Construction of a Genome-Wide Mutant Library in Rice Using CRISPR/Cas9

OsMPK4 promotes phosphorylation and degradation of IPA1 in response to salt stress to confer salt tolerance in rice

Salt stress adversely affects plant growth, development, and crop yield. Rice (Oryza sativa L.) is one of the most salt-sensitive cereal crops, especially at the early seedling stage. Mitogen-activated protein kinase (MAPK/MPK) cascades have been shown to play critical roles in salt response in Arabidopsis. However, the roles of the MPK cascade signaling in rice salt response and substrates of OsMPK remain largely unknown. Here, we report that the salt-induced OsMPK4-Ideal Plant Architecture 1 (IPA1) signaling pathway regulates the salt tolerance in rice. Under salt stress, OsMPK4 could interact with IPA1 and phosphorylate IPA1 at Thr180, leading to degradation of IPA1. Genetic evidence shows that IPA1 is a negative regulator of salt tolerance in rice, whereas OsMPK4 promotes salt response in an IPA1-dependent manner. Taken together, our results uncover an OsMPK4-IPA1 signal cascade that modulates the salt stress response in rice and sheds new light on the breeding of salt-tolerant rice varieties.

Highly efficient CRISPR systems for loss-of-function and gain-of-function research in pear calli

CRISPR/Cas systems have been widely used for genome engineering in many plant species. However, their potentials have remained largely untapped in fruit crops, particularly in pear, due to the high levels of genomic heterozygosity and difficulties in tissue culture and stable transformation. To date, only a few reports on the application of the CRISPR/Cas9 system in pear have been documented, and have shown very low editing efficiency. Here we report a highly efficient CRISPR toolbox for loss-of-function and gain-of-function research in pear. We compared four different CRISPR/Cas9 expression systems for loss-of-function analysis and identified a potent system that showed nearly 100% editing efficiency for multi-site mutagenesis. To expand the targeting scope, we further tested different CRISPR/Cas12a and Cas12b systems in pear for the first time, albeit with low editing efficiency. In addition, we established a CRISPR activation (CRISPRa) system for multiplexed gene activation in pear calli for gain-of-function analysis. Furthermore, we successfully engineered the anthocyanin and lignin biosynthesis pathways using both CRISPR/Cas9 and CRISPRa systems in pear calli. Taking these results together, we have built a highly efficient CRISPR toolbox for genome editing and gene regulation, paving the way for functional genomics studies as well as molecular breeding in pear.

Covering the Combinatorial Design Space of Multiplex CRISPR/Cas Experiments in Plants

Over the past years, CRISPR/Cas-mediated genome editing has revolutionized plant genetic studies and crop breeding. Specifically, due to its ability to simultaneously target multiple genes, the multiplex CRISPR/Cas system has emerged as a powerful technology for functional analysis of genetic pathways. As such, it holds great potential for application in plant systems to discover genetic interactions and to improve polygenic agronomic traits in crop breeding. However, optimal experimental design regarding coverage of the combinatorial design space in multiplex CRISPR/Cas screens remains largely unexplored. To contribute to well-informed experimental design of such screens in plants, we first establish a representation of the design space at different stages of a multiplex CRISPR/Cas experiment. We provide two independent computational approaches yielding insights into the plant library size guaranteeing full coverage of all relevant multiplex combinations of gene knockouts in a specific multiplex CRISPR/Cas screen. These frameworks take into account several design parameters (e.g., the number of target genes, the number of gRNAs designed per gene, and the number of elements in the combinatorial array) and efficiencies at subsequent stages of a multiplex CRISPR/Cas experiment (e.g., the distribution of gRNA/Cas delivery, gRNA-specific mutation efficiency, and knockout efficiency). With this work, we intend to raise awareness about the limitations regarding the number of target genes and order of genetic interaction that can be realistically analyzed in multiplex CRISPR/Cas experiments with a given number of plants. Finally, we establish guidelines for designing multiplex CRISPR/Cas experiments with an optimal coverage of the combinatorial design space at minimal plant library size.

Reverse genetic approaches for breeding nutrient-rich and climate-resilient cereal and food legume crops

In the last decade, advancements in genomics tools and techniques have led to the discovery of many genes. Most of these genes still need to be characterized for their associated function and therefore, such genes remain underutilized for breeding the next generation of improved crop varieties. The recent developments in different reverse genetic approaches have made it possible to identify the function of genes controlling nutritional, biochemical, and metabolic traits imparting drought, heat, cold, salinity tolerance as well as diseases and insect-pests. This article focuses on reviewing the current status and prospects of using reverse genetic approaches to breed nutrient-rich and climate resilient cereal and food legume crops.

Improving environmental stress resilience in crops by genome editing: insights from extremophile plants

In basic and applied sciences, genome editing has become an indispensable tool, especially the versatile and adaptable CRISPR/Cas9 system. Using CRISPR/Cas9 in plants has enabled modifications of many valuable traits, including environmental stress tolerance, an essential aspect when it comes to ensuring food security under climate change pressure. The CRISPR toolbox enables faster and more precise plant breeding by facilitating: multiplex gene editing, gene pyramiding, and de novo domestication. In this paper, we discuss the most recent advances in CRISPR/Cas9 and alternative CRISPR-based systems, along with the technical challenges that remain to be overcome. A revision of the latest proof-of-concept and functional characterization studies has indeed provided more insight into the quantitative traits affecting crop yield and stress tolerance. Additionally, we focus on the applications of CRISPR/Cas9 technology in regard to extremophile plants, due to their significance on: industrial, ecological and economic levels. These still unexplored genetic resources could provide the means to harden our crops against the threat of climate change, thus ensuring food security over the next century.

Boosting plant genome editing with a versatile CRISPR-Combo system

CRISPR-Cas9, its derived base editors and CRISPR activation systems have greatly aided genome engineering in plants. However, these systems are mostly used separately, leaving their combinational potential largely untapped. Here we develop a versatile CRISPR-Combo platform, based on a single Cas9 protein, for simultaneous genome editing (targeted mutagenesis or base editing) and gene activation in plants. We showcase the powerful applications of CRISPR-Combo for boosting plant genome editing. First, CRISPR-Combo is used to shorten the plant life cycle and reduce the efforts in screening transgene-free genome-edited plants by activation of a florigen gene in Arabidopsis. Next, we demonstrate accelerated regeneration and propagation of genome-edited plants by activation of morphogenic genes in poplar. Furthermore, we apply CRISPR-Combo to achieve rice regeneration without exogenous plant hormones, which is established as a new method to predominately enrich heritable targeted mutations. In conclusion, CRISPR-Combo is a versatile genome engineering tool with promising applications in crop breeding.

