Reducing rice seed storage protein accumulation leads to changes in nutrient quality and storage organelle formation

Application of CRISPR/Cas9 system to knock out GluB gene for developing low glutelin rice mutant

The nutritional quality improvement is among the most integral objective for any rice molecular breeding programs. The seed storage proteins (SSPs) have greater role to determine the nutritional quality of any cereal grains. Rice contains relatively balanced amino acid composition and the SSPs are fractioned into albumins (ALB), globulins (GLO), prolamins (PRO) and glutelins (GLU) according to differences in solubility. GLUs are further divided into subfamilies: GluA, GluB, GluC, and GluD depending on resemblance in amino acid. The GLU protein accounts for 60-80% of total protein contents, encoded by 15 genes located on different chromosomes of rice genome. The clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) system was employed to knockout Glu-B (LOC-Os02g15070) gene in non-basmati rice PK386 cultivar. The mutant displayed two base pair and three base pair mutation in the targeted regions. The homozygous mutant plant displayed reduction for both in total protein contents and GLU contents whereas, elevation in GLO, ALB and PRO. Moreover, the mutant plant also displayed reduction in physio-chemical properties e.g., total starch, amylose and gel consistency. The agronomic characteristics of both mutant and wild type displayed non-significant differences along with increase in higher percentage of chalkiness in mutant plants. The results obtained from scanning electron microscopy showed the loosely packed starch granules compared to wild type. The gene expression analysis displayed the lower expression of gene at 5 days after flowering (DAF), 10 DAF, 15 DAF and 20 DAF compared to wild type. GUS sub-cellular localization showed the staining in seed which further validated the results obtained from gene expression. Based on these findings it can be concluded Glu-B gene have significant role in controlling GLU contents and can be utilized in breeding programs to enhance the nutritional quality of rice, and may serve as healthy diet for patient allergic with high GLU contents.

Cloning and functional characterization of the legumin A gene (EuLEGA) from Eucommia ulmoides Oliver

Legumin A is a seed storage protein that provides nutrients for seed germination. The purpose of this study was to describe the structure and expression pattern of the EuLEGA gene in Eucommia ulmoides Oliver (E. ulmoides) and to infer its functional role. The 1287 bp coding sequence of the EuLEGA CDS of the EuLEGA gene, encoding a protein containing 428 amino acid residues, was cloned. The structure predicted that the protein belonged to the RmlC (deoxythymidine diphosphates, dTDP)-4-dehydrorhamnose 3,5-epimerase)-like cupin conserved domain family, which contains both RmlC, a key enzyme for the synthesis of rhamnose and legumin A. The overexpression (OE) vector of the EuLEGA gene was constructed and genetically transformed into tobacco and E. ulmoides; the RNA interference (RNAi) vector of the EuLEGA gene was constructed and genetically transformed into E. ulmoides; and the contents of legumin A and rhamnose were detected. The results showed that the EuLEGA gene could significantly increase the content of legumin A in transgenic tobacco leaves and transgenic E. ulmoides regenerative buds, and the OE of this gene in E. ulmoides could promote an increase in rhamnose content. RNAi caused a significant decrease in the legumin A content in the regenerated buds of E. ulmoides. These was a significant increase in legumin A in the transgenic tobacco seeds, and these results indicate that the expression of the EuLEGA gene is closely related to the accumulation of legumin A. Subcellular localization studies revealed that EuLEGA is localized to the cytoplasm with the vacuolar membrane. Analysis of the EuLEGA gene expression data revealed that the expression level of the EuLEGA gene in the samaras was significantly greater than that in the leaves and stems. In addition, the study also demonstrated that GA3 can upregulate the expression levels of the EuLEGA gene, while ABA and MeJA can downregulate its expression levels.

Compensatory Modulation of Seed Storage Protein Synthesis and Alteration of Starch Accumulation by Selective Editing of 13 kDa Prolamin Genes by CRISPR-Cas9 in Rice

Rice prolamins are categorized into three groups by molecular size (10, 13, or 16 kDa), while the 13 kDa prolamins are assigned to four subgroups (Pro13a-I, Pro13a-II, Pro13b-I, and Pro13b-II) based on cysteine residue content. Since lowering prolamin content in rice is essential to minimize indigestion and allergy risks, we generated four knockout lines using CRISPR-Cas9, which selectively reduced the expression of a specific subgroup of the 13 kDa prolamins. These four mutant rice lines also showed the compensatory expression of glutelins and non-targeted prolamins and were accompanied by low grain weight, altered starch content, and atypically-shaped starch granules and protein bodies. Transcriptome analysis identified 746 differentially expressed genes associated with 13 kDa prolamins during development. Correlation analysis revealed negative associations between genes in Pro13a-I and those in Pro13a-II and Pro13b-I/II subgroups. Furthermore, alterations in the transcription levels of 9 ER stress and 17 transcription factor genes were also observed in mutant rice lines with suppressed expression of 13 kDa prolamin. Our results provide profound insight into the functional role of 13 kDa rice prolamins in the regulatory mechanisms underlying rice seed development, suggesting their promising potential application to improve nutritional and immunological value.

Biofortified Rice Provides Rich Sakuranetin in Endosperm

Sakuranetin plays a key role as a phytoalexin in plant resistance to biotic and abiotic stresses, and possesses diverse health-promoting benefits. However, mature rice seeds do not contain detectable levels of sakuranetin. In the present study, a transgenic rice plant was developed in which the promoter of an endosperm-specific glutelin gene OsGluD-1 drives the expression of a specific enzyme naringenin 7-O-methyltransferase (NOMT) for sakuranetin biosynthesis. The presence of naringenin, which serves as the biosynthetic precursor of sakuranetin made this modification feasible in theory. Liquid chromatography tandem mass spectrometry (LC-MS/MS) validated that the seeds of transgenic rice accumulated remarkable sakuranetin at the mature stage, and higher at the filling stage. In addition, the panicle blast resistance of transgenic rice was significantly higher than that of the wild type. Specially, the matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) imaging was performed to detect the content and spatial distribution of sakuranetin and other nutritional metabolites in transgenic rice seeds. Notably, this genetic modification also did not change the nutritional and quality indicators such as soluble sugars, total amino acids, total flavonoids, amylose, total protein, and free amino acid content in rice. Meanwhile, the phenotypes of the transgenic plant during the whole growth and developmental periods and agricultural traits such as grain width, grain length, and 1000-grain weight exhibited no significant differences from the wild type. Collectively, the study provides a conceptual advance on cultivating sakuranetin-rich biofortified rice by metabolic engineering. This new breeding idea may not only enhance the disease resistance of cereal crop seeds but also improve the nutritional value of grains for human health benefits.

Generation of major glutelin-deficient (GluA, GluB, and GluC) semi-dwarf Koshihikari rice line

KEY MESSAGE

We generated a new Koshihikari rice line with a drastically reduced content of glutelin proteins and higher lodging resistance by using new and conventional plant breeding techniques. Using CRISPR/Cas9-mediated genome editing, we generated mutant rice with drastically decreased contents of major glutelins. A Koshihikari rice mutant line, a123, lacking four glutelins (GluA1, GluA2, GluB4, and GluB5) was used as a host, and another five major glutelin genes (GluA3, GluB1a, GluB1b, GluB2, and GluC) were knocked out through two iterations of Agrobacterium-mediated transformation. Mutant seeds were deficient in the GluA family, GluB family, and GluC, and the line obtained was named GluABC KO. Glutelin content was much lower in GluABC KO than in the existing low-glutelin rice mutant LGC-1. A null segregant of GluABC KO was selected using new-generation sequencing and backcrossing, and the sd-1 allele for the semi-dwarf trait was introduced to increase lodging resistance.

