Transcriptomic analyses indicate that maize ligule development recapitulates gene expression patterns that occur during lateral organ initiation

Histological and single-nucleus transcriptome analyses reveal the specialized functions of ligular sclerenchyma cells and key regulators of leaf angle in maize

Leaf angle (LA) is a crucial factor that affects planting density and yield in maize. However, the regulatory mechanisms underlying LA formation remain largely unknown. In this study, we performed a comparative histological analysis of the ligular region across various maize inbred lines and revealed that LA is significantly influenced by a two-step regulatory process involving initial cell elongation followed by subsequent lignification in the ligular adaxial sclerenchyma cells (SCs). Subsequently, we performed both bulk and single-nucleus RNA sequencing, generated a comprehensive transcriptomic atlas of the ligular region, and identified numerous genes enriched in the hypodermal cells that may influence their specialization into SCs. Furthermore, we functionally characterized two genes encoding atypical basic-helix-loop-helix (bHLH) transcription factors, bHLH30 and its homolog bHLH155, which are highly expressed in the elongated adaxial cells. Genetic analyses revealed that bHLH30 and bHLH155 positively regulate LA expansion, and molecular experiments demonstrated their ability to activate the transcription of genes involved in cell elongation and lignification of SCs. These findings highlight the specialized functions of ligular adaxial SCs in LA regulation by restricting further extension of ligular cells and enhancing mechanical strength. The transcriptomic atlas of the ligular region at single-nucleus resolution not only deepens our understanding of LA regulation but also enables identification of numerous potential targets for optimizing plant architecture in modern maize breeding.

DNA- and Selectable-Marker-Free Genome-Editing System Using Zygotes from Recalcitrant Maize Inbred B73

Genome-editing tools such as the clustered regularly interspaced short palindromic repeats/Cas9 (CRISPR/Cas9) system have become essential tools for increasing the efficiency and accuracy of plant breeding. Using such genome-editing tools on maize, one of the most important cereal crops of the world, will greatly benefit the agriculture and the mankind. Conventional genome-editing methods typically used for maize involve insertion of a Cas9-guide RNA expression cassette and a selectable marker in the genome DNA; however, using such methods, it is essential to eliminate the inserted DNA cassettes to avoid legislative concerns on gene-modified organisms. Another major hurdle for establishing an efficient and broadly applicable DNA-free genome-editing system for maize is presented by recalcitrant genotypes/cultivars, since cell/tissue culture and its subsequent regeneration into plantlets are crucial for producing transgenic and/or genome-edited maize. In this study, to establish a DNA-free genome-editing system for recalcitrant maize genotypes/cultivars, Cas9-gRNA ribonucleoproteins were directly delivered into zygotes isolated from the pollinated flowers of the maize-B73 cultivar. The zygotes successfully developed and were regenerated into genome-edited plantlets by co-culture with phytosulfokine, a peptide phytohormone. The method developed herein made it possible to obtain DNA- and selectable-marker-free genome-edited recalcitrant maize genotypes/cultivars with high efficiency. This method can advance the molecular breeding of maize and other important cereals, regardless of their recalcitrant characteristics.

Genetic regulation of self-organizing azimuthal canopy orientations and their impacts on light interception in maize

The efficiency of solar radiation interception contributes to the photosynthetic efficiency of crop plants. Light interception is a function of canopy architecture, including plant density; leaf number, length, width, and angle; and azimuthal canopy orientation. We report on the ability of some maize (Zea mays) genotypes to alter the orientations of their leaves during development in coordination with adjacent plants. Although the upper canopies of these genotypes retain the typical alternate-distichous phyllotaxy of maize, their leaves grow parallel to those of adjacent plants. A genome-wide association study (GWAS) on this parallel canopy trait identified candidate genes, many of which are associated with shade avoidance syndrome, including phytochromeC2. GWAS conducted on the fraction of photosynthetically active radiation (PAR) intercepted by canopies also identified multiple candidate genes, including liguleless1 (lg1), previously defined by its role in ligule development. Under high plant densities, mutants of shade avoidance syndrome and liguleless genes (lg1, lg2, and Lg3) exhibit altered canopy patterns, viz, the numbers of interrow leaves are greatly reduced as compared to those of nonmutant controls, resulting in dramatically decreased PAR interception. In at least the case of lg2, this phenotype is not a consequence of abnormal ligule development. Instead, liguleless gene functions are required for normal light responses, including azimuth canopy re-orientation.

Roles of auxin pathways in maize biology

Phytohormones play a central role in plant development and environmental responses. Auxin is a classical hormone that is required for organ formation, tissue patterning, and defense responses. Auxin pathways have been extensively studied across numerous land plant lineages, including bryophytes and eudicots. In contrast, our understanding of the roles of auxin in maize morphogenesis and immune responses is limited. Here, we review evidence for auxin-mediated processes in maize and describe promising areas for future research in the auxin field. Several recent transcriptomic and genetic studies have demonstrated that auxin is a key influencer of both vegetative and reproductive development in maize (namely roots, leaves, and kernels). Auxin signaling has been implicated in both maize shoot architecture and immune responses through genetic and molecular analyses of the conserved co-repressor RAMOSA ENHANCER LOCUS2. Polar auxin transport is linked to maize drought responses, root growth, shoot formation, and leaf morphogenesis. Notably, maize has been a key system for delineating auxin biosynthetic pathways and offers many opportunities for future investigations on auxin metabolism. In addition, crosstalk between auxin and other phytohormones has been uncovered through gene expression studies and is important for leaf and root development in maize. Collectively these studies point to auxin as a cornerstone for maize biology that could be leveraged for improved crop resilience and yield.

Maize Feature Store: A centralized resource to manage and analyze curated maize multi-omics features for machine learning applications

The big-data analysis of complex data associated with maize genomes accelerates genetic research and improves agronomic traits. As a result, efforts have increased to integrate diverse datasets and extract meaning from these measurements. Machine learning models are a powerful tool for gaining knowledge from large and complex datasets. However, these models must be trained on high-quality features to succeed. Currently, there are no solutions to host maize multi-omics datasets with end-to-end solutions for evaluating and linking features to target gene annotations. Our work presents the Maize Feature Store (MFS), a versatile application that combines features built on complex data to facilitate exploration, modeling and analysis. Feature stores allow researchers to rapidly deploy machine learning applications by managing and providing access to frequently used features. We populated the MFS for the maize reference genome with over 14 000 gene-based features based on published genomic, transcriptomic, epigenomic, variomic and proteomics datasets. Using the MFS, we created an accurate pan-genome classification model with an AUC-ROC score of 0.87. The MFS is publicly available through the maize genetics and genomics database. Database URL  https://mfs.maizegdb.org/.

