Modulation of kernel storage proteins in grain sorghum (Sorghum bicolor (L.) Moench)

Manifestation of agronomically valuable traits in the progeny of a sorghum mutant carrying the genetic construct for RNA silencing of the γ-kafirin gene

Improving the nutritional value of grain sorghum, a drought- and heat-tolerant grain crop, is an important task in the context of global warming. One of the reasons for the low nutritional value of sorghum grain is the resistance of its storage proteins (kafirins) to proteolytic digestion, which is due, among other things, to the structural organization of protein bodies, in which γ-kafirin, the most resistant to proteases, is located on the periphery, encapsulating more easily digested α-kafirins. The introduction of genetic constructs capable of inducing RNA silencing of the γ-kafirin (gKAF1) gene opens up prospects for solving this problem. Using Agrobacterium-mediated genetic transformation of immature embryos of the grain sorghum cv. Avans we have obtained a mutant with improved digestibility of endosperm proteins (up to 92 %) carrying a genetic construct for RNA silencing of the gKAF1 gene. The goal of this work was to study the stability of inheritance of the introduced genetic construct in T2-T4 generations, to identify the number of its copies, as well as to trace the manifestation of agronomically valuable traits in the offspring of the mutant. The mutant lines were grown in experimental plots in three randomized blocks. The studied lines were characterized by improved digestibility of kafirins, a modified type of endosperm, completely or partially devoid of the vitreous layer, an increased percentage of lysine (by 75 %), reduced plant height, peduncle length, 1000-grains weight, and grain yield from the panicle. In T2, a line with monogenic control of GA resistance was selected. qPCR analysis showed that in different T3 and T4 plants, the genetic construct was present in 2-4 copies. In T3, a line with a high digestibility of endosperm proteins (81 %) and a minimal decrease in agronomically valuable traits (by 5-7 %) was selected.

Effects of coarse cereals on dough and Chinese steamed bread - a review

Chinese steamed breads (CSBs) are long-established staple foods in China. To enhance the nutritional value, coarse cereals such as oats, buckwheat, and quinoa have been added to the formulation for making CSBs. This review presents the nutritional value of various coarse cereals and analyses the interactions between the functional components of coarse cereals in the dough. The addition of coarse cereals leads to changes in the rheological, fermentation, and pasting aging properties of the dough, which further deteriorates the appearance and texture of CSBs. This review can provide some suggestions and guidelines for the production of staple and nutritious staple foods.

Large and stable genome edits at the sorghum alpha kafirin locus result in changes in chromatin accessibility and globally increased expression of genes encoding lysine enrichment

INTRODUCTION

Sorghum is a resilient and widely cultivated grain crop used for feed and food. However, it's grain is deficient in lysine, an essential amino acid. This is due to the primary seed storage proteins, the alpha-kafirins, lacking lysine. It has been observed that reductions in alpha-kafirin protein results in rebalancing of the seed proteome and a corresponding increase in non-kafirin proteins which leads to an increased lysine content. However, the mechanisms underlying proteome rebalancing are unclear. This study characterizes a previously developed gene edited sorghum line, with deletions at the alpha kafirin locus.

METHODS

A single consensus guide RNA leads to tandem deletion of multiple members of the gene family in addition to the small target site mutations in remaining genes. RNA-seq and ATAC-seq were utilized to identify changes in gene expression and chromatin accessibility in developing kernels in the absence of most alpha-kafirin expression.

RESULTS

Several differentially accessible chromatin regions and differentially expressed genes were identified. Additionally, several genes upregulated in the edited sorghum line were common with their syntenic orthologues differentially expressed in maize prolamin mutants. ATAC-seq showed enrichment of the binding motif for ZmOPAQUE 11, perhaps indicating the transcription factor's involvement in the kernel response to reduced prolamins.

DISCUSSION

Overall, this study provides a resource of genes and chromosomal regions which may be involved in sorghum's response to reduced seed storage proteins and the process of proteome rebalancing.

