological conditions in a standardised consistent interface. Alongside these data, new features in Single Cell Expression Atlas allow users to query metadata through our new cell type wheel search. At the experiment level data can be explored through two types of dimensionality reduction plots, t-distributed Stochastic Neighbor Embedding (tSNE) and Uniform Manifold Approximation and Projection (UMAP), overlaid with either clustering or metadata information to assist users' understanding. Data are also visualised as marker gene heatmaps identifying genes that help confer cluster identity. For some data, additional visualisations are available as interactive cell level anatomograms and cell type gene expression heatmaps.

Genotype and phenotype data standardization, utilization and integration in the big data era for agricultural sciences

Large-scale genotype and phenotype data have been increasingly generated to identify genetic markers, understand gene function and evolution and facilitate genomic selection. These datasets hold immense value for both current and future studies, as they are vital for crop breeding, yield improvement and overall agricultural sustainability. However, integrating these datasets from heterogeneous sources presents significant challenges and hinders their effective utilization. We established the Genotype-Phenotype Working Group in November 2021 as a part of the AgBioData Consortium (https://www.agbiodata.org) to review current data types and resources that support archiving, analysis and visualization of genotype and phenotype data to understand the needs and challenges of the plant genomic research community. For 2021-22, we identified different types of datasets and examined metadata annotations related to experimental design/methods/sample collection, etc. Furthermore, we thoroughly reviewed publicly funded repositories for raw and processed data as well as secondary databases and knowledgebases that enable the integration of heterogeneous data in the context of the genome browser, pathway networks and tissue-specific gene expression. Based on our survey, we recommend a need for (i) additional infrastructural support for archiving many new data types, (ii) development of community standards for data annotation and formatting, (iii) resources for biocuration and (iv) analysis and visualization tools to connect genotype data with phenotype data to enhance knowledge synthesis and to foster translational research. Although this paper only covers the data and resources relevant to the plant research community, we expect that similar issues and needs are shared by researchers working on animals. Database URL: https://www.agbiodata.org.

A de novo evolved gene contributes to rice grain shape difference between indica and japonica

The role of de novo evolved genes from non-coding sequences in regulating morphological differentiation between species/subspecies remains largely unknown. Here, we show that a rice de novo gene GSE9 contributes to grain shape difference between indica/xian and japonica/geng varieties. GSE9 evolves from a previous non-coding region of wild rice Oryza rufipogon through the acquisition of start codon. This gene is inherited by most japonica varieties, while the original sequence (absence of start codon, gse9) is present in majority of indica varieties. Knockout of GSE9 in japonica varieties leads to slender grains, whereas introgression to indica background results in round grains. Population evolutionary analyses reveal that gse9 and GSE9 are derived from wild rice Or-I and Or-III groups, respectively. Our findings uncover that the de novo GSE9 gene contributes to the genetic and morphological divergence between indica and japonica subspecies, and provide a target for precise manipulation of rice grain shape.

Loss of a Premature Stop Codon in the Rice Wall-Associated Kinase 91 (WAK91) Gene Is a Candidate for Improving Leaf Sheath Blight Disease Resistance

Leaf sheath blight disease (SB) of rice caused by the soil-borne fungus Rhizoctonia solani results in 10-30% global yield loss annually and can reach 50% under severe outbreaks. Many disease resistance genes and receptor-like kinases (RLKs) are recruited early on by the host plant to respond to pathogens. Wall-associated receptor kinases (WAKs), a subfamily of receptor-like kinases, have been shown to play a role in fungal defense. The rice gene WAK91 (OsWAK91), co-located in the major SB resistance QTL region on chromosome 9, was identified by us as a candidate in defense against rice sheath blight. An SNP mutation T/C in the WAK91 gene was identified in the susceptible rice variety Cocodrie (CCDR) and the resistant line MCR010277 (MCR). The consequence of the resistant allele C is a stop codon loss, resulting in an open reading frame with extra 62 amino acid carrying a longer protein kinase domain and additional phosphorylation sites. Our genotype and phenotype analysis of the parents CCDR and MCR and the top 20 individuals of the double haploid SB population strongly correlate with the SNP. The susceptible allele T is present in the japonica subspecies and most tropical and temperate japonica lines. Multiple US commercial rice varieties with a japonica background carry the susceptible allele and are known for SB susceptibility. This discovery opens the possibility of introducing resistance alleles into high-yielding commercial varieties to reduce yield losses incurred by the sheath blight disease.

