OsINV3 and Its Homolog, OsINV2, Control Grain Size in Rice

Molecular dissection of an intronic enhancer governing cold-induced expression of the vacuolar invertase gene in potato

Potato (Solanum tuberosum) is the third most important food crop in the world. Potato tubers must be stored at cold temperatures to minimize sprouting and losses due to disease. However, cold temperatures strongly induce the expression of the potato vacuolar invertase gene (VInv) and cause reducing sugar accumulation. This process, referred to as "cold-induced sweetening," is a major postharvest problem for the potato industry. We discovered that the cold-induced expression of VInv is controlled by a 200 bp enhancer, VInvIn2En, located in its second intron. We identified several DNA motifs in VInvIn2En that bind transcription factors involved in the plant cold stress response. Mutation of these DNA motifs abolished VInvIn2En function as a transcriptional enhancer. We developed VInvIn2En deletion lines in both diploid and tetraploid potato using clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated nuclease 9 (Cas9)-mediated gene editing. VInv transcription in cold-stored tubers was significantly reduced in the deletion lines. Interestingly, the VInvIn2En sequence is highly conserved among distantly related Solanum species, including tomato (Solanum lycopersicum) and other non-tuber-bearing species. We conclude that the VInv gene and the VInvIn2En enhancer have adopted distinct roles in the cold stress response in tubers of tuber-bearing Solanum species.

SMS2, a Novel Allele of OsINV3, Regulates Grain Size in Rice

Grain size has an important effect on rice yield. Although several key genes that regulate seed size have been reported in rice, their molecular mechanisms remain unclear. In this study, a rice small grain size 2 (sms2) mutant was identified, and MutMap resequencing analysis results showed that a 2 bp insertion in the second exon of the LOC_Os02g01590 gene resulted in a grain length and width lower than those of the wild-type Teqing (TQ). We found that SMS2 encoded vacuolar acid invertase, a novel allele of OsINV3, which regulates grain size. GO and KEGG enrichment analyses showed that SMS2 was involved in endoplasmic reticulum protein synthesis, cysteine and methionine metabolism, and propionic acid metabolism, thereby regulating grain size. An analysis of sugar content in young panicles showed that SMS2 reduced sucrose, fructose, and starch contents, thus regulating grain size. A haplotype analysis showed that Hap2 of SMS2 had a longer grain and was widely present in indica rice varieties. Our results provide a new theoretical basis for the molecular and physiological mechanisms by which SMS2 regulates grain size.

Identification, characterisation and expression analysis of peanut sugar invertase genes reveal their vital roles in response to abiotic stress

KEY MESSAGE

Sucrose invertase activity is positively related to osmotic and salt stress resistance in peanut. Sucrose invertases (INVs) have important functions in plant growth and response to environmental stresses. However, their biological roles in peanut are still not fully revealed. In this research, we identified 42 AhINV genes in the peanut genome. They were highly conserved and clustered into three groups with 24 segmental duplication events occurred under purifying selection. Transcriptional expression analysis exhibited that they were all ubiquitously expressed, and most of them were up-regulated by osmotic and salt stresses, with AhINV09, AhINV23 and AhINV19 showed the most significant up-regulation. Further physiochemical analysis showed that the resistance of peanut to osmotic and salt stress was positively related to the high sugar content and sucrose invertase activity. Our results provided fundamental information on the structure and evolutionary relationship of INV gene family in peanut and gave theoretical guideline for further functional study of AhINV genes in response to abiotic stress.

CRISPR/Cas9-mediated genome editing techniques and new breeding strategies in cereals - current status, improvements, and perspectives

Cereal crops, including triticeae species (barley, wheat, rye), as well as edible cereals (wheat, corn, rice, oat, rye, sorghum), are significant suppliers for human consumption, livestock feed, and breweries. Over the past half-century, modern varieties of cereal crops with increased yields have contributed to global food security. However, presently cultivated elite crop varieties were developed mainly for optimal environmental conditions. Thus, it has become evident that taking into account the ongoing climate changes, currently a priority should be given to developing new stress-tolerant cereal cultivars. It is necessary to enhance the accuracy of methods and time required to generate new cereal cultivars with the desired features to adapt to climate change and keep up with the world population expansion. The CRISPR/Cas9 system has been developed as a powerful and versatile genome editing tool to achieve desirable traits, such as developing high-yielding, stress-tolerant, and disease-resistant transgene-free lines in major cereals. Despite recent advances, the CRISPR/Cas9 application in cereals faces several challenges, including a significant amount of time required to develop transgene-free lines, laboriousness, and a limited number of genotypes that may be used for the transformation and in vitro regeneration. Additionally, developing elite lines through genome editing has been restricted in many countries, especially Europe and New Zealand, due to a lack of flexibility in GMO regulations. This review provides a comprehensive update to researchers interested in improving cereals using gene-editing technologies, such as CRISPR/Cas9. We will review some critical and recent studies on crop improvements and their contributing factors to superior cereals through gene-editing technologies.

