Zein Storage Proteins

A multi-omics approach reveals a link between ribosomal protein alterations and proteome rebalancing in Arabidopsis thaliana seeds

The ability of seeds to restore their amino acid content and composition after the elimination of the most abundant seed storage proteins (SSPs) is well-documented, yet the underlying mechanisms remain unclear. To better understand how seeds compensate for major proteomic disruptions, we conducted a comprehensive analysis on an Arabidopsis mutant lacking the three most abundant SSPs, the cruciferins. Our initial findings indicated that carbon, nitrogen, and sulfur levels, as well as total protein and oil content, remained unchanged in these mutants suggesting rebalanced seeds. Transcriptomics and proteomics performed during seed maturation of Col-0 and the triple mutant revealed significant modulation in many components of the translational machinery, especially ribosomal proteins (RPs), and in the antioxidation response in the mutant. These findings suggest that RPs play a critical role in facilitating proteomic homeostasis during seed maturation when proteomic perturbation occurs. Biochemical and metabolic analyses of the triple mutant dry seeds revealed increased protein carbonylation and elevated glutathione levels further supporting the link between SSP accumulation and seed redox homeostasis. Overall, we propose that in response to significant proteomic perturbations, changes in the proteome and amino acid composition of seeds are accompanied by a broad remodeling of the translation apparatus. We postulate that these alterations are key elements in seed adaptability and robustness to large proteomic perturbations during seed maturation.

Nitrogen-dependent binding of the transcription factor PBF1 contributes to the balance of protein and carbohydrate storage in maize endosperm

Fluctuations in nitrogen (N) availability influence protein and starch levels in maize (Zea mays) seeds, yet the underlying mechanism is not well understood. Here, we report that N limitation impacted the expression of many key genes in N and carbon (C) metabolism in the developing endosperm of maize. Notably, the promoter regions of those genes were enriched for P-box sequences, the binding motif of the transcription factor prolamin-box binding factor 1 (PBF1). Loss of PBF1 altered accumulation of starch and proteins in endosperm. Under different N conditions, PBF1 protein levels remained stable but PBF1 bound different sets of target genes, especially genes related to the biosynthesis and accumulation of N and C storage products. Upon N-starvation, the absence of PBF1 from the promoters of some zein genes coincided with their reduced expression, suggesting that PBF1 promotes zein accumulation in the endosperm. In addition, PBF1 repressed the expression of sugary1 (Su1) and starch branching enzyme 2b (Sbe2b) under normal N supply, suggesting that, under N-deficiency, PBF1 redirects the flow of C skeletons for zein toward the formation of C compounds. Overall, our study demonstrates that PBF1 modulates C and N metabolism during endosperm development in an N-dependent manner.

A novel NAC family transcription factor SPR suppresses seed storage protein synthesis in wheat

The synthesis of seed storage protein (SSP) is mainly regulated at the transcriptional level. However, few transcriptional regulators of SSP synthesis have been characterized in common wheat (Triticum aestivum) owing to the complex genome. As the A genome donor of common wheat, Triticum urartu could be an elite model in wheat research considering its simple genome. Here, a novel NAC family transcription factor TuSPR from T. urartu was found preferentially expressed in developing endosperm during grain-filling stages. In common wheat transgenically overexpressing TuSPR, the content of total SSPs was reduced by c. 15.97% attributed to the transcription declines of SSP genes. Both in vitro and in vivo assays showed that TuSPR bound to the cis-element 5'-CANNTG-3' distributed in SSP gene promoters and suppressed the transcription. The homolog in common wheat TaSPR shared a conserved function with TuSPR on SSP synthesis suppression. The knock-down of TaSPR in common wheat resulted in 7.07%-20.34% increases in the total SSPs. Both TuSPR and TaSPR could be superior targets in genetic engineering to manipulate SSP content in wheat, and this work undoubtedly expands our knowledge of SSP gene regulation.

The regulation of zein biosynthesis in maize endosperm

We review the current knowledge regarding the regulation of zein storage proteins biosynthesis and protein body formation, which are crucial processes for the successful accumulation of nutrients in maize kernels. Storage proteins in the seeds of crops in the grass family (Poaceae) are a major source of dietary protein for humans. In maize (Zea mays), proteins are the second largest nutrient component in the kernels, accounting for ~ 10% of the kernel weight. Over half of the storage proteins in maize kernels are zeins, which lack two essential amino acids, lysine and tryptophan. This deficiency limits the use of maize proteins in the food and feed industries. Zeins are encoded by a large super-gene family. During endosperm development, zeins accumulate in protein bodies, which are derived from the rough endoplasmic reticulum. In recent years, our knowledge of the pathways of zein biosynthesis and their deposition within the endosperm has been greatly expanded. In this review, we summarize the current understanding of zeins, including the genes encoding these proteins, their expression patterns and transcriptional regulation, the process of protein body formation, and other biological processes affecting zein accumulation.

