Brachypodium distachyon and Setaria viridis: Model Genetic Systems for the Grasses

Adaptive optical third-harmonic generation microscopy for in vivo imaging of tissues

Third-harmonic generation microscopy is a powerful label-free nonlinear imaging technique, providing essential information about structural characteristics of cells and tissues without requiring external labelling agents. In this work, we integrated a recently developed compact adaptive optics module into a third-harmonic generation microscope, to measure and correct for optical aberrations in complex tissues. Taking advantage of the high sensitivity of the third-harmonic generation process to material interfaces and thin membranes, along with the 1,300-nm excitation wavelength used here, our adaptive optical third-harmonic generation microscope enabled high-resolution in vivo imaging within highly scattering biological model systems. Examples include imaging of myelinated axons and vascular structures within the mouse spinal cord and deep cortical layers of the mouse brain, along with imaging of key anatomical features in the roots of the model plant Brachypodium distachyon. In all instances, aberration correction led to enhancements in image quality.

Adaptive optical third-harmonic generation microscopy for in vivo imaging of tissues

Third-harmonic generation microscopy is a powerful label-free nonlinear imaging technique, providing essential information about structural characteristics of cells and tissues without requiring external labelling agents. In this work, we integrated a recently developed compact adaptive optics module into a third-harmonic generation microscope, to measure and correct for optical aberrations in complex tissues. Taking advantage of the high sensitivity of the third-harmonic generation process to material interfaces and thin membranes, along with the 1,300-nm excitation wavelength used here, our adaptive optical third-harmonic generation microscope enabled high-resolution in vivo imaging within highly scattering biological model systems. Examples include imaging of myelinated axons and vascular structures within the mouse spinal cord and deep cortical layers of the mouse brain, along with imaging of key anatomical features in the roots of the model plant Brachypodium distachyon. In all instances, aberration correction led to significant enhancements in image quality.

Trehalose 6-phosphate metabolism in C4 species

Trehalose 6-phosphate (Tre6P), the intermediate of trehalose biosynthesis, is an essential signal metabolite in plants, linking growth and development to carbon status. Our current understanding of Tre6P metabolism and signaling pathways in plants is based almost entirely on studies performed with Arabidopsis thaliana, a model plant that performs C₃ photosynthesis. Conversely, our knowledge on the molecular mechanisms involved in Tre6P regulation of carbon partitioning and metabolism in C₄ plants is scarce. This topic is especially relevant due to the agronomic importance of crops performing C₄ photosynthesis, such as maize, sorghum and sugarcane. In this review, we focused our attention on recent developments related to Tre6P metabolism in C₄ species and raised some open questions that should be addressed in the near future to improve the yield of economically important crops.

A Survey of the Transcriptomic Resources in Durum Wheat: Stress Responses, Data Integration and Exploitation

Durum wheat (Triticum turgidum subsp. durum (Desf.) Husn.) is an allotetraploid cereal crop of worldwide importance, given its use for making pasta, couscous, and bulgur. Under climate change scenarios, abiotic (e.g., high and low temperatures, salinity, drought) and biotic (mainly exemplified by fungal pathogens) stresses represent a significant limit for durum cultivation because they can severely affect yield and grain quality. The advent of next-generation sequencing technologies has brought a huge development in transcriptomic resources with many relevant datasets now available for durum wheat, at various anatomical levels, also focusing on phenological phases and environmental conditions. In this review, we cover all the transcriptomic resources generated on durum wheat to date and focus on the corresponding scientific insights gained into abiotic and biotic stress responses. We describe relevant databases, tools and approaches, including connections with other "omics" that could assist data integration for candidate gene discovery for bio-agronomical traits. The biological knowledge summarized here will ultimately help in accelerating durum wheat breeding.

The CC-NB-LRR protein BSR1 from Brachypodium confers resistance to Barley stripe mosaic virus in gramineous plants by recognising TGB1 movement protein

Although some nucleotide binding, leucine-rich repeat immune receptor (NLR) proteins conferring resistance to specific viruses have been identified in dicot plants, NLR proteins involved in viral resistance have not been described in monocots. We have used map-based cloning to isolate the CC-NB-LRR (CNL) Barley stripe mosaic virus (BSMV) resistance gene barley stripe resistance 1 (BSR1) from Brachypodium distachyon Bd3-1 inbred line. Stable BSR1 transgenic Brachypodium line Bd21-3, barley (Golden Promise) and wheat (Kenong 199) plants developed resistance against BSMV ND18 strain. Allelic variation analyses indicated that BSR1 is present in several Brachypodium accessions collected from countries in the Middle East. Protein domain swaps revealed that the intact LRR domain and the C-terminus of BSR1 are required for resistance. BSR1 interacts with the BSMV ND18 TGB1 protein in planta and shows temperature-sensitive antiviral resistance. The R³⁹⁰ and T³⁹² residues of TGB1_ND (ND18 strain) and the G¹⁹⁶ and K¹⁹⁷ residues within the BSR1 P-loop motif are key amino acids required for immune activation. BSR1 is the first cloned virus resistance gene encoding a typical CNL protein in monocots, highlighting the utility of the Brachypodium model for isolation and analysis of agronomically important genes for crop improvement.

Genome-Wide Identification of Maize Protein Arginine Methyltransferase Genes and Functional Analysis of ZmPRMT1 Reveal Essential Roles in Arabidopsis Flowering Regulation and Abiotic Stress Tolerance

Histone methylation, as one of the important epigenetic regulatory mechanisms, plays a significant role in growth and developmental processes and stress responses of plants, via altering the methylation status or ratio of arginine and lysine residues of histone tails, which can affect the regulation of gene expression. Protein arginine methyltransferases (PRMTs) have been revealed to be responsible for histone methylation of specific arginine residues in plants, which is important for maintaining pleiotropic development and adaptation to abiotic stresses in plants. Here, for the first time, a total of eight PRMT genes in maize have been identified and characterized in this study, named as ZmPRMT1-8. According to comparative analyses of phylogenetic relationship and structural characteristics among PRMT gene family members from several representative species, all maize 8 PRMT proteins were categorized into three distinct subfamilies. Further, schematic structure and chromosome location analyses displayed evolutionarily conserved structure features and an unevenly distribution on maize chromosomes of ZmPRMT genes, respectively. The expression patterns of ZmPRMT genes in different tissues and under various abiotic stresses (heat, drought, and salt) were determined. The expression patterns of ZmPRMT genes indicated that they play a role in regulating growth and development and responses to abiotic stress. Eventually, to verify the biological roles of ZmPRMT genes, the transgenic Arabidopsis plants overexpressing ZmPRMT1 gene was constructed as a typical representative. The results demonstrated that overexpression of ZmPRMT1 can promote earlier flowering time and confer enhanced heat tolerance in transgenic Arabidopsis. Taken together, our results are the first to report the roles of ZmPRMT1 gene in regulating flowering time and resisting heat stress response in plants and will provide a vital theoretical basis for further unraveling the functional roles and epigenetic regulatory mechanism of ZmPRMT genes in maize growth, development and responses to abiotic stresses.

MicroRNAs expression profiles in early responses to different levels of water deficit in Setaria viridis

Water deficit is a major constraint for crops of economic importance in almost all agricultural regions. However, plants have an active defense system to adapt to these adverse conditions, acting in the reprogramming of gene expression responsible for encoding microRNAs (miRNAs). These miRNAs promote the regulation to the target gene expression by the post-transcriptional (PTGS) and transcriptional gene silencing (TGS), modulating several pathways including defense response to water deficit. The broader knowledge of the miRNA expression profile and its regulatory networks in response to water deficit can provide evidence for the development of new biotechnological tools for genetic improvement of several important crops. In this study, we used Setaria viridis accession A10.1 as a C4 model plant to widely investigate the miRNA expression profile in early responses to different levels of water deficit. Ecophysiological studies in Setaria viridis under water deficit and after rewatering demonstrated a drought tolerant accession, capable of a rapid recovery from the stress. Deep small RNA sequencing and degradome studies were performed in plants submitted to drought to identify differentially expressed miRNA genes and their predicted targets, using in silico analysis. Our findings showed that several miRNAs were differentially modulated in response to distinctive levels of water deficit and after rewatering. The predicted mRNA targets mainly corresponded to genes related to cell wall remodeling, antioxidant system and drought-related transcription factors, indicating that these genes are rapidly regulated in early responses to drought stress. The implications of these modulations are extensively discussed, and higher-effect miRNAs are suggested as major players for potential use in genetic engineering to improve drought tolerance in economically important crops, such as sugarcane, maize, and sorghum.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-022-01226-z.

