Positional cloning in maize (Zea mays subsp. mays, Poaceae)

Maize mutant screens: from classical methods to new CRISPR-based approaches

Mutations play a pivotal role in shaping the trajectory and outcomes of a species evolution and domestication. Maize (Zea mays) has been a major staple crop and model for genetic research for more than 100 yr. With the arrival of site-directed mutagenesis and genome editing (GE) driven by the Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR), maize mutational research is once again in the spotlight. If we combine the powerful physiological and genetic characteristics of maize with the already available and ever increasing toolbox of CRISPR-Cas, prospects for its future trait engineering are very promising. This review aimed to give an overview of the progression and learnings of maize screening studies analyzing forward genetics, natural variation and reverse genetics to focus on recent GE approaches. We will highlight how each strategy and resource has contributed to our understanding of maize natural and induced trait variability and how this information could be used to design the next generation of mutational screenings.

Advancements in genetic techniques and functional genomics for enhancing crop traits and agricultural sustainability

Genetic variability is essential for the development of new crop varieties with economically beneficial traits. The traits can be inherited from wild relatives or induced through mutagenesis. Novel genetic elements can then be identified and new gene functions can be predicted. In this study, forward and reverse genetics approaches were described, in addition to their applications in modern crop improvement programs and functional genomics. By using heritable phenotypes and linked genetic markers, forward genetics searches for genes by using traditional genetic mapping and allele frequency estimation. Despite recent advances in sequencing technology, omics and computation, genetic redundancy remains a major challenge in forward genetics. By analyzing close-related genes, we will be able to dissect their functional redundancy and predict possible traits and gene activity patterns. In addition to these predictions, sophisticated reverse gene editing tools can be used to verify them, including TILLING, targeted insertional mutagenesis, gene silencing, gene targeting and genome editing. By using gene knock-down, knock-up and knock-out strategies, these tools are able to detect genetic changes in cells. In addition, epigenome analysis and editing enable the development of novel traits in existing crop cultivars without affecting their genetic makeup by increasing epiallelic variants. Our understanding of gene functions and molecular dynamics of various biological phenomena has been revised by all of these findings. The study also identifies novel genetic targets in crop species to improve yields and stress tolerances through conventional and non-conventional methods. In this article, genetic techniques and functional genomics are specifically discussed and assessed for their potential in crop improvement.

The maize PLASTID TERMINAL OXIDASE (PTOX) locus controls the carotenoid content of kernels

Carotenoids perform a broad range of important functions in humans; therefore, carotenoid biofortification of maize (Zea mays L.), one of the most highly produced cereal crops worldwide, would have a global impact on human health. PLASTID TERMINAL OXIDASE (PTOX) genes play an important role in carotenoid metabolism; however, the possible function of PTOX in carotenoid biosynthesis in maize has not yet been explored. In this study, we characterized the maize PTOX locus by forward- and reverse-genetic analyses. While most higher plant species possess a single copy of the PTOX gene, maize carries two tandemly duplicated copies. Characterization of mutants revealed that disruption of either copy resulted in a carotenoid-deficient phenotype. We identified mutations in the PTOX genes as being causal of the classic maize mutant, albescent1. Remarkably, overexpression of ZmPTOX1 significantly improved the content of carotenoids, especially β-carotene (provitamin A), which was increased by ~threefold, in maize kernels. Overall, our study shows that maize PTOX locus plays an important role in carotenoid biosynthesis in maize kernels and suggests that fine-tuning the expression of this gene could improve the nutritional value of cereal grains.

Mapping of qChalk1 controlling grain chalkiness in japonica rice

BACKGROUND

Rice grain chalkiness is an undesirable characteristic that affects grain quality. The aim of this study was to map QTLs controlling grain chalkiness in japonica rice.