CRISPR-knockout of β-kafirin in sorghum does not recapitulate the grain quality of natural mutants

When gene editing was applied to knockout beta-kafirin, there was a compensatory increase of gamma-kafirin which does not occur in domesticated null varieties, so enhanced grain quality was not achieved. Sorghum bicolor is an important animal feedstock cereal crop throughout Australia and the southern United States, where its use as a food product is limited by issues with low calorific and nutritive value. Qualities such as reduced digestibility and low essential amino acid content are directly attributed to the kafirin grain storage proteins, the major components of protein bodies within the endosperm. Specifically, the β- and γ-kafirins have few protease cleavage sites and high levels of cysteine residues which lead to a highly cross-linked shell of intra- and inter-molecular disulphide linkages that encapsulate the more digestible α- and δ-kafirins in the core of the protein bodies. Naturally occurring β-kafirin mutants exist and are known to have improved grain quality, with enhanced protein contents and digestibility, traits which are often attributed to the lack of this cysteine-rich kafirin in the mature grain. However, when CRISPR/Cas9 editing was used to create β-kafirin knockout lines, there was no improvement to grain quality in the Tx430 background, although they did have unique protein composition and changes to protein body morphology in the vitreous endosperm. One explanation of the divergence in quality traits found the lines lacking β-kafirin are due to a drastic increase of γ-kafirin which was only found in the gene edited lines. This study highlights that in some germplasm, there is a level of redundancy between the peripheral kafirins, and that improvement of grain protein digestibility cannot be achieved by simply removing the β-kafirin protein in all genetic backgrounds.

Genome Editing for Sustainable Agriculture in Africa

Sustainable intensification of agriculture in Africa is essential for accomplishing food and nutritional security and addressing the rising concerns of climate change. There is an urgent need to close the yield gap in staple crops and enhance food production to feed the growing population. In order to meet the increasing demand for food, more efficient approaches to produce food are needed. All the tools available in the toolbox, including modern biotechnology and traditional, need to be applied for crop improvement. The full potential of new breeding tools such as genome editing needs to be exploited in addition to conventional technologies. Clustered regularly interspaced short palindromic repeats/CRISPR-associated protein (CRISPR/Cas)-based genome editing has rapidly become the most prevalent genetic engineering approach for developing improved crop varieties because of its simplicity, efficiency, specificity, and easy to use. Genome editing improves crop variety by modifying its endogenous genome free of any foreign gene. Hence, genome-edited crops with no foreign gene integration are not regulated as genetically modified organisms (GMOs) in several countries. Researchers are using CRISPR/Cas-based genome editing for improving African staple crops for biotic and abiotic stress resistance and improved nutritional quality. Many products, such as disease-resistant banana, maize resistant to lethal necrosis, and sorghum resistant to the parasitic plant Striga and enhanced quality, are under development for African farmers. There is a need for creating an enabling environment in Africa with science-based regulatory guidelines for the release and adoption of the products developed using CRISPR/Cas9-mediated genome editing. Some progress has been made in this regard. Nigeria and Kenya have recently published the national biosafety guidelines for the regulation of gene editing. This article summarizes recent advances in developments of tools, potential applications of genome editing for improving staple crops, and regulatory policies in Africa.

Progress and challenges in sorghum biotechnology, a multipurpose feedstock for the bioeconomy

Sorghum [Sorghum bicolor (L.) Moench] is the fifth most important cereal crop globally by harvested area and production. Its drought and heat tolerance allow high yields with minimal input. It is a promising biomass crop for the production of biofuels and bioproducts. In addition, as an annual diploid with a relatively small genome compared with other C4 grasses, and excellent germplasm diversity, sorghum is an excellent research species for other C4 crops such as maize. As a result, an increasing number of researchers are looking to test the transferability of findings from other organisms such as Arabidopsis thaliana and Brachypodium distachyon to sorghum, as well as to engineer new biomass sorghum varieties. Here, we provide an overview of sorghum as a multipurpose feedstock crop which can support the growing bioeconomy, and as a monocot research model system. We review what makes sorghum such a successful crop and identify some key traits for future improvement. We assess recent progress in sorghum transformation and highlight how transformation limitations still restrict its widespread adoption. Finally, we summarize available sorghum genetic, genomic, and bioinformatics resources. This review is intended for researchers new to sorghum research, as well as those wishing to include non-food and forage applications in their research.