SvSTL1 in the large subunit family of ribonucleotide reductases plays a major role in chloroplast development of Setaria viridis

Ribonucleotide reductases (RNRs) are essential enzymes in DNA synthesis. However, little is known about the RNRs in plants. Here, we identified a svstl1 mutant from the self-created ethyl methanesulfonate (EMS) mutant library of Setaria viridis. The mutant leaves exhibited a bleaching phenotype at the heading stage. Paraffin section analysis showed the destruction of the C₄ Kranz anatomy. Transmission electron microscopy results further demonstrated the severely disturbed development of some chloroplasts. MutMap analysis revealed that the SvSTL1 gene is the primary candidate, encoding a large subunit of RNRs. Complementation experiments confirmed that SvSTL1 is responsible for the phenotype of svstl1. There are two additional RNR large subunit homologs in S. viridis, SvSTL2 and SvSTL3. To further understand the functions of these three RNR large subunit genes, a series of mutants were generated via CRISPR/Cas9 technology. In striking contrast to the finding that all three SvSTLs interact with the RNR small subunit, the phenotype varied along with the copies of chloroplast genome among different svstl single mutants: the svstl1 mutant exhibited pronounced chloroplast development and significantly fewer copies of the chloroplast genome than the svstl2 or svstl3 single mutants. These results suggested that SvSTL1 plays a major role in the optimal function of RNRs and is essential for chloroplast development. Furthermore, through the analysis of double and triple mutants, the study provides new insights into the finely tuned coordination among SvSTLs to maintain normal chloroplast development in the emerging C₄ model plant S. viridis.

Mass spectral imaging showing the plant growth-promoting rhizobacteria's effect on the Brachypodium awn

The plant growth-promoting rhizobacteria (PGPR) on the host plant surface play a key role in biological control and pathogenic response in plant functions and growth. However, it is difficult to elucidate the PGPR effect on plants. Such information is important in biomass production and conversion. Brachypodium distachyon (Brachypodium), a genomics model for bioenergy and native grasses, was selected as a C3 plant model; and the Gram-negative Pseudomonas fluorescens SBW25 (P.) and Gram-positive Arthrobacter chlorophenolicus A6 (A.) were chosen as representative PGPR strains. The PGPRs were introduced to the Brachypodium seed's awn prior to germination, and their possible effects on the seeding and growth were studied using different modes of time-of-flight secondary ion mass spectrometry (ToF-SIMS) measurements, including a high mass-resolution spectral collection and delayed image extraction. We observed key plant metabolic products and biomarkers, such as flavonoids, phenolic compounds, fatty acids, and auxin indole-3-acetic acid in the Brachypodium awns. Furthermore, principal component analysis and two-dimensional imaging analysis reveal that the Brachypodium awns are sensitive to the PGPR, leading to chemical composition and morphology changes on the awn surface. Our results show that ToF-SIMS can be an effective tool to probe cell-to-cell interactions at the biointerface. This work provides a new approach to studying the PGPR effects on awn and shows its potential for the research of plant growth in the future.

Improvement of small seed for big nutritional feed

Exploding global population, rapid urbanization, salinization of soils, decreasing arable land availability, groundwater resources, and dynamic climatic conditions pose impending damage to our food security by reducing the grain quality and quantity. This issue is further compounded in arid and semi-arid regions due to the shortage of irrigation water and erratic rainfalls. Millets are gluten (a family of proteins)-free and cultivated all over the globe for human consumption, fuel, feed, and fodder. They provide nutritional security for the under- and malnourished. With the deployment of strategies like foliar spray, traditional/marker-assisted breeding, identification of candidate genes for the translocation of important minerals, and genome-editing technologies, it is now tenable to biofortify important millets. Since the bioavailability of iron and zinc has been proven in human trials, the challenge is to make such grains accessible. This review encompasses nutritional benefits, progress made, challenges being encountered, and prospects of enriching millet crops with essential minerals.

Differential Gene Expression of Brachypodium distachyon Roots Colonized by Gluconacetobacter diazotrophicus and the Role of BdCESA8 in the Colonization

Alternatives to synthetic nitrogen fertilizer are needed to reduce the costs of crop production and offset environmental damage. Nitrogen-fixing bacterium Gluconacetobacter diazotrophicus has been proposed as a possible biofertilizer for monocot crop production. However, the colonization of G. diazotrophicus in most monocot crops is limited and deep understanding of the response of host plants to G. diazotrophicus colonization is still lacking. In this study, the molecular response of the monocot plant model Brachypodium distachyon was studied during G. diazotrophicus root colonization. The gene expression profiles of B. distachyon root tissues colonized by G. diazotrophicus were generated via next-generation RNA sequencing, and investigated through gene ontology and metabolic pathway analysis. The RNA sequencing results indicated that Brachypodium is actively involved in G. diazotrophicus colonization via cell wall synthesis. Jasmonic acid, ethylene, gibberellin biosynthesis. nitrogen assimilation, and primary and secondary metabolite pathways are also modulated to accommodate and control the extent of G. diazotrophicus colonization. Cellulose synthesis is significantly downregulated during colonization. The loss of function mutant for Brachypodium cellulose synthase 8 (BdCESA8) showed decreased cellulose content in xylem and increased resistance to G. diazotrophicus colonization. This result suggested that the cellulose synthesis of the secondary cell wall is involved in G. diazotrophicus colonization. The results of this study provide insights for future research in regard to gene manipulation for efficient colonization of nitrogen-fixing bacteria in Brachypodium and monocot crops.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Efficient genome editing and gene knockout in Setaria viridis with CRISPR/Cas9 directed gene editing by the non-homologous end-joining pathway

The CRISPR/Cas9 system has been used for genome editing in several organisms, including higher plants. This system induces site-specific mutations in the genome based on the nucleotide sequence of engineered guide RNAs. The complex genomes of C4 grasses makes genome editing a challenge in key grass crops like maize (Zea mays), sorghum (Sorghum bicolor), Brachiaria spp., switchgrass (Panicum virgatum), and sugarcane (Saccharum spp.). Setaria viridis is a diploid C4 grass widely used as a model for these C4 crop plants. Here, an optimized CRISPR/Cas9 binary vector that exploits the non-homologous end joining (NHEJ) system was used to knockout a green fluorescent protein (gfp) transgene in S. viridis accession A10.1. Transformation of embryogenic callus by A. tumefaciens generated ten glufosinate-ammonium resistant transgenic events. In the T0 generation, 60% of the events were biallelic mutants in the gfp transgene with no detectable accumulation of GFP protein and without insertions or deletions in predicted off-target sites. The gfp mutations generated by CRISPR/Cas9 were stable and displayed Mendelian segregation in the T1 generation. Altogether, the system described here is a highly efficient genome editing system for S. viridis, an important model plant for functional genomics studies in C4 grasses. Also, this system is a potential tool for improvement of agronomic traits in C4 crop plants with complex genomes.