The maize preligule band is subdivided into distinct domains with contrasting cellular properties prior to ligule outgrowth

The maize ligule is an epidermis-derived structure that arises from the preligule band (PLB) at a boundary between the blade and sheath. A hinge-like auricle also develops immediately distal to the ligule and contributes to blade angle. Here, we characterize the stages of PLB and early ligule development in terms of topography, cell area, division orientation, cell wall rigidity and auxin response dynamics. Differential thickening of epidermal cells and localized periclinal divisions contributed to the formation of a ridge within the PLB, which ultimately produces the ligule fringe. Patterns in cell wall rigidity were consistent with the subdivision of the PLB into two regions along a distinct line positioned at the nascent ridge. The proximal region produces the ligule, while the distal region contributes to one epidermal face of the auricles. Although the auxin transporter PIN1 accumulated in the PLB, observed differential auxin transcriptional response did not underlie the partitioning of the PLB. Our data demonstrate that two zones with contrasting cellular properties, the preligule and preauricle, are specified within the ligular region before ligule outgrowth.

A non-JA producing oxophytodienoate reductase functions in salicylic acid-mediated antagonism with jasmonic acid during pathogen attack

Peroxisome-localized oxo-phytodienoic acid (OPDA) reductases (OPR) are enzymes converting 12-OPDA into jasmonic acid (JA). However, the biochemical and physiological functions of the cytoplasmic non-JA producing OPRs remain largely unknown. Here, we generated Mutator-insertional mutants of the maize OPR2 gene and tested its role in resistance to pathogens with distinct lifestyles. Functional analyses showed that the opr2 mutants were more susceptible to the (hemi)biotrophic pathogens Colletotrichum graminicola and Ustilago maydis, but were more resistant to the necrotrophic fungus Cochliobolus heterostrophus. Hormone profiling revealed that increased susceptibility to C. graminicola was associated with decreased salicylic acid (SA) but increased JA levels. Mutation of the JA-producing lipoxygenase 10 (LOX10) reversed this phenotype in the opr2 mutant background, corroborating the notion that JA promotes susceptibility to this pathogen. Exogenous SA did not rescue normal resistance levels in opr2 mutants, suggesting that this SA-inducible gene is the key downstream component of the SA-mediated defences against C. graminicola. Disease assays of the single and double opr2 and lox10 mutants and the JA-deficient opr7opr8 mutants showed that OPR2 negatively regulates JA biosynthesis, and that JA is required for resistance against C. heterostrophus. Overall, this study uncovers a novel function of a non-JA producing OPR as a major negative regulator of JA biosynthesis during pathogen infection, a function that leads to its contrasting contribution to either resistance or susceptibility depending on pathogen lifestyle.

Molecular and functional dissection of LIGULELESS1 (LG1) in plants

Plant architecture is a culmination of the features necessary for capturing light energy and adapting to the environment. An ideal architecture can promote an increase in planting density, light penetration to the lower canopy, airflow as well as heat distribution to achieve an increase in crop yield. A number of plant architecture-related genes have been identified by map cloning, quantitative trait locus (QTL) and genome-wide association study (GWAS) analysis. LIGULELESS1 (LG1) belongs to the squamosa promoter-binding protein (SBP) family of transcription factors (TFs) that are key regulators for plant growth and development, especially leaf angle (LA) and flower development. The DRL1/2-LG1-RAVL pathway is involved in brassinosteroid (BR) signaling to regulate the LA in maize, which has facilitated the regulation of plant architecture. Therefore, exploring the gene regulatory functions of LG1, especially its relationship with LA genes, can help achieve the precise regulation of plant phenotypes adapted to varied environments, thereby increasing the yield. This review comprehensively summarizes the advances in LG1 research, including its effect on LA and flower development. Finally, we discuss the current challenges and future research goals associate with LG1.

A Wox3-patterning module organizes planar growth in grass leaves and ligules

Grass leaves develop from a ring of primordial initial cells within the periphery of the shoot apical meristem, a pool of organogenic stem cells that generates all of the organs of the plant shoot. At maturity, the grass leaf is a flattened, strap-like organ comprising a proximal supportive sheath surrounding the stem and a distal photosynthetic blade. The sheath and blade are partitioned by a hinge-like auricle and the ligule, a fringe of epidermally derived tissue that grows from the adaxial (top) leaf surface. Together, the ligule and auricle comprise morphological novelties that are specific to grass leaves. Understanding how the planar outgrowth of grass leaves and their adjoining ligules is genetically controlled can yield insight into their evolutionary origins. Here we use single-cell RNA-sequencing analyses to identify a 'rim' cell type present at the margins of maize leaf primordia. Cells in the leaf rim have a distinctive identity and share transcriptional signatures with proliferating ligule cells, suggesting that a shared developmental genetic programme patterns both leaves and ligules. Moreover, we show that rim function is regulated by genetically redundant Wuschel-like homeobox3 (WOX3) transcription factors. Higher-order mutations in maize Wox3 genes greatly reduce leaf width and disrupt ligule outgrowth and patterning. Together, these findings illustrate the generalizable use of a rim domain during planar growth of maize leaves and ligules, and suggest a parsimonious model for the homology of the grass ligule as a distal extension of the leaf sheath margin.

Novel insights into maize (Zea mays) development and organogenesis for agricultural optimization

In maize, intrinsic hormone activities and sap fluxes facilitate organogenesis patterning and plant holistic development; these hormone movements should be a primary focus of developmental biology and agricultural optimization strategies. Maize (Zea mays) is an important crop plant with distinctive life history characteristics and structural features. Genetic studies have extended our knowledge of maize developmental processes, genetics, and molecular ecophysiology. In this review, the classical life cycle and life history strategies of maize are analyzed to identify spatiotemporal organogenesis properties and develop a definitive understanding of maize development. The actions of genes and hormones involved in maize organogenesis and sex determination, along with potential molecular mechanisms, are investigated, with findings suggesting central roles of auxin and cytokinins in regulating maize holistic development. Furthermore, investigation of morphological and structural characteristics of maize, particularly node ubiquity and the alternate attachment pattern of lateral organs, yields a novel regulatory model suggesting that maize organ initiation and subsequent development are derived from the stimulation and interaction of auxin and cytokinin fluxes. Propositions that hormone activities and sap flow pathways control organogenesis are thoroughly explored, and initiation and development processes of distinctive maize organs are discussed. Analysis of physiological factors driving hormone and sap movement implicates cues of whole-plant activity for hormone and sap fluxes to stimulate maize inflorescence initiation and organ identity determination. The physiological origins and biogenetic mechanisms underlying maize floral sex determination occurring at the tassel and ear spikelet are thoroughly investigated. The comprehensive outline of maize development and morphogenetic physiology developed in this review will enable farmers to optimize field management and will provide a reference for de novo crop domestication and germplasm improvement using genome editing biotechnologies, promoting agricultural optimization.