Targeting a gene regulatory element enhances rice grain yield by decoupling panicle number and size

Crop genetic improvement requires balancing complex tradeoffs caused by gene pleiotropy and linkage drags, as exemplified by IPA1 (Ideal Plant Architecture 1), a typical pleiotropic gene in rice that increases grains per panicle but reduces tillers. In this study, we identified a 54-base pair cis-regulatory region in IPA1 via a tiling-deletion-based CRISPR-Cas9 screen that, when deleted, resolves the tradeoff between grains per panicle and tiller number, leading to substantially enhanced grain yield per plant. Mechanistic studies revealed that the deleted fragment is a target site for the transcription factor An-1 to repress IPA1 expression in panicles and roots. Targeting gene regulatory regions should help dissect tradeoff effects and provide a rich source of targets for breeding complementary beneficial traits.

Twenty years of rice genomics research: From sequencing and functional genomics to quantitative genomics

Since the completion of the rice genome sequencing project in 2005, we have entered the era of rice genomics, which is still in its ascendancy. Rice genomics studies can be classified into three stages: structural genomics, functional genomics, and quantitative genomics. Structural genomics refers primarily to genome sequencing for the construction of a complete map of rice genome sequence. This is fundamental for rice genetics and molecular biology research. Functional genomics aims to decode the functions of rice genes. Quantitative genomics is large-scale sequence- and statistics-based research to define the quantitative traits and genetic features of rice populations. Rice genomics has been a transformative influence on rice biological research and contributes significantly to rice breeding, making rice a good model plant for studying crop sciences.

Genome Editing and Designer Crops for the Future

Domestication spanning over thousands of years led to the evolution of crops that are being cultivated in recent times. Later, selective breeding methods were practiced by human to produce improved cultivars/germplasm. Classical breeding was further transformed into molecular- and genomics-assisted breeding strategies, however, these approaches are labor-intensive and time-consuming. The advent of omics technologies has facilitated the identification of genes and genetic determinants that regulate particular traits allowing the direct manipulation of target genes and genomic regions to achieve desirable phenotype. Recently, genome editing technologies such as meganucleases (MN), zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and CRISPR (clustered regularly interspaced short palindromic repeats)/CRISPR-Associated protein 9 (Cas9) have gained popularity for precise editing of genes to develop crop varieties with superior agronomic, physiological, climate-resilient, and nutritional traits. Owing to the efficiency and precision, genome editing approaches have been widely used to design the crops that can survive the challenges posed by changing climate, and also cater the food and nutritional requirements for ever-growing population. Here, we briefly review different genome editing technologies deployed for crop improvement, and the fundamental differences between GE technology and transgene-based approach. We also summarize the recent advances in genome editing and how this radical expansion can complement the previously established technologies along with breeding for creating designer crops.

Deficiency in flavonoid biosynthesis genes CHS, CHI, and CHIL alters rice flavonoid and lignin profiles

Lignin is a complex phenylpropanoid polymer deposited in the secondary cell walls of vascular plants. Unlike most gymnosperm and eudicot lignins that are generated via the polymerization of monolignols, grass lignins additionally incorporate the flavonoid tricin as a natural lignin monomer. The biosynthesis and functions of tricin-integrated lignin (tricin-lignin) in grass cell walls and its effects on the utility of grass biomass remain largely unknown. We herein report a comparative analysis of rice (Oryza sativa) mutants deficient in the early flavonoid biosynthetic genes encoding CHALCONE SYNTHASE (CHS), CHALCONE ISOMERASE (CHI), and CHI-LIKE (CHIL), with an emphasis on the analyses of disrupted tricin-lignin formation and the concurrent changes in lignin profiles and cell wall digestibility. All examined CHS-, CHI-, and CHIL-deficient rice mutants were largely depleted of extractable flavones, including tricin, and nearly devoid of tricin-lignin in the cell walls, supporting the crucial roles of CHS and CHI as committed enzymes and CHIL as a noncatalytic enhancer in the conserved biosynthetic pathway leading to flavone and tricin-lignin formation. In-depth cell wall structural analyses further indicated that lignin content and composition, including the monolignol-derived units, were differentially altered in the mutants. However, regardless of the extent of the lignin alterations, cell wall saccharification efficiencies of all tested rice mutants were similar to that of the wild-type controls. Together with earlier studies on other tricin-depleted grass mutant and transgenic plants, our results reflect the complexity in the metabolic consequences of tricin pathway perturbations and the relationships between lignin profiles and cell wall properties.

High-throughput methods for genome editing: the more the better

During the last decade, targeted genome-editing technologies, especially clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated protein (Cas) technologies, have permitted efficient targeting of genomes, thereby modifying these genomes to offer tremendous opportunities for deciphering gene function and engineering beneficial traits in many biological systems. As a powerful genome-editing tool, the CRISPR/Cas systems, combined with the development of next-generation sequencing and many other high-throughput techniques, have thus been quickly developed into a high-throughput engineering strategy in animals and plants. Therefore, here, we review recent advances in using high-throughput genome-editing technologies in animals and plants, such as the high-throughput design of targeted guide RNA (gRNA), construction of large-scale pooled gRNA, and high-throughput genome-editing libraries, high-throughput detection of editing events, and high-throughput supervision of genome-editing products. Moreover, we outline perspectives for future applications, ranging from medication using gene therapy to crop improvement using high-throughput genome-editing technologies.