Genetic resources and precise gene editing for targeted improvement of barley abiotic stress tolerance

Abiotic stresses, predominately drought, heat, salinity, cold, and waterlogging, adversely affect cereal crops. They limit barley production worldwide and cause huge economic losses. In barley, functional genes under various stresses have been identified over the years and genetic improvement to stress tolerance has taken a new turn with the introduction of modern gene-editing platforms. In particular, clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) is a robust and versatile tool for precise mutation creation and trait improvement. In this review, we highlight the stress-affected regions and the corresponding economic losses among the main barley producers. We collate about 150 key genes associated with stress tolerance and combine them into a single physical map for potential breeding practices. We also overview the applications of precise base editing, prime editing, and multiplexing technologies for targeted trait modification, and discuss current challenges including high-throughput mutant genotyping and genotype dependency in genetic transformation to promote commercial breeding. The listed genes counteract key stresses such as drought, salinity, and nutrient deficiency, and the potential application of the respective gene-editing technologies will provide insight into barley improvement for climate resilience.

Genome Context Viewer (GCV) version 2: enhanced visual exploration of multiple annotated genomes

The Genome Context Viewer is a web application for identifying, aligning, and visualizing genomic regions based on their micro and macrosyntenic structures. By using functional elements such as gene annotations as the unit of search and comparison, the Genome Context Viewer can compute and display relationships between regions across many assemblies from federated data sources in real-time, enabling users to rapidly explore multiple annotated genomes and identify divergence and structural events that can help provide insight into evolutionary mechanisms associated with functional consequences. In this work, we introduce version 2 of the Genome Context Viewer and highlight new features that enhance usability, performance, and ease of deployment.

Inference of differentially expressed genes using generalized linear mixed models in a pairwise fashion

Background

Technological advances involving RNA-Seq and Bioinformatics allow quantifying the transcriptional levels of genes in cells, tissues, and cell lines, permitting the identification of Differentially Expressed Genes (DEGs). DESeq2 and edgeR are well-established computational tools used for this purpose and they are based upon generalized linear models (GLMs) that consider only fixed effects in modeling. However, the inclusion of random effects reduces the risk of missing potential DEGs that may be essential in the context of the biological phenomenon under investigation. The generalized linear mixed models (GLMM) can be used to include both effects.

Methods

We present DEGRE (Differentially Expressed Genes with Random Effects), a user-friendly tool capable of inferring DEGs where fixed and random effects on individuals are considered in the experimental design of RNA-Seq research. DEGRE preprocesses the raw matrices before fitting GLMMs on the genes and the derived regression coefficients are analyzed using the Wald statistical test. DEGRE offers the Benjamini-Hochberg or Bonferroni techniques for P-value adjustment.

Results

The datasets used for DEGRE assessment were simulated with known identification of DEGs. These have fixed effects, and the random effects were estimated and inserted to measure the impact of experimental designs with high biological variability. For DEGs’ inference, preprocessing effectively prepares the data and retains overdispersed genes. The biological coefficient of variation is inferred from the counting matrices to assess variability before and after the preprocessing. The DEGRE is computationally validated through its performance by the simulation of counting matrices, which have biological variability related to fixed and random effects. DEGRE also provides improved assessment measures for detecting DEGs in cases with higher biological variability. We show that the preprocessing established here effectively removes technical variation from those matrices. This tool also detects new potential candidate DEGs in the transcriptome data of patients with bipolar disorder, presenting a promising tool to detect more relevant genes.

Conclusions

DEGRE provides data preprocessing and applies GLMMs for DEGs’ inference. The preprocessing allows efficient remotion of genes that could impact the inference. Also, the computational and biological validation of DEGRE has shown to be promising in identifying possible DEGs in experiments derived from complex experimental designs. This tool may help handle random effects on individuals in the inference of DEGs and presents a potential for discovering new interesting DEGs for further biological investigation.