ZmDWF1 regulates leaf angle in maize

Leaf angle (LA) is a critical agronomic trait enhancing grain yield under high-density planting in maize. A number of researches have been conducted in recent years to investigate the quantitative trait loci/genes responsible for LA variation, while only a few genes were identified through map-based cloning. Here we cloned the ZmDWF1 gene, which was previously reported to encode Δ24-sterol reductase in the brassinosteroids (BRs) biosynthesis pathway. Overexpression of ZmDWF1 resulted in enlarged LA, indicating that ZmDWF1 is a positive regulator of LA in maize. To reveal the regulatory framework of ZmDWF1, we conducted RNA-Sequencing and yeast-two hybrid (Y2H) screening analysis. RNA-Sequencing analyzing results indicate ZmDWF1 mainly affected expression level of genes involved in cell wall associated metabolism and hormone metabolism including BR, gibberellin, and auxin. Y2H screening with Bi-FC assay confirmed three proteins (ZmPP2C-1, ZmROF1, and ZmTWD1) interacting with ZmDWF1. We revealed a new regulatory network of ZmDWF1 gene in controlling plant architecture in maize.

Co-option of the SHOOT MERISTEMLESS network regulates protocorm-like body development in Phalaenopsis aphrodite

The protocorm is a structure that is formed upon germination of an orchid seed. It lacks cotyledons and is ovoid in shape. The protocorm-like body (PLB), on the other hand, is a protocorm-like organ induced from somatic tissues. PLBs have been widely used for orchid micropropagation. Because of its unique structure and its application in the orchid industry, PLB development has drawn considerable interest from orchid and developmental biologists. Our previous genome-wide comparative transcriptome study demonstrated that protocorms and PLBs share similar molecular signatures and suggested that SHOOT MERISTEMLESS (STM)-dependent organogenesis is important for PLB development. Here, we show that overexpression of Phalaenopsis aphrodite STM (PaSTM) greatly enhances PLB regeneration from vegetative tissue-based explants of Phalaenopsis orchids, confirming its regulatory role in PLB development. Expression of PaSTM restored shoot meristem function of the Arabidopsis (Arabidopsis thaliana) stm-2 mutant. Moreover, we identified class S11 MYB transcription factors (TFs) as targets downstream of PaSTM. A cis-acting element, TTGACT, identified in the promoters of S11 MYB TFs was found to be important for PaSTM binding and activation. Overexpression of PaSTM or its downstream targets, PaMYB13, PaMYB14, and PaMYB17, enhanced de novo shoot regeneration in Arabidopsis, indicating the active role of the PaSTM-S11 PaMYB module in organogenesis. In summary, our data demonstrate that PaSTM is important for PLB development. The STM-S11 MYB regulatory module is evolutionarily conserved and may regulate shoot or shoot-related organ development in plants.

The power of classic maize mutants: Driving forward our fundamental understanding of plants

Since Mendel, maize has been a powerhouse of fundamental genetics research. From testing the Mendelian laws of inheritance, to the first genetic and cytogenetic maps, to the use of whole-genome sequencing data for crop improvement, maize is at the forefront of genetics advances. Underpinning much of this revolutionary work are the classic morphological mutants; the "freaks" that stood out in the field to even the untrained eye. Here we review some of these classic developmental mutants and their importance in the history of genetics, as well as their key role in our fundamental understanding of plant development.

Boundary domain genes were recruited to suppress bract growth and promote branching in maize

Grass inflorescence development is diverse and complex and involves sophisticated but poorly understood interactions of genes regulating branch determinacy and leaf growth. Here, we use a combination of transcript profiling and genetic and phylogenetic analyses to investigate tasselsheath1 (tsh1) and tsh4, two maize genes that simultaneously suppress inflorescence leaf growth and promote branching. We identify a regulatory network of inflorescence leaf suppression that involves the phase change gene tsh4 upstream of tsh1 and the ligule identity gene liguleless2 (lg2). We also find that a series of duplications in the tsh1 gene lineage facilitated its shift from boundary domain in nongrasses to suppressed inflorescence leaves of grasses. Collectively, these results suggest that the boundary domain genes tsh1 and lg2 were recruited to inflorescence leaves where they suppress growth and regulate a nonautonomous signaling center that promotes inflorescence branching, an important component of yield in cereal grasses.

Leaf angle: a target of genetic improvement in cereal crops tailored for high-density planting

High-density planting is an effective measure for increasing crop yield per unit land area. Leaf angle (LA) is a key trait of plant architecture and a target for genetic improvement of crops. Upright leaves allow better light capture in canopy under high-density planting, thus enhancing photosynthesis efficiency, ventilation and stress resistance, and ultimately higher grain yield. Here, we summarized the latest progress on the cellular and molecular mechanisms regulating LA formation in rice and maize. We suggest several standing out questions for future studies and then propose some promising strategies to manipulate LA for breeding of cereal crops tailored for high-density planting.

Tissue-specific transcriptomics reveal functional differences in floral development

Flowers are produced by floral meristems, groups of stem cells that give rise to floral organs. In grasses, including the major cereal crops, flowers (florets) are contained in spikelets, which contain one to many florets, depending on the species. Importantly, not all grass florets are developmentally equivalent, and one or more florets are often sterile or abort in each spikelet. Members of the Andropogoneae tribe, including maize (Zea mays), produce spikelets with two florets; the upper and lower florets are usually dimorphic, and the lower floret is greatly reduced compared to the upper floret. In maize ears, early development appears identical in both florets but the lower floret ultimately aborts. To gain insight into the functional differences between florets with different fates, we used laser capture microdissection coupled with RNA-sequencing to globally examine gene expression in upper and lower floral meristems in maize. Differentially expressed genes were involved in hormone regulation, cell wall, sugar, and energy homeostasis. Furthermore, cell wall modifications and sugar accumulation differed between the upper and lower florets. Finally, we identified a boundary domain between upper and lower florets, which we hypothesize is important for floral meristem activity. We propose a model in which growth is suppressed in the lower floret by limiting sugar availability and upregulating genes involved in growth repression. This growth repression module may also regulate floret fertility in other grasses and potentially be modulated to engineer more productive cereal crops.

An in situ sequencing approach maps PLASTOCHRON1 at the boundary between indeterminate and determinate cells

The plant shoot apex houses the shoot apical meristem, a highly organized and active stem-cell tissue where molecular signaling in discrete cells determines when and where leaves are initiated. We optimized a spatial transcriptomics approach, in situ sequencing (ISS), to colocalize the transcripts of 90 genes simultaneously on the same section of tissue from the maize (Zea mays) shoot apex. The RNA ISS technology reported expression profiles that were highly comparable with those obtained by in situ hybridizations (ISHs) and allowed the discrimination between tissue domains. Furthermore, the application of spatial transcriptomics to the shoot apex, which inherently comprised phytomers that are in gradual developmental stages, provided a spatiotemporal sequence of transcriptional events. We illustrate the power of the technology through PLASTOCHRON1 (PLA1), which was specifically expressed at the boundary between indeterminate and determinate cells and partially overlapped with ROUGH SHEATH1 and OUTER CELL LAYER4 transcripts. Also, in the inflorescence, PLA1 transcripts localized in cells subtending the lateral primordia or bordering the newly established meristematic region, suggesting a more general role of PLA1 in signaling between indeterminate and determinate cells during the formation of lateral organs. Spatial transcriptomics builds on RNA ISH, which assays relatively few transcripts at a time and provides a powerful complement to single-cell transcriptomics that inherently removes cells from their native spatial context. Further improvements in resolution and sensitivity will greatly advance research in plant developmental biology.