Inheritance of the genetic construct for RNA-silencing of the γ-kafirin gene (gKAF1) in the progeny of transgenic sorghum plants

Sorghum is one of the most important cereal crops in drought regions of the Globe. However, a number of constrains, such as poor nutritional value compared to other cereals, which is caused by resistance of its grain storage proteins (kafirins) to protease digestion, makes it less popular. The use of modern genetic technologies, such as RNA interference, opens up prospects for solving this problem. In this paper, we describe the inheritance of the genetic construct for RNAi silencing of the γ-kafirin gene (gKAF1) in the progeny of the mutant Avans-1/18, that we obtained previously by Agrobacterium-mediated genetic transformation of immature embryos of commercial cv. Avans. Inheritance of the genetic construct was traced by PCR-analysis to the nos-promoter governing expression of the marker gene bar and to the ubi1-intron, which is part of the genetic construct for silencing. It was found that the genetic construct inherited in T1 and T2 generations of the mutant grown both in the growth chamber and in the field plot, and in the F1 hybrids of the mutant with CMS-lines A2 KVV-181, A2 KVV-114, A2 O-1237, and fertile lines Volzhskoe-4v, Volzhskoe-615, O-1237. The studied plants from T1 and T2 generations had significantly higher in vitro protein digestibility than original non-transgenic cv. Avans. The vitreous endosperm in these plants was reduced either completely or manifested a faint layer. These data demonstrated that genetic construct for gKAF1 silencing was not only stably integrated in genome of the mutant plants but also expressed and improved the digestibility of endosperm proteins. In addition, in T1 progeny we found the plant, PCR positive for nos-promoter but PCR-negative for ubi1-intron, that had vitreous endosperm and was characterized by poor in vitro protein digestibility. These data are an example of instability of the genetic construct for RNAi-silencing in genome of some transgenic plants; they show close correlation of the floury endosperm type and high in vitro protein digestibility.

Protein bodies of the endoplasmic reticulum in Arabidopsis thaliana (Brassicaceae): origin, structural and biochemical features, functional significance

History of the discovery, formation, structural and biochemical traits of the protein bodies, derivatives of the granular endoplasmic reticulum (GER) that are known as ER-bodies, are reviewed. The functions of ER-bodies in cell vital activity mainly in Arabidopsis thaliana are reported. The highly specific component of ER-bodies, β-glucosidase enzyme, is described and its protecting role for plants under effect of abiotic and biotic factors is characterized. Based on the analytical review of the literature, it is shown that ER-bodies and the transcription factor NAI2 are unique to species of the family Brassicaceae. The specificity of the system GER – ER-bodies for Brassicaceae and thus the fundamental and applied importance of future research of mechanisms of its functioning in A. thaliana and other Brassicaceae species are emphasized.

Is the kafirin profile capable of modulating the ileal digestibility of amino acids in a soybean meal-sorghum diet fed to pigs?