High-depth resequencing of 312 accessions reveals the local adaptation of foxtail millet

Based on the high-density variation map, we identified genome-level evidence for local adaptation and demonstrated that Siprr37 with transposon insertion contributes to the fitness of foxtail millet in the northeastern ecoregion. Adaptation is a robust way through which plants are able to overcome environmental constraints. The mechanisms of adaptation in heterogeneous natural environments are largely unknown. Deciphering the genomic basis of local adaptation will contribute to further improvement in domesticated plants. To this end, we describe a high-depth (19.4 ×) haplotype map of 3.02 million single nucleotide polymorphisms in foxtail millet (Setaria italica) from whole-genome resequencing of 312 accessions. In the genome-wide scan, we identified a set of improvement signals (including the homologous gene of OsIPA1, a key gene controlling ideal plant architecture) related to the geographical adaptation to four ecoregions in China. In particular, based on the genome-wide association analysis results, we identified the contribution of a pseudo-response regulator gene, SiPRR37, to heading date adaptation in foxtail millet. We observed the expression changes of SiPRR37 resulted from a key Tc1-Mariner transposon insertion in the first intron. Positive selection analyses revealed that SiPRR37 mainly contributed to the adaptation of northeastern ecoregions. Taken together, foxtail millet adapted to the northeastern region by regulating the function of SiPRR37, which sheds lights on genome-level evidence for adaptive geographical divergence. Besides, our data provide a nearly complete catalog of genomic variation aiding the identification of functionally important variants.

Diazotrophic Bacteria and Their Mechanisms to Interact and Benefit Cereals

Plant-growth-promoting bacteria (PGPB) stimulate plant growth through diverse mechanisms. In addition to biological nitrogen fixation, diazotrophic PGPB can improve nutrient uptake efficiency from the soil, produce and release phytohormones to the host, and confer resistance against pathogens. The genetic determinants that drive the success of biological nitrogen fixation in nonlegume plants are understudied. These determinants include recognition and signaling pathways, bacterial colonization, and genotype specificity between host and bacteria. This review presents recent discoveries of how nitrogen-fixing PGPB interact with cereals and promote plant growth. We suggest adopting an experimental model system, such as the Setaria-diazotrophic bacteria association, as a reliable way to better understand the associated mechanisms and, ultimately, increase the use of PGPB inoculants for sustainable agriculture.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Population genomic structure of Eurasian and African foxtail millet landrace accessions inferred from genotyping-by-sequencing

Foxtail millet [Setaria italica (L.) P. Beauv.] is the second most important millet species globally and is adapted to cultivation in diverse environments. Like its wild progenitor, green foxtail [S. viridis (L.) P. Beauv.], it is a model species for C4 photosynthetic pathways and stress tolerance genes in related bioenergy crops. We addressed questions regarding the evolution and spread of foxtail millet through a population genomic study of landraces from across its cultivated range in Europe, Asia, and Africa. We sought to determine population genomic structure and the relationship of domesticated lineages relative to green foxtail. Further, we aimed to identify genes involved in environmental stress tolerance that have undergone differential selection between geographical and genetic groups. Foxtail millet landrace accessions (n = 328) and green foxtail accessions (n = 12) were sequenced by genotyping-by-sequencing (GBS). After filtering, 5,677 single nucleotide polymorphisms (SNPs) were retained for the combined foxtail millet-green foxtail dataset and 5,020 for the foxtail millet dataset. We extended geographic coverage of green foxtail by including previously published GBS sequence tags, yielding a 4,515-SNP dataset for phylogenetic reconstruction. All foxtail millet samples were monophyletic relative to green foxtail, suggesting a single origin of foxtail millet, although no group of foxtail millet was clearly the most ancestral. Four genetic clusters were found within foxtail millet, each with a distinctive geographical distribution. These results, together with archaeobotanical evidence, suggest plausible routes of spread of foxtail millet. Selection scans identified nine candidate genes potentially involved in environmental adaptations, particularly to novel climates encountered, as domesticated foxtail millet spread to new altitudes and latitudes.

Comparative Analysis of Transcriptome and sRNAs Expression Patterns in the Brachypodium distachyon- Magnaporthe oryzae Pathosystems

The hemibiotrophic fungus Magnaporthe oryzae (Mo) is the causative agent of rice blast and can infect aerial and root tissues of a variety of Poaceae, including the model Brachypodium distachyon (Bd). To gain insight in gene regulation processes occurring at early disease stages, we comparatively analyzed fungal and plant mRNA and sRNA expression in leaves and roots. A total of 310 Mo genes were detected consistently and differentially expressed in both leaves and roots. Contrary to Mo, only minor overlaps were observed in plant differentially expressed genes (DEGs), with 233 Bd-DEGs in infected leaves at 2 days post inoculation (DPI), compared to 4978 at 4 DPI, and 138 in infected roots. sRNA sequencing revealed a broad spectrum of Mo-sRNAs that accumulated in infected tissues, including candidates predicted to target Bd mRNAs. Conversely, we identified a subset of potential Bd-sRNAs directed against fungal cell wall components, virulence genes and transcription factors. We also show a requirement of operable RNAi genes from the DICER-like (DCL) and ARGONAUTE (AGO) families for fungal virulence. Overall, our work elucidates the extensive reprogramming of transcriptomes and sRNAs in both plant host (Bd) and fungal pathogen (Mo), further corroborating the critical role played by sRNA species in the establishment of the interaction and its outcome.

Setaria viridis chlorotic and seedling-lethal mutants define critical functions for chloroplast gene expression

Deep insights into chloroplast biogenesis have been obtained by mutant analysis; however, in C₄ plants a relevant mutant collection has only been developed and exploited for maize. Here, we report the initial characterization of an ethyl methyl sulfonate-induced mutant population for the C₄ model Setaria viridis. Approximately 1000 M₂ families were screened for the segregation of pale-green seedlings in the M₃ generation, and a subset of these was identified to be deficient in post-transcriptional steps of chloroplast gene expression. Causative mutations were identified for three lines using deep sequencing-based bulked segregant analysis, and in one case confirmed by transgenic complementation. Using chloroplast RNA-sequencing and other molecular assays, we describe phenotypes of mutants deficient in PSRP7, a plastid-specific ribosomal protein, OTP86, an RNA editing factor, and cpPNP, the chloroplast isozyme of polynucleotide phosphorylase. The psrp mutant is globally defective in chloroplast translation, and has varying deficiencies in the accumulation of chloroplast-encoded proteins. The otp86 mutant, like its Arabidopsis counterpart, is specifically defective in editing of the rps14 mRNA; however, the conditional pale-green mutant phenotype contrasts with the normal growth of the Arabidopsis mutant. The pnp mutant exhibited multiple defects in 3' end maturation as well as other qualitative changes in the chloroplast RNA population. Overall, our collection opens the door to global analysis of photosynthesis and early seedling development in an emerging C₄ model.

Photosynthetic and transcriptomic responses of two C4 grass species with different NaCl tolerance

This report reveals the effects of salt on the photosynthetic electron transport and transcriptome of the glycophyte Setaria viridis (S. viridis) and its salt-tolerant close relative halophyte Spartina alterniflora (S. alterniflora). S. viridis was unable to survive exposed to sodium chloride (NaCl) levels higher than 100 mM, in contrast, S. alterniflora could tolerate NaCl up to 550 mM, with negligible effect on gas exchange related parameters and conductance of electrons transport chain (g_ETC). Under salt, the prompt fluorescence (OJIP-curves) exhibits an increase in the O- and J-steps in S. viridis and much less for S. alterniflora. Flowing NaCl stress, a dramatic decline in the photosystem II (PSII) primary photochemistry was observed for S. viridis, as reflected by the drastic drop in F_v/F_m, F_v/F_o and Φ_PSII; however, no substantial change was recorded for these parameters in S. alterniflora. Interestingly, we found an increase in the primary PSII photochemistry (Φ_PSII) for S. alterniflora with increasing either NaCl concentration or NaCl treatment duration. The NPQ magnitude was strongly enhanced for S. viridis even at a low NaCl (50 mM); however, it remains unchangeable or slightly increased for S. alterniflora at NaCl levels above 400 mM. After NaCl treatment, we found an increase in both the proportion of oxidized P₇₀₀ and the amount of active P₇₀₀ in S. viridis and almost no change for S. alterniflora. Under salt, the net photosynthetic rate (A) and stomatal conductance (gs) measurements demonstrate that A decreases earlier in S. viridis, even after one week exposure to only 50 mM NaCl; in contrast, in S. alterniflora, the effect of NaCl on A and g_s was minor even after exposure for two weeks to high NaCl levels. For S. viridis exposed to 50 mM NaCl for 12 d, carbon dioxide (CO₂) at a concentration of 2000 μL L^-1 could not fully restore A to the control (Ctrl) level. Conversely, in S. alterniflora, high CO₂ can fully restore A for all NaCl treatments except at 550 mM. RNA-seq data shows a major impact of NaCl on metabolic pathways in S. viridis and we found a number of transcription factors potentially related to NaCl responses. For S. alterniflora, no major changes in the transcriptomic levels were recorded under NaCl stress. To confirm our data analysis of RNA-seq, we performed quantitative reverse transcription polymerase chain reaction (qRT-PCR) analysis for randomly selected four genes for each species (8 genes in total) and we found that our results (up- and/or down-regulated genes) are fully consistent and match well our RNA-seq data. Overall, this study showed drastically different photosynthetic and transcriptomic responses of a salt-tolerant C₄ grass species and one salt-sensitive C₄ grass species to NaCl stress, which suggests that S. alterniflora could be used as a promising model species to study salt tolerance in C₄ or monocot species.