Genetic control of leaf angle in sorghum and its effect on light interception

Developing sorghum genotypes adapted to different light environments requires understanding of a plant's ability to capture light, determined through leaf angle specifically. This study dissected the genetic basis of leaf angle in 3 year field trials at two sites, using a sorghum diversity panel (729 accessions). A wide range of variation in leaf angle with medium heritability was observed. Leaf angle explained 36% variation in canopy light extinction coefficient, highlighting the extent to which variation in leaf angle influences light interception at the whole-canopy level. This study also found that the sorghum races of Guinea and Durra consistently having the largest and smallest leaf angle, respectively, highlighting the potential role of leaf angle in adaptation to distinct environments. The genome-wide association study detected 33 quantitative trait loci (QTLs) associated with leaf angle. Strong synteny was observed with previously detected leaf angle QTLs in maize (70%) and rice (40%) within 10 cM, among which the overlap was significantly enriched according to χ2 tests, suggesting a highly consistent genetic control in grasses. A priori leaf angle candidate genes identified in maize and rice were found to be enriched within a 1-cM window around the sorghum leaf angle QTLs. Additionally, protein domain analysis identified the WD40 protein domain as being enriched within a 1-cM window around the QTLs. These outcomes show that there is sufficient heritability and natural variation in the angle of upper leaves in sorghum which may be exploited to change light interception and optimize crop canopies for different contexts.

qTeller: a tool for comparative multi-genomic gene expression analysis

MOTIVATION

Over the last decade, RNA-Seq whole-genome sequencing has become a widely used method for measuring and understanding transcriptome-level changes in gene expression. Since RNA-Seq is relatively inexpensive, it can be used on multiple genomes to evaluate gene expression across many different conditions, tissues and cell types. Although many tools exist to map and compare RNA-Seq at the genomics level, few web-based tools are dedicated to making data generated for individual genomic analysis accessible and reusable at a gene-level scale for comparative analysis between genes, across different genomes and meta-analyses.

RESULTS

To address this challenge, we revamped the comparative gene expression tool qTeller to take advantage of the growing number of public RNA-Seq datasets. qTeller allows users to evaluate gene expression data in a defined genomic interval and also perform two-gene comparisons across multiple user-chosen tissues. Though previously unpublished, qTeller has been cited extensively in the scientific literature, demonstrating its importance to researchers. Our new version of qTeller now supports multiple genomes for intergenomic comparisons, and includes capabilities for both mRNA and protein abundance datasets. Other new features include support for additional data formats, modernized interface and back-end database and an optimized framework for adoption by other organisms' databases.

AVAILABILITY AND IMPLEMENTATION

The source code for qTeller is open-source and available through GitHub (https://github.com/Maize-Genetics-and-Genomics-Database/qTeller). A maize instance of qTeller is available at the Maize Genetics and Genomics database (MaizeGDB) (https://qteller.maizegdb.org/), where we have mapped over 200 unique datasets from GenBank across 27 maize genomes.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

Evolution of the grass leaf by primordium extension and petiole-lamina remodeling

[Figure: see text].

SDG128 is involved in maize leaf inclination

Maize leaf angle (LA) is a complex quantitative trait that is controlled by developmental signals, hormones, and environmental factors. However, the connection between histone methylation and LAs in maize remains unclear. Here, we reported that SET domain protein 128 (SDG128) is involved in leaf inclination in maize. Knockdown of SDG128 using an RNA interference approach resulted in an expanded architecture, less large vascular bundles, more small vascular bundles, and larger spacing of large vascular bundles in the auricles. SDG128 interacts with ZmGID2 both in vitro and in vivo. Knockdown of ZmGID2 also showed a larger LA with less large vascular bundles and larger spacing of vascular bundles. In addition, the transcription level of cell wall expansion family genes ZmEXPA1, ZmEXPB2, and GRMZM2G005887; transcriptional factor genes Lg1, ZmTAC1, and ZmCLA4; and auxin pathway genes ZmYUCCA7, ZmYUCCA8, and ZmARF22 was reduced in SDG128 and ZmGID2 knockdown plants. SDG128 directly targets ZmEXPA1, ZmEXPB2, LG1, and ZmTAC1 and is required for H3K4me3 deposition at these genes. Together, the results of the present study suggest that SDG128 and ZmGID2 are involved in the maize leaf inclination.

The dynamics of maize leaf development: Patterned to grow while growing a pattern

Leaves are a significant component of the shoot system in grasses, functioning in light capture and photosynthesis. Leaf width, length, and angle are expressions of development that collectively define canopy architecture. Thus, the distinctive morphology of grass leaves is an interdependent readout of developmental patterning and growth along the proximal-distal, medial-lateral, and adaxial-abaxial axes. Here, we review the chronology of patterning and growth, namely along the proximal-distal axis, during maize leaf development. We underscore that patterning and growth occur simultaneously, making use of shared developmental gradients and molecular pathways.

An RNA in situ hybridization protocol optimized for monocot tissue

RNA in situ hybridization can be time-consuming and difficult to troubleshoot. Here, we provide an optimized protocol for maize leaf tissue, though it can be applied to other plant tissues such as shoot apical meristems, embryos, and floral organs. We generate three >100 bp unique antisense probes for each gene of interest and hybridize them to tissue sections. For complete details on the use and execution of this protocol, please refer to Bezrutczyk et al. (2021).

Modification of cereal plant architecture by genome editing to improve yields

We summarize recent genome editing studies that have focused on the examination (or reexamination) of plant architectural phenotypes in cereals and the modification of these traits for crop improvement. Plant architecture is defined as the three-dimensional organization of the entire plant. Shoot architecture refers to the structure and organization of the aboveground components of a plant, reflecting the developmental patterning of stems, branches, leaves and inflorescences/flowers. Root system architecture is essentially determined by four major shape parameters-growth, branching, surface area and angle. Interest in plant architecture has arisen from the profound impact of many architectural traits on agronomic performance, and the genetic and hormonal regulation of these traits which makes them sensitive to both selective breeding and agronomic practices. This is particularly important in staple crops, and a large body of literature has, therefore, accumulated on the control of architectural phenotypes in cereals, particularly rice due to its twin role as one of the world's most important food crops as well as a model organism in plant biology and biotechnology. These studies have revealed many of the molecular mechanisms involved in the regulation of tiller/axillary branching, stem height, leaf and flower development, root architecture and the grain characteristics that ultimately help to determine yield. The advent of genome editing has made it possible, for the first time, to introduce precise mutations into cereal crops to optimize their architecture and close in on the concept of the ideotype. In this review, we consider recent genome editing studies that have focused on the examination (or reexamination) of plant architectural phenotypes in cereals and the modification of these traits for crop improvement.