METHODS AND RESULTS

In this study, two japonica rice cultivars with similar grain shapes but different grain chalkiness rates were crossed and the F₂ and BC₁F₂ populations were subjected to QTL-seq analysis to map the QTLs controlling the grain chalkiness rate. QTL-seq analysis revealed SNP index differences on chromosome 1 in both of the segregating populations. Using polymorphic markers between the two parents, QTL mapping was conducted on 213 individual plants in the BC₁F₂ population. QTL mapping confined a QTL controlling grain chalkiness, qChalk1, to a 1.1 Mb genomic region on chromosome 1. qChalk1 explained 19.7% of the phenotypic variation.

CONCLUSION

A QTL controlling grain chalkiness qChalk1 was detected in both F₂ and BC₁F₂ segregating populations by QTL-Seq and QTL mapping methods. This result would be helpful for further cloning of the genes controlling grain chalkiness in japonica rice.

Mapping Maize Mutants Using Bulked-Segregant Analysis and Next-Generation Sequencing

Forward genetics is used to identify the genetic basis for a phenotype. The approach involves identifying a mutant organism exhibiting a phenotype of interest and then mapping the causative locus or gene. Bulked-segregant analysis (BSA) is a quick and effective approach to map mutants using pools of mutants and wild-type plants from a segregating population to identify linkage of the mutant phenotype, and this approach has been successfully used in plants. Traditional linkage mapping approaches are outdated and time intensive, and can be very difficult. With the highly evolved development and reduction in cost of high-throughput sequencing, this new approach combined with BSA has become extremely effective in multiple plant species, including Zea mays (maize). While the approach is incredibly powerful, careful experimental design, bioinformatic mapping techniques, and interpretation of results are important to obtain the desired results in an effective and timely manner. Poor design of a mapping population, limitations in bioinformatic experience, and inadequate understanding of sequence data are limitations of these approaches for the researcher. Here, we describe a straightforward protocol for mapping mutations responsible for a phenotype of interest in maize, using high-throughput sequencing and BSA. Specifically, we discuss relevant aspects of developing a mutant mapping population. This is followed by a detailed protocol for DNA preparation and analysis of short-read sequences to map and identify candidate causative mutations responsible for the mutant phenotype of interest. We provide command-line and perl scripts to complete the bioinformatic analysis of the mutant sequence data. This protocol lays out the design of the BSA, bioinformatic approaches, and interpreting the sequencing data. These methods are very adaptable to any forward genetics experiment and provide a step-by-step approach to identifying the genetic basis of a maize mutant phenotype. © 2022 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol: Bulked-segregant analysis and high-throughput sequencing to map maize mutants.

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.

Chromosome-Scale Genome Assembly of Gilia yorkii Enables Genetic Mapping of Floral Traits in an Interspecies Cross

Substantial morphological variation in land plants remains inaccessible to genetic analysis because current models lack variation in important ecological and agronomic traits. The genus Gilia was historically a model for biosystematics studies and includes variation in morphological traits that are poorly understood at the genetic level. We assembled a chromosome-scale reference genome of G. yorkii and used it to investigate genome evolution in the Polemoniaceae. We performed QTL (quantitative trait loci) mapping in a G. yorkii×G. capitata interspecific population for traits related to inflorescence architecture and flower color. The genome assembly spans 2.75 Gb of the estimated 2.80-Gb genome, with 96.7% of the sequence contained in the nine largest chromosome-scale scaffolds matching the haploid chromosome number. Gilia yorkii experienced at least one round of whole-genome duplication shared with other Polemoniaceae after the eudicot paleohexaploidization event. We identified QTL linked to variation in inflorescence architecture and petal color, including a candidate for the major flower color QTL-a tandem duplication of flavanol 3',5'-hydroxylase. Our results demonstrate the utility of Gilia as a forward genetic model for dissecting the evolution of development in plants including the causal loci underlying inflorescence architecture transitions.