A mini foxtail millet with an Arabidopsis-like life cycle as a C4 model system

Foxtail millet (Setaria italica) is an important crop species and an emerging model plant for C₄ grasses. However, functional genomics research on foxtail millet is challenging because of its long generation time, relatively large stature and recalcitrance to genetic transformation. Here we report the development of xiaomi, a rapid-cycling mini foxtail millet mutant as a C₄ model system. Five to six generations of xiaomi can be grown in a year in growth chambers due to its short life cycle and small plant size, similar to Arabidopsis. A point mutation in the Phytochrome C (PHYC) gene was found to be causal for these characteristics. PHYC encodes a light receptor essential for photoperiodic flowering. A reference-grade xiaomi genome comprising 429.94 Mb of sequence was assembled and a gene-expression atlas from 11 different tissues was developed. These resources, together with an established highly efficient transformation system and a multi-omics database, make xiaomi an ideal model system for functional studies of C₄ plants.

Defensins of Grasses: A Systematic Review

The grass family (Poaceae) is one of the largest families of flowering plants, growing in all climatic zones of all continents, which includes species of exceptional economic importance. The high adaptability of grasses to adverse environmental factors implies the existence of efficient resistance mechanisms that involve the production of antimicrobial peptides (AMPs). Of plant AMPs, defensins represent one of the largest and best-studied families. Although wheat and barley seed γ-thionins were the first defensins isolated from plants, the functional characterization of grass defensins is still in its infancy. In this review, we summarize the current knowledge of the characterized defensins from cultivated and selected wild-growing grasses. For each species, isolation of defensins or production by heterologous expression, peptide structure, biological activity, and structure-function relationship are described, along with the gene expression data. We also provide our results on in silico mining of defensin-like sequences in the genomes of all described grass species and discuss their potential functions. The data presented will form the basis for elucidation of the mode of action of grass defensins and high adaptability of grasses to environmental stress and will provide novel potent molecules for practical use in medicine and agriculture.

Plant science's next top models

BACKGROUND

Model organisms are at the core of life science research. Notable examples include the mouse as a model for humans, baker's yeast for eukaryotic unicellular life and simple genetics, or the enterobacteria phage λ in virology. Plant research was an exception to this rule, with researchers relying on a variety of non-model plants until the eventual adoption of Arabidopsis thaliana as primary plant model in the 1980s. This proved to be an unprecedented success, and several secondary plant models have since been established. Currently, we are experiencing another wave of expansion in the set of plant models.

SCOPE

Since the 2000s, new model plants have been established to study numerous aspects of plant biology, such as the evolution of land plants, grasses, invasive and parasitic plant life, adaptation to environmental challenges, and the development of morphological diversity. Concurrent with the establishment of new plant models, the advent of the 'omics' era in biology has led to a resurgence of the more complex non-model plants. With this review, we introduce some of the new and fascinating plant models, outline why they are interesting subjects to study, the questions they will help to answer, and the molecular tools that have been established and are available to researchers.

CONCLUSIONS

Understanding the molecular mechanisms underlying all aspects of plant biology can only be achieved with the adoption of a comprehensive set of models, each of which allows the assessment of at least one aspect of plant life. The model plants described here represent a step forward towards our goal to explore and comprehend the diversity of plant form and function. Still, several questions remain unanswered, but the constant development of novel technologies in molecular biology and bioinformatics is already paving the way for the next generation of plant models.

Mechanical stimulation in Brachypodium distachyon: Implications for fitness, productivity, and cell wall properties

Mechanical stimulation, including exposure to wind, is a common environmental variable for plants. However, knowledge about the morphogenetic response of the grasses (Poaceae) to mechanical stimulation and impact on relevant agronomic traits is very limited. Two natural accessions of Brachypodium distachyon were exposed to wind and mechanical treatments. We surveyed a wide range of stem-related traits to determine the effect of the two treatments on plant growth, development, and stem biomass properties. Both treatments induced significant quantitative changes across multiple scales, from the whole plant down to cellular level. The two treatments resulted in shorter stems, reduced biomass, increased tissue rigidity, delayed flowering, and reduced seed yield in both accessions. Among changes in cell wall-related features, a substantial increase in lignin content and pectin methylesterase activity was most notable. Mechanical stimulation also reduced the enzymatic sugar release from the cell wall, thus increasing biomass recalcitrance. Notably, treatments had a distinct and opposite effect on vascular bundle area in the two accessions, suggesting genetic variation in modulating these responses to mechanical stimulation. Our findings highlight that exposure of grasses to mechanical stimulation is a relevant environmental factor affecting multiple traits important for their utilization in food, feed, and bioenergy applications.

Drought-inducible changes in the histone modification H3K9ac are associated with drought-responsive gene expression in Brachypodium distachyon

H3K9ac, an epigenetic marker, is widely distributed in plant genomes. H3K9ac enhances gene expression, which is highly conserved in eukaryotes. However, genome-wide studies of H3K9ac in monocot species are limited, and the changes in H3K9ac under drought stress for individual genes are still not clear. We analysed changes in the H3K9ac level of Brachypodium distachyon under 20% PEG-6000-simulated drought stress conditions. We also performed chromatin immunoprecipitation, followed by next generation sequencing (ChIP-seq) on H3K9ac to reveal changes in H3K9ac for individual genes at the genome-wide level. Our study showed that H3K9ac was mainly enriched in gene exon regions. Drought increased or decreased the H3K9ac level at specific genomic loci. We identified 40 genes associated with increased H3K9ac levels and 36 genes associated with decreased H3K9ac levels under drought stress. Further, RT-qPCR analyses showed that H3K9ac was positively associated with gene expression of those drought-responsive genes. We conclude that H3K9ac enhances the expression level of a large number of drought-responsive genes under drought stress in B. distachyon. The data presented here will help to reveal the correlation of some specific drought-responsive genes and their enriched H3K9ac levels in the model plant B. distachyon.

An approach for visualizing the spatial metabolome of an entire plant root system inspired by the Swiss-rolling technique

The spatial configuration and morphology of roots are commonly monitored for a better understanding of plant health and development. However, this approach provides minimal details about the biochemistry regulating the observable traits. Therefore, the ability to metabolically map the entire root structure would be of major value. Here, we developed a sample preparation approach that enables imaging of the entire root within a restricted space (width of microscope slide), which was influenced by the Swiss-rolling technique. We were able to image and confidently identify molecules along the entire root structure from rolled-root tissue sections using multiple spatially resolved mass spectrometry approaches.