Sulfate transport and metabolism: strategies to improve the seed protein quality

Sulfur-containing amino acids (SAA), namely methionine, and cysteine are crucial essential amino acids (EAA) considering the dietary requirements of humans and animals. However, a few crop plants, especially legumes, are characterized with suboptimal levels of these EAA thereby limiting their nutritive value. Hence, improved comprehension of the mechanistic perspective of sulfur transport and assimilation into storage reserve, seed storage protein (SSP), is imperative. Efforts to augment the level of SAA in seed storage protein form an integral component of strategies to balance nutritive quality and quantity. In this review, we highlight the emerging trends in the sulfur biofortification approaches namely transgenics, genetic and molecular breeding, and proteomic rebalancing with sulfur nutrition. The transgenic 'push and pull strategy' could enhance sulfur capture and storage by expressing genes that function as efficient transporters, sulfate assimilatory enzymes, sulfur-rich foreign protein sinks, or by suppressing catabolic enzymes. Modern molecular breeding approaches that adopt high throughput screening strategies and machine learning algorithms are invaluable in identifying candidate genes and alleles associated with SAA content and developing improved crop varieties. Sulfur is an essential plant nutrient and its optimal uptake is crucial for seed sulfur metabolism, thereby affecting seed quality and yields through proteomic rebalance between sulfur-rich and sulfur-poor seed storage proteins.

OsSHMT4 Is Required for Synthesis of Rice Storage Protein and Storage Organelle Formation in Endosperm Cells

Storage proteins are essential for seed germination and seedling growth, as they provide an indispensable nitrogen source and energy. Our previous report highlighted the defective endosperm development in the serine hydroxymethyltransferase 4 (OsSHMT4) gene mutant, floury endosperm20-1 (flo20-1). However, the alterations in storage protein content and distribution within the flo20-1 endosperm remained unclear. Here, the immunocytochemistry analyses revealed a deficiency in storage protein accumulation in flo20-1. Electron microscopic observation uncovered abnormal morphological structures in protein bodies (PBI and PBII) in flo20-1. Immunofluorescence labeling demonstrated that aberrant prolamin composition could lead to the subsequent formation and deposition of atypical structures in protein body I (PBI), and decreased levels of glutelins and globulin resulted in protein body II (PBII) malformation. Further RNA-seq data combined with qRT-PCR results indicated that altered transcription levels of storage protein structural genes were responsible for the abnormal synthesis and accumulation of storage protein, which further led to non-concentric ring structural PBIs and amorphous PBIIs. Collectively, our findings further underscored that OsSHMT4 is required for the synthesis and accumulation of storage proteins and storage organelle formation in endosperm cells.

QTL detection for rice grain storage protein content and genetic effect verifications

Rice grain quality is a multifarious attribute mainly governed by multiple nutritional factors. Grain protein is the central component of rice grain nutrition dominantly affecting eating-cooking qualities. Grain protein content is quantitatively influenced by its protein fractions. Genetic quantification of five protein fractions-albumins, globulins, prolamins, glutelin, and grain protein content-were evaluated by exploiting two BC3F2 mapping populations, derived from Kongyu131/TKM9 (population-I) and Kongyu131/Bg94-1 (population-II), which were grown in a single environment. Correlation studies among protein fractions and grain protein content were thoroughly investigated. A genetic linkage map was developed by using 146 single sequence repeat (SSR) markers in population-I and 167 markers in population-II. In total, 40 QTLs were delineated for five traits in both populations. Approximately 22 QTLs were dissected in population-I, derived from Kongyu131/TKM9, seven QTLs for albumin content, four QTLs for globulin content, three QTLs for prolamin content, four QTLs for glutelin content, and four QTLs for grain protein content. In total, 18 QTLs were detected in population-II, derived from Kongyu131/Bg94-1, five QTLs for albumin content, three QTLs for globulin content, four QTLs for prolamin content, two QTLs for glutelin content, and four QTLs for grain protein content. Three QTLs, qAlb7.1, Alb7.2, and qGPC7.2, derived from population-II (Kongyu131/Bg94-1) for albumin and grain protein content were successfully validated in the near isogenic line (NIL) populations. The localized chromosomal locus of the validated QTLs could be helpful for fine mapping via map-based cloning to discover underlying candidate genes. The functional insights of the underlying candidate gene would furnish novel perceptivity for the foundation of rice grain protein content and trigger the development of nutritionally important rice cultivars by combining marker-assisted selection (MAS) breeding.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-023-01436-7.

Improving rice eating and cooking quality by enhancing endogenous expression of a nitrogen-dependent floral regulator

Improving rice eating and cooking quality (ECQ) is one of the primary tasks in rice production to meet the rising demands of consumers. However, improving grain ECQ without compromising yield faces a great challenge under varied nitrogen (N) supplies. Here, we report the approach to upgrade rice ECQ by native promoter-controlled high expression of a key N-dependent floral and circadian clock regulator Nhd1. The amplification of endogenous Nhd1 abundance alters rice heading date but does not affect the entire length of growth duration, N use efficiency and grain yield under both low and sufficient N conditions. Enhanced expression of Nhd1 reduces amylose content, pasting temperature and protein content while increasing gel consistence in grains. Metabolome and transcriptome analyses revealed that increased expression of Nhd1 mainly regulates the metabolism of carbohydrates and amino acids in the grain filling stage. Moreover, expression level of Nhd1 shows a positive relationship with grain ECQ in some local main cultivars. Thus, intensifying endogenous abundance of Nhd1 is a promising strategy to upgrade grain ECQ in rice production.

Down-Regulation of Rice Glutelin by CRISPR-Cas9 Gene Editing Decreases Carbohydrate Content and Grain Weight and Modulates Synthesis of Seed Storage Proteins during Seed Maturation

The glutelins are a family of abundant plant proteins comprised of four glutelin subfamilies (GluA, GluB, GluC, and GluD) encoded by 15 genes. In this study, expression of subsets of rice glutelins were suppressed using CRISPR-Cas9 gene-editing technology to generate three transgenic rice variant lines, GluA1, GluB2, and GluC1. Suppression of the targeted glutelin genes was confirmed by SDS-PAGE, Western blot, and q-RT-PCR. Transgenic rice variants GluA1, GluB2, and GluC1 showed reduced amylose and starch content, increased prolamine content, reduced grain weight, and irregularly shaped protein aggregates/protein bodies in mature seeds. Targeted transcriptional profiling of immature seeds was performed with a focus on genes associated with grain quality, starch content, and grain weight, and the results were analyzed using the Pearson correlation test (requiring correlation coefficient absolute value ≥ 0.7 for significance). Significantly up- or down-regulated genes were associated with gene ontology (GO) and KEGG pathway functional annotations related to RNA processing (spliceosomal RNAs, group II catalytic introns, small nucleolar RNAs, microRNAs), as well as protein translation (transfer RNA, ribosomal RNA and other ribosome and translation factors). These results suggest that rice glutelin genes may interact during seed development with genes that regulate synthesis of starch and seed storage proteins and modulate their expression via post-transcriptional and translational mechanisms.

Influence on Accumulation Levels and Subcellular Localization of Prolamins by Fusion with the Functional Peptide in Transgenic Rice Seeds

To exploit the rice seed-based oral vaccine against Sjögren's syndrome, altered peptide ligand of N-terminal 1 (N1-APL7) from its M3 muscarinic acetylcholine receptor (M3R) autoantigen was expressed as fusion protein with the representative four types of rice prolamins (16 kDa, 14 kDa, 13 kDa, and 10 kDa prolamins) under the control of the individual native prolamin promoter. The 10kD:N1-APL7 and 14kD:N1-APL7 accumulated at high levels (287 and 58 µg/grain), respectively, whereas production levels of the remaining ones were remarkably low. Co-expression of these fusion proteins did not enhance the accumulation level of N1-APL7 in an additive manner. Downregulation of endogenous seed storage proteins by RNAi-mediated suppression also did not lead to substantial elevation of the co-expressed prolamin:N1-APL7 products. When transgenic rice seeds were subjected to in vitro proteolysis with pepsin, the 10kD:N1-APL7 was digested more quickly than the endogenous 10 kDa prolamin and the 14kD:N1-APL7 deposited in PB-Is. This difference could be explained by the finding that the 10kD:N1-APL7 was unexpectedly localized in the PB-IIs containing glutelins. These results indicated that not only accumulation level but also subcellular localization of inherent prolamins were highly influenced by the liked N1-APL7 peptide.