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

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

Genome-wide association study to identify genomic loci associated with early vigor in bread wheat under simulated water deficit complemented with quantitative trait loci meta-analysis

A genome-wide association study (GWAS) was used to identify associated loci with early vigor under simulated water deficit and grain yield under field drought in a diverse collection of Iranian bread wheat landraces. In addition, a meta-quantitative trait loci (MQTL) analysis was used to further expand our approach by retrieving already published quantitative trait loci (QTL) from recombinant inbred lines, double haploids, back-crosses, and F2 mapping populations. In the current study, around 16%, 14%, and 16% of SNPs were in significant linkage disequilibrium (LD) in the A, B, and D genomes, respectively, and varied between 5.44% (4A) and 21.85% (6A). Three main subgroups were identified among the landraces with different degrees of admixture, and population structure was further explored through principal component analysis. Our GWAS identified 54 marker-trait associations (MTAs) that were located across the wheat genome but with the highest number found in the B sub-genome. The gene ontology (GO) analysis of MTAs revealed that around 75% were located within or closed to protein-coding genes. In the MQTL analysis, 23 MQTLs, from a total of 215 QTLs, were identified and successfully projected onto the reference map. MQT-YLD4, MQT-YLD9, MQT-YLD13, MQT-YLD17, MQT-YLD18, MQT-YLD19, and MQTL-RL1 contributed to the highest number of projected QTLs and were therefore regarded as the most reliable and stable QTLs under water deficit conditions. These MQTLs greatly facilitate the identification of putative candidate genes underlying at each MQTL interval due to the reduced confidence of intervals associated with MQTLs. These findings provide important information on the genetic basis of early vigor traits and grain yield under water deficit conditions and set the foundation for future investigations into adaptation to water deficit in bread wheat.

Developing Genetic Engineering Techniques for Control of Seed Size and Yield

Many signaling pathways regulate seed size through the development of endosperm and maternal tissues, which ultimately results in a range of variations in seed size or weight. Seed size can be determined through the development of zygotic tissues (endosperm and embryo) and maternal ovules. In addition, in some species such as rice, seed size is largely determined by husk growth. Transcription regulator factors are responsible for enhancing cell growth in the maternal ovule, resulting in seed growth. Phytohormones induce significant effects on entire features of growth and development of plants and also regulate seed size. Moreover, the vegetative parts are the major source of nutrients, including the majority of carbon and nitrogen-containing molecules for the reproductive part to control seed size. There is a need to increase the size of seeds without affecting the number of seeds in plants through conventional breeding programs to improve grain yield. In the past decades, many important genetic factors affecting seed size and yield have been identified and studied. These important factors constitute dynamic regulatory networks governing the seed size in response to environmental stimuli. In this review, we summarized recent advances regarding the molecular factors regulating seed size in Arabidopsis and other crops, followed by discussions on strategies to comprehend crops' genetic and molecular aspects in balancing seed size and yield.

Understanding complex genetic architecture of rice grain weight through QTL-meta analysis and candidate gene identification

Quantitative trait loci (QTL) for rice grain weight identified using bi-parental populations in various environments were found inconsistent and have a modest role in marker assisted breeding and map-based cloning programs. Thus, the identification of a consistent consensus QTL region across populations is critical to deploy in marker aided breeding programs. Using the QTL meta-analysis technique, we collated rice grain weight QTL information from numerous studies done across populations and in diverse environments to find constitutive QTL for grain weight. Using information from 114 original QTL in meta-analysis, we discovered three significant Meta-QTL (MQTL) for grain weight on chromosome 3. According to gene ontology, these three MQTL have 179 genes, 25 of which have roles in developmental functions. Amino acid sequence BLAST of these genes indicated their orthologue conservation among core cereals with similar functions. MQTL3.1 includes the OsAPX1, PDIL, SAUR, and OsASN1 genes, which are involved in grain development and have been discovered to play a key role in asparagine biosynthesis and metabolism, which is crucial for source-sink regulation. Five potential candidate genes were identified and their expression analysis indicated a significant role in early grain development. The gene sequence information retrieved from the 3 K rice genome project revealed the deletion of six bases coding for serine and alanine in the last exon of OsASN1 led to an interruption in the synthesis of α-helix of the protein, which negatively affected the asparagine biosynthesis pathway in the low grain weight genotypes. Further, the MQTL3.1 was validated using linked marker RM7197 on a set of genotypes with extreme phenotypes. MQTL that have been identified and validated in our study have significant scope in MAS breeding and map-based cloning programs for improving rice grain weight.