Meta-QTL and haplo-pheno analysis reveal superior haplotype combinations associated with low grain chalkiness under high temperature in rice

Chalk, an undesirable grain quality trait in rice, is primarily formed due to high temperatures during the grain-filling process. Owing to the disordered starch granule structure, air spaces and low amylose content, chalky grains are easily breakable during milling thereby lowering head rice recovery and its market price. Availability of multiple QTLs associated with grain chalkiness and associated attributes, provided us an opportunity to perform a meta-analysis and identify candidate genes and their alleles contributing to enhanced grain quality. From the 403 previously reported QTLs, 64 Meta-QTLs encompassing 5262 non-redundant genes were identified. MQTL analysis reduced the genetic and physical intervals and nearly 73% meta-QTLs were narrower than 5cM and 2Mb, revealing the hotspot genomic regions. By investigating expression profiles of 5262 genes in previously published datasets, 49 candidate genes were shortlisted on the basis of their differential regulation in at least two of the datasets. We identified non-synonymous allelic variations and haplotypes in 39 candidate genes across the 3K rice genome panel. Further, we phenotyped a subset panel of 60 rice accessions by exposing them to high temperature stress under natural field conditions over two Rabi cropping seasons. Haplo-pheno analysis uncovered haplotype combinations of two starch synthesis genes, GBSSI and SSIIa, significantly contributing towards the formation of grain chalk in rice. We, therefore, report not only markers and pre-breeding material, but also propose superior haplotype combinations which can be introduced using either marker-assisted breeding or CRISPR-Cas based prime editing to generate elite rice varieties with low grain chalkiness and high HRY traits.

Oryzalexin S biosynthesis: a cross-stitched disappearing pathway

Rice produces many diterpenoid phytoalexins and, reflecting the importance of these natural products in this important cereal crop plant, its genome contains three biosynthetic gene clusters (BGCs) for such metabolism. The chromosome 4 BGC (c4BGC) is largely associated with momilactone production, in part due to the presence of the initiating syn-copalyl diphosphate (CPP) synthase gene (OsCPS4). Oryzalexin S is also derived from syn-CPP. However, the relevant subsequently acting syn-stemarene synthase gene (OsKSL8) is not located in the c4BGC. Production of oryzalexin S further requires hydroxylation at carbons 2 and 19 (C2 and C19), presumably catalyzed by cytochrome P450 (CYP) monooxygenases. Here it is reported the closely related CYP99A2 and CYP99A3, whose genes are also found in the c4BGC catalyze the necessary C19-hydroxylation, while the closely related CYP71Z21 and CYP71Z22, whose genes are found in the recently reported chromosome 7 BGC (c7BGC), catalyze subsequent hydroxylation at C2α. Thus, oryzalexin S biosynthesis utilizes two distinct BGCs, in a pathway cross-stitched together by OsKSL8. Notably, in contrast to the widely conserved c4BGC, the c7BGC is subspecies (ssp.) specific, being prevalent in ssp. japonica and only rarely found in the other major ssp. indica. Moreover, while the closely related syn-stemodene synthase OsKSL11 was originally considered to be distinct from OsKSL8, it has now been reported to be a ssp. indica derived allele at the same genetic loci. Intriguingly, more detailed analysis indicates that OsKSL8(j) is being replaced by OsKSL11 (OsKSL8i), suggesting introgression from ssp. indica to (sub)tropical japonica, with concurrent disappearance of oryzalexin S production.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42994-022-00092-3.

Genomic Survey of Flavin Monooxygenases in Wild and Cultivated Rice Provides Insight into Evolution and Functional Diversities

The flavin monooxygenase (FMO) enzyme was discovered in mammalian liver cells that convert a carcinogenic compound, N-N'-dimethylaniline, into a non-carcinogenic compound, N-oxide. Since then, many FMOs have been reported in animal systems for their primary role in the detoxification of xenobiotic compounds. In plants, this family has diverged to perform varied functions like pathogen defense, auxin biosynthesis, and S-oxygenation of compounds. Only a few members of this family, primarily those involved in auxin biosynthesis, have been functionally characterized in plant species. Thus, the present study aims to identify all the members of the FMO family in 10 different wild and cultivated Oryza species. Genome-wide analysis of the FMO family in different Oryza species reveals that each species has multiple FMO members in its genome and that this family is conserved throughout evolution. Taking clues from its role in pathogen defense and its possible function in ROS scavenging, we have also assessed the involvement of this family in abiotic stresses. A detailed in silico expression analysis of the FMO family in Oryza sativa subsp. japonica revealed that only a subset of genes responds to different abiotic stresses. This is supported by the experimental validation of a few selected genes using qRT-PCR in stress-sensitive Oryza sativa subsp. indica and stress-sensitive wild rice Oryza nivara. The identification and comprehensive in silico analysis of FMO genes from different Oryza species carried out in this study will serve as the foundation for further structural and functional studies of FMO genes in rice as well as other crop types.