Network analyses identify a transcriptomic proximodistal prepattern in the maize leaf primordium

The formation of developmental boundaries is a common feature of multicellular plants and animals, and impacts the initiation, structure and function of all organs. Maize leaves comprise a proximal sheath that encloses the stem, and a distal photosynthetic blade that projects away from the plant axis. An epidermally derived ligule and a joint-like auricle develop at the blade/sheath boundary of maize leaves. Mutations disturbing the ligule/auricle region disrupt leaf patterning and impact plant architecture, yet it is unclear how this developmental boundary is established. Targeted microdissection followed by transcriptomic analyses of young leaf primordia were utilized to construct a co-expression network associated with development of the blade/sheath boundary. Evidence is presented for proximodistal gradients of gene expression that establish a prepatterned transcriptomic boundary in young leaf primordia, before the morphological initiation of the blade/sheath boundary in older leaves. This work presents a conceptual model for spatiotemporal patterning of proximodistal leaf domains, and provides a rich resource of candidate gene interactions for future investigations of the mechanisms of blade/sheath boundary formation in maize.

OsmiR164-targeted OsNAM, a boundary gene, plays important roles in rice leaf and panicle development

The CUP-SHAPED COTYLEDON (CUC) genes (CUC1, CUC2 and CUC3) regulate organ boundary formation in Arabidopsis. However, the functions of their homologous genes in rice (Oryza sativa) are still unknown. Here, we have identified an orthologous gene of CUC1 and CUC2 in rice, named OsNAM. Subcellular localization and yeast two-hybrid assay results have suggested that OsNAM encodes a conserved nuclear NAC (NAM/ATAF1/CUC2) protein with a transcriptional activator. The null mutant osnam-1 presented a fused leaf structure, small panicles, reduced branches and aberrant floral organ identities when compared with those of the wild type. Beta-glucuronidase staining and GFP reporter lines indicated that OsNAM was expressed in young tissues and that its boundary enrichment expression was regulated by OsmiR164. Loss-of-function mutants for OsCUC3 resulted in no obvious defects throughout rice development. The osnam oscuc3 double mutant, however, resulted in severe leaf fusion of the first two leaves, while the osnam single mutant showed a similar phenotype from the seventh leaf. These results indicated that OsNAM and OsCUC3 act redundantly for boundary specification during post-embryonic development. Overall, we describe the biological functions of OsNAM and OsCUC3 in rice development and the expression characteristics of OsNAM. This work reveals the important role of CUC genes in rice.

Genome-wide analysis of spatiotemporal expression patterns during rice leaf development

BACKGROUND

Rice leaves consist of three distinct regions along a proximal-distal axis, namely the leaf blade, sheath, and blade-sheath boundary region. Each region has a unique morphology and function, but the genetic programs underlying the development of each region are poorly understood. To fully elucidate rice leaf development and discover genes with unique functions in rice and grasses, it is crucial to explore genome-wide transcriptional profiles during the development of the three regions.

RESULTS

In this study, we performed microarray analysis to profile the spatial and temporal patterns of gene expression in the rice leaf using dissected parts of leaves sampled in broad developmental stages. The dynamics in each region revealed that the transcriptomes changed dramatically throughout the progress of tissue differentiation, and those of the leaf blade and sheath differed greatly at the mature stage. Cluster analysis of expression patterns among leaf parts revealed groups of genes that may be involved in specific biological processes related to rice leaf development. Moreover, we found novel genes potentially involved in rice leaf development using a combination of transcriptome data and in situ hybridization, and analyzed their spatial expression patterns at high resolution. We successfully identified multiple genes that exhibit localized expression in tissues characteristic of rice or grass leaves.

CONCLUSIONS

Although the genetic mechanisms of leaf development have been elucidated in several eudicots, direct application of that information to rice and grasses is not appropriate due to the morphological and developmental differences between them. Our analysis provides not only insights into the development of rice leaves but also expression profiles that serve as a valuable resource for gene discovery. The genes and gene clusters identified in this study may facilitate future research on the unique developmental mechanisms of rice leaves.

Developmental genetics of maize vegetative shoot architecture

More than 1.1 billion tonnes of maize grain were harvested across 197 million hectares in 2019 (FAOSTAT 2020). The vast global productivity of maize is largely driven by denser planting practices, higher yield potential per area of land, and increased yield potential per plant. Shoot architecture, the three-dimensional structural arrangement of the above-ground plant body, is critical to maize grain yield and biomass. Structure of the shoot is integral to all aspects of modern agronomic practices. Here, the developmental genetics of the maize vegetative shoot is reviewed. Plant architecture is ultimately determined by meristem activity, developmental patterning, and growth. The following topics are discussed: shoot apical meristem, leaf architecture, axillary meristem and shoot branching, and intercalary meristem and stem activity. Where possible, classical and current studies in maize developmental genetics, as well as recent advances leveraged by "-omics" analyses, are highlighted within these sections.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-021-01208-1.

High planting density induces the expression of GA3-oxidase in leaves and GA mediated stem elongation in bioenergy sorghum

The stems of bioenergy sorghum hybrids at harvest are > 4 m long, contain > 40 internodes and account for ~ 80% of harvested biomass. In this study, bioenergy sorghum hybrids were grown at four planting densities (~ 20,000 to 132,000 plants/ha) under field conditions for 60 days to investigate the impact shading has on stem growth and biomass accumulation. Increased planting density induced a > 2-fold increase in sorghum internode length and a ~ 22% decrease in stem diameter, a typical shade avoidance response. Shade-induced internode elongation was due to an increase in cell length and number of cells spanning the length of internodes. SbGA3ox2 (Sobic.003G045900), a gene encoding the last step in GA biosynthesis, was expressed ~ 20-fold higher in leaf collar tissue of developing phytomers in plants grown at high vs. low density. Application of GA3 to bioenergy sorghum increased plant height, stem internode length, cell length and the number of cells spanning internodes. Prior research showed that sorghum plants lacking phytochrome B, a key photoreceptor involved in shade signaling, accumulated more GA1 and displayed shade avoidance phenotypes. These results are consistent with the hypothesis that increasing planting density induces expression of GA3-oxidase in leaf collar tissue, increasing synthesis of GA that stimulates internode elongation.