Cloning and functional analysis of soluble acid invertase 2 gene (SbSAI-2) in sorghum

The sorghum soluble acid invertase gene SbSAI-2 was cloned and the function verified in Pichia pastoris and rice, showing the SbSAI-2 affects composition and content of sugar in stem juice. Sugar metabolism is one of the most important metabolic processes in plants, in which soluble acid invertase plays a key role. However, the structure and function of the soluble acid transferase gene in sorghum are still fully unclear. In this study, SbSAI-2 was cloned from the sorghum variety BTx623, and two transcripts were found through sequence analysis, with only one transcript translated into an active protein. There is 72% homology between SbSAI-2 and OsVIN2. The construction of Osvin2 mutant lines and SbSAI-2-1 overexpression lines in Oryza sativa L. japonica. cv. Nipponbare were produced to clarify the invertase functionality. While the invertase activity in the stem of the Osvin2 mutant line was reduced, with no significant difference (P  > 0.05), and the contents of fructose and glucose in stem tissue did not change significantly (P  > 0.05), and the content of sucrose increased by 38.89% (P  < 0.01). In SbSAI-2-1 overexpression lines, the invertase activity in stem was increased by more than 20 times (P  < 0.01). The contents of glucose and fructose in stem tissues were increased by two and three times, respectively (P  < 0.01), while the content of sucrose was significantly decreased, which was below the detection limit (P  < 0.01). This study indicated that SbSAI-2 is a key enzyme related to sucrose metabolism and affects the composition and content of sugar in stems. The result provided further the gene function verification and laid a foundation for the development of molecular markers.

Comparative Genomic Analysis of Quantitative Trait Loci Associated With Micronutrient Contents, Grain Quality, and Agronomic Traits in Wheat (Triticum aestivum L.)

Comparative genomics and meta-quantitative trait loci (MQTLs) analysis are important tools for the identification of reliable and stable QTLs and functional genes controlling quantitative traits. We conducted a meta-analysis to identify the most stable QTLs for grain yield (GY), grain quality traits, and micronutrient contents in wheat. A total of 735 QTLs retrieved from 27 independent mapping populations reported in the last 13 years were used for the meta-analysis. The results showed that 449 QTLs were successfully projected onto the genetic consensus map which condensed to 100 MQTLs distributed on wheat chromosomes. This consolidation of MQTLs resulted in a three-fold reduction in the confidence interval (CI) compared with the CI for the initial QTLs. Projection of QTLs revealed that the majority of QTLs and MQTLs were in the non-telomeric regions of chromosomes. The majority of micronutrient MQTLs were located on the A and D genomes. The QTLs of thousand kernel weight (TKW) were frequently associated with QTLs for GY and grain protein content (GPC) with co-localization occurring at 55 and 63%, respectively. The co- localization of QTLs for GY and grain Fe was found to be 52% and for QTLs of grain Fe and Zn, it was found to be 66%. The genomic collinearity within Poaceae allowed us to identify 16 orthologous MQTLs (OrMQTLs) in wheat, rice, and maize. Annotation of promising candidate genes (CGs) located in the genomic intervals of the stable MQTLs indicated that several CGs (e.g., TraesCS2A02G141400, TraesCS3B02G040900, TraesCS4D02G323700, TraesCS3B02G077100, and TraesCS4D02G290900) had effects on micronutrients contents, yield, and yield-related traits. The mapping refinements leading to the identification of these CGs provide an opportunity to understand the genetic mechanisms driving quantitative variation for these traits and apply this information for crop improvement programs.

Genome editing in cereal crops: an overview

Genome-editing technologies offer unprecedented opportunities for crop improvement with superior precision and speed. This review presents an analysis of the current state of genome editing in the major cereal crops- rice, maize, wheat and barley. Genome editing has been used to achieve important agronomic and quality traits in cereals. These include adaptive traits to mitigate the effects of climate change, tolerance to biotic stresses, higher yields, more optimal plant architecture, improved grain quality and nutritional content, and safer products. Not all traits can be achieved through genome editing, and several technical and regulatory challenges need to be overcome for the technology to realize its full potential. Genome editing, however, has already revolutionized cereal crop improvement and is poised to shape future agricultural practices in conjunction with other breeding innovations.