Comparative Proteome-wide Characterization of Three Different Tissues of High-Protein Mutant and Wild Type Unravels Protein Accumulation Mechanisms in Rice Seeds

Improving the proteins and amino acid contents of rice seeds is one of the prime objectives of plant breeders. We recently developed an EMS mutant/high-protein mutant (HPM) of rice that exhibits 14.8% of the total protein content as compared to its parent Dharial (wild-type), which shows only 9.3% protein content in their mature seeds. However, the mechanisms underlying the higher protein accumulation in these HPM seeds remain largely elusive. Here, we utilized high-throughput proteomics to examine the differences in the proteome profiles of the embryo, endosperm, and bran tissues of Dharial and HPM seeds. Utilizing a label-free quantitative proteomic and subsequent functional analyses of the identified proteins revealed that nitrogen compound biosynthesis, intracellular transport, protein/amino acid synthesis, and photosynthesis-related proteins were specifically enriched in the endosperm and bran of the high-protein mutant seed. Our data have uncovered proteome-wide changes highlighting various functions of metabolic pathways associated with protein accumulation in rice seeds.

Regulation of seed storage protein synthesis in monocot and dicot plants: A comparative review

Seeds are a major source of nutrients for humans and animal livestock worldwide. With improved living standards, high nutritional quality has become one of the main targets for breeding. Storage protein content in seeds, which is highly variable depending on plant species, serves as a pivotal criterion of seed nutritional quality. In the last few decades, our understanding of the molecular genetics and regulatory mechanisms of storage protein synthesis has greatly advanced. Here, we systematically and comprehensively summarize breakthroughs on the conservation and divergence of storage protein synthesis in dicot and monocot plants. With regard to storage protein accumulation, we discuss evolutionary origins, developmental processes, characteristics of main storage protein fractions, regulatory networks, and genetic modifications. In addition, we discuss potential breeding strategies to improve storage protein accumulation and provide perspectives on some key unanswered problems that need to be addressed.

Post-anthesis supplementary irrigation improves grain yield and nutritional quality of drip-irrigated rice (Oryza sativa L.)

Introduction

Approximately 50% of irrigation water is saved during drip-irrigation of rice, which has tremendous potential for water-saving agriculture, particularly in areas where water resources are scarce. However, the grain yield and quality of drip-irrigated rice are adversely affected.

Methods

In this study, we investigated the effects of different irrigation strategies on the grain yield and quality of drip-irrigated rice using field experiments. Four irrigation treatments were studied: whole growing season flooding (FI), whole growing season normal drip irrigation (DI, soil relative moisture (RSM) was maintained in the range of 90-100%), pre-anthesis drip irrigation and post-anthesis water stress (SAF, the RSM was maintained in the range of 80-90% after anthesis), pre-anthesis drip irrigation, and post-anthesis flooding (FAF).

Results

The results showed that grain yield, harvest index, seed setting rate and 1000 grain weight in DI and SAF were significantly lower than in FI and FAF. These parameters were not significantly different between FI and FAF but were significantly greater in DI than in SAF. Compared with FI and FAF, the source capacity, source activity time, and sink activity of DI and SAF decreased, and the sink-source difference increased. The sink-source difference had a significant negative correlation with rice yield and 1000 grain weight. The activities of ADP-glucose pyrophosphorylase, starch branching enzyme, and amylopectin content in grains in the middle panicles of FAF were significantly higher than those of DI and SAF. SAF resulted in increased amylose/amylopectin ratio and total protein content in grains but decreased proportion of glutenin in total protein. Irrigation after anthesis of drip-irrigated rice narrowed the difference between sink sources in rice plants, increased the grain yield and harvest index by 29.2% and 11%, respectively, compared to DI, increased water productivity by 19% compared to FI, and improved the grain quality of drip-irrigated rice.

Discussion

This study highlights that post-anthesis sufficient irrigation of drip-irrigated rice plays a positive role in maintaining the source-sink balance. This study serves as a foundation for the development of more effective rice farming methods that conserve water, while increasing the grain yield and quality of drip-irrigated rice.

A new demethylase gene, OsDML4, is involved in high temperature-increased grain chalkiness in rice

High temperature (HT) can affect the accumulation of seed storage materials and cause adverse effects on the yield and quality of rice. DNA methylation plays an important role in plant growth and development. Here, we identified a new demethylase gene OsDML4 and discovered its function in cytosine demethylation to affect endosperm formation. Loss of function of OsDML4 induced chalky endosperm only under HT and dramatically reduced the transcription and accumulation of glutelins and 16 kDa prolamin. The expression of two transcription factor genes RISBZ1 and RPBF was significantly decreased in the osdml4 mutants, which caused adverse effects on the formation of protein bodies (PBs) with greatly decreased PB-II number, and incomplete and abnormally shaped PB-IIs. Whole-genome bisulfite sequencing analysis of seeds at 15 d after pollination revealed much higher global methylation levels of CG, CHG, and CHH contexts in the osdml4 mutants compared with the wild type. Moreover, the RISBZ1 promoter was hypermethylated but the RPBF promoter was almost unchanged under HT. No significant difference was detected between the wild type and osdml4 mutants under normal temperature. Our study demonstrated a novel OsDML4-mediated DNA methylation involved in the formation of chalky endosperm only under HT and provided a new perspective in regulating endosperm development and the accumulation of seed storage proteins in rice.

Efficient breeding of low glutelin content rice germplasm by simultaneous editing multiple glutelin genes via CRISPR/Cas9

Chronic kidney disease (CKD) and phenylketonuria (PKU) patients need to eat rice with low glutelin content. Therefore, breeding low glutelin content rice varieties with high yield and delicious taste is one of the major goals of rice breeders due to the high demand for the product. In this study, we designed three sgRNAs targeting nine glutelin genes and generated nine T-DNA-free homozygous editing lines with reduced glutelin content compared with the wild-type due to simultaneous mutation(s) in 5-7 glutelin genes. The glutelin content of two lines is even significantly lower than that of the low glutelin content cultivar, LGC-1. Compared to the wild-type, these low glutelin lines showed similar agronomic traits, including yield components and viscosity properties, and can be used as new varieties or parental materials for further breeding.

Rapid improvement of rice eating and cooking quality through gene editing toward glutelin as target

Rice eating and cooking quality (ECQ) is a major concern of breeders and consumers, determining market competitiveness worldwide. Rice grain protein content (GPC) is negatively related to ECQ, making it possible to improve ECQ by manipulating GPC. However, GPC is genetically complex and sensitive to environmental conditions; therefore, little progress has been made in traditional breeding for ECQ. Here, we report that CRISPR/Cas9-mediated knockout of genes encoding the grain storage protein glutelin rapidly produced lines with downregulated GPC and improved ECQ. Our finding provides a new strategy for improving rice ECQ.