The effects of kafirins on protein and amino acid ileal digestibility have not been evaluated in vivo in pigs. The aim of this study was to determine the effects of protein profile on apparent ileal digestibility (AID) of amino acids. We used a sorghum hybrid with low tannin content (<0.5%). The same hybrid was harvested from 2 different plots with different kafirin profile. Sorghum with greater content of total kafirins had less content of γ- and α1-kafirins and higher content of β- and α2-kafirins than that with lower content of total kafirins. Two sorghum-soybean meal (SBM) diets were formulated: 1) low kafirin (LK) content (32.2 g/kg) and 2) high kafirin (HK) content (48.1 g/kg). A control diet (maize-SBM) and a reference SBM-diet were also prepared. The reference diet was fed to all pigs following the experimental period and was used to estimate the AID of cereals by the difference method. "T" cannulas were fixed in the distal ileum of 18 barrows (6 by treatment), divided into 2 groups of 9 pigs. The pigs were fed 2.5 times their maintenance requirement of digestible energy (110 kcal/kg BW^0.75). The AID of dry matter, protein, amino acids, and energy of the experimental diets was measured; the AID of cereals (maize, LK sorghum and HK sorghum) was estimated by the difference method. The maize-SBM diet was more digestible than the sorghum-SBM diets, only with respect to valine (P < 0.05). The AID of valine in the maize-SBM diet was higher than that in sorghum-SBM diets. The changes in kafirin profile between the diets only affected the AID of threonine (P < 0.01), which decreased by 9.5 percentage units in LK diet compared with HK diet. Regarding the AID of cereals, maize exhibited greater AID than sorghum, with respect to valine (P < 0.01) and serine (P < 0.10). A comparison of sorghum with LK and HK content showed that the AID of threonine and serine increased by 50.5 (P < 0.001) and 19.2 percentage units (P < 0.05) in the latter, respectively. The higher content of γ-kafirins in LK sorghum negatively affected threonine and serine digestibility, implying that the AID of amino acids is affected more by the profile than the content of kafirins.

Exploring the nutritional and phytochemical potential of sorghum in food processing for food security

Purpose

Sorghum is quite comparable to wheat, rich source of nutrients with various health benefits, and therefore considered as a grain of future. The purpose of this paper is to review the bioactive active compounds, health benefits and processing of the sorghum. Sorghum is utilized for animal feeding rather than the human food usage. Therefore, this paper focuses on the emerging new health foods with benefits of the sorghum.

Design/methodology/approach

Major well-known bibliometric information sources searched were the Web of Science, Google Scholar, Scopus and PubMed. Several keywords like nutritional value of sorghum, bioactive compounds present in sorghum, health benefits of sorghum and processing of sorghum were chosen to obtain a large range of papers to be analyzed. A final inventory of 91 scientific sources was made after sorting and classifying them according to different criteria based on topic, academic field country of origin and year of publication.

Findings

From the literature reviewed, sorghum processing through various methods, including milling, malting, fermentation and blanching, bioactive compounds, as well as health benefits of sorghum were found and discussed.

Originality/value

Through this paper, possible processing methods and health benefits of sorghum are discussed after detailed studies of literature from journal articles.

A Comparative Study of Selected Physical and Biochemical Traits of Wild-Type and Transgenic Sorghum to Reveal Differences Relevant to Grain Quality

Transgenic sorghum featuring RNAi suppression of certain kafirins was developed recently, to address the problem of poor protein digestibility in the grain. However, it was not firmly established if other important quality parameters were adversely affected by this genetic intervention. In the present study several quality parameters were investigated by surveying several important physical and biochemical grain traits. Important differences in grain weight, density and endosperm texture were found that serve to differentiate the transgenic grains from their wild-type counterpart. In addition, ultrastructural analysis of the protein bodies revealed a changed morphology that is indicative of the effect of suppressed kafirins. Importantly, lysine was found to be significantly increased in one of the transgenic lines in comparison to wild-type; while no significant changes in anti-nutritional factors could be detected. The results have been insightful for demonstrating some of the corollary changes in transgenic sorghum grain, that emerge from imposed kafirin suppression.

Sorghum (Sorghum bicolor L.): Nutrients, bioactive compounds, and potential impact on human health

Sorghum is the fifth most produced cereal in the world and is a source of nutrients and bioactive compounds for the human diet. We summarize the recent findings concerning the nutrients and bioactive compounds of sorghum and its potential impact on human health, analyzing the limitations and positive points of the studies and proposing directions for future research. Sorghum is basically composed of starch, which is more slowly digested than that of other cereals, has low digestibility proteins and unsaturated lipids, and is a source of some minerals and vitamins. Furthermore, most sorghum varieties are rich in phenolic compounds, especially 3-deoxyanthocyanidins and tannins. The results obtained in vitro and in animals have shown that phenolics compounds and fat soluble compounds (polycosanols) isolated from sorghum benefit the gut microbiota and parameters related to obesity, oxidative stress, inflammation, diabetes, dyslipidemia, cancer, and hypertension. The effects of whole sorghum and its fractions on human health need to be evaluated. In conclusion, sorghum is a source of nutrients and bioactive compounds, especially 3-deoxyanthocyanidins, tannins, and polycosanols, which beneficially modulate, in vitro and in animals, parameters related to noncommunicable diseases. Studies should be conducted to evaluate the effects of different processing on protein and starch digestibility of sorghum as well as on the profile and bioavailability of its bioactive compounds, especially 3-deoxyanthocyanidins and tannins. Furthermore, the benefits resulting from the interaction of bioactive compounds in sorghum and human microbiota should be studied.