Genetic and Molecular Factors Determining Grain Weight in Rice

Grain weight is one of the major factors determining single plant yield production of rice and other cereal crops. Research has begun to reveal the regulatory mechanisms underlying grain weight as well as grain size, highlighting the importance of this research for plant molecular biology. The developmental trait of grain weight is affected by multiple molecular and genetic aspects that lead to dynamic changes in cell division, expansion and differentiation. Additionally, several important biological pathways contribute to grain weight, such as ubiquitination, phytohormones, G-proteins, photosynthesis, epigenetic modifications and microRNAs. Our review integrates early and more recent findings, and provides future perspectives for how a more complete understanding of grain weight can optimize strategies for improving yield production. It is surprising that the acquired wealth of knowledge has not revealed more insights into the underlying molecular mechanisms. To accelerating molecular breeding of rice and other cereals is becoming an emergent and critical task for agronomists. Lastly, we highlighted the importance of leveraging gene editing technologies as well as structural studies for future rice breeding applications.

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.

Transcriptome Analysis Reveals Photoperiod-Associated Genes Expressed in Rice Anthers

Environmental conditions, such as photoperiod and temperature, can affect male fertility in plants. While this feature is heavily exploited in rice to generate male-sterile lines for hybrid breeding, the underlying molecular mechanisms remain largely unknown. In this study, we use a transcriptomics approach to identify key genes and regulatory networks affecting pollen maturation in rice anthers in response to different day lengths. A total of 11,726 differentially expressed genes (DEGs) were revealed, of which 177 were differentially expressed at six time points over a 24-h period. GO enrichment analysis revealed that genes at all time points were enriched in transport, carbohydrate, and lipid metabolic processes, and signaling pathways, particularly phytohormone signaling. In addition, co-expression network analysis revealed four modules strongly correlated with photoperiod. Within these four modules, 496 hub genes were identified with a high degree of connectivity to other photoperiod-sensitive DEGs, including two previously reported photoperiod- and temperature-sensitive genes affecting male fertility, Carbon Starved Anther and UDP-glucose pyrophosphorylase, respectively. This work provides a new understanding on photoperiod-sensitive pollen development in rice, and our gene expression data will provide a new, comprehensive resource to identify new environmentally sensitive genes regulating male fertility for use in crop improvement.

GhN/AINV13 positively regulates cotton stress tolerance by interacting with the 14-3-3 protein

Neutral/alkaline invertases (N/AINVs) are sucrose hydrolases with important roles in plants. In this study, 15, 15, 15, 29, and 30 N/AINVs were identified in the Gossypium species, G. raimondii, G. herbaceum, G. arboreum, G. hirsutum, and G. barbadense, respectively. Along with two previously discovered branches, α and β, a new clade γ was first discovered in our study. Investigation of gene collinearity showed that whole-genome duplication (WGD) and polyploidization were responsible for the expansion of the N/AINV gene family in allopolyploid Gossypium. Moreover, expression patterns revealed that GhN/AINV3/13/17/23/24/28 from the β clade is highly expressed during the period of fiber initiation. The invertase activity of GhN/AINV13 and GhN/AINV23 were confirmed by restoring defects of invertase-deficient yeast mutant SEY2102. Treatments of abiotic stress showed that most GhN/AINVs were induced in response to polyethylene glycol (PEG) or salt stress. A virus-induced gene-silencing (VIGS) experiment and yeast two-hybrid assay demonstrated that GhN/AINV13 may interact with their positive regulators Gh14-3-3 proteins and participate in the fiber initiation or stress tolerance of cotton. Our results provided fundamental information regarding N/AINVs and highlight their potential functions in cotton stress tolerance.

The Molecular Regulation of Carbon Sink Strength in Grapevine (Vitis vinifera L.)

Sink organs, the net receivers of resources from source tissues, provide food and energy for humans. Crops yield and quality are improved by increased sink strength and source activity, which are affected by many factors, including sugars and hormones. With the growing global population, it is necessary to increase photosynthesis into crop biomass and yield on a per plant basis by enhancing sink strength. Sugar translocation and accumulation are the major determinants of sink strength, so understanding molecular mechanisms and sugar allocation regulation are conducive to develop biotechnology to enhance sink strength. Grapevine (Vitis vinifera L.) is an excellent model to study the sink strength mechanism and regulation for perennial fruit crops, which export sucrose from leaves and accumulates high concentrations of hexoses in the vacuoles of fruit mesocarp cells. Here recent advances of this topic in grape are updated and discussed, including the molecular biology of sink strength, including sugar transportation and accumulation, the genes involved in sugar mobilization and their regulation of sugar and other regulators, and the effects of hormones on sink size and sink activity. Finally, a molecular basis model of the regulation of sugar accumulation in the grape is proposed.