Unprocessed wheat γ-gliadin reduces gluten accumulation associated with the endoplasmic reticulum stress and elevated cell death

Along with increasing demands for high yield, elite processing quality and improved nutrient value in wheat, concerns have emerged around the effects of gluten in wheat-based foods on human health. However, knowledge of the mechanisms regulating gluten accumulation remains largely unexplored. Here we report the identification and characterization of a wheat low gluten protein 1 (lgp1) mutant that shows extremely low levels of gliadins and glutenins. The lgp1 mutation in a single γ-gliadin gene causes defective signal peptide cleavage, resulting in the accumulation of an excessive amount of unprocessed γ-gliadin and a reduced level of gluten, which alters the endoplasmic reticulum (ER) structure, forms the autophagosome-like structures, leads to the delivery of seed storage proteins to the extracellular space and causes a reduction in starch biosynthesis. Physiologically, these effects trigger ER stress and cell death. This study unravels a unique mechanism that unprocessed γ-gliadin reduces gluten accumulation associated with ER stress and elevated cell death in wheat. Moreover, the reduced gluten level in the lgp1 mutant makes it a good candidate for specific diets for patients with diabetes or kidney diease.

Endomembrane-mediated storage protein trafficking in plants: Golgi-dependent or Golgi-independent?

Seed storage proteins (SSPs) accumulated within plant seeds constitute the major protein nutrition sources for human and livestock. SSPs are synthesized on the endoplasmic reticulum (ER) and then deposited in plant-specific protein bodies (PBs), including ER-derived PBs and protein storage vacuoles (PSVs). Plant seeds have evolved a distinct endomembrane system to accomplish SSP transport. There are two distinct types of trafficking pathways contributing to SSP delivery to PSVs, one Golgi-dependent and the other Golgi-independent. In recent years, molecular, genetic and biochemical studies have shed light on the complex network controlling SSP trafficking, to which both evolutionarily conserved molecular machineries and plant-unique regulators contribute. In this review, we discuss current knowledge of PB biogenesis and endomembrane-mediated SSP transport, focusing on ER export and post-Golgi traffic. These knowledges support a dominant role for the Golgi-dependent pathways in SSP transport in Arabidopsis and rice. In addition, we describe cutting-edge strategies to dissect the endomembrane trafficking system in plant seeds to advance the field.

Molecular farming using transgenic rice endosperm

Plant expression platforms are low-cost, scalable, safe, and environmentally friendly systems for the production of recombinant proteins and bioactive metabolites. Rice (Oryza sativa L.) endosperm is an ideal bioreactor for the production and storage of high-value active substances, including pharmaceutical proteins, oral vaccines, vitamins, and nutraceuticals such as flavonoids and carotenoids. Here, we explore the use of molecular farming from producing medicines to developing functional food crops (biofortification). We review recent progress in producing pharmaceutical proteins and bioactive substances in rice endosperm and compare this platform with other plant expression systems. We describe how rice endosperm could be modified to design metabolic pathways and express and store stable products and discuss the factors restricting the commercialization of transgenic rice products and future prospects.

Endomembrane mediated-trafficking of seed storage proteins: from Arabidopsis to cereal crops

Seed storage proteins (SSPs) are of great importance in plant science and agriculture, particularly in cereal crops, due to their nutritional value and their impact on food properties. During seed maturation, massive amounts of SSPs are synthesized and deposited either within protein bodies derived from the endoplasmic reticulum, or into specialized protein storage vacuoles (PSVs). The processing and trafficking of SSPs vary among plant species, tissues, and even developmental stages, as well as being influenced by SSP composition. The different trafficking routes, which affect the amount of SSPs that seeds accumulate and their composition and modifications, rely on a highly dynamic and functionally specialized endomembrane system. Although the general steps in SSP trafficking have been studied in various plants, including cereals, the detailed underlying molecular and regulatory mechanisms are still elusive. In this review, we discuss the main endomembrane routes involved in SSP trafficking to the PSV in Arabidopsis and other eudicots, and compare and contrast the SSP trafficking pathways in major cereal crops, particularly in rice and maize. In addition, we explore the challenges and strategies for analyzing the endomembrane system in cereal crops.

Characterization of the bark storage protein gene (JcBSP) family in the perennial woody plant Jatropha curcas and the function of JcBSP1 in Arabidopsis thaliana

BACKGROUND

Bark storage protein (BSP) plays an important role in seasonal nitrogen cycling in perennial deciduous trees. However, there is no report on the function of BSP in the perennial woody oil plant Jatropha curcas.

METHODS

In this study, we identified six members of JcBSP gene family in J. curcas genome. The patterns, seasonal changes, and responses to nitrogen treatment in gene expression of JcBSPs were detected by quantitative reverse transcription-polymerase chain reaction (qRT-PCR). Overexpression of JcBSP1 in transgenic Arabidopsis thaliana was driven by a constitutive cauliflower mosaic virus (CaMV) 35S RNA promoter.

RESULTS

JcBSP members were found to be expressed in various tissues, except seeds. The seasonal changes in the total protein concentration and JcBSP1 expression in the stems of J. curcas were positively correlated, as both increased in autumn and winter and decreased in spring and summer. In addition, the JcBSP1 expression in J. curcas seedlings treated with different concentrations of an NH₄NO₃ solution was positively correlated with the NH₄NO₃ concentration and application duration. Furthermore, JcBSP1 overexpression in Arabidopsis resulted in a phenotype of enlarged rosette leaves, flowers, and seeds, and significantly increased the seed weight and yield in transgenic plants.

A Genome Wide Association Study Revealed Key Single Nucleotide Polymorphisms/Genes Associated With Seed Germination in Gossypium hirsutum L

Fast and uniform seed germination is essential to stabilize crop yields in agricultural production. It is important to understand the genetic basis of seed germination for improving the vigor of crop seeds. However, little is known about the genetic basis of seed vigor in cotton. In this study, we evaluated four seed germination-related traits of a core collection consisting of 419 cotton accessions, and performed a genome-wide association study (GWAS) to explore important loci associated with seed vigor using 3.66 million high-quality single nucleotide polymorphisms (SNPs). The results showed that four traits, including germination potential, germination rate, germination index, and vigor index, exhibited broad variations and high correlations. A total of 92 significantly associated SNPs located within or near 723 genes were identified for these traits, of which 13 SNPs could be detected in multiple traits. Among these candidate genes, 294 genes were expressed at seed germination stage. Further function validation of the two genes of higher expression showed that Gh_A11G0176 encoding Hsp70-Hsp90 organizing protein negatively regulated Arabidopsis seed germination, while Gh_A09G1509 encoding glutathione transferase played a positive role in regulating tobacco seed germination and seedling growth. Furthermore, Gh_A09G1509 might promote seed germination and seedling establishment through regulating glutathione metabolism in the imbibitional seeds. Our findings provide unprecedented information for deciphering the genetic basis of seed germination and performing molecular breeding to improve field emergence through genomic selection in cotton.

Rice seed storage proteins: Biosynthetic pathways and the effects of environmental factors

Rice (Oryza sativa L.) is the most important food crop for at least half of the world's population. Due to improved living standards, the cultivation of high-quality rice for different purposes and markets has become a major goal. Rice quality is determined by the presence of many nutritional components, including seed storage proteins (SSPs), which are the second most abundant nutrient components of rice grains after starch. Rice SSP biosynthesis requires the participation of multiple organelles and is influenced by the external environment, making it challenging to understand the molecular details of SSP biosynthesis and improve rice protein quality. In this review, we highlight the current knowledge of rice SSP biosynthesis, including a detailed description of the key molecules involved in rice SSP biosynthetic processes and the major environmental factors affecting SSP biosynthesis. The effects of these factors on SSP accumulation and their contribution to rice quality are also discussed based on recent findings. This recent knowledge suggests not only new research directions for exploring rice SSP biosynthesis but also innovative strategies for breeding high-quality rice varieties.