Computational tools and resources for CRISPR/Cas genome editing

The past decade has witnessed a rapid evolution in identifying more versatile clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein (Cas) nucleases and their functional variants as well as in developing precise CRISPR/Cas-derived genome editors. The programmable and robust features of the genomic editors provide an effective RNA-guided platform for fundamental life science research and subsequent applications in diverse scenarios, including biomedical innovation and targeted crop improvement. One of the most essential principles is to guide alterations in genomic sequences or genes in the intended manner without undesired off-target impacts, which strongly depends on the efficiency and specificity of single guide RNA (sgRNA)-directed recognition of targeted DNA sequences. Recent advances in empirical scoring algorithms and machine learning models have facilitated sgRNA design and off-target prediction. In this review, we first briefly introduced the different features of CRISPR/Cas tools that should be taken into consideration to achieve specific purposes. Secondly, we focused on the computer-assisted tools and resources that are widely used in designing sgRNAs and analyzing CRISPR/Cas-induced on- and off-target mutations. Thirdly, we provide insights on the limitations of available computational tools that surely help researchers of this field for further optimization. Lastly, we suggested a simple but effective workflow for choosing and applying web-based resources and tools for CRISPR/Cas genome editing.

Expanding Gene-Editing Potential in Crop Improvement with Pangenomes

Pangenomes aim to represent the complete repertoire of the genome diversity present within a species or cohort of species, capturing the genomic structural variance between individuals. This genomic information coupled with phenotypic data can be applied to identify genes and alleles involved with abiotic stress tolerance, disease resistance, and other desirable traits. The characterisation of novel structural variants from pangenomes can support genome editing approaches such as Clustered Regularly Interspaced Short Palindromic Repeats and CRISPR associated protein Cas (CRISPR-Cas), providing functional information on gene sequences and new target sites in variant-specific genes with increased efficiency. This review discusses the application of pangenomes in genome editing and crop improvement, focusing on the potential of pangenomes to accurately identify target genes for CRISPR-Cas editing of plant genomes while avoiding adverse off-target effects. We consider the limitations of applying CRISPR-Cas editing with pangenome references and potential solutions to overcome these limitations.

A FLASH pipeline for arrayed CRISPR library construction and the gene function discovery of rice receptor-like kinases

Clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR-associated protein 9 (Cas9)-mediated gene editing is revolutionizing plant research and crop breeding. Here, we present an effective and streamlined pipeline for arrayed CRISPR library construction and demonstrate it is suitable for small- to large-scale genome editing in plants. This pipeline introduces artificial PCR fragment-length markers for distinguishing guide RNAs (gRNAs) (FLASH), and a group of 12 constructs harboring different FLASH tags are co-transformed into plants each time. The identities of gRNAs in Agrobacterium mixtures and transgenic plants can therefore be read out by detecting the FLASH tags, a process that requires only conventional PCR and gel electrophoresis rather than sequencing. We generated an arrayed CRISPR library targeting all 1,072 members of the receptor-like kinase (RLK) family in rice. One-shot transformation generated a mutant population that covers gRNAs targeting 955 RLKs, and 74.3% (710/955) of the target genes had three or more independent T₀ lines. Our results indicate that the FLASH tags act as bona fide surrogates for the gRNAs and are tightly (92.1%) associated with frameshift mutations in the target genes. In addition, the FLASH pipeline allows for rapid identification of unintended editing events without corresponding T-DNA integrations and generates high-order mutants of closely related RLK genes. Furthermore, we showed that the RLK mutant library enables rapid discovery of defense-related RLK genes. This study introduces an effective pipeline for arrayed CRISPR library construction and provides genome-wide rice RLK mutant resources for functional genomics.

Cold responses in rice: From physiology to molecular biology

As rice originated in tropical or subtropical areas, it is generally sensitive to cold stress. Understanding the physiological and molecular mechanisms underlying rice responses to cold stress can provide new power for engineering cold-tolerant and high-yielding rice varieties.

RIR1 represses plant immunity by interacting with mitochondrial complex I subunit in rice

We previously observed decreased expression of rice OsmiR159a.1 on infection with the bacterial blight-causing pathogen Xanthomonas oryzae pv. oryzae (Xoo), and identified the OsLRR_RLK (leucine-rich repeat_ receptor like kinase) gene as an authentic target of OsmiR159a.1. Here, we found that a Tos17 insertion mutant of LRR_RLK displayed increasing temporal resistance to Xoo, whereas the LRR_RLK overexpression lines were susceptible to the pathogen early on in the infection, indicating that LRR_RLK encodes a repressor of rice resistance to Xoo infection, and it was renamed as RIR1 (Rice Immunity Repressor 1). RIR1 overexpression plants were more susceptible to Xoo at late growth stage, suggesting that RIR1 mRNA levels are negatively correlated with the resistance of rice against Xoo. We discovered that OsmiR159a.1 repression in Xoo-infected plants was largely dependent on the pathogen's type III secretion system. Co-immunoprecipitation, bimolecular fluoresence complementation, and pull-down assays indicated that RIR1 interacted with the NADH-ubiquinone oxidoreductase (NUO) 51-kDa subunit of the mitochondrial complex I through its kinase domain. Notably, impairment of RIR1 or overexpression of NUO resulted in reactive oxygen species accumulation and enhanced expression of pathogen-resistance genes, including jasmonic acid pathway genes. We propose that pathogens may inhibit OsmiR159 to interfere with the RIR1-NUO interaction, and subsequently depression of rice immune signalling pathways. The resistance genes manipulated by Xoo can be a probe to explore the regulatory network during host-pathogen interactions.

Chilling tolerance in rice: Past and present

Rice is generally sensitive to chilling stress, which seriously affects growth and yield. Since early in the last century, considerable efforts have been made to understand the physiological and molecular mechanisms underlying the response to chilling stress and improve rice chilling tolerance. Here, we review the research trends and advances in this field. The phenotypic and biochemical changes caused by cold stress and the physiological explanations are briefly summarized. Using published data from the past 20 years, we reviewed the past progress and important techniques in the identification of quantitative trait loci (QTL), novel genes, and cellular pathways involved in rice chilling tolerance. The advent of novel technologies has significantly advanced studies of cold tolerance, and the characterization of QTLs, key genes, and molecular modules have sped up molecular design breeding for cold tolerance in rice varieties. In addition to gene function studies based on overexpression or artificially generated mutants, elucidating natural allelic variation in specific backgrounds is emerging as a novel approach for the study of cold tolerance in rice, and the superior alleles identified using this approach can directly facilitate breeding.