Maize bHLH55 functions positively in salt tolerance through modulation of AsA biosynthesis by directly regulating GDP-mannose pathway genes

Ascorbic acid (AsA) is an antioxidant and enzyme co-factor that is vital to plant development and abiotic stress tolerance. However, the regulation mechanisms of AsA biosynthesis in plants remain poorly understood. Here, we report a basic helix-loop-helix 55 (ZmbHLH55) transcription factor that regulates AsA biosynthesis in maize. Analysis of publicly available transcriptomic data revealed that ZmbHLH55 is co-expressed with several genes of the GDP-mannose pathway. Experimental data showed that ZmbHLH55 forms homodimers localized to the cell nuclei, and it exhibits DNA binding and transactivation activity in yeast. Under salt stress conditions, knock down mutant (zmbhlh55) in maize accumulated lower levels of AsA compared with wild type, accompanied by lower antioxidant enzymes activity, shorter root length, and higher malondialdehyde (MDA) level. Gene expression data from the WT and zmbhlh55 mutant, showed that ZmbHLH55 positively regulates the expression of ZmPGI2, ZmGME1, and ZmGLDH, but negatively regulates ZmGMP1 and ZmGGP. Furthermore, ZmbHLH55-overexpressing Arabidopsis, under salt conditions, showed higher AsA levels, increased rates of germination, and elevated antioxidant enzyme activities. In conclusion, these results have identified previously unknown regulation mechanisms for AsA biosynthesis, indicating that ZmbHLH55 may be a potential candidate to enhance plant salt stress tolerance in the future.

Plant stem-cell organization and differentiation at single-cell resolution

Plants maintain populations of pluripotent stem cells in shoot apical meristems (SAMs), which continuously produce new aboveground organs. We used single-cell RNA sequencing (scRNA-seq) to achieve an unbiased characterization of the transcriptional landscape of the maize shoot stem-cell niche and its differentiating cellular descendants. Stem cells housed in the SAM tip are engaged in genome integrity maintenance and exhibit a low rate of cell division, consistent with their contributions to germline and somatic cell fates. Surprisingly, we find no evidence for a canonical stem-cell organizing center subtending these cells. In addition, trajectory inference was used to trace the gene expression changes that accompany cell differentiation, revealing that ectopic expression of KNOTTED1 (KN1) accelerates cell differentiation and promotes development of the sheathing maize leaf base. These single-cell transcriptomic analyses of the shoot apex yield insight into the processes of stem-cell function and cell-fate acquisition in the maize seedling and provide a valuable scaffold on which to better dissect the genetic control of plant shoot morphogenesis at the cellular level.

Plant stem-cell organization and differentiation at single-cell resolution

Plants possess the remarkable ability to grow and produce new organs throughout their lifespan, owing to the activities of persistent populations of pluripotent stem cells within their meristematic tips. Here we isolated individual cells from the microscopic shoot apical meristem (SAM) of maize and provide single-cell transcriptomic analysis of a plant shoot meristem. This study enabled an unbiased analysis of the developmental genetic organization of the maize shoot apex and uncovered evolutionarily divergent and conserved signatures of SAM homeostasis. The fine-scale resolution of single-cell analysis was used to reconstruct the process of shoot cell differentiation, whereby stem cells acquire diverse and distinct cell fates over developmental time in wild-type and mutant maize seedlings.

Plants maintain populations of pluripotent stem cells in shoot apical meristems (SAMs), which continuously produce new aboveground organs. We used single-cell RNA sequencing (scRNA-seq) to achieve an unbiased characterization of the transcriptional landscape of the maize shoot stem-cell niche and its differentiating cellular descendants. Stem cells housed in the SAM tip are engaged in genome integrity maintenance and exhibit a low rate of cell division, consistent with their contributions to germline and somatic cell fates. Surprisingly, we find no evidence for a canonical stem-cell organizing center subtending these cells. In addition, trajectory inference was used to trace the gene expression changes that accompany cell differentiation, revealing that ectopic expression of KNOTTED1 (KN1) accelerates cell differentiation and promotes development of the sheathing maize leaf base. These single-cell transcriptomic analyses of the shoot apex yield insight into the processes of stem-cell function and cell-fate acquisition in the maize seedling and provide a valuable scaffold on which to better dissect the genetic control of plant shoot morphogenesis at the cellular level.

A Missense Mutation in a Large Subunit of Ribonucleotide Reductase Confers Temperature-Gated Tassel Formation

Temperature is a major factor regulating plant growth. To reproduce at extreme temperatures, plants must develop normal reproductive organs when exposed to temperature changes. However, little is known about the underlying molecular mechanisms. Here, we identified the maize (Zea mays) mutant thermosensitive vanishing tassel1-R (tvt1-R), which lacks tassels at high (restrictive) temperatures due to shoot apical meristem (SAM) arrest, but forms normal tassels at moderate (permissive) temperatures. The critical stage for phenotypic conversion in tvt1-R mutants is V2 to V6 (Vn, where "n" is the number of leaves with collars visible). Positional cloning and allelism and complementation tests revealed that a G-to-A mutation causing a Arg₂₇₇-to-His₂₇₇ substitution in ZmRNRL1, a ribonucleotide reductase (RNR) large subunit (RNRL), confers the tvt1-R mutant phenotype. RNR regulates the rate of deoxyribonucleoside triphosphate (dNTP) production for DNA replication and damage repair. By expression, yeast two-hybrid, RNA sequencing, and flow cytometric analyses, we found that ZmRNRL1-tvt1-R failed to interact with all three RNR small subunits at 34°C due to the Arg₂₇₇-to-His₂₇₇ substitution, which could impede RNR holoenzyme (α₂β₂) formation, thereby decreasing the dNTP supply for DNA replication. Decreased dNTP supply may be especially severe for the SAM that requires a continuous, sufficient dNTP supply for rapid division, as demonstrated by the SAM arrest and tassel absence in tvt1-R mutants at restrictive temperatures. Our study reveals a novel mechanism of temperature-gated tassel formation in maize and provides insight into the role of RNRL in SAM maintenance.

Genome-wide mapping and prediction of plant architecture in a sorghum nested association mapping population

Modifying plant architecture is often necessary for yield improvement and climate adaptation, but we lack understanding of the genotype-phenotype map for plant morphology in sorghum. Here, we use a nested association mapping (NAM) population that captures global allelic diversity of sorghum to characterize the genetics of leaf erectness, leaf width (at two stages), and stem diameter. Recombinant inbred lines (n = 2200) were phenotyped in multiple environments (35,200 observations) and joint linkage mapping was performed with ∼93,000 markers. Fifty-four QTL of small to large effect were identified for trait BLUPs (9-16 per trait) each explaining 0.4-4% of variation across the NAM population. While some of these QTL colocalize with sorghum homologs of grass genes (e.g., those involved in transcriptional regulation of hormone synthesis [rice SPINDLY] and transcriptional regulation of development [rice Ideal plant architecture1]), most QTL did not colocalize with an a priori candidate gene (92%). Genomic prediction accuracy was generally high in five-fold cross-validation (0.65-0.83), and varied from low to high in leave-one-family-out cross-validation (0.04-0.61). The findings provide a foundation to identify the molecular basis of architecture variation in sorghum and establish genomic-enabled breeding for improved plant architecture.