Structural variation at the maize WUSCHEL1 locus alters stem cell organization in inflorescences

Structural variation in plant genomes is a significant driver of phenotypic variability in traits important for the domestication and productivity of crop species. Among these are traits that depend on functional meristems, populations of stem cells maintained by the CLAVATA-WUSCHEL (CLV-WUS) negative feedback-loop that controls the expression of the WUS homeobox transcription factor. WUS function and impact on maize development and yield remain largely unexplored. Here we show that the maize dominant Barren inflorescence3 (Bif3) mutant harbors a tandem duplicated copy of the ZmWUS1 gene, ZmWUS1-B, whose novel promoter enhances transcription in a ring-like pattern. Overexpression of ZmWUS1-B is due to multimerized binding sites for type-B RESPONSE REGULATORs (RRs), key transcription factors in cytokinin signaling. Hypersensitivity to cytokinin causes stem cell overproliferation and major rearrangements of Bif3 inflorescence meristems, leading to the formation of ball-shaped ears and severely affecting productivity. These findings establish ZmWUS1 as an essential meristem size regulator in maize and highlight the striking effect of cis-regulatory variation on a key developmental program.

The WUSCHEL transcription factor promotes plant stem cell proliferation. Here the authors show that the maize Bif3 mutant contains a duplication of the ZmWUS1 locus leading to cytokinin hypersensitivity and overproliferation at the shoot meristem demonstrating the role of WUSCHEL in maize and how structural variation can impact plant morphology.

Maize kernel development

Maize (Zea mays) is a leading cereal crop in the world. The maize kernel is the storage organ and the harvest portion of this crop and is closely related to its yield and quality. The development of maize kernel is initiated by the double fertilization event, leading to the formation of a diploid embryo and a triploid endosperm. The embryo and endosperm are then undergone independent developmental programs, resulting in a mature maize kernel which is comprised of a persistent endosperm, a large embryo, and a maternal pericarp. Due to the well-characterized morphogenesis and powerful genetics, maize kernel has long been an excellent model for the study of cereal kernel development. In recent years, with the release of the maize reference genome and the development of new genomic technologies, there has been an explosive expansion of new knowledge for maize kernel development. In this review, we overviewed recent progress in the study of maize kernel development, with an emphasis on genetic mapping of kernel traits, transcriptome analysis during kernel development, functional gene cloning of kernel mutants, and genetic engineering of kernel traits.

The barren stalk2 Gene Is Required for Axillary Meristem Development in Maize

The diversity of plant architecture is determined by axillary meristems (AMs). AMs are produced from small groups of stem cells in the axils of leaf primordia and generate vegetative branches and reproductive inflorescences. Previous studies identified genes critical for AM development that function in auxin biosynthesis, transport, and signaling. barren stalk1 (ba1), a basic helix-loop-helix transcription factor, acts downstream of auxin to control AM formation. Here, we report the cloning and characterization of barren stalk2 (ba2), a mutant that fails to produce ears and has fewer branches and spikelets in the tassel, indicating that ba2 functions in reproductive AM development. Furthermore, the ba2 mutation suppresses tiller growth in the teosinte branched1 mutant, indicating that ba2 also plays an essential role in vegetative AM development. The ba2 gene encodes a protein that co-localizes and heterodimerizes with BA1 in the nucleus. Characterization of the genetic interaction between ba2 and ba1 demonstrates that ba1 shows a gene dosage effect in ba2 mutants, providing further evidence that BA1 and BA2 act together in the same pathway. Characterization of the molecular and genetic interaction between ba2 and additional genes required for the regulation of ba1 further supports this finding. The ba1 and ba2 genes are orthologs of rice genes, LAX PANICLE1 (LAX1) and LAX2, respectively, hence providing insights into pathways controlling AMs development in grasses.