Gene Editing Technologies for Sugarcane Improvement: Opportunities and Limitations

Plant-based biofuels present a promising alternative to depleting non-renewable fuel resources. One of the benefits of biofuel is reduced environmental impact, including reduction in greenhouse gas emission which causes climate change. Sugarcane is one of the most important bioenergy crops. Sugarcane juice is used to produce table sugar and first-generation biofuel (e.g., bioethanol). Sugarcane bagasse is also a potential material for second-generation cellulosic biofuel production. Researchers worldwide are striving to improve sugarcane biomass yield and quality by a variety of means including biotechnological tools. This paper reviews the use of sugarcane as a feedstock for biofuel production, and gene manipulation tools and approaches, including RNAi and genome-editing tools, such as TALENs and CRISPR-Cas9, for improving its quality. The specific focus here is on CRISPR system because it is low cost, simple in design and versatile compared to other genome-editing tools. The advance of CRISPR-Cas9 technology has transformed plant research with its ability to precisely delete, insert or replace genes in recent years. Lignin is the primary material responsible for biomass recalcitrance in biofuel production. The use of genome editing technology to modify lignin composition and distribution in sugarcane cell wall has been realized. The current and potential applications of genome editing technology for sugarcane improvement are discussed. The advantages and limitations of utilizing RNAi and TALEN techniques in sugarcane improvement are discussed as well.

A Critical Review: Recent Advancements in the Use of CRISPR/Cas9 Technology to Enhance Crops and Alleviate Global Food Crises

Genome editing (GE) has revolutionized the biological sciences by creating a novel approach for manipulating the genomes of living organisms. Many tools have been developed in recent years to enable the editing of complex genomes. Therefore, a reliable and rapid approach for increasing yield and tolerance to various environmental stresses is necessary to sustain agricultural crop production for global food security. This critical review elaborates the GE tools used for crop improvement. These tools include mega-nucleases (MNs), such as zinc-finger nucleases (ZFNs), and transcriptional activator-like effector nucleases (TALENs), and clustered regularly interspaced short palindromic repeats (CRISPR). Specifically, this review addresses the latest advancements in the role of CRISPR/Cas9 for genome manipulation for major crop improvement, including yield and quality development of biotic stress- and abiotic stress-tolerant crops. Implementation of this technique will lead to the production of non-transgene crops with preferred characteristics that can result in enhanced yield capacity under various environmental stresses. The CRISPR/Cas9 technique can be combined with current and potential breeding methods (e.g., speed breeding and omics-assisted breeding) to enhance agricultural productivity to ensure food security. We have also discussed the challenges and limitations of CRISPR/Cas9. This information will be useful to plant breeders and researchers in the thorough investigation of the use of CRISPR/Cas9 to boost crops by targeting the gene of interest.

Integrated approach for the molecular characterization of edited plants obtained via Agrobacterium tumefaciens-mediated gene transfer

Agrobacterium tumefaciens-mediated gene transfer—actually the most used method to engineer plants—may lead to integration of multiple copies of T-DNA in the plant genome, as well as to chimeric tissues composed of modified cells and wild type cells. A molecular characterization of the transformed lines is thus a good practice to select the best ones for further investigation. Nowadays, several quantitative and semi-quantitative techniques are available to estimate the copy number (CN) of the T-DNA in genetically modified plants. In this study, we compared three methods based on (1) real-time polymerase chain reaction (qPCR), (2) droplet digital PCR (ddPCR), and (3) next generation sequencing (NGS), to carry out a molecular characterization of grapevine edited lines. These lines contain a knock-out mutation, obtained via CRISPR/Cas9 technology, in genes involved in plant susceptibility to two important mildew diseases of grapevine. According to our results, qPCR and ddPCR outputs are largely in agreement in terms of accuracy, especially for low CN values, while ddPCR resulted more precise than qPCR. With regard to the NGS analysis, the CNs detected with this method were often not consistent with those calculated by qPCR and ddPCR, and NGS was not able to discriminate the integration points in three out of ten lines. Nevertheless, the NGS method can positively identify T-DNA truncations or the presence of tandem/inverted repeats, providing distinct and relevant information about the transgene integration asset. Moreover, the expression analysis of Cas9 and single guide RNA (sgRNA), and the sequencing of the target site added new information to be related to CN data. This work, by reporting a practical case-study on grapevine edited lines, explores pros and cons of the most advanced diagnostic techniques available for the precocious selection of the proper transgenic material. The results may be of interest both to scientists developing new transgenic lines, and to laboratories in charge of GMO control.

Systems-based rice improvement approaches for sustainable food and nutritional security

An integrated research approach to ensure sustainable rice yield increase of a crop grown by 25% of the world's farmers in 10% of cropland is essential for global food security. Rice, being a global staple crop, feeds about 56% of the world population and sustains 40% of the world's poor. At ~ $200 billion, it also accounts for 13% of the annual crop value. With hunger and malnutrition rampant among the poor, rice research for development is unique in global food and nutrition security. A systems-based, sustainable increase in rice quantity and quality is imperative for environmental and biodiversity benefits. Upstream 'discovery' through biotechnology, midstream 'development' through breeding and agronomy, downstream 'dissemination and deployment' must be 'demand-driven' for 'distinct socio-economic transformational impacts'. Local agro-ecology and livelihood nexus must drive the research agenda for targeted benefits. This necessitates sustained long-term investments by government, non-government and private sectors to secure the future food, nutrition, environment, prosperity and equity status.

Advances in genomics and genome editing for breeding next generation of fruit and nut crops

Fruit tree crops are an essential part of the food production systems and are key to achieve food and nutrition security. Genetic improvement of fruit trees by conventional breeding has been slow due to the long juvenile phase. Advancements in genomics and molecular biology have paved the way for devising novel genetic improvement tools like genome editing, which can accelerate the breeding of these perennial crops to a great extent. In this article, advancements in genomics of fruit trees covering genome sequencing, transcriptome sequencing, genome editing technologies (GET), CRISPR-Cas system based genome editing, potential applications of CRISPR-Cas9 in fruit tree crops improvement, the factors influencing the CRISPR-Cas editing efficiency and the challenges for CRISPR-Cas9 applications in fruit tree crops improvement are reviewed. Besides, base editing, a recently emerging more precise editing system, and the future perspectives of genome editing in the improvement of fruit and nut crops are covered.