Deciphering Macromolecular Interactions Involved in Abiotic Stress Signaling: A Review of Bioinformatics Analysis

Plant functioning and responses to abiotic stresses largely involve regulations at the transcriptomic level via complex interactions of signal molecules, signaling cascades, and regulators. Nevertheless, all the signaling networks involved in responses to abiotic stresses have not yet been fully established. The in-depth analysis of transcriptomes in stressed plants has become a relevant state-of-the-art methodology to study these regulations and signaling pathways that allow plants to cope with or attempt to survive abiotic stresses. The plant science and molecular biology community has developed databases about genes, proteins, protein-protein interactions, protein-DNA interactions and ontologies, which are valuable sources of knowledge for deciphering such regulatory and signaling networks. The use of these data and the development of bioinformatics tools help to make sense of transcriptomic data in specific contexts, such as that of abiotic stress signaling, using functional biological approaches. The aim of this chapter is to present and assess some of the essential online tools and resources that will allow novices in bioinformatics to decipher transcriptomic data in order to characterize the cellular processes and functions involved in abiotic stress responses and signaling. The analysis of case studies further describes how these tools can be used to conceive signaling networks on the basis of transcriptomic data. In these case studies, particular attention was paid to the characterization of abiotic stress responses and signaling related to chemical and xenobiotic stressors.

Coexpression network and trans-activation analyses of maize reproductive phasiRNA loci

The anther-enriched phased, small interfering RNAs (phasiRNAs) play vital roles in sustaining male fertility in grass species. Their long non-coding precursors are synthesized by RNA polymerase II and are likely regulated by transcription factors (TFs). A few putative transcriptional regulators of the 21- or 24-nucleotide phasiRNA loci (referred to as 21- or 24-PHAS loci) have been identified in maize (Zea mays), but whether any of the individual TFs or TF combinations suffice to activate any PHAS locus is unclear. Here, we identified the temporal gene coexpression networks (modules) associated with maize anther development, including two modules highly enriched for the 21- or 24-PHAS loci. Comparisons of these coexpression modules and gene sets dysregulated in several reported male sterile TF mutants provided insights into TF timing with regard to phasiRNA biogenesis, including antagonistic roles for OUTER CELL LAYER4 and MALE STERILE23. Trans-activation assays in maize protoplasts of individual TFs using bulk-protoplast RNA-sequencing showed that two of the TFs coexpressed with 21-PHAS loci could activate several 21-nucleotide phasiRNA pathway genes but not transcription of 21-PHAS loci. Screens for combinatorial activities of these TFs and, separately, the recently reported putative transcriptional regulators of 24-PHAS loci using single-cell (protoplast) RNA-sequencing, did not detect reproducible activation of either 21-PHAS or 24-PHAS loci. Collectively, our results suggest that the endogenous transcriptional machineries and/or chromatin states in the anthers are necessary to activate reproductive PHAS loci.

SbCASP-LP1C1 improves salt exclusion by enhancing the root apoplastic barrier

Sweet sorghum [Sorghum bicolor (L.) Moench], a C4 crop with high biomass and strong resistance to multiple stresses, can grow and reproduce in saline-alkaline soil and is an ideal raw material for biofuels. Under high-salinity conditions, sweet sorghum shows extensive salt exclusion. However, the specific molecular mechanism of the apoplastic barrier in salt exclusion is unknown. In this study, SbCASP-LP1C1 (a CASP-like protein1C1) was localized in the plasma membrane of sweet sorghum root endodermal cells, and its function was further studied by heterologous expression in Arabidopsis (35 S:SbCASP-LP1C1-GFP). When germinated and grown on 50 mM NaCl, the SbCASP-LP1C1-expressing lines had longer roots and a higher salinity threshold compared with wild-type (Col-0) plant and the casp-lp T-DNA insertion mutant in Arabidopsis. The 35 S:SbCASP-LP1C1-GFP lines also suffered less oxidative damage as determined by DAB and NBT staining, and the expression levels of several antioxidant genes were higher in these lines. Moreover, the stele of 35 S:SbCASP-LP1C1-GFP lines was less permeable to propidium iodide, and these plants contained less Na⁺ in their shoots and roots compared to wild type and casp-lp. In the 35 S:SbCASP-LP1C1-GFP lines, the expression levels of two Casparian strip synthesis genes, MYB36 and ESB1, were increased. These results indicate that SbCASP-LP1C1 may be involved in the polymerization of lignin monomers in the Casparian strip of sweet sorghum, thereby regulating salt tolerance. These results provide a theoretical basis to understand the role of plant roots in salt exclusion and a means by which to improve the salt tolerance of crops.