Heat-Moisture Treatment Further Reduces In Vitro Digestibility and Enhances Resistant Starch Content of a High-Resistant Starch and Low-Glutelin Rice

A novel rice germplasm sbeIIb/Lgc1 producing grains rich in resistant starch (RS) but low in glutelin has been developed through CRISPR/Cas9-mediated targeted mutagenesis for its potential benefits to patients with diabetes and kidney diseases. In this study, a hydrothermal approach known as heat-moisture treatment (HMT) was identified as a simple and effective method in reinforcing the nutritional benefits of sbeIIb/Lgc1 rice. As a result of HMT treatment at 120 °C for 2 h, significant reductions in in vitro digestibility and enhancements in RS content were observed in sbeIIb/Lgc1 rice flour when the rice flour mass fraction was 80% and 90%. The low-glutelin feature of sbeIIb/Lgc1 rice was not compromised by HMT. The potential impacts of HMT on a range of physicochemical properties of sbeIIb/Lgc1 rice flour have also been analyzed. HMT resulted in a darker color of rice flour, alteration in the semi-crystalline structure, an increase in gelatinization temperatures, and reductions in the pasting viscosities as the moisture content increased. This study provides vital data for the food industry to facilitate the application of this dual-functional rice flour as a health food ingredient.

Nitrogen Fertilizer Regulated Grain Storage Protein Synthesis and Reduced Chalkiness of Rice Under Actual Field Warming

Our previous study has shown that nitrogen plays an important role in dealing with significantly increased chalkiness caused by elevated temperature. However, the role of nitrogen metabolites has not been given sufficient attention, and its regulatory mechanism is not clear. This study investigated the effects of high temperature and nitrogen fertilizer on the synthesis of grain storage protein and further explored the quality mechanism under the actual scenario of field warming. Results showed that increased temperature and nitrogen fertilizer could affect the activities of nitrogen metabolism enzymes, namely, glutamate synthetase, glutamine synthetase, glutamic pyruvic transaminase, and glutamic oxaloacetic transaminase, and the expressions of storage protein synthesis factor genes, namely, GluA and GluB, and subfamily genes, namely, pro14, BiP1, and PDIL1, which co-induced the changes of storage protein synthesis in rice grains. Furthermore, the increased temperature changed the balance of grain storage substances which may lead to the significantly increased chalky rate (197.67%) and chalkiness (532.92%). Moreover, there was a significant negative correlation between prolamin content and chalkiness, indicating that nitrogen fertilizer might regulate the formation of chalkiness by affecting the synthesis of prolamin. Results suggested that nitrogen application could regulate the related core factors involved in nitrogen metabolism pathways, which, in turn, affects the changes in the storage protein components in the grain and further affects quality. Therefore, as a conventional cultivation measure, nitrogen application would have a certain value in future rice production in response to climate warming.

Endoplasmic reticulum stress pathway mediates the early heat stress response of developing rice seeds

A transient heat stress occurring during early seed development in rice (Oryza sativa) reduces seed size by altering endosperm development. However, the relationship between the timing of the stress and specific developmental stage on heat sensitivity is not well-understood. To address this, we imposed a series of non-overlapping heat stress treatments and found that young seeds are most sensitive during the first two days after flowering. Temporal transcriptome analysis of developing, heat stressed (35°C) seeds during this window shows that Inositol-requiring enzyme 1 (IRE1)-mediated endoplasmic reticulum (ER) stress response and jasmonic acid (JA) pathways are the early (1-3 h) drivers of heat stress response. We propose that increased JA levels under heat stress may precede ER stress response as JA application promotes the spliced form of OsbZIP50, an ER response marker gene linked to IRE1-specific pathway. This study presents temporal and mechanistic insights into the role of JA and ER stress signalling during early heat stress response of rice seeds that impact both grain size and quality. Modulating the heat sensitivity of the early sensing pathways and downstream endosperm development genes can enhance rice resilience to transient heat stress events.

Unique Glutelin Expression Patterns and Seed Endosperm Structure Facilitate Glutelin Accumulation in Polyploid Rice Seed

BACKGROUND

Rice is not only an essential food but also a source of high quality protein. Polyploidy is an evolutionary trajectory in plants, and enhancing glutelin by polyploidization is an attractive strategy for improving the nutritional value of rice seeds and presents a great potential for enhancing the commercial value of rice. Elucidating the mechanisms underlying glutelin synthesis and accumulation in tetraploid rice is of great significance.

RESULTS

To enhance the nutritional value of rice, we developed tetraploid rice and evaluated the contents of various nutrient elements in mature seeds. The results revealed a significant increase in protein contents, including the total seed storage proteins, glutelins, and amino acids in tetraploid rice when compared with those in diploid rice. Tandem mass tag-based quantitative proteomic analyses of seeds revealed that glutelins regulated by several glutelin genes in 9311-4x were significantly up-regulated (≥1.5-fold), which was further verified by immunoblot analyses. In addition, temporal expression patterns of various glutelin subunits in different rice lines were investigated. The results revealed significant differences in the expression patterns between diploid and tetraploid rice seeds. Cytohistological analyses results revealed that the thickness of aleurone cell layers increased significantly by 32% in tetraploid rice, the structures of protein storage vacuoles (PSVs) in sub-aleurone cells were more diverse and abundant than those of diploid rice. Temporal expression and proteomic analyses results revealed that protein disulfide isomerase-like 1-1 expression levels were higher in tetraploid rice than in diploid rice, and that the gene responded to oxidative folding with increased levels of proglutelin and appropriate distribution of seed glutelins in tetraploid rice.

CONCLUSION

The results of the present study revealed that polyploidization increased glutelin content by influencing glutelin biosynthesis, transport, and deposition, while variations in glutelin accumulation between tetraploid and diploid rice were largely manifested in the initial time, duration, and relative levels of various glutelin gene expressions during seed filling stages. These findings provide novel insights into improving the protein quality and nutritional value of rice seeds by polyploid breeding.

Improvement of production yield and extraction efficacy of recombinant protein by high endosperm-specific expression along with simultaneous suppression of major seed storage proteins

Human transforming growth factor-β1 (hTGF-β1)　was produced in transgenic rice seeds. To boost its production yield and to extract it simply, it was expressed under the control of seed-specific promoters along with the simultaneous suppression of endogenous seed storage proteins　(SSPs)　through RNA interference (RNAi). When driven by the 26 kDa α-globulin endosperm-specific promoter, it accumulated up to the markedly high level of 452 μg/grain. However, exchange with other seed-specific promoters such as 18 kDa oleosin and AGPase promoters resulted in remarkable reduction to the levels of 62 and 48 μg/grain, respectively, even though endogenous SSPs were reduced to the similar level. These production levels were almost similar to those (42 and 108 μg/grain) produced by the glutelin GluB-1 endosperm-specific promoter and the maize ubiquitin constitutive promoter without reduction of SSPs, respectively. When extracted from these transgenic rice seeds with reduced SSPs with various buffers, it could be solubilized with denaturant solution, which was in remarkable contrast with those without depressed SSPs which required further supplementation of reducing agent for extraction. This difference was associated with the fact that it was mainly deposited to ER-derived structures though self-aggregation or interaction with remaining prolamin via intermolecular disulfide bonds.

The NAC Transcription Factors OsNAC20 and OsNAC26 Regulate Starch and Storage Protein Synthesis

Starch and storage proteins determine the weight and quality of cereal grains. Synthesis of these two grain components has been comprehensively investigated, but the transcription factors responsible for their regulation remain largely unknown. In this study, we investigated the roles of NAM, ATAF, and CUC (NAC) transcription factors, OsNAC20, and OsNAC26 in starch and storage protein synthesis in rice (Oryza sativa) endosperm. OsNAC20 and OsNAC26 showed high levels of amino acid sequence similarity. Both were localized in the aleurone layer, starchy endosperm, and embryo. Mutation of OsNAC20 or OsNAC26 alone had no effect on the grain, while the osnac20/26 double mutant had significantly decreased starch and storage protein content. OsNAC20 and OsNAC26 alone could directly transactivate the expression of starch synthaseI (SSI), pullulanase (Pul), glutelin A1 (GluA1), glutelin B4/5 (GluB4/5), α-globulin, and 16 kD prolamin and indirectly influenced plastidial disproportionating enzyme1 (DPE1) expression to regulate starch and storage protein synthesis. Although they could also bind to the promoters of ADP-Glc pyrophosphorylase small subunit 2b (AGPS2b), ADP-Glc pyrophosphorylase large subunit 2 (AGPL2), and starch branching enzymeI (SBEI), and the expression of the three genes was largely decreased in the osnac20/26 mutant, ADP-Glc pyrophosphorylase and starch branching enzyme activities were unchanged in this double mutant. In addition, OsNAC20 and OsNAC26 are main regulators of Pul, GluB4, α-globulin, and 16 kD prolamin In conclusion, OsNAC20 and OsNAC26 play an essential and redundant role in the regulation of starch and storage protein synthesis.