Proteome rebalancing in transgenic Camelina occurs within the enlarged proteome induced by β-carotene accumulation and storage protein suppression

Oilseed crops are global commodities for their oil and protein seed content. We have engineered the oilseed Camelina sativa to exhibit increased protein content with a slight decrease in oil content. The introduction of a phytoene synthase gene with an RNAi cassette directed to suppress the storage protein 2S albumin resulted in seeds with an 11-24 % elevation in overall protein. The phytoene synthase cassette alone produced enhanced β-carotene content of an average 275 ± 6.10 μg/g dry seed and an overall altered seed composition of 11 % less protein and comparable nontransgenic amounts of both oil and carbohydrates. Stacking an RNAi to suppress the major 2S storage protein resulted in seeds that contain elevated protein and slight decrease in oil and carbohydrate amounts showing that Camelina rebalances its proteome within an enlarged protein content genotype. In both β-carotene enhanced seeds with/without RNAi2S suppression, the seed size was noticeably enlarged compared to nontransgenic counterpart seeds. Metabolic analysis of maturing seeds indicate that the enhanced β-carotene trait had the larger effect than the RNAi2S suppression on the seed metabolome. The use of a GRAS (generally regarded as safe) β-carotene as a visual marker in a floral dip transformation system, such as Camelina, might eliminate the need for costly regulatory and controversial antibiotic resistance markers. β-carotene enhanced RNAi2S suppressed Camelina seeds could be further developed as a rapid heterologous protein production platform in a nonfood crop leveraging its enlarged protein content and visual marker.

Molecular Approaches to Understand Nutritional Potential of Coarse Cereals

Coarse grains are important group of crops that constitutes staple food for large population residing primarily in the arid and semi-arid regions of the world. Coarse grains are designated as nutri-cereals as they are rich in essential amino acids, minerals and vitamins. In spite of having several nutritional virtues in coarse grain as mentioned above, there is still scope for improvement in quality parameters such as cooking qualities, modulation of nutritional constituents and reduction or elimination of anti-nutritional factors. Besides its use in traditional cooking, coarse grains have been used mainly in the weaning food preparation and other malted food production. Improvement in quality parameters will certainly increase consumer's preference for coarse grains and increase their demand. The overall genetic gain in quality traits of economic importance in the cultivated varieties will enhance their industrial value and simultaneously increase income of farmers growing these varieties. The urgent step for improvement of quality traits in coarse grains requires a detailed understanding of molecular mechanisms responsible for varied level of different nutritional contents in different genotypes of these crops. In this review we have discussed the progresses made in understanding of coarse grain biology with various omics tool coupled with modern breeding approaches and the current status with regard to our effort towards dissecting traits related to improvement of quality and nutritional constituents of grains.

Co-suppression of synthesis of major α-kafirin sub-class together with γ-kafirin-1 and γ-kafirin-2 required for substantially improved protein digestibility in transgenic sorghum

KEY MESSAGE

Co-suppressing major kafirin sub-classes is fundamental to improved protein digestibility and nutritional value of sorghum. The improvement is linked to an irregularly invaginated phenotype of protein bodies.