BiomeNet: a database for construction and analysis of functional interaction networks for any species with a sequenced genome

Motivation

Owing to advanced DNA sequencing and genome assembly technology, the number of species with sequenced genomes is rapidly increasing. The aim of the recently launched Earth BioGenome Project is to sequence genomes of all eukaryotic species on Earth over the next 10 years, making it feasible to obtain genomic blueprints of the majority of animal and plant species by this time. Genetic models of the sequenced species will later be subject to functional annotation, and a comprehensive molecular network should facilitate functional analysis of individual genes and pathways. However, network databases are lagging behind genome sequencing projects as even the largest network database provides gene networks for less than 10% of sequenced eukaryotic genomes, and the knowledge gap between genomes and interactomes continues to widen.

Results

We present BiomeNet, a database of 95 scored networks comprising over 8 million co-functional links, which can build and analyze gene networks for any species with the sequenced genome. BiomeNet transfers functional interactions between orthologous proteins from source networks to the target species within minutes and automatically constructs gene networks with the quality comparable to that of existing networks. BiomeNet enables assembly of the first-in-species gene networks not available through other databases, which are highly predictive of diverse biological processes and can also provide network analysis by extracting subnetworks for individual biological processes and network-based gene prioritizations. These data indicate that BiomeNet could enhance the benefits of decoding the genomes of various species, thus improving our understanding of the Earth’ biodiversity.

Availability and implementation

The BiomeNet is freely available at http://kobic.re.kr/biomenet/.

Supplementary information

Supplementary data are available at Bioinformatics online.

QTL mapping of yield component traits on bin map generated from resequencing a RIL population of foxtail millet (Setaria italica)

Background

Foxtail millet (Setaria italica) has been developed into a model genetical system for deciphering architectural evolution, C₄ photosynthesis, nutritional properties, abiotic tolerance and bioenergy in cereal grasses because of its advantageous characters with the small genome size, self-fertilization, short growing cycle, small growth stature, efficient genetic transformation and abundant diverse germplasm resources. Therefore, excavating QTLs of yield component traits, which are closely related to aspects mentioned above, will further facilitate genetic research in foxtail millet and close cereal species.

Results

Here, 164 Recombinant inbreed lines from a cross between Longgu7 and Yugu1 were created and 1,047,978 SNPs were identified between both parents via resequencing. A total of 3413 bin markers developed from SNPs were used to construct a binary map, containing 3963 recombinant breakpoints and totaling 1222.26 cM with an average distance of 0.36 cM between adjacent markers. Forty-seven QTLs were identified for four traits of straw weight, panicle weight, grain weight per plant and 1000-grain weight. These QTLs explained 5.5–14.7% of phenotypic variance. Thirty-nine favorable QTL alleles were found to inherit from Yugu1. Three stable QTLs were detected in multi-environments, and nine QTL clusters were identified on Chromosome 3, 6, 7 and 9.

Conclusions

A high-density genetic map with 3413 bin markers was constructed and three stable QTLs and 9 QTL clusters for yield component traits were identified. The results laid a powerful foundation for fine mapping, identifying candidate genes, elaborating molecular mechanisms and application in foxtail millet breeding programs by marker-assisted selection.

Form, development and function of grass stomata

Stomata are cellular breathing pores on leaves that open and close to absorb photosynthetic carbon dioxide and to restrict water loss through transpiration, respectively. Grasses (Poaceae) form morphologically innovative stomata, which consist of two dumbbell-shaped guard cells flanked by two lateral subsidiary cells (SCs). This 'graminoid' morphology is associated with faster stomatal movements leading to more water-efficient gas exchange in changing environments. Here, we offer a genetic and mechanistic perspective on the unique graminoid form of grass stomata and the developmental innovations during stomatal cell lineage initiation, recruitment of SCs and stomatal morphogenesis. Furthermore, the functional consequences of the four-celled, graminoid stomatal morphology are summarized. We compile the identified players relevant for stomatal opening and closing in grasses, and discuss possible mechanisms leading to cell-type-specific regulation of osmotic potential and turgor. In conclusion, we propose that the investigation of functionally superior grass stomata might reveal routes to improve water-stress resilience of agriculturally relevant plants in a changing climate.

Genome-wide characterization of the laccase gene family in Setaria viridis reveals members potentially involved in lignification

Five laccase genes are potentially involved in developmental lignification in the model C4 grass Setaria viridis and their different tissue specificities suggest subfunctionalization events. Plant laccases are copper-containing glycoproteins involved in monolignol oxidation and, therefore, their activity is essential for lignin polymerization. Although these enzymes belong to large multigene families with highly redundant members, not all of them are thought to be involved in lignin metabolism. Here, we report on the genome-wide characterization of the laccase gene family in the model C4 grass Setaria viridis and further identification of the members potentially involved in monolignol oxidation. A total of 52 genes encoding laccases (SvLAC1 to SvLAC52) were found in the genome of S. viridis, and phylogenetic analyses showed that these genes were heterogeneously distributed among the characteristic six subclades of the family and are under relaxed selective constraints. The observed expansion in the total number of genes in this species was mainly caused by tandem duplications within subclade V, which accounts for 68% of the whole family. Comparative phylogenetic analyses showed that the expansion of subclade V is specifically observed for the Paniceae tribe within the Panicoideae subfamily in grasses. Five SvLAC genes (SvLAC9, SvLAC13, SvLAC15, SvLAC50, and SvLAC52) fulfilled the criteria established to identify lignin-related candidates: (1) phylogenetic proximity to previously characterized lignin-related laccases from other species, (2) similar expression pattern to that observed for lignin biosynthetic genes in the S. viridis elongating internode, and (3) high expression in S. viridis tissues undergoing active lignification. In addition, in situ hybridization experiments not only confirmed that these selected SvLAC genes were expressed in lignifying cells, but also that their expression showed different tissue specificities, suggesting subfunctionalization events within the family. These five laccase genes are strong candidates to be involved in lignin polymerization in S. viridis and might be good targets for lignin bioengineering strategies.

Metabolic shifts associated with drought-induced senescence in Brachypodium

The metabolic underpinnings of plant survival under severe drought-induced senescence conditions are poorly understood. In this study, we assessed the morphological, physiological and metabolic responses to sustained water deficit in Brachypodium distachyon, a model organism for research on temperate grasses. Relative to control plants, fresh biomass, leaf water potential, and chlorophyll levels decreased rapidly in plants grown under drought conditions, demonstrating an early onset of senescence. The leaf C/N ratio and protein content showed an increase in plants subjected to drought stress. The concentrations of several small molecule carbohydrates and amino acid-derived metabolites previously implicated in osmotic protection increased rapidly in plants experiencing water deficit. Malic acid, a low molecular weight organic acid with demonstrated roles in stomatal closure, also increased rapidly as a response to drought treatment. The concentrations of prenyl lipids, such as phytol and α-tocopherol, increased early during the drought treatment but then dropped dramatically. Surprisingly, continued changes in the quantities of metabolites were observed, even in samples harvested from visibly senesced plants. The data presented here provide insights into the processes underlying persistent metabolic activity during sustained water deficit and can aid in identifying mechanisms of drought tolerance in plants.

Biochemical and physiological flexibility accompanies reduced cellulose biosynthesis in Brachypodium cesa1 S830N

Here, we present a study into the mechanisms of primary cell wall cellulose formation in grasses, using the model cereal grass Brachypodium distachyon. The exon found adjacent to the BdCESA1 glycosyltransferase QXXRW motif was targeted using Targeting Induced Local Lesions in Genomes (TILLING) and sequencing candidate amplicons in multiple parallel reactions (SCAMPRing) leading to the identification of the Bdcesa1 ^S830N allele. Plants carrying this missense mutation exhibited a significant reduction in crystalline cellulose content in tissues that rely on the primary cell wall for biomechanical support. However, Bdcesa1 ^S830N plants failed to exhibit the predicted reduction in plant height. In a mechanism unavailable to eudicotyledons, B. distachyon plants homozygous for the Bdcesa1 ^S830N allele appear to overcome the loss of internode expansion anatomically by increasing the number of nodes along the stem. Stem biomechanics were resultantly compromised in Bdcesa1 ^S830N . The Bdcesa1 ^S830N missense mutation did not interfere with BdCESA1 gene expression. However, molecular dynamic simulations of the CELLULOSE SYNTHASE A (CESA) structure with modelled membrane interactions illustrated that Bdcesa1 ^S830N exhibited structural changes in the translated gene product responsible for reduced cellulose biosynthesis. Molecular dynamic simulations showed that substituting S830N resulted in a stabilizing shift in the flexibility of the class specific region arm of the core catalytic domain of CESA, revealing the importance of this motion to protein function.