Establishment of an efficient seed fluorescence reporter-assisted CRISPR/Cas9 gene editing in maize

Genome editing by clustered regularly interspaced short palindromic sequences (CRISPR)/CRISPR-associated protein 9 (Cas9) has revolutionized functional gene analysis and genetic improvement. While reporter-assisted CRISPR/Cas systems can greatly facilitate the selection of genome-edited plants produced via stable transformation, this approach has not been well established in seed crops. Here, we established the seed fluorescence reporter (SFR)-assisted CRISPR/Cas9 systems in maize (Zea mays L.), using the red fluorescent DsRED protein expressed in the endosperm (En-SFR/Cas9), embryos (Em-SFR/Cas9), or both tissues (Em/En-SFR/Cas9). All three SFRs showed distinct fluorescent patterns in the seed endosperm and embryo that allowed the selection of seeds carrying the transgene of having segregated the transgene out. We describe several case studies of the implementation of En-SFR/Cas9, Em-SFR/Cas9, and Em/En- SFR/Cas9 to identify plants not harboring the genome-editing cassette but carrying the desired mutations at target genes in single genes or in small-scale mutant libraries, and report on the successful generation of single-target mutants and/or mutant libraries with En-SFR/Cas9, Em-SFR/Cas9, and Em/En-SFR/Cas9. SFR-assisted genome editing may have particular value for application scenarios with a low transformation frequency and may be extended to other important monocot seed crops.

Exploring the gene pool of Brassica napus by genomics-based approaches

De novo allopolyploidization in Brassica provides a very successful model for reconstructing polyploid genomes using progenitor species and relatives to broaden crop gene pools and understand genome evolution after polyploidy, interspecific hybridization and exotic introgression. B. napus (AACC), the major cultivated rapeseed species and the third largest oilseed crop in the world, is a young Brassica species with a limited genetic base resulting from its short history of domestication, cultivation, and intensive selection during breeding for target economic traits. However, the gene pool of B. napus has been significantly enriched in recent decades that has been benefit from worldwide effects by the successful introduction of abundant subgenomic variation and novel genomic variation via intraspecific, interspecific and intergeneric crosses. An important question in this respect is how to utilize such variation to breed crops adapted to the changing global climate. Here, we review the genetic diversity, genome structure, and population-level differentiation of the B. napus gene pool in relation to known exotic introgressions from various species of the Brassicaceae, especially those elucidated by recent genome-sequencing projects. We also summarize progress in gene cloning, trait-marker associations, gene editing, molecular marker-assisted selection and genome-wide prediction, and describe the challenges and opportunities of these techniques as molecular platforms to exploit novel genomic variation and their value in the rapeseed gene pool. Future progress will accelerate the creation and manipulation of genetic diversity with genomic-based improvement, as well as provide novel insights into the neo-domestication of polyploid crops with novel genetic diversity from reconstructed genomes.

Using precision phenotyping to inform de novo domestication

An update on the use of precision phenotyping to assess the potential of lesser cultivated species as candidates for de novo domestication or similar development for future agriculture.

Genome-wide association studies: assessing trait characteristics in model and crop plants

GWAS involves testing genetic variants across the genomes of many individuals of a population to identify genotype–phenotype association. It was initially developed and has proven highly successful in human disease genetics. In plants genome-wide association studies (GWAS) initially focused on single feature polymorphism and recombination and linkage disequilibrium but has now been embraced by a plethora of different disciplines with several thousand studies being published in model and crop species within the last decade or so. Here we will provide a comprehensive review of these studies providing cases studies on biotic resistance, abiotic tolerance, yield associated traits, and metabolic composition. We also detail current strategies of candidate gene validation as well as the functional study of haplotypes. Furthermore, we provide a critical evaluation of the GWAS strategy and its alternatives as well as future perspectives that are emerging with the emergence of pan-genomic datasets.

Engineering microalgae: transition from empirical design to programmable cells

Domesticated microalgae hold great promise for the sustainable provision of various bioresources for human domestic and industrial consumption. Efforts to exploit their potential are far from being fully realized due to limitations in the know-how of microalgal engineering. The associated technologies are not as well developed as those for heterotrophic microbes, cyanobacteria, and plants. However, recent studies on microalgal metabolic engineering, genome editing, and synthetic biology have immensely helped to enhance transformation efficiencies and are bringing new insights into this field. Therefore, this article, summarizes recent developments in microalgal biotechnology and examines the prospects for generating specialty and commodity products through the processes of metabolic engineering and synthetic biology. After a brief examination of empirical engineering methods and vector design, this article focuses on quantitative transformation cassette design, elaborates on target editing methods and emerging digital design of algal cellular metabolism to arrive at high yields of valuable products. These advances have enabled a transition of manners in microalgal engineering from single-gene and enzyme-based metabolic engineering to systems-level precision engineering, from cells created with genetically modified (GM) tags to that without GM tags, and ultimately from proof of concept to tangible industrial applications. Finally, future trends are proposed in microalgal engineering, aiming to establish individualized transformation systems in newly identified species for strain-specific specialty and commodity products, while developing sophisticated universal toolkits in model algal species.

Enhancing rice grain production by manipulating the naturally evolved cis-regulatory element-containing inverted repeat sequence of OsREM20

Grain number per panicle (GNP) is an important agronomic trait that contributes to rice grain yield. Despite its importance in rice breeding, the molecular mechanism underlying GNP regulation remains largely unknown. In this study, we identified a previously unrecognized regulatory gene that controls GNP in rice, Oryza sativa REPRODUCTIVE MERISTEM 20 (OsREM20), which encodes a B3 domain transcription factor. Through genetic analysis and transgenic validation we found that genetic variation in the CArG box-containing inverted repeat (IR) sequence of the OsREM20 promoter alters its expression level and contributes to GNP variation among rice varieties. Furthermore, we revealed that the IR sequence regulates OsREM20 expression by affecting the direct binding of OsMADS34 to the CArG box within the IR sequence. Interestingly, the divergent pOsREM20^IR and pOsREM20^ΔIR alleles were found to originate from different Oryza rufipogon accessions, and were independently inherited into the japonica and indica subspecies, respectively, during domestication. Importantly, we demonstrated that IR sequence variations in the OsREM20 promoter can be utilized for germplasm improvement through either genome editing or traditional breeding. Taken together, our study characterizes novel genetic variations responsible for GNP diversity in rice, reveals the underlying molecular mechanism in the regulation of agronomically important gene expression, and provides a promising strategy for improving rice production by manipulating the cis-regulatory element-containing IR sequence.