Candidate gene identification of existing or induced mutations with pipelines applicable to large genomes

Bulked segregant analysis (BSA) is used to identify existing or induced variants that are linked to phenotypes. Although it is widely used in Arabidopsis and rice, it remains challenging for crops with large genomes, such as maize. Moreover, analysis of huge data sets can present a bottleneck linking phenotypes to their molecular basis, especially for geneticists without programming experience. Here, we identified two genes of maize defective kernel mutants with newly developed analysis pipelines that require no programing skills and should be applicable to any large genome. In the 1970s, Neuffer and Sheridan generated a chemically induced defective kernel (dek) mutant collection with the potential to uncover critical genes for seed development. To locate such mutations, the dek phenotypes were introgressed into two inbred lines to take advantage of maize haplotype variations and their sequenced genomes. We generated two pipelines that take fastq files derived from next-generation (nextGen) paired-end DNA and cDNA sequencing as input, call on several well established and freely available genomic analysis tools to call SNPs and INDELs, and generate lists of the most likely causal mutations together with variant index plots to locate the mutation to a specific sequence position on a chromosome. The pipelines were validated with a known strawberry mutation before cloning the dek mutants, thereby enabling phenotypic analysis of large genomes by next-generation sequencing.

RAMOSA1 ENHANCER LOCUS2-Mediated Transcriptional Repression Regulates Vegetative and Reproductive Architecture

Transcriptional repression in multicellular organisms orchestrates dynamic and precise gene expression changes that enable complex developmental patterns. Here, we present phenotypic and molecular characterization of the maize (Zea mays) transcriptional corepressor RAMOSA1 ENHANCER LOCUS2 (REL2), a unique member of the highly conserved TOPLESS (TPL) family. Analysis of single recessive mutations in rel2 revealed an array of vegetative and reproductive phenotypes, many related to defects in meristem initiation and maintenance. To better understand how REL2-mediated transcriptional complexes relate to rel2 phenotypes, we performed protein interaction assays and transcriptional profiling of mutant inflorescences, leading to the identification of different maize transcription factors and regulatory pathways that employ REL2 repression to control traits directly impacting maize yields. In addition, we used our REL2 interaction data to catalog conserved repression motifs present on REL2 interactors and showed that two of these, RLFGV- and DLN-type motifs, interact with the C-terminal WD40 domain of REL2 rather than the N terminus, which is known to bind LxLxL EAR motifs. These findings establish that the WD40 domain of TPL family proteins is an independent protein interaction surface that may work together with the N-terminal domain to allow the formation of large macromolecular complexes of functionally related transcription factors.

Whole Genome Resequencing from Bulked Populations as a Rapid QTL and Gene Identification Method in Rice

Most Quantitative Trait Loci (QTL) and gene isolation approaches, such as positional- or map-based cloning, are time-consuming and low-throughput methods. Understanding and detecting the genetic material that controls a phenotype is a key means to functionally analyzing genes as well as to enhance crop agronomic traits. In this regard, high-throughput technologies have great prospects for changing the paradigms of DNA marker revealing, genotyping, and for discovering crop genetics and genomic study. Bulk segregant analysis, based on whole genome resequencing approaches, permits the rapid isolation of the genes or QTL responsible for the causative mutation of the phenotypes. MutMap, MutMap Gap, MutMap+, modified MutMap, and QTL-seq methods are among those approaches that have been confirmed to be fruitful gene mapping approaches for crop plants, such as rice, irrespective of whether the characters are determined by polygenes. As a result, in the present study we reviewed the progress made by all these methods to identify QTL or genes in rice.

Bulked-Segregant Analysis Coupled to Whole Genome Sequencing (BSA-Seq) for Rapid Gene Cloning in Maize

Forward genetics remains a powerful method for revealing the genes underpinning organismal form and function, and for revealing how these genes are tied together in gene networks. In maize, forward genetics has been tremendously successful, but the size and complexity of the maize genome made identifying mutant genes an often arduous process with traditional methods. The next generation sequencing revolution has allowed for the gene cloning process to be significantly accelerated in many organisms, even when genomes are large and complex. Here, we describe a bulked-segregant analysis sequencing (BSA-Seq) protocol for cloning mutant genes in maize. Our simple strategy can be used to quickly identify a mapping interval and candidate single nucleotide polymorphisms (SNPs) from whole genome sequencing of pooled F2 individuals. We employed this strategy to identify narrow odd dwarf as an enhancer of teosinte branched1, and to identify a new allele of defective kernel1 Our method provides a quick, simple way to clone genes in maize.