Deletion of maize RDM4 suggests a role in endosperm maturation as well as vegetative and stress-responsive growth

Opaque kernels in maize may result from mutations in many genes, such as OPAQUE-2. In this study, a maize null mutant of RNA-DIRECTED DNA METHYLATION 4 (RDM4) showed an opaque kernel phenotype, as well as plant developmental delay, male sterility, and altered response to cold stress. We found that in opaque kernels, all zein proteins were reduced and amino acid content was changed, including increased lysine. Transcriptomic and proteomic analysis confirmed the zein reduction and proteomic rebalancing of non-zein proteins, which was quantitatively and qualitatively different from opaque-2. Global transcriptional changes were found in endosperm and leaf, including many transcription factors and tissue-specific expressed genes. Furthermore, of the more than 8000 significantly differentially expressed genes in wild type in response to cold, a significant proportion (25.9% in moderate cold stress and 40.8% in near freezing stress) were not differentially expressed in response to cold in rdm4, suggesting RDM4 may participate in regulation of abiotic stress tolerance. This initial characterization of maize RDM4 provides a basis for further investigating its function in endosperm and leaf, and as a regulator of normal and stress-responsive development.

Evaluation of the Quality of a High-Resistant Starch and Low-Glutelin Rice (Oryza sativa L.) Generated through CRISPR/Cas9-Mediated Targeted Mutagenesis

A high-resistant starch (RS) and low-glutelin diet is beneficial for the health of patients with diabetes and kidney diseases. Rice is an important food crop worldwide. Previous studies have demonstrated that downregulating the expression of rice starch branching enzyme IIb (SBEIIb) affected the composition and the structure of starch. However, there has been no report about generating the loss-of-function mutants of SBEIIb using low-glutelin rice cultivars as recipients. In this study, we adopted a CRISPR/Cas9 system to induce site-specific mutations at the SBEIIb locus in an elite low-glutelin japonica rice cultivar derived from Low Glutelin Content-1 (LGC-1) and successfully obtained two independent transgene-free sbeIIb/Lgc1 mutant lines. In the mutant lines, the apparent amylose content (AAC) was increased by approximately 1.8-fold and the RS content reached approximately 6%. The glutelin content was approximately 2%, maintaining the low-glutelin trait of the recipient cultivar. The formation mechanism of RS was explored by analyzing the fine structures and the properties of starch. According to the X-ray diffraction pattern and the increased lipid content, the high RS content of the sbeIIb/Lgc1 lines was attributed to the increased content of amylose-lipid complex. Further analyses of the nutritional quality revealed that the soluble sugar and lipid contents, especially sucrose and unsaturated fatty acids, increased in the sbeIIb/Lgc1 lines significantly. This research is expected to facilitate the cultivation and the application of functional rice suitable for patients with diabetes and kidney diseases.

Genetic Manipulation for Improved Nutritional Quality in Rice

Food with higher nutritional value is always desired for human health. Rice is the prime staple food in more than thirty developing countries, providing at least 20% of dietary protein, 3% of dietary fat and other essential nutrients. Several factors influence the nutrient content of rice which includes agricultural practices, post-harvest processing, cultivar type as well as manipulations followed by selection through breeding and genetic means. In addition to mutation breeding, genetic engineering approach also contributed significantly for the generation of nutrition added varieties of rice in the last decade or so. In the present review, we summarize the research update on improving the nutritional characteristics of rice by using genetic engineering and mutation breeding approach. We also compare the conventional breeding techniques of rice with modern molecular breeding techniques toward the generation of nutritionally improved rice variety as compared to other cereals in areas of micronutrients and availability of essential nutrients such as folate and iron. In addition to biofortification, our focus will be on the efforts to generate low phytate in seeds, increase in essential fatty acids or addition of vitamins (as in golden rice) all leading to the achievements in rice nutrition science. The superiority of biotechnology over conventional breeding being already established, it is essential to ascertain that there are no serious negative agronomic consequences for consumers with any difference in grain size or color or texture, when a nutritionally improved variety of rice is generated through genetic engineering technology.

Comparison of wheat, soybean, rice, and pea protein properties for effective applications in food products

Pea and rice proteins are promising to substitute allergenic proteins, and increasingly, play important roles in the food industry because of their hypoallergenic characteristics and nutritional value. However, manufacturers generally provide limited functionality information on these proteins. Therefore, this study comprehensively compared functional properties of wheat, soybean, rice, and pea proteins for their industrial applications and illustrated correlation among various functionalities. Results showed that protein solubility (PS) was highly related to its water absorption (WA) capacity, emulsifying activity index (EAI), and emulsion stability index (ESI). The overall functionality of pea protein was close to that of soybean protein while rice protein cannot match with all other proteins. sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis indicated the composition of each protein was unique. While the deconvolution of the amide I band of the Raman spectra indicated soybean and pea proteins that shared similar features, but they were different from that of wheat and rice proteins. PRACTICAL APPLICATIONS: Due to the allergenicity of wheat and soybean proteins, food manufacturers are looking for alternative protein sources. Rice and Pea proteins are promising substitutes because of their "allergen-friendly" as well as their emergence in the food market. This study provided a comprehensive comparison of the functionality of commercially available wheat, soybean, rice, and pea proteins. The information presented in this study would be helpful to food scientists, scholars, or engineers when they develop appropriate application of various proteins in food products.

Proteomic analysis of reserve proteins in commercial rice cultivars

Rice consumption is rising in western countries with the adoption of new nutritional styles, which require the avoidance of gluten. Nevertheless, there are reports of rice allergic reactions. Rice grains contain a low amount of proteins most of which are storage proteins represented by glutelins, prolamins, albumins, and globulins. Some of these proteins are seed allergenic proteins as α-amylase/trypsin inhibitor, globulins, β-glyoxylase, and several glutelins. Italy is the major rice producer in Europe, and for this, seed reserve proteins of four Italian rice cultivars were characterized by 2D-GE analysis. Some differentially abundant proteins were identified and classified as allergenic proteins, prompting a further characterization of the genes encoding some of these proteins. In particular, a deletion in the promoter region of the 19 KDa globulin gene has been identified, which may be responsible for the different abundance of the protein in the Karnak cultivar. This polymorphism can be applied for cultivar identification in commercial samples. Seed proteome was characterized by a variable combination of several proteins, which may determine a different allergenic potential. Proteomic and genomic allowed to identify the protein profile of four commercial cultivars and to develop a molecular marker useful for the analysis of commercial products.

Genome-Wide Association Analysis Dissects the Genetic Basis of the Grain Carbon and Nitrogen Contents in Milled Rice

BACKGROUND

Carbon (C) and nitrogen (N) are two fundamental components of starch and protein, which are important determinants of grain yield and quality. The food preferences of consumers and the expected end-use of grains in different rice-growing regions require diverse varieties that differ in terms of the grain N content (GNC) and grain C content (GCC) of milled rice. Thus, it is important that quantitative trait loci (QTLs)/genes with large effects on the variation of GNC and GCC are identified in breeding programs.