CRISPR screens in plants: approaches, guidelines, and future prospects

Clustered regularly interspaced short palindromic repeat (CRISPR)-associated systems have revolutionized genome engineering by facilitating a wide range of targeted DNA perturbations. These systems have resulted in the development of powerful new screens to test gene functions at the genomic scale. While there is tremendous potential to map and interrogate gene regulatory networks at unprecedented speed and scale using CRISPR screens, their implementation in plants remains in its infancy. Here we discuss the general concepts, tools, and workflows for establishing CRISPR screens in plants and analyze the handful of recent reports describing the use of this strategy to generate mutant knockout collections or to diversify DNA sequences. In addition, we provide insight into how to design CRISPR knockout screens in plants given the current challenges and limitations and examine multiple design options. Finally, we discuss the unique multiplexing capabilities of CRISPR screens to investigate redundant gene functions in highly duplicated plant genomes. Combinatorial mutant screens have the potential to routinely generate higher-order mutant collections and facilitate the characterization of gene networks. By integrating this approach with the numerous genomic profiles that have been generated over the past two decades, the implementation of CRISPR screens offers new opportunities to analyze plant genomes at deeper resolution and will lead to great advances in functional and synthetic biology.

New genomic approaches for enhancing maize genetic improvement

Maize (Zea mays) is one of the most widely grown crops in the world, with an annual global production of over 1147 million tons. Genomics approaches are thought to be the best solution for accelerating yield improvement to meet the challenges of a growing population and global climate change. Here, we review current approaches to the exploration of novel genetic variation in genomes, DNA modifications, and transcription levels of cultivated maize, landraces, and wild relatives. We discuss applications of genetic engineering to maize yield improvement and highlight future directions for maize genomics studies.

Exploiting Genic Male Sterility in Rice: From Molecular Dissection to Breeding Applications

Rice (Oryza sativa L.) occupies a very salient and indispensable status among cereal crops, as its vast production is used to feed nearly half of the world's population. Male sterile plants are the fundamental breeding materials needed for specific propagation in order to meet the elevated current food demands. The development of the rice varieties with desired traits has become the ultimate need of the time. Genic male sterility is a predominant system that is vastly deployed and exploited for crop improvement. Hence, the identification of new genetic elements and the cognizance of the underlying regulatory networks affecting male sterility in rice are crucial to harness heterosis and ensure global food security. Over the years, a variety of genomics studies have uncovered numerous mechanisms regulating male sterility in rice, which provided a deeper and wider understanding on the complex molecular basis of anther and pollen development. The recent advances in genomics and the emergence of multiple biotechnological methods have revolutionized the field of rice breeding. In this review, we have briefly documented the recent evolution, exploration, and exploitation of genic male sterility to the improvement of rice crop production. Furthermore, this review describes future perspectives with focus on state-of-the-art developments in the engineering of male sterility to overcome issues associated with male sterility-mediated rice breeding to address the current challenges. Finally, we provide our perspectives on diversified studies regarding the identification and characterization of genic male sterility genes, the development of new biotechnology-based male sterility systems, and their integrated applications for hybrid rice breeding.

Genetic and molecular control of grain yield in maize

Understanding the genetic and molecular basis of grain yield is important for maize improvement. Here, we identified 49 consensus quantitative trait loci (cQTL) controlling maize yield-related traits using QTL meta-analysis. Then, we collected yield-related traits associated SNPs detected by association mapping and identified 17 consensus significant loci. Comparing the physical positions of cQTL with those of significant SNPs revealed that 47 significant SNPs were located within 20 cQTL regions. Furthermore, intensive reviews of 31 genes regulating maize yield-related traits found that the functions of many genes were conservative in maize and other plant species. The functional conservation indicated that some of the 575 maize genes (orthologous to 247 genes controlling yield or seed traits in other plant species) might be functionally related to maize yield-related traits, especially the 49 maize orthologous genes in cQTL regions, and 41 orthologous genes close to the physical positions of significant SNPs. In the end, we prospected on the integration of the public sources for exploring the genetic and molecular mechanisms of maize yield-related traits, and on the utilization of genetic and molecular mechanisms for maize improvement.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-021-01214-3.

Genome engineering for crop improvement and future agriculture

Feeding the ever-growing population is a major challenge, especially in light of rapidly changing climate conditions. Genome editing is set to revolutionize plant breeding and could help secure the global food supply. Here, I review the development and application of genome editing tools in plants while highlighting newly developed techniques. I describe new plant breeding strategies based on genome editing and discuss their impact on crop production, with an emphasis on recent advancements in genome editing-based plant improvements that could not be achieved by conventional breeding. I also discuss challenges facing genome editing that must be overcome before realizing the full potential of this technology toward future crops and food production.

Efficient CRISPR/Cas9 mediated Pooled-sgRNAs assembly accelerates targeting multiple genes related to male sterility in cotton

BACKGROUND

Upland cotton (Gossypium hirsutum), harboring a complex allotetraploid genome, consists of A and D sub-genomes. Every gene has multiple copies with high sequence similarity that makes genetic, genomic and functional analyses extremely challenging. The recent accessibility of CRISPR/Cas9 tool provides the ability to modify targeted locus efficiently in various complicated plant genomes. However, current cotton transformation method targeting one gene requires a complicated, long and laborious regeneration process. Hence, optimizing strategy that targeting multiple genes is of great value in cotton functional genomics and genetic engineering.

RESULTS

To target multiple genes in a single experiment, 112 plant development-related genes were knocked out via optimized CRISPR/Cas9 system. We optimized the key steps of pooled sgRNAs assembly method by which 116 sgRNAs pooled together into 4 groups (each group consisted of 29 sgRNAs). Each group of sgRNAs was compiled in one PCR reaction which subsequently went through one round of vector construction, transformation, sgRNAs identification and also one round of genetic transformation. Through the genetic transformation mediated Agrobacterium, we successfully generated more than 800 plants. For mutants identification, Next Generation Sequencing technology has been used and results showed that all generated plants were positive and all targeted genes were covered. Interestingly, among all the transgenic plants, 85% harbored a single sgRNA insertion, 9% two insertions, 3% three different sgRNAs insertions, 2.5% mutated sgRNAs. These plants with different targeted sgRNAs exhibited numerous combinations of phenotypes in plant flowering tissues.