Restorer-of-Fertility Mutations Recovered in Transposon-Active Lines of S Male-Sterile Maize

Mitochondria execute key pathways of central metabolism and serve as cellular sensing and signaling entities, functions that depend upon interactions between mitochondrial and nuclear genetic systems. This is exemplified in cytoplasmic male sterility type S (CMS-S) of Zea mays, where novel mitochondrial open reading frames are associated with a pollen collapse phenotype, but nuclear restorer-of-fertility (restorer) mutations rescue pollen function. To better understand these genetic interactions, we screened Activator-Dissociation (Ac-Ds), Enhancer/Suppressor-mutator (En/Spm), and Mutator (Mu) transposon-active CMS-S stocks to recover new restorer mutants. The frequency of restorer mutations increased in transposon-active stocks compared to transposon-inactive stocks, but most mutants recovered from Ac-Ds and En/Spm stocks were unstable, reverting upon backcrossing to CMS-S inbred lines. However, 10 independent restorer mutations recovered from CMS-S Mu transposon stocks were stable upon backcrossing. Many restorer mutations condition seed-lethal phenotypes that provide a convenient test for allelism. Eight such mutants recovered in this study included one pair of allelic mutations that were also allelic to the previously described rfl2-1 mutant. Targeted analysis of mitochondrial proteins by immunoblot identified two features that consistently distinguished restored CMS-S pollen from comparably staged, normal-cytoplasm, nonmutant pollen: increased abundance of nuclear-encoded alternative oxidase relative to mitochondria-encoded cytochrome oxidase and decreased abundance of mitochondria-encoded ATP synthase subunit 1 compared to nuclear-encoded ATP synthase subunit 2. CMS-S restorer mutants thus revealed a metabolic plasticity in maize pollen, and further study of these mutants will provide new insights into mitochondrial functions that are critical to pollen and seed development.

Identification of downy mildew resistance gene candidates by positional cloning in maize (Zea mays subsp. mays; Poaceae)

PREMISE OF THE STUDY

Positional cloning in combination with phenotyping is a general approach to identify disease-resistance gene candidates in plants; however, it requires several time-consuming steps including population or fine mapping. Therefore, in the present study, we suggest a new combined strategy to improve the identification of disease-resistance gene candidates.

METHODS AND RESULTS

Downy mildew (DM)-resistant maize was selected from five cultivars using a spreader row technique. Positional cloning and bioinformatics tools were used to identify the DM-resistance quantitative trait locus marker (bnlg1702) and 47 protein-coding gene annotations. Eventually, five DM-resistance gene candidates, including bZIP34, Bak1, and Ppr, were identified by quantitative reverse-transcription PCR (RT-PCR) without fine mapping of the bnlg1702 locus.

CONCLUSIONS

The combined protocol with the spreader row technique, quantitative trait locus positional cloning, and quantitative RT-PCR was effective for identifying DM-resistance candidate genes. This cloning approach may be applied to other whole-genome-sequenced crops or resistance to other diseases.

Next-Generation Sequencing from Bulked-Segregant Analysis Accelerates the Simultaneous Identification of Two Qualitative Genes in Soybean

Next-generation sequencing (NGS)-based bulked-segregant analysis (BSA) approaches have been proven successful for rapidly mapping genes in plant species. However, most such methods are based on mutants and usually only one gene controlling the mutant phenotype is identified. In this study, NGS-based BSA was employed to map simultaneously two qualitative genes controlling cotyledon color of seed in soybean. Yellow-cotyledon (YC) and green-cotyledon (GC) bulks from progenies of a biparental population (Zhonghuang 30 × Jiyu 102) were sequenced. The SNP-index of each SNP locus in YC and GC bulks was calculated and two genomic regions on chromosomes 1 and 11 harboring, respectively, loci qCC1 and qCC2 were identified by Δ(SNP-index) analysis. These two BSA-seq-derived loci were further validated with SSR markers and fine-mapped. qCC1 was mapped to a 30.7-kb region containing four annotated genes and qCC2 was mapped to a 67.7-kb region with nine genes. These two regions contained, respectively, genes D1 and D2, which had previously been identified by homology-based cloning as being associated with cotyledon color. Sequence analysis of the NGS data also identified a frameshift deletion in the coding region of D1. These results suggested that BSA-seq could accelerate the mapping of loci controlling qualitative traits, even if a trait is controlled by more than one locus.