RESULTS

To dissect the genetic basis of the variation of GNC and GCC in rice, the Dumas combustion method was used to analyze 751 diverse accessions regarding the GNC, GCC, and C/N ratio of the milled grains. The GCC and GNC differed significantly among the rice subgroups, especially between Xian/Indica (XI) and Geng/Japonica (GJ). Interestingly, in the GJ subgroup, the GNC was significantly lower in modern varieties (MV) than in landraces (LAN). In the XI subgroup, the GCC was significantly higher in MV than in LAN. One, six, and nine QTLs, with 55 suggestively associated single nucleotide polymorphisms, were detected for the GNC, GCC, and C/N ratio in three panels during a single-locus genome-wide association study (GWAS). Three of these QTLs were also identified in a multi-locus GWAS. We screened 113 candidate genes in the 16 QTLs in gene-based haplotype analyses. Among these candidate genes, LOC_Os01g06240 at qNC-1.1, LOC_Os05g33300 at qCC-5.1, LOC_Os01g04360 at qCN-1.1, and LOC_Os05g43880 at qCN-5.2 may partially explain the significant differences between the LAN and MV. These candidate genes should be cloned and may be useful for molecular breeding to rapidly improve the GNC, GCC, and C/N ratio of rice.

CONCLUSIONS

Our findings represent valuable information regarding the genetic basis of the GNC and GCC and may be relevant for enhancing the application of favorable haplotypes of candidate genes for the molecular breeding of new rice varieties with specific grain N and C contents.

Comprehensive Analysis of the Lysine Succinylome and Protein Co-modifications in Developing Rice Seeds

Lysine succinylation has been recognized as a post-translational modification (PTM) in recent years. It is plausible that succinylation may have a vaster functional impact than acetylation because of bulkier structural changes and more significant charge differences on the modified lysine residue. Currently, however, the quantity and identity of succinylated proteins and their corresponding functions in cereal plants remain largely unknown. In this study, we estimated the native succinylation occupancy on lysine was between 2% to 10% in developing rice seeds. Eight hundred fifty-four lysine succinylation sites on 347 proteins have been identified by a thorough investigation in developing rice seeds. Six motifs were revealed as preferred amino acid sequence arrangements for succinylation sites, and a noteworthy motif preference was identified in proteins associated with different biological processes, molecular functions, pathways, and domains. Remarkably, heavy succinylation was detected on major seed storage proteins, in conjunction with critical enzymes involved in central carbon metabolism and starch biosynthetic pathways for rice seed development. Meanwhile, our results showed that the modification pattern of in vitro nonenzymatically succinylated proteins was different from those of the proteins isolated from cells in Western blots, suggesting that succinylation is not generated via nonenzymatic reaction in the cells, at least not completely. Using the acylation data obtained from the same rice tissue, we mapped many sites harboring lysine succinylation, acetylation, malonylation, crotonylation, and 2-hydroxisobutyrylation in rice seed proteins. A striking number of proteins with multiple modifications were shown to be involved in critical metabolic events. Given that these modification moieties are intermediate products of multiple cellular metabolic pathways, these targeted lysine residues may mediate the crosstalk between different metabolic pathways via modifications by different moieties. Our study exhibits a platform for extensive investigation of molecular networks administrating cereal seed development and metabolism via PTMs.

NF-YC12 is a key multi-functional regulator of accumulation of seed storage substances in rice

Starch and storage proteins, the primary storage substances of cereal endosperm, are a major source of food for humans. However, the transcriptional regulatory networks of the synthesis and accumulation of storage substances remain largely unknown. Here, we identified a rice endosperm-specific gene, NF-YC12, that encodes a putative nuclear factor-Y transcription factor subunit C. NF-YC12 is expressed in the aleurone layer and starchy endosperm during grain development. Knockout of NF-YC12 significantly decreased grain weight as well as altering starch and protein accumulation and starch granule formation. RNA-sequencing analysis revealed that in the nf-yc12 mutant genes related to starch biosynthesis and the metabolism of energy reserves were enriched in the down-regulated category. In addition, starch and protein contents in seeds differed between NF-YC12-overexpression lines and the wild-type. NF-YC12 was found to interact with NF-YB1. ChIP-qPCR and yeast one-hybrid assays showed that NF-YC12 regulated the rice sucrose transporter OsSUT1 in coordination with NF-YB1 in the aleurone layer. In addition, NF-YC12 was directly bound to the promoters of FLO6 (FLOURY ENDOSPERM6) and OsGS1;3 (glutamine synthetase1) in developing endosperm. This study demonstrates a transcriptional regulatory network involving NF-YC12, which coordinates multiple pathways to regulate endosperm development and the accumulation of storage substances in rice seeds.

CRISPR/Cas9 editing of three CRUCIFERIN C homoeologues alters the seed protein profile in Camelina sativa

BACKGROUND

The oilseed Camelina sativa is grown for a range of applications, including for biofuel, biolubricants, and as a source of omega-3 fatty acids for the aquaculture feed industry. The seed meal co-product is used as a source of protein for animal feed; however, the low value of the meal hinders profitability and more widespread application of camelina. The nutritional quality of the seed meal is largely determined by the abundance of specific seed storage proteins and their amino acid composition. Manipulation of seed storage proteins has been shown to be an effective means for either adjustment of nutritional content of seeds or for enhancing accumulation of high-value recombinant proteins in seeds.

RESULTS

CRISPR/Cas9 gene editing technology was used to generate deletions in the first exon of the three homoeologous genes encoding the seed storage protein CRUCIFERIN C (CsCRUC), creating an identical premature stop-codon in each and resulting in a CsCRUC knockout line. The mutant alleles were detected by applying a droplet digital PCR drop-off assay. The quantitative nature of this technique is particularly valuable when applied to polyploid species because it can accurately determine the number of mutated alleles in a gene family. Loss of CRUC protein did not alter total seed protein content; however, the abundance of other cruciferin isoforms and other seed storage proteins was altered. Consequently, seed amino acid content was significantly changed with an increase in the proportion of alanine, cysteine and proline, and decrease of isoleucine, tyrosine and valine. CsCRUC knockout seeds did not have changed total oil content, but the fatty acid profile was significantly altered with increased relative abundance of all saturated fatty acids.

CONCLUSIONS

This study demonstrates the plasticity of the camelina seed proteome and establishes a CRUC-devoid line, providing a framework for modifying camelina seed protein composition. The results also illustrate a possible link between the composition of the seed proteome and fatty acid profile.

Glutelin subtype-dependent protein localization in rice grain evidenced by immunodetection analyses

GluA and GluB-4/5 glutelin subfamilies are mainly localized to outer region of the endosperm, particularly in its ventral side, in rice grain, but GluC is localized to throughout the endosperm. The major seed storage protein in rice (Oryza sativa) is glutelin, which forms a vacuole-derived protein body type-II. Glutelins are encoded by multiple genes, and generally comprise four protein subfamilies, namely, GluA, GluB, GluC, and GluD: however, the localization pattern of glutelin in rice grains remains obscure. In this study, we investigated the localization pattern of five subtypes of the glutelin protein in rice grains using glutelin-subtype specific antibodies. Immunoblot analysis against sequentially polished rice flour fractions from three crop years and seven japonica rice varieties revealed that GluA was strongly localized in the outer region of the endosperm, including the subaleurone layer, whereas GluC was distributed throughout the endosperm. Among the glutelin subtypes, GluA was mostly found in the outer region of the rice grain, followed by GluB-4/5, GluB-1, GluD, and GluC. Immunofluorescence labeling microscopy analysis using immature rice seeds clearly revealed that the localization pattern of GluC and GluD was completely different from that of GluA and GluB. Expression levels of all glutelins, particularly GluA, GluB-1, and GluB-4/5, were stronger on the ventral than dorsal side in rice grains. These results provide strong and consistent evidence that glutelins localize to the rice grain in a subfamily-dependent manner.