Resource allocation to growth or luxury consumption drives mycorrhizal responses

Highly variable phenotypic responses in mycorrhizal plants challenge our functional understanding of plant-fungal mutualisms. Using non-invasive high-throughput phenotyping, we observed that arbuscular mycorrhizal (AM) fungi relieved phosphorus (P) limitation and enhanced growth of Brachypodium distachyon under P-limited conditions, while photosynthetic limitation under low nitrogen (N) was exacerbated by the fungus. However, these responses were strongly dependent on host genotype: only the faster growing genotype (Bd3-1) utilised P transferred from the fungus to achieve improved growth under P-limited conditions. Under low N, the slower growing genotype (Bd21) had a carbon and N surplus that was linked to a less negative growth response compared with the faster growing genotype. These responses were linked to the regulation of N : P stoichiometry, couples resource allocation to growth or luxury consumption in diverse plant lineages. Our results attest strongly to a mechanism in plants by which plant genotype-specific resource economics drive phenotypic outcomes during AM symbioses.

Modified expression of ZmMYB167 in Brachypodium distachyon and Zea mays leads to increased cell wall lignin and phenolic content

One of the challenges to enable targeted modification of lignocellulosic biomass from grasses for improved biofuel and biochemical production lies within our limited understanding of the transcriptional control of secondary cell wall biosynthesis. Here, we investigated the role of the maize MYB transcription factor ZmMYB167 in secondary cell wall biosynthesis and how modified ZmMYB167 expression in two distinct grass model species affects plant biomass and growth phenotypes. Heterologous expression of ZmMYB167 in the C3 model system Brachypodium led to mild dwarf phenotypes, increased lignin (~7% to 13%) and S-lignin monomer (~11% to 16%) content, elevated concentrations of cell wall-bound p-coumaric acid (~15% to 24%) and reduced biomass sugar release (~20%) compared to controls. Overexpression of ZmMYB167 in the C4 model system Zea mays increased lignin (~4% to 13%), p-coumaric acid (~8% to 52%) and ferulic acid (~13% to 38%) content but did not affect plant growth and development nor biomass recalcitrance. Taken together, modifying ZmMYB167 expression represents a target to alter lignin and phenolic content in grasses. The ZmMYB167 expression-induced discrepancies in plant phenotypic and biomass properties between the two grass model systems highlight the challenges and opportunities for MYB transcription factor-based genetic engineering approaches of grass biomass.

Never the Two Shall Mix: Robust Indel Markers to Ensure the Fidelity of Two Pivotal and Closely-Related Accessions of Brachypodium distachyon

Brachypodium distachyon is an established model for monocotyledonous plants. Numerous markers intended for gene discovery and population genetics have been designed. However to date, very few indel markers with larger and easily scored length polymorphism differences, that distinguish between the two morphologically similar and highly utilized B. distachyon accessions, Bd21, the reference genome accession, and Bd21-3, the transformation-optimal accession, are publically available. In this study, 22 indel markers were designed and utilized to produce length polymorphism differences of 150 bp or more, for easy discrimination between Bd21 and Bd21-3. When tested on four other B. distachyon accessions, one case of multiallelism was observed. It was also shown that the markers could be used to determine homozygosity and heterozygosity at specific loci in a Bd21 x Bd3-1 F2 population. The work done in this study allows researchers to maintain the fidelity of Bd21 and Bd21-3 stocks for both transgenic and nontransgenic studies. It also provides markers that can be utilized in conjunction with others already available for further research on population genetics, gene discovery and gene characterization, all of which are necessary for the relevance of B. distachyon as a model species.

Providing carbon skeletons to sustain amide synthesis in roots underlines the suitability of Brachypodium distachyon for the study of ammonium stress in cereals

Plants mainly acquire N from the soil in the form of nitrate (NO₃ ^-) or ammonium (NH₄ ⁺). Ammonium-based nutrition is gaining interest because it helps to avoid the environmental pollution associated with nitrate fertilization. However, in general, plants prefer NO₃ ^- and indeed, when growing only with NH₄ ⁺ they can encounter so-called ammonium stress. Since Brachypodium distachyon is a useful model species for the study of monocot physiology and genetics, we chose it to characterize performance under ammonium nutrition. Brachypodium distachyon Bd21 plants were grown hydroponically in 1 or 2.5 mM NO₃ ^- or NH₄ ⁺. Nitrogen and carbon metabolism associated with NH₄ ⁺ assimilation was evaluated in terms of tissue contents of NO₃ ^-, NH₄ ⁺, K, Mg, Ca, amino acids and organic acids together with tricarboxylic acid (TCA) cycle and NH₄ ⁺-assimilating enzyme activities and RNA transcript levels. The roots behaved as a physiological barrier preventing NH₄ ⁺ translocation to aerial parts, as indicated by a sizeable accumulation of NH₄ ⁺, Asn and Gln in the roots. A continuing high NH₄ ⁺ assimilation rate was made possible by a tuning of the TCA cycle and its associated anaplerotic pathways to match 2-oxoglutarate and oxaloacetate demand for Gln and Asn synthesis. These results show B. distachyon to be a highly suitable tool for the study of the physiological, molecular and genetic basis of ammonium nutrition in cereals.

Multilab EcoFAB study shows highly reproducible physiology and depletion of soil metabolites by a model grass

There is a dynamic reciprocity between plants and their environment: soil physiochemical properties influence plant morphology and metabolism, and root morphology and exudates shape the environment surrounding roots. Here, we investigate the reproducibility of plant trait changes in response to three growth environments. We utilized fabricated ecosystem (EcoFAB) devices to grow the model grass Brachypodium distachyon in three distinct media across four laboratories: phosphate-sufficient and -deficient mineral media allowed assessment of the effects of phosphate starvation, and a complex, sterile soil extract represented a more natural environment with yet uncharacterized effects on plant growth and metabolism. Tissue weight and phosphate content, total root length, and root tissue and exudate metabolic profiles were consistent across laboratories and distinct between experimental treatments. Plants grown in soil extract were morphologically and metabolically distinct, with root hairs four times longer than with other growth conditions. Further, plants depleted half of the metabolites investigated from the soil extract. To interact with their environment, plants not only adapt morphology and release complex metabolite mixtures, but also selectively deplete a range of soil-derived metabolites. The EcoFABs utilized here generated high interlaboratory reproducibility, demonstrating their value in standardized investigations of plant traits.

Global Diversity of the Brachypodium Species Complex as a Resource for Genome-Wide Association Studies Demonstrated for Agronomic Traits in Response to Climate

The development of model systems requires a detailed assessment of standing genetic variation across natural populations. The Brachypodium species complex has been promoted as a plant model for grass genomics with translation to small grain and biomass crops. To capture the genetic diversity within this species complex, thousands of Brachypodium accessions from around the globe were collected and genotyped by sequencing. Overall, 1897 samples were classified into two diploid or allopolyploid species, and then further grouped into distinct inbred genotypes. A core set of diverse B. distachyon diploid lines was selected for whole genome sequencing and high resolution phenotyping. Genome-wide association studies across simulated seasonal environments was used to identify candidate genes and pathways tied to key life history and agronomic traits under current and future climatic conditions. A total of 8, 22, and 47 QTL were identified for flowering time, early vigor, and energy traits, respectively. The results highlight the genomic structure of the Brachypodium species complex, and the diploid lines provided a resource that allows complex trait dissection within this grass model species.