Plant homeodomain proteins provide a mechanism for how leaves grow wide

The mechanisms whereby leaf anlagen undergo proliferative growth and expansion to form wide, flat leaves are unclear. The maize gene NARROWSHEATH1 (NS1) is a WUSCHEL-related homeobox3 (WOX3) homolog expressed at the margins of leaf primordia, and is required for mediolateral outgrowth. To investigate the mechanisms of NS1 function, we used chromatin immunoprecipitation and laser-microdissection RNA-seq of leaf primordial margins to identify gene targets bound and modulated by NS1. Microscopic analyses of cell division and gene expression in expanding leaves, and reverse genetic analyses of homologous NS1 target genes in Arabidopsis, reveal that NS1 controls mediolateral outgrowth by repression of a growth inhibitor and promotion of cell division at primordial leaf margins. Intriguingly, homologous WOX gene products are expressed in stem cell-organizing centers and traffic to adjoining cells to activate stem-cell identity non-autonomously. In contrast, WOX3/NS1 does not traffic, and stimulates cell divisions in the same cells in which it is transcribed.

Highlighted Article: The NS1 homeodomain transcription factor regulates lateral organ outgrowth from shoot meristems and leaf primordial margins by repressing the expression of negative growth regulators.

Multi-Trait Genome-Wide Association Studies Reveal Loci Associated with Maize Inflorescence and Leaf Architecture

Maize inflorescence is a complex phenotype that involves the physical and developmental interplay of multiple traits. Given the evidence that genes could pleiotropically contribute to several of these traits, we used publicly available maize data to assess the ability of multivariate genome-wide association study (GWAS) approaches to identify pleiotropic quantitative trait loci (pQTL). Our analysis of 23 publicly available inflorescence and leaf-related traits in a diversity panel of n = 281 maize lines genotyped with 376,336 markers revealed that the two multivariate GWAS approaches we tested were capable of identifying pQTL in genomic regions coinciding with similar associations found in previous studies. We then conducted a parallel simulation study on the same individuals, where it was shown that multivariate GWAS approaches yielded a higher true-positive quantitative trait nucleotide (QTN) detection rate than comparable univariate approaches for all evaluated simulation settings except for when the correlated simulated traits had a heritability of 0.9. We therefore conclude that the implementation of state-of-the-art multivariate GWAS approaches is a useful tool for dissecting pleiotropy and their more widespread implementation could facilitate the discovery of genes and other biological mechanisms underlying maize inflorescence.

Control of cell fate during axillary meristem initiation

Axillary meristems (AMs) are located in the leaf axil and can establish new growth axes. Whereas their neighboring cells are differentiated, the undifferentiated cells in the AM endow the AM with the same developmental potential as the shoot apical meristem. The AM is, therefore, an excellent system to study stem cell fate maintenance in plants. In this review, we summarize the current knowledge of AM initiation. Recent findings have shown that AMs derive from a stem cell lineage that is maintained in the leaf axil. This review covers AM progenitor cell fate maintenance, reactivation, and meristem establishment. We also highlight recent work that links transcription factors, phytohormones, and epigenetic regulation to AM initiation.

ZmIBH1-1 regulates plant architecture in maize

Leaf angle (LA) is a critical agronomic trait in maize, with more upright leaves allowing higher planting density, leading to more efficient light capture and higher yields. A few genes responsible for variation in LA have been identified by map-based cloning. In this study, we cloned maize ZmIBH1-1, which encodes a bHLH transcription factor with both a basic binding region and a helix-loop-helix domain, and the results of qRT-PCR showed that it is a negative regulator of LA. Histological analysis indicated that changes in LA were mainly caused by differential cell wall lignification and cell elongation in the ligular region. To determine the regulatory framework of ZmIBH1-1, we conducted RNA-seq and DNA affinity purification (DAP)-seq analyses. The combined results revealed 59 ZmIBH1-1-modulated target genes with annotations, and they were mainly related to the cell wall, cell development, and hormones. Based on the data, we propose a regulatory model for the control of plant architecture by ZmIBH1-1 in maize.

Transcriptomic network analyses shed light on the regulation of cuticle development in maize leaves

Plant cuticles provide barriers to water loss and arose as aquatic plants adapted to the dry terrestrial environment. The cuticle components, waxes and the fatty acid-based polymer cutin, are synthesized in the plant epidermis, exported across the cell wall, and deposited on the plant surface. This study suggests a role for PHYTOCHROME light receptors during cuticle development in leaves of maize and moss, diverse species that are separated by more than 400 million y of land plant evolution. We hypothesize that phytochrome-mediated light signaling contributed to the evolution of cuticles in land plants.

Plant cuticles are composed of wax and cutin and evolved in the land plants as a hydrophobic boundary that reduces water loss from the plant epidermis. The expanding maize adult leaf displays a dynamic, proximodistal gradient of cuticle development, from the leaf base to the tip. Laser microdissection RNA Sequencing (LM-RNAseq) was performed along this proximodistal gradient, and complementary network analyses identified potential regulators of cuticle biosynthesis and deposition. A weighted gene coexpression network (WGCN) analysis suggested a previously undescribed function for PHYTOCHROME-mediated light signaling during the regulation of cuticular wax deposition. Genetic analyses reveal that phyB1 phyB2 double mutants of maize exhibit abnormal cuticle composition, supporting the predictions of our coexpression analysis. Reverse genetic analyses also show that phy mutants of the moss Physcomitrella patens exhibit abnormal cuticle composition, suggesting an ancestral role for PHYTOCHROME-mediated, light-stimulated regulation of cuticle development during plant evolution.

The Maize Hairy Sheath Frayed1 (Hsf1) Mutation Alters Leaf Patterning through Increased Cytokinin Signaling

Leaf morphogenesis requires growth polarized along three axes-proximal-distal (P-D) axis, medial-lateral axis, and abaxial-adaxial axis. Grass leaves display a prominent P-D polarity consisting of a proximal sheath separated from the distal blade by the auricle and ligule. Although proper specification of the four segments is essential for normal morphology, our knowledge is incomplete regarding the mechanisms that influence P-D specification in monocots such as maize (Zea mays). Here, we report the identification of the gene underlying the semidominant, leaf patterning maize mutant Hairy Sheath Frayed1 (Hsf1). Hsf1 plants produce leaves with outgrowths consisting of proximal segments-sheath, auricle, and ligule-emanating from the distal blade margin. Analysis of three independent Hsf1 alleles revealed gain-of-function missense mutations in the ligand binding domain of the maize cytokinin (CK) receptor Z. mays Histidine Kinase1 (ZmHK1) gene. Biochemical analysis and structural modeling suggest the mutated residues near the CK binding pocket affect CK binding affinity. Treatment of the wild-type seedlings with exogenous CK phenocopied the Hsf1 leaf phenotypes. Results from expression and epistatic analyses indicated the Hsf1 mutant receptor appears to be hypersignaling. Our results demonstrate that hypersignaling of CK in incipient leaf primordia can reprogram developmental patterns in maize.