Natural variation of OsGluA2 is involved in grain protein content regulation in rice

Grain protein content (GPC) affects rice nutrition quality. Here, we identify two stable quantitative trait loci (QTLs), qGPC-1 and qGPC-10, controlling GPC in a mapping population derived from indica and japonica cultivars crossing. Map-based cloning reveals that OsGluA2, encoding a glutelin type-A2 precursor, is the candidate gene underlying qGPC-10. It functions as a positive regulator of GPC and has a pleiotropic effect on rice grain quality. One SNP located in OsGluA2 promoter region is associated with its transcript expression level and GPC diversity. Polymorphisms of this nucleotide can divide all haplotypes into low (OsGluA2^LET) and high (OsGluA2^HET) expression types. Population genetic and evolutionary analyses reveal that OsGluA2^LET, mainly present in japonica accessions, originates from wild rice. However, OsGluA2^HET, the dominant type in indica, is acquired through mutation of OsGluA2^LET. Our results shed light on the understanding of natural variations of GPC between indica and japonica subspecies.

Grain protein content determines rice nutrition quality. Here, the authors show that a single nucleotide polymorphism in the promoter region of OsGluA2, encoding a glutelin type-A2 precursor, is responsible for glutelin content difference between the indica and japonica rice subspecies.

Applications of the CRISPR/Cas9 System for Rice Grain Quality Improvement: Perspectives and Opportunities

Grain quality improvement is a key target for rice breeders, along with yield. It is a multigenic trait that is simultaneously influenced by many factors. Over the past few decades, breeding for semi-dwarf cultivars and hybrids has significantly contributed to the attainment of high yield demands but reduced grain quality, which thus needs the attention of researchers. The availability of rice genome sequences has facilitated gene discovery, targeted mutagenesis, and revealed functional aspects of rice grain quality attributes. Some success has been achieved through the application of molecular markers to understand the genetic mechanisms for better rice grain quality; however, researchers have opted for novel strategies. Genomic alteration employing genome editing technologies (GETs) like clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) for reverse genetics has opened new avenues of research in the life sciences, including for rice grain quality improvement. Currently, CRISPR/Cas9 technology is widely used by researchers for genome editing to achieve the desired biological objectives, because of its simple targeting. Over the past few years many genes that are related to various aspects of rice grain quality have been successfully edited via CRISPR/Cas9 technology. Interestingly, studies on functional genomics at larger scales have become possible because of the availability of GETs. In this review, we discuss the progress made in rice by employing the CRISPR/Cas9 editing system and its eminent applications. We also elaborate possible future avenues of research with this system, and our understanding regarding the biological mechanism of rice grain quality improvement.

Overexpressing wheat low-molecular-weight glutenin subunits in rice (Oryza sativa L. japonica cv. Koami) seeds

Genes encoding wheat low-molecular-weight glutenin subunits (LMW-GSs) that confer dough strength and extensibility were previously identified from Korean wheat cultivars. To improve low viscoelasticity of rice (Oryza sativa L.) dough caused by the lack of seed storage proteins comparable to wheat gluten, two genes, LMW03 and LMW28, encoding LMW-GSs are cloned from Korean wheat cultivar Jokyoung. The LMW genes are inserted into binary vectors under the control of the rice endosperm-specific Glu-B1 promoter. Transgenic rice plants expressing LMW03 or LMW28 in their seeds are generated using Agrobacterium-mediated transformation. The expression of recombinant wheat LMW-GS in the transgenic rice seeds was confirmed by SDS-PAGE and immunoblot analysis. Their accumulation in the endosperm and aleurone layers of rice seeds was observed through in situ immuno-hybridization.

Phenotypic and molecular dissection of grain quality using the USDA rice mini-core collection

Grain quality is a major breeding objective and paramount to food production. This study was aimed to phenotypically and molecularly dissect the rice grain quality, especially amylose content (AC), grain protein content (GPC) and alkali spreading value (ASV), using the USDA rice mini-core collection representing the world-wide rice germplasm lines. Grain chemical analysis combined with genome-wide association study (GWAS) was used for the study. A wide genetic variation was observed for these grain quality traits in the mini-core collection. Germplasm lines unique in AC, GPC and ASV and desirable for grain quality improvement were identified. The genetic diversity of the collection was re-analyzed using new SNPs, thus providing a more precise genotypic information about the collection. Furthermore, ten loci significantly associated with these grain quality traits were identified through GWAS using 22947 high-quality SNPs. These results, therefore, provide knowledge, resources and molecular tools for efficient rice grain quality improvement.

Russell-Like Bodies in Plant Seeds Share Common Features With Prolamin Bodies and Occur Upon Recombinant Protein Production

Although many recombinant proteins have been produced in seeds at high yields without adverse effects on the plant, endoplasmic reticulum (ER) stress and aberrant localization of endogenous or recombinant proteins have also been reported. The production of murine interleukin-10 (mIL-10) in Arabidopsis thaliana seeds resulted in the de novo formation of ER-derived structures containing a large fraction of the recombinant protein in an insoluble form. These bodies containing mIL-10 were morphologically similar to Russell bodies found in mammalian cells. We confirmed that the compartment containing mIL-10 was enclosed by ER membranes, and 3D electron microscopy revealed that these structures have a spheroidal shape. Another feature shared with Russell bodies is the continued viability of the cells that generate these organelles. To investigate similarities in the formation of Russell-like bodies and the plant-specific protein bodies formed by prolamins in cereal seeds, we crossed plants containing ectopic ER-derived prolamin protein bodies with a line accumulating mIL-10 in Russell-like bodies. This resulted in seeds containing only one population of protein bodies in which mIL-10 inclusions formed a central core surrounded by the prolamin-containing matrix, suggesting that both types of protein aggregates are together removed from the secretory pathway by a common mechanism. We propose that, like mammalian cells, plant cells are able to form Russell-like bodies as a self-protection mechanism, when they are overloaded with a partially transport-incompetent protein, and we discuss the resulting challenges for recombinant protein production.

Compensatory rebalancing of rice prolamins by production of recombinant prolamin/bioactive peptide fusion proteins within ER-derived protein bodies

Bioactive peptide was produced by fusion to rice prolamins in transgenic rice seeds. Their accumulation levels were affected by their deposition sites and by compensatory rebalancing between prolamins within PB-Is. Peptide immunotherapy using analogue peptide ligands (APLs) is one of promising treatments against autoimmune diseases. Use of seed storage protein as a fusion carrier is reasonable strategy for production of such small size bioactive peptides. In this study, to examine the efficacy of various rice prolamins deposited in ER-derived protein bodies (PB-Is), the APL12 from the Glucose-6-phosphate isomerase (GPI325-339) was expressed by fusion to four types of representative prolamins under the control of the individual native promoters. When the 14 and 16 kDa Cys-rich prolamins, which were localized in middle layer of PB-Is, were used for production of the APL12, they highly accumulated in transgenic rice seeds (~ 200 µg/grain). By contrast, fusion to the 10 and 13 kDa prolamins, which were localized in the core and outermost layer of PB-Is, resulted in lower levels of accumulation (~ 40 µg/grain). These results suggest that accumulation levels were highly affected by their deposition sites. Next, when different prolamin/APL12 fusion proteins were co-expressed to increase accumulation levels, they could not be increased so much as their expected additive levels. High accumulation of one type prolamin/APL12 led to reduction of other type(s) prolamin/APL12 to maintain the limited amounts of prolamins that can be deposited in PB-Is. Moreover, suppression of endogenous seed proteins by RNA interference also did not significantly enhance the accumulation levels of prolamin/APL12. These findings suggest that there may be compensatory rebalancing mechanism that controls the accumulation levels of prolamins deposited within PB-Is.