CONCLUSION

All targeted genes were successfully edited with high specificity. Our pooled sgRNAs assembly offers a simple, fast and efficient method/strategy to target multiple genes in one time and surely accelerated the study of genes function in cotton.

A route to de novo domestication of wild allotetraploid rice

Cultivated rice varieties are all diploid, and polyploidization of rice has long been desired because of its advantages in genome buffering, vigorousness, and environmental robustness. However, a workable route remains elusive. Here, we describe a practical strategy, namely de novo domestication of wild allotetraploid rice. By screening allotetraploid wild rice inventory, we identified one genotype of Oryza alta (CCDD), polyploid rice 1 (PPR1), and established two important resources for its de novo domestication: (1) an efficient tissue culture, transformation, and genome editing system and (2) a high-quality genome assembly discriminated into two subgenomes of 12 chromosomes apiece. With these resources, we show that six agronomically important traits could be rapidly improved by editing O. alta homologs of the genes controlling these traits in diploid rice. Our results demonstrate the possibility that de novo domesticated allotetraploid rice can be developed into a new staple cereal to strengthen world food security.

Using Interactome Big Data to Crack Genetic Mysteries and Enhance Future Crop Breeding

The functional genes underlying phenotypic variation and their interactions represent "genetic mysteries". Understanding and utilizing these genetic mysteries are key solutions for mitigating the current threats to agriculture posed by population growth and individual food preferences. Due to advances in high-throughput multi-omics technologies, we are stepping into an Interactome Big Data era that is certain to revolutionize genetic research. In this article, we provide a brief overview of current strategies to explore genetic mysteries. We then introduce the methods for constructing and analyzing the Interactome Big Data and summarize currently available interactome resources. Next, we discuss how Interactome Big Data can be used as a versatile tool to dissect genetic mysteries. We propose an integrated strategy that could revolutionize genetic research by combining Interactome Big Data with machine learning, which involves mining information hidden in Big Data to identify the genetic models or networks that control various traits, and also provide a detailed procedure for systematic dissection of genetic mysteries,. Finally, we discuss three promising future breeding strategies utilizing the Interactome Big Data to improve crop yields and quality.

Crop breeding - From experience-based selection to precision design

Crops are the foundation of human society, not only by providing needed nutrition, but also by feeding livestock and serving as raw materials for industry. Cereal crops, which supply most of our calories, have been supporting humans for thousands of years. However food security is facing many challenges nowadays, including growing populations, water shortage, and increased incidence of biotic and abiotic stresses. According to statistical data from the Food and Agriculture Organization of the United Nations (FAO, http://www.fao.org/), the people suffering severe food insecurity increased from 7.9 % in 2015 to 9.7 % in 2019 and the number of people exposed to moderate or severe food insecurity have increased by 400 million over the same time period. Although there are many ways to cope with these challenges, crop breeding remains the most crucial and direct manner. With the development of molecular genetics, the speed of cloning genetic variations underlying corresponding phenotypes of agricultural importance is considerably more rapid. As a consequence breeding methods have evolved from phenotype-based to genome-based selection. In the future, knowledge-driven crop design, which integrates multi-omics data to reveal the connections between genotypes and phenotypes and to build selection models, will undoubtedly become the most efficient way to shape plants, to improve crops, and to ensure food security.

ζ-Carotene Isomerase Suppresses Tillering in Rice through the Coordinated Biosynthesis of Strigolactone and Abscisic Acid

Rice tillering is an important agronomic trait affecting grain yield. Here, we identified a high-tillering mutant tillering20 (t20), which could be restored to the wild type by treatment with the strigolactone (SL) analog rac-GR24. T20 encodes a chloroplast ζ-carotene isomerase (Z-ISO), which is involved in the biosynthesis of carotenoids and their metabolites, SL and abscisic acid (ABA). The t20 mutant has reduced SL and ABA, raising the question of how SL and ABA biosynthesis is coordinated, and whether they have overlapping functions in tillering. We discovered that rac-GR24 stimulated T20 expression and enhanced all-trans-β-carotene biosynthesis. Importantly, rac-GR24 also stimulated expression of Oryza sativa 9-CIS-EPOXYCAROTENOID DIOXYGENASE 1 (OsNCED1) through induction of Oryza sativa HOMEOBOX12 (OsHOX12), promoting ABA biosynthesis in shoot base. On the other hand, ABA treatment significantly repressed SL biosynthesis and the ABA biosynthetic mutants displayed elevated SL biosynthesis. ABA treatment reduced the number of basal tillers in both t20 and wild-type plants. Furthermore, while ABA-deficient mutants aba1 and aba2 had the same number of basal tillers as wild type, they had more unproductive upper tillers at maturity. This work demonstrates complex interactions in the biosynthesis of carotenoid, SLs and ABA, and reveals a role for ABA in the regulation of rice tillering.

Optimization of multiplexed CRISPR/Cas9 system for highly efficient genome editing in Setaria viridis

In recent years, Setaria viridis has been developed as a model plant to better understand the C4 photosynthetic pathway in major crops. With the increasing availability of genomic resources for S. viridis research, highly efficient genome editing technologies are needed to create genetic variation resources for functional genomics. Here, we developed a protoplast assay to rapidly optimize the multiplexed clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated (Cas9) system in S. viridis. Targeted mutagenesis efficiency was further improved by an average of 1.4-fold with the exonuclease, Trex2. Distinctive mutation profiles were found in the Cas9_Trex2 samples, with 94% of deletions larger than 10 bp, and essentially no insertions at all tested target sites. Further analyses indicated that 52.2% of deletions induced by Cas9_Trex2, as opposed to 3.5% by Cas9 alone, were repaired through microhomology-mediated end joining (MMEJ) rather than the canonical non-homologous end joining DNA repair pathway. Combined with a robust Agrobacterium-mediated transformation method with more than 90% efficiency, the multiplex CRISPR/Cas9_Trex2 system was demonstrated to induce targeted mutations in two tightly linked genes, svDrm1a and svDrm1b, at a frequency ranging from 73% to 100% in T0 plants. These mutations were transmitted to at least 60% of the transgene-free T1 plants, with 33% of them containing bi-allelic or homozygous mutations in both genes. This highly efficient multiplex CRISPR/Cas9_Trex2 system makes it possible to create a large mutant resource for S. viridis in a rapid and high throughput manner, and has the potential to be widely applicable in achieving more predictable and deletion-only MMEJ-mediated mutations in many plant species.