Wax crystal-sparse leaf 3 encoding a β-ketoacyl-CoA reductase is involved in cuticular wax biosynthesis in rice

WSL3 encodes β-ketoacyl-CoA reductase (KCR) in rice, in a similar way to YBR159w in yeast, and is essential for VLCFA biosynthesis and leaf wax accumulation. Cuticular waxes on plant surfaces limit non-stomatal water loss, protect plants against deposits of dust and impose a physical barrier to pathogen infection. We identified a wax-deficient mutant of rice, wax crystal-sparse leaf 3 (wsl3), which exhibits a pleiotropic phenotype that includes reduced epicuticular wax crystals on the leaf surface and altered wax composition. Map-based cloning demonstrated that defects in the mutant were caused by two adjacent single-nucleotide changes in a gene encoding β-ketoacyl-CoA reductase (KCR) that catalyzes the second step of the fatty acid elongation reaction. The identity of WSL3 was further confirmed by genetic complementation. Transient assays of fluorescent protein-tagged WSL3 in tobacco protoplasts showed that WSL3 localizes to the endoplasmic reticulum, the compartment of fatty acid elongation in cells. Quantitative PCR and histochemical staining indicated that WSL3 is universally expressed in tissues. RNA interference of WSL3 caused a phenotype that mimicked the wsl3 mutant. Very long-chain fatty acids (VLCFAs) 20:0 and 22:0, or 20:1Δ(11) and 22:1Δ(13), were detected when WSL3 and Arabidopsis fatty acid elongation 1 (FAE1) were co-expressed in a yeast ybr159wΔ mutant strain. Our results indicated that WSL3 affects rice cuticular wax production by participating in VLCFA elongation.

Wax crystal-sparse leaf 3 encoding a β-ketoacyl-CoA reductase is involved in cuticular wax biosynthesis in rice

WSL3 encodes β-ketoacyl-CoA reductase (KCR) in rice, in a similar way to YBR159w in yeast, and is essential for VLCFA biosynthesis and leaf wax accumulation. Cuticular waxes on plant surfaces limit non-stomatal water loss, protect plants against deposits of dust and impose a physical barrier to pathogen infection. We identified a wax-deficient mutant of rice, wax crystal-sparse leaf 3 (wsl3), which exhibits a pleiotropic phenotype that includes reduced epicuticular wax crystals on the leaf surface and altered wax composition. Map-based cloning demonstrated that defects in the mutant were caused by two adjacent single-nucleotide changes in a gene encoding β-ketoacyl-CoA reductase (KCR) that catalyzes the second step of the fatty acid elongation reaction. The identity of WSL3 was further confirmed by genetic complementation. Transient assays of fluorescent protein-tagged WSL3 in tobacco protoplasts showed that WSL3 localizes to the endoplasmic reticulum, the compartment of fatty acid elongation in cells. Quantitative PCR and histochemical staining indicated that WSL3 is universally expressed in tissues. RNA interference of WSL3 caused a phenotype that mimicked the wsl3 mutant. Very long-chain fatty acids (VLCFAs) 20:0 and 22:0, or 20:1Δ(11) and 22:1Δ(13), were detected when WSL3 and Arabidopsis fatty acid elongation 1 (FAE1) were co-expressed in a yeast ybr159wΔ mutant strain. Our results indicated that WSL3 affects rice cuticular wax production by participating in VLCFA elongation.