Exploring natural variation for rice sheath blight resistance in Brachypodium distachyon

Sheath blight caused by the soil borne fungus Rhizoctonia solani AG1-IA is one of the major diseases of rice in the world. Genetic resistance in rice against this disease has not been very successful. Brachypodium distachyon is considered as a model species for several cereal crops and it has been studied in the past to identify novel sources of disease resistance against cereal crop diseases. Therefore, the current study was designed to explore nonhost disease resistance in Brachypodium accessions against sheath blight pathogen of rice, Rhizoctonia solani. A total of 19 Brachypodium distachyon accessions were screened for resistance against Rhizoctonia solani AG1-IA. Different levels of resistance reactions were observed among accessions. Quantification of jasmonic acid (JA) and salicylic acid (SA) concentration in selected resistant (Bd3-1), moderately susceptible (Bd21), and susceptible (Bd30-1) inbred accessions revealed that Bd3-1 accumulated more JA upon pathogen infection compared to Bd21 or Bd30-1. In contrary, no differences were observed for SA accumulation in tested accessions suggesting that the resistance to R. solani in Brachypodium is due to an SA-independent defense pathway. Our study provides a new foundation to explore this area for more durable resistance against this disease.

Recent Activity in Expanding Populations and Purifying Selection Have Shaped Transposable Element Landscapes across Natural Accessions of the Mediterranean Grass Brachypodium distachyon

Transposable element (TE) activity has emerged as a major cause of variation in genome size and structure among species. To what extent TEs contribute to genetic variation and divergence within species, however, is much less clear, mainly because population genomic data have so far only been available for the classical model organisms. In this study, we use the annual Mediterranean grass Brachypodium distachyon to investigate TE dynamics in natural populations. Using whole-genome sequencing data for 53 natural accessions, we identified more than 5,400 TE polymorphisms across the studied genomes. We found, first, that while population bottlenecks and expansions have shaped genetic diversity in B. distachyon, these events did not lead to lineage-specific activations of TE families, as observed in other species. Instead, the same families have been active across the species range and TE activity is homogeneous across populations, indicating the presence of conserved regulatory mechanisms. Second, almost half of the TE insertion polymorphisms are accession-specific, most likely because of recent activity in expanding populations and the action of purifying selection. And finally, although TE insertion polymorphisms are underrepresented in and around genes, more than 1,000 of them occur in genic regions and could thus contribute to functional divergence. Our study shows that while TEs in B. distachyon are “well-behaved” compared with TEs in other species with larger genomes, they are an abundant source of lineage-specific genetic variation and may play an important role in population divergence and adaptation.

Live Confocal Imaging of Brachypodium Spikelet Meristems

Live confocal imaging of fluorescent reporters and stains in plant meristems provides valuable measurements of gene expression, protein dynamics, cell polarity, cell division, and growth. The spikelet meristem in the grass Brachypodium distachyon (Brachypodium) is well suited to live imaging because of the ease of dissection, small meristem size, simple arrangement of organs, and because each plant provides abundant spikelet meristems. Brachypodium is also far easier to genetically transform than other grass species. Presented here is a protocol for the growth, staging, dissection, mounting, and imaging of Brachypodium spikelet meristems for live confocal imaging.

Species-Associated Differences in the Below-Ground Microbiomes of Wild and Domesticated Setaria

The rhizosphere microbiome is known to play a crucial role in promoting plant growth, partly by countering soil-borne phytoparasites and by improving nutrient uptake. The abundance and composition of the rhizosphere and root-associated microbiota are influenced by several factors, including plant species and genotype. We hypothesize that crop domestication might influence the composition and diversity of plant-associated microbiomes. We tested the contribution of domestication to the bacterial and archaeal root and soil composition associated with six genotypes of domesticated Setaria italica and four genotypes of its wild ancestor, S. viridis. The bacterial microbiome in the rhizoplane and root endophyte compartments, and the archaea in the endophyte compartment, showed major composition differences. For instance, members of the Betaproteobacteria and Firmicutes were overrepresented in S. italica root samples compared to S. viridis. Metagenomic analysis of samples that contained both root surface-bound (rhizoplane) and inside-root (endophytic) bacteria defined two unique microbial communities only associated with S. italica roots and one only associated with S. viridis roots. Root endophytic bacteria were found in six discernible communities, of which four were primarily on S. italica and two primarily on S. viridis. Among archaea, Methanobacteria, and Methanomicrobia exhibited species-associated differences in the rhizosphere and root compartments, but most detected archaea were not classified more specifically than at the level of phylum. These results indicate a host genetic contribution to the microbial composition in Setaria, and suggest that domestication has selected for specific associations in the root and in the rhizosphere.

Climate-smart crops with enhanced photosynthesis

The potential of enhanced photosynthetic efficiency to help achieve the sustainable yield increases required to meet future demands for food and energy has spurred intense research towards understanding, modeling, and engineering photosynthesis. These current efforts, largely focused on the C3 model Arabidopsis thaliana or crop plants (e.g. rice, sorghum, maize, and wheat), could be intensified and broadened using model systems closely related to our food, feed, and energy crops and that allow rapid design-build-test-learn cycles. In this outlooking Opinion, we advocate for a concerted effort to expand our understanding and improve our ability to redesign carbon uptake, allocation, and utilization. We propose two specific research directions that combine enhanced photosynthesis with climate-smart metabolic attributes: (i) engineering pathways for flexible (facultative) C3-C4 metabolism where plants will operate either C3 or C4 photosynthesis based on environmental conditions such as temperature, light, and atmospheric CO2 levels; and (ii) increasing rhizospheric sink strength for carbon utilization, including strategies that allow for augmented transport of carbon to the soil for improved soil properties and carbon storage without jeopardizing aboveground crop biomass. We argue that such ambitious undertakings be first approached and demonstrated by exploring the full genomic potential of two model grasses, the C3Brachypodium distachyon and the C4Setaria viridis. The development of climate-smart crops could provide novel and bold solutions to increase crop productivity while reducing atmospheric carbon and nitrogen emissions.

Domestication and Improvement in the Model C4 Grass, Setaria

Setaria viridis (green foxtail) and its domesticated relative S. italica (foxtail millet) are diploid C4 panicoid grasses that are being developed as model systems for studying grass genomics, genetics, development, and evolution. According to archeological evidence, foxtail millet was domesticated from green foxtail approximately 9,000 to 6,000 YBP in China. Under long-term human selection, domesticated foxtail millet developed many traits adapted to human cultivation and agricultural production. In comparison with its wild ancestor, foxtail millet has fewer vegetative branches, reduced grain shattering, delayed flowering time and less photoperiod sensitivity. Foxtail millet is the only present-day crop in the genus Setaria, although archeological records suggest that other species were domesticated and later abandoned in the last 10,000 years. We present an overview of domestication in foxtail millet, by reviewing recent studies on the genetic regulation of several domesticated traits in foxtail millet and discuss how the foxtail millet and green foxtail system could be further developed to both better understand its domestication history, and to provide more tools for future breeding efforts.

The Status of Setaria viridis Transformation: Agrobacterium-Mediated to Floral Dip

Setaria viridis has many attributes, including small stature and simple growth requirements, that make it attractive as a model species for monocots. Genetic engineering (transformation) methodology is a key prerequisite for adoption of plant species as models. Various transformation approaches have been reported for S. viridis including tissue culture-based and in planta by Agrobacterium tumefaciens infection of floral organs referred to as the floral dip method. The tissue culture-based method utilizes A. tumefaciens infection of mature seed-derived callus with subsequent recovery of stable transgenic lines. Vectors found to be most effective contain the hygromycin phosphotransferase selectable marker gene driven by either Panicum virgatum or Zea mays ubiquitin promoters. As for the floral dip method, there are two reports based on Agrobacterium infection of young S. viridis inflorescences. Plants were allowed to mature, seeds were collected, and analysis of the progeny verified the presence of transgenes. Each transformation approach, tissue culture-based and floral dip, has advantages and disadvantages depending on the expertise of personnel and resources available. While the tissue culture-based method results in a higher transformation efficiency than floral dip, implementation requires a specific technical skillset that limits availability of experienced personnel to successfully perform transformations. Less technical experience is required for floral dip; however, a lack of high-quality growth chambers or greenhouses that provide the necessary optimum growing conditions would reduce an already low transformation efficiency or would not result in recovery of transgenic lines. An overview of transformation methods reported for S. viridis is presented in this review.