Long non-coding RNAs in plants: emerging modulators of gene activity in development and stress responses

MAIN CONCLUSION

Long non-coding RNAs modulate gene activity in plant development and stress responses by various molecular mechanisms. Long non-coding RNAs (lncRNAs) are transcripts larger than 200 nucleotides without protein coding potential. Computational approaches have identified numerous lncRNAs in different plant species. Research in the past decade has unveiled that plant lncRNAs participate in a wide range of biological processes, including regulation of flowering time and morphogenesis of reproductive organs, as well as abiotic and biotic stress responses. LncRNAs execute their functions by interacting with DNA, RNA and protein molecules, and by modulating the expression level of their targets through epigenetic, transcriptional, post-transcriptional or translational regulation. In this review, we summarize characteristics of plant lncRNAs, discuss recent progress on understanding of lncRNA functions, and propose an experimental framework for functional characterization.

CAFRI-Rice: CRISPR applicable functional redundancy inspector to accelerate functional genomics in rice

Rice (Oryza sativa L.) is a staple crop with agricultural traits that have been intensively investigated. However, despite the variety of mutant population and multi-omics data that have been generated, rice functional genomic research has been bottlenecked due to the functional redundancy in the genome. This phenomenon has masked the phenotypes of knockout mutants by functional compensation and redundancy. Here, we present an intuitive tool, CRISPR applicable functional redundancy inspector to accelerate functional genomics in rice (CAFRI-Rice; cafri-rice.khu.ac.kr). To create this tool, we generated a phylogenetic heatmap that can estimate the similarity between protein sequences and expression patterns, based on 2,617 phylogenetic trees and eight tissue RNA-sequencing datasets. In this study, 33,483 genes were sorted into 2,617 families, and about 24,980 genes were tested for functional redundancy using a phylogenetic heatmap approach. It was predicted that 7,075 genes would have functional redundancy, according to the threshold value validated by an analysis of 111 known genes functionally characterized using knockout mutants and 5,170 duplicated genes. In addition, our analysis demonstrated that an anther/pollen-preferred gene cluster has more functional redundancy than other clusters. Finally, we showed the usefulness of the CAFRI-Rice-based approach by overcoming the functional redundancy between two root-preferred genes via loss-of-function analyses as well as confirming the functional dominancy of three genes through a literature search. This CAFRI-Rice-based target selection for CRISPR/Cas9-mediated mutagenesis will not only accelerate functional genomic studies in rice but can also be straightforwardly expanded to other plant species.

Applications of CRISPR-Cas in agriculture and plant biotechnology

The prokaryote-derived CRISPR-Cas genome editing technology has altered plant molecular biology beyond all expectations. Characterized by robustness and high target specificity and programmability, CRISPR-Cas allows precise genetic manipulation of crop species, which provides the opportunity to create germplasms with beneficial traits and to develop novel, more sustainable agricultural systems. Furthermore, the numerous emerging biotechnologies based on CRISPR-Cas platforms have expanded the toolbox of fundamental research and plant synthetic biology. In this Review, we first briefly describe gene editing by CRISPR-Cas, focusing on the newest, precise gene editing technologies such as base editing and prime editing. We then discuss the most important applications of CRISPR-Cas in increasing plant yield, quality, disease resistance and herbicide resistance, breeding and accelerated domestication. We also highlight the most recent breakthroughs in CRISPR-Cas-related plant biotechnologies, including CRISPR-Cas reagent delivery, gene regulation, multiplexed gene editing and mutagenesis and directed evolution technologies. Finally, we discuss prospective applications of this game-changing technology.

Global Role of Crop Genomics in the Face of Climate Change

The development of climate change resilient crops is necessary if we are to meet the challenge of feeding the growing world's population. We must be able to increase food production despite the projected decrease in arable land and unpredictable environmental conditions. This review summarizes the technological and conceptual advances that have the potential to transform plant breeding, help overcome the challenges of climate change, and initiate the next plant breeding revolution. Recent developments in genomics in combination with high-throughput and precision phenotyping facilitate the identification of genes controlling critical agronomic traits. The discovery of these genes can now be paired with genome editing techniques to rapidly develop climate change resilient crops, including plants with better biotic and abiotic stress tolerance and enhanced nutritional value. Utilizing the genetic potential of crop wild relatives (CWRs) enables the domestication of new species and the generation of synthetic polyploids. The high-quality crop plant genome assemblies and annotations provide new, exciting research targets, including long non-coding RNAs (lncRNAs) and cis-regulatory regions. Metagenomic studies give insights into plant-microbiome interactions and guide selection of optimal soils for plant cultivation. Together, all these advances will allow breeders to produce improved, resilient crops in relatively short timeframes meeting the demands of the growing population and changing climate.

Targeted, random mutagenesis of plant genes with dual cytosine and adenine base editors

Targeted saturation mutagenesis of crop genes could be applied to produce genetic variants with improved agronomic performance. However, tools for directed evolution of plant genes, such as error-prone PCR or DNA shuffling, are limited1. We engineered five saturated targeted endogenous mutagenesis editors (STEMEs) that can generate de novo mutations and facilitate directed evolution of plant genes. In rice protoplasts, STEME-1 edited cytosine and adenine at the same target site with C > T efficiency up to 61.61% and simultaneous C > T and A > G efficiency up to 15.10%. STEME-NG, which incorporates the nickase Cas9-NG protospacer-adjacent motif variant, was used with 20 individual single guide RNAs in rice protoplasts to produce near-saturated mutagenesis (73.21%) for a 56-amino-acid portion of the rice acetyl-coenzyme A carboxylase (OsACC). We also applied STEME-1 and STEME-NG for directed evolution of the OsACC gene in rice and obtained herbicide resistance mutations. This set of two STEMEs will accelerate trait development and should work in any plants amenable to CRISPR-based editing.