ABSTRACT

The combined suppression of only two genes, γ kafirin-1 (25 kDa) and γ-kafirin-2 (50 kDa), significantly increases sorghum kafirin in vitro digestibility. Co-suppression of a third gene, α-kafirin A1 (25 kDa), in addition to the two genes increases the digestibility further. The high-digestibility trait has previously only been obtained either through the co-suppression of six kafirin genes (α-A1, 25 kDa; α-B1, 19 kDa; α-B2, 22 kDa; γ-kaf1, 27 kDa; γ-kaf 2, 50 kDa; and δ-kaf 2, 18 kDa) or through random chemical-induced mutations (for example, the high protein digestibility mutant). We present further evidence that suppressing just three of these genes alters kafirin protein cross-linking and protein body microstructure to an irregularly invaginated phenotype. The irregular invaginations are consistent with high pepsin enzyme accessibility and hence high digestibility. The approach we adopted towards increasing sorghum protein digestibility appears to be an effective tool in improving the status of sorghum as a principal supplier of energy and protein in poor communities residing in marginal agro-ecological zones of Africa.

Recent advances in the study of prolamin storage protein organization and function

Prolamin storage proteins are the main repository for nitrogen in the endosperm of cereal seeds. These stable proteins accumulate at massive levels due to the high level expression from extensively duplicated genes in endoreduplicated cells. Such abundant accumulation is achieved through efficient packaging in endoplasmic reticulum localized protein bodies in a process that is not completely understood. Prolamins are also a key determinant of hard kernel texture in the mature seed; an essential characteristic of cereal grains like maize. However, deficiencies of key essential amino acids in prolamins result in relatively poor grain protein quality. The inverse relationship between prolamin accumulation and protein quality has fueled an interest in understanding the role of prolamins and other proteins in endosperm maturation. This article reviews recent technological advances that have enabled dissection of overlapping and non-redundant roles of prolamins, particularly the maize zeins. This has come through molecular characterization of mutants first identified many decades ago, selective down-regulation of specific zein genes or entire zein gene families, and most recently through combining deletion mutagenesis with current methods in genome and transcriptome profiling. Works aimed at understanding prolamin deposition and function as well as creating novel variants with improved nutritional and digestibility characteristics, are reported.

The wheat HMW-glutenin 1Dy10 gene promoter controls endosperm expression in Brachypodium distachyon

The grass species Brachypodium distachyon has emerged as a model system for the study of gene structure and function in temperate cereals. As a first demonstration of the utility of Brachypodium to study wheat gene promoter function, we transformed it with a T-DNA that included the uidA reporter gene under control of a wheat High-Molecular-Weight Glutenin Subunit (HMW-GS) gene promoter and transcription terminator. For comparison, the same expression cassette was introduced into wheat by biolistics. Histochemical staining for β-glucuronidase (GUS) activity showed that the wheat promoter was highly expressed in the endosperms of all the seeds of Brachypodium and wheat homozygous plants. It was not active in any other tissue of transgenic wheat, but showed variable and sporadic activity in a minority of styles of the pistils of four homozygous transgenic Brachypodium lines. The ease of obtaining transgenic Brachypodium plants and the overall faithfulness of expression of the wheat HMW-GS promoter in those plants make it likely that this model system can be used for studies of other promoters from cereal crop species that are difficult to transform.

Role of γ-kafirin in the formation and organization of kafirin microstructures

The possible importance of the cysteine-rich γ-prolamin in kafirin and zein functionality has been neglected. The role of γ-kafirin in organized microstructures was investigated in microparticles. Residual kafirin (total kafirin minus γ-kafirin) "microparticles" were non-discrete (amorphous mass of material), as viewed by electron microscopy and atomic force microscopy. Adding 15% γ-kafirin to residual kafirin resulted in the formation of a mixture of non-discrete material and nanosize discrete spherical structures. Adding 30% γ-kafirin to the residual kafirin resulted in discrete spherical nanosize particles. Fourier transform infrared spectroscopy indicated that γ-kafirin had a mixture of random-coil and β-sheet conformations, in contrast to total kafirin, which is mainly α-helical conformation. γ-Kafirin also had a very high glass transition temperature (Tg) (≈270 °C). The conformation and high Tg of γ-kafirin probably confer structural stability to kafirin microstructures. Because of its ability to form disulfide cross-links, γ-kafirin appears to be essential to form and stabilize organized microstructures.