The Challenge of Analyzing the Sugarcane Genome

Reference genome sequences have become key platforms for genetics and breeding of the major crop species. Sugarcane is probably the largest crop produced in the world (in weight of crop harvested) but lacks a reference genome sequence. Sugarcane has one of the most complex genomes in crop plants due to the extreme level of polyploidy. The genome of modern sugarcane hybrids includes sub-genomes from two progenitors Saccharum officinarum and S. spontaneum with some chromosomes resulting from recombination between these sub-genomes. Advancing DNA sequencing technologies and strategies for genome assembly are making the sugarcane genome more tractable. Advances in long read sequencing have allowed the generation of a more complete set of sugarcane gene transcripts. This is supporting transcript profiling in genetic research. The progenitor genomes are being sequenced. A monoploid coverage of the hybrid genome has been obtained by sequencing BAC clones that cover the gene space of the closely related sorghum genome. The complete polyploid genome is now being sequenced and assembled. The emerging genome will allow comparison of related genomes and increase understanding of the functioning of this polyploidy system. Sugarcane breeding for traditional sugar and new energy and biomaterial uses will be enhanced by the availability of these genomic resources.

Centromeric DNA characterization in the model grass Brachypodium distachyon provides insights on the evolution of the genus

Brachypodium distachyon is a well-established model monocot plant, and its small and compact genome has been used as an accurate reference for the much larger and often polyploid genomes of cereals such as Avena sativa (oats), Hordeum vulgare (barley) and Triticum aestivum (wheat). Centromeres are indispensable functional units of chromosomes and they play a core role in genome polyploidization events during evolution. As the Brachypodium genus contains about 20 species that differ significantly in terms of their basic chromosome numbers, genome size, ploidy levels and life strategies, studying their centromeres may provide important insight into the structure and evolution of the genome in this interesting and important genus. In this study, we isolated the centromeric DNA of the B. distachyon reference line Bd21 and characterized its composition via the chromatin immunoprecipitation of the nucleosomes that contain the centromere-specific histone CENH3. We revealed that the centromeres of Bd21 have the features of typical multicellular eukaryotic centromeres. Strikingly, these centromeres contain relatively few centromeric satellite DNAs; in particular, the centromere of chromosome 5 (Bd5) consists of only ~40 kb. Moreover, the centromeric retrotransposons in B. distachyon (CRBds) are evolutionarily young. These transposable elements are located both within and adjacent to the CENH3 binding domains, and have similar compositions. Moreover, based on the presence of CRBds in the centromeres, the species in this study can be grouped into two distinct lineages. This may provide new evidence regarding the phylogenetic relationships within the Brachypodium genus.

Tissue Culture (Somatic Embryogenesis)-Induced Tnt1 Retrotransposon-Based Mutagenesis in Brachypodium distachyon

Brachypodium distachyon is a model grass species for economically important cereal crops. Efforts are in progress to develop useful functional genomic resources in Brachypodium. A tobacco retrotransposon, Tnt1, has been used successfully in recent past to generate insertional mutagenesis in several dicot plant species. Tnt1 retrotransposon replicates, transposes, and inserts at multiple random genomic locations in the plant genome. Transposition occurs only during somatic embryogenesis but not during seed transmission. We developed Brachypodium transgenic plants that can express the Tnt1 element. Here, we describe an efficient tissue culture-based approach to generate Tnt1 insertional mutant population using transgenic Brachypodium line expressing the Tnt1 retrotransposon.

Making Roots, Shoots, and Seeds: IDD Gene Family Diversification in Plants

The INDETERMINATE DOMAIN (IDD) family of transcriptional regulators controls a diversity of processes in a variety of plant tissues and organs and at different stages of plant development. Several recent reports describe the genetic characterization of IDD family members, including those that are likely to regulate C₄ kranz anatomy, with implications for the engineering of C₄ traits into C₃ crops. In this review we summarize the reported functions of IDD members in the regulation of metabolic sensing and leaf, root, seed, and inflorescence development. We also provide an IDD phylogeny for the grasses and suggest future directions and strategies to define the function of IDDs in C₄ photosynthesis and other developmental processes.

Neural Net Classification Combined With Movement Analysis to Evaluate Setaria viridis as a Model System for Time of Day of Anther Appearance

In many plant species, the time of day at which flowers open to permit pollination is tightly regulated. Proper time of flower opening, or Time of Day of Anther Appearance (TAA), may coordinate flowering opening with pollinator activity or may shift temperature sensitive developmental processes to cooler times of the day. The genetic mechanisms that regulate the timing of this process in cereal crops are unknown. To address this knowledge gap, it is necessary to establish a monocot model system that exhibits variation in TAA. Here, we examine the suitability of Setaria viridis, the model for C4 photosynthesis, for such a role. We developed an imaging system to monitor the temporal regulation of growth, flower opening time, and other physiological characteristics in Setaria. This system enabled us to compare Setaria varieties Ames 32254, Ames 32276, and PI 669942 variation in growth and daily flower opening time. We observed that TAA occurs primarily at night in these three Setaria accessions. However, significant variation between the accessions was observed for both the ratio of flowers that open in the day vs. night and the specific time of day where the rate is maximal. Characterizing this physiological variation is a requisite step toward uncovering the molecular mechanisms regulating TAA. Leveraging the regulation of TAA could provide researchers with a genetic tool to improve crop productivity in new environments.

Imaging cellulose synthase motility during primary cell wall synthesis in the grass Brachypodium distachyon

The mechanism of cellulose synthesis has been studied by characterizing the motility of cellulose synthase complexes tagged with a fluorescent protein; however, this approach has been used exclusively on the hypocotyl of Arabidopsis thaliana. Here we characterize cellulose synthase motility in the model grass, Brachypodium distachyon. We generated lines in which mEGFP is fused N-terminal to BdCESA3 or BdCESA6 and which grew indistinguishably from the wild type (Bd21-3) and had dense fluorescent puncta at or near the plasma membrane. Measured with a particle tracking algorithm, the average speed of GFP-BdCESA3 particles in the mesocotyl was 164 ± 78 nm min⁻¹ (error gives standard deviation [SD], n = 1451 particles). Mean speed in the root appeared similar. For comparison, average speed in the A. thaliana hypocotyl expressing GFP-AtCESA6 was 184 ± 86 nm min⁻¹ (n = 2755). For B. distachyon, we quantified root diameter and elongation rate in response to inhibitors of cellulose (dichlorobenylnitrile; DCB), microtubules (oryzalin), or actin (latrunculin B). Neither oryzalin nor latrunculin affected the speed of CESA complexes; whereas, DCB reduced average speed by about 50% in B. distachyon and by about 35% in A. thaliana. Evidently, between these species, CESA motility is well conserved.

Genetic engineering of grass cell wall polysaccharides for biorefining

Grasses represent an abundant and widespread source of lignocellulosic biomass, which has yet to fulfil its potential as a feedstock for biorefining into renewable and sustainable biofuels and commodity chemicals. The inherent recalcitrance of lignocellulosic materials to deconstruction is the most crucial limitation for the commercial viability and economic feasibility of biomass biorefining. Over the last decade, the targeted genetic engineering of grasses has become more proficient, enabling rational approaches to modify lignocellulose with the aim of making it more amenable to bioconversion. In this review, we provide an overview of transgenic strategies and targets to tailor grass cell wall polysaccharides for biorefining applications. The bioengineering efforts and opportunities summarized here rely primarily on (A) reprogramming gene regulatory networks responsible for the biosynthesis of lignocellulose, (B) remodelling the chemical structure and substitution patterns of cell wall polysaccharides and (C) expressing lignocellulose degrading and/or modifying enzymes in planta. It is anticipated that outputs from the rational engineering of grass cell wall polysaccharides by such strategies could help in realizing an economically sustainable, grass-derived lignocellulose processing industry.