Wheat TaSPL8 Modulates Leaf Angle Through Auxin and Brassinosteroid Signaling

In grass crops, leaf angle is determined by development of the lamina joint, the tissue connecting the leaf blade and sheath, and is closely related to crop architecture and yield. In this study, we identified a mutant generated by fast neutron radiation that exhibited an erect leaf phenotype caused by defects in lamina joint development. Map-based cloning revealed that the gene TaSPL8, encoding a SQUAMOSA PROMOTER BINDING-LIKE (SPL) protein, is deleted in this mutant. TaSPL8 knock-out mutants exhibit erect leaves due to loss of the lamina joint, compact architecture, and increased spike number especially in high planting density, suggesting similarity with its LIGULESS1 homologs in maize (Zea mays) and rice (Oryza sativa). Hence, LG1 could be a robust target for plant architecture improvement in grass species. Common wheat (Triticum aestivum, 2n = 6× = 42; BBAADD) is an allohexaploid containing A, B, and D subgenomes and the homeologous gene of TaSPL8 from the D subgenome contributes to the length of the lamina joint to a greater extent than that from the A and B subgenomes. Comparison of the transcriptome between the Taspl8 mutant and the wild type revealed that TaSPL8 is involved in the activation of genes related to auxin and brassinosteroid pathways and cell elongation. TaSPL8 binds to the promoters of the AUXIN RESPONSE FACTOR gene and of the brassinosteroid biogenesis gene CYP90D2 and activates their expression. These results indicate that TaSPL8 might regulate lamina joint development through auxin signaling and the brassinosteroid biosynthesis pathway.

Tomato locule number and fruit size controlled by natural alleles of lc and fas

Improving yield by increasing the size of produce is an important selection criterion during the domestication of fruit and vegetable crops. Genes controlling meristem organization and organ formation work in concert to regulate the size of reproductive organs. In tomato, lc and fas control locule number, which often leads to enlarged fruits compared to the wild progenitors. LC is encoded by the tomato ortholog of WUSCHEL (WUS), whereas FAS is encoded by the tomato ortholog of CLAVATA3 (CLV3). The critical role of the WUS-CLV3 feedback loop in meristem organization has been demonstrated in several plant species. We show that mutant alleles for both loci in tomato led to an expansion of the SlWUS expression domain in young floral buds 2-3 days after initiation. Single and double mutant alleles of lc and fas maintain higher SlWUS expression during the development of the carpel primordia in the floral bud. This augmentation and altered spatial expression of SlWUS provided a mechanistic basis for the formation of multilocular and large fruits. Our results indicated that lc and fas are gain-of-function and partially loss-of-function alleles, respectively, while both mutations positively affect the size of tomato floral meristems. In addition, expression profiling showed that lc and fas affected the expression of several genes in biological processes including those involved in meristem/flower development, patterning, microtubule binding activity, and sterol biosynthesis. Several differentially expressed genes co-expressed with SlWUS have been identified, and they are enriched for functions in meristem regulation. Our results provide new insights into the transcriptional regulation of genes that modulate meristem maintenance and floral organ determinacy in tomato.

Transcriptomic Analysis of Leaf Sheath Maturation in Maize

The morphological development of the leaf greatly influences plant architecture and crop yields. The maize leaf is composed of a leaf blade, ligule and sheath. Although extensive transcriptional profiling of the tissues along the longitudinal axis of the developing maize leaf blade has been conducted, little is known about the transcriptional dynamics in sheath tissues, which play important roles in supporting the leaf blade. Using a comprehensive transcriptome dataset, we demonstrated that the leaf sheath transcriptome dynamically changes during maturation, with the construction of basic cellular structures at the earliest stages of sheath maturation with a transition to cell wall biosynthesis and modifications. The transcriptome again changes with photosynthesis and lignin biosynthesis at the last stage of sheath tissue maturation. The different tissues of the maize leaf are highly specialized in their biological functions and we identified 15 genes expressed at significantly higher levels in the leaf sheath compared with their expression in the leaf blade, including the BOP2 homologs GRMZM2G026556 and GRMZM2G022606, DOGT1 (GRMZM2G403740) and transcription factors from the B3 domain, C2H2 zinc finger and homeobox gene families, implicating these genes in sheath maturation and organ specialization.

Evolutionarily informed deep learning methods for predicting relative transcript abundance from DNA sequence

Deep learning methodologies have revolutionized prediction in many fields and show potential to do the same in molecular biology and genetics. However, applying these methods in their current forms ignores evolutionary dependencies within biological systems and can result in false positives and spurious conclusions. We developed two approaches that account for evolutionary relatedness in machine learning models: (i) gene-family-guided splitting and (ii) ortholog contrasts. The first approach accounts for evolution by constraining model training and testing sets to include different gene families. The second approach uses evolutionarily informed comparisons between orthologous genes to both control for and leverage evolutionary divergence during the training process. The two approaches were explored and validated within the context of mRNA expression level prediction and have the area under the ROC curve (auROC) values ranging from 0.75 to 0.94. Model weight inspections showed biologically interpretable patterns, resulting in the hypothesis that the 3' UTR is more important for fine-tuning mRNA abundance levels while the 5' UTR is more important for large-scale changes.

Characterization of a dominant mutation for the liguleless trait: Aegilops tauschii liguleless (Lgt)

BACKGROUND

Leaves of Poaceae have a unique morphological feature: they consist of a proximal sheath and a distal blade separated by a ligular region. The sheath provides structural support and protects young developing leaves, whereas the main function of the blade is photosynthesis. The auricles allow the blade to tilt back for optimal photosynthesis and determine the angle of a leaf, whereas the ligule protects the stem from the entry of water, microorganisms, and pests. Liguleless variants have an upright leaf blade that wraps around the culm. Research on liguleless mutants of maize and other cereals has led to identification of genes that are involved in leaf patterning and differentiation.

RESULTS

We characterized an induced liguleless mutant (LM) of Aegilops tauschii Coss., a donor of genome D of bread wheat Triticum aestivum L.. The liguleless phenotype of LM is under dominant monogenic control (Lg^t). To determine precise position of Lg^t on the Ae. tauschii genetic map, highly saturated genetic maps were constructed containing 887 single-nucleotide polymorphism (SNP) markers derived via diversity arrays technology (DArT)seq. The Lg^t gene was mapped to chromosome 5DS. Taking into account coordinates of the SNP markers, flanking Lg^t, on the pseudomolecule 5D, a chromosomal region that contains this gene was determined, and a list of candidate genes was identified. Morphological features of the LM phenotype suggest that Lg^t participates in the control of leaf development, mainly, in leaf proximal-distal patterning, and its dominant mutation causes abnormal ligular region but does not affect reproductive development.

CONCLUSIONS

Here we report characterization of a liguleless Ae. tauschii mutant, whose phenotype is under control of a dominant mutation of Lg^t. The dominant mode of inheritance of the liguleless trait in a Triticeae species is reported for the first time. The position of the Lg^t locus on chromosome 5DS allowed us to identify a list of candidate genes. This list does not contain Ae. tauschii orthologs of any well-characterized cereal genes whose mutations cause liguleless phenotypes. Thus, the characterized Lg^t mutant represents a new model for further investigation of plant leaf patterning and differentiation.