Transcriptional Regulation of Zein Gene Expression in Maize through the Additive and Synergistic Action of opaque2, Prolamine-Box Binding Factor, and O2 Heterodimerizing Proteins

Maize (Zea mays) zeins are some of the most abundant cereal seed storage proteins (SSPs). Their abundance influences kernel hardness but compromises its nutritional quality. Transcription factors regulating the expression of zein and other SSP genes in cereals are endosperm-specific and homologs of maize opaque2 (O2) and prolamine-box binding factor (PBF). This study demonstrates that the ubiquitously expressed transcription factors, O2 heterodimerizing proteins (OHPs), specifically regulate 27-kD γ-zein gene expression (through binding to an O2-like box in its promoter) and interact with PBF. The zein content of double mutants OhpRNAi;o2 and PbfRNAi;o2 and the triple mutant PbfRNAi;OhpRNAi;o2 is reduced by 83, 89, and 90%, respectively, compared with the wild type. The triple mutant developed the smallest zein protein bodies, which were merely one-tenth the wild type's size. Total protein levels in these mutants were maintained in a relatively constant range through proteome rebalancing. These data show that OHPs, O2, and PBF are master regulators of zein storage protein synthesis, acting in an additive and synergistic mode. The differential expression patterns of OHP and O2 genes may cause the slight differences in the timing of 27-kD γ-zein and 22-kD α-zein accumulation during protein body formation.

Proteome balancing of the maize seed for higher nutritional value

Most flowering plant seeds are composed of the embryo and endosperm, which are surrounded by maternal tissue, in particular the seed coat. Whereas the embryo is the dormant progeny, the endosperm is a terminal organ for storage of sugars and amino acids in proteins and carbohydrates, respectively. Produced in maternal leaves during photosynthesis, sugars, and amino acids are transported to developing seeds after flowering, and during germination they nourish early seedlings growth. Maize endosperm usually contains around 10% protein and 70% starch, and their composition ratio is rather stable, because it is strictly regulated through a pre-set genetic program that is woven by networks of many interacting or counteracting genes and pathways. Endosperm protein, however, is of low nutritional value due mainly to the high expression of the α-zein gene family, which encodes lysine-free proteins. Reduced levels of these proteins in the opaque 2 (o2) mutant and α-zein RNAi (RNA interference) transgenic seed is compensated by an increase of non-zein proteins, leading to the rebalancing of the nitrogen sink and producing more or less constant levels of total proteins in the seed. The same rebalancing of zeins and non-zeins has been observed for maize seeds bred for 30% protein. In contrast to the nitrogen sink, storage of sulfur is controlled through the accumulation of specialized sulfur-rich proteins in maize endosperm. Silencing the synthesis of α-zeins through RNAi fails to raise sulfur-rich proteins. Although overexpression of the methionine-rich δ-zein can increase the methionine level in seeds, it occurs at least in part at the expense of the cysteine-rich β- and γ-zeins, demonstrating a balance between cysteine and methionine in sulfur storage. Therefore, we propose that the throttle for the flow of sulfur is placed before the synthesis of sulfur amino acids when sulfur is taken up and reduced during photosynthesis.

Mutational breeding and genetic engineering in the development of high grain protein content

Cereals are the most important crops in the world for both human consumption and animal feed. Improving their nutritional values, such as high protein content, will have significant implications, from establishing healthy lifestyles to helping remediate malnutrition problems worldwide. Besides providing a source of carbohydrate, grain is also a natural source of dietary fiber, vitamins, minerals, specific oils, and other disease-fighting phytocompounds. Even though cereal grains contain relatively little protein compared to legume seeds, they provide protein for the nutrition of humans and livestock that is about 3 times that of legumes. Most cereal seeds lack a few essential amino acids; therefore, they have imbalanced amino acid profiles. Lysine (Lys), threonine (Thr), methionine (Met), and tryptophan (Trp) are among the most critical and are a limiting factor in many grain crops for human nutrition. Tremendous research has been put into the efforts to improve these essential amino acids. Development of high protein content can be outlined in four different approaches through manipulating seed protein bodies, modulating certain biosynthetic pathways to overproduce essential and limiting amino acids, increasing nitrogen relocation to the grain through the introduction of transgenes, and exploiting new genetic variance. Various technologies have been employed to improve protein content including conventional and mutational breeding, genetic engineering, marker-assisted selection, and genomic analysis. Each approach involves a combination of these technologies. Advancements in nutrigenomics and nutrigenetics continue to improve public knowledge at a rapid pace on the importance of specific aspects of food nutrition for optimum fitness and health. An understanding of the molecular basis for human health and genetic predisposition to certain diseases through human genomes enables individuals to personalize their nutritional requirements. It is critically important, therefore, to improve grain protein quality. Highly nutritious grain can be tailored to functional foods to meet the needs for both specific individuals and human populations as a whole.

Understanding the dissolution of α-zein in aqueous ethanol and acetic acid solutions

Zein is a corn prolamin that has broad industrial applications because of its unique physical properties. Currently, the high cost of extraction and purification, which is directly related to the dispersion of zein in different solvents, is the major bottleneck of the zein industry. Solution behaviors of zein have been studied for a long time. However, the physical nature of zein in different solvents remains unclear. In this study, small-angle X-ray scattering (SAXS), static light scattering (SLS), and rheology were combined to study the structure and protein-solvent interaction of α-zein in both acetic acid and aqueous ethanol solutions. We found that the like-dissolve-like rule, the partial unfolding, and the protonation of zein are all critical to understanding the solution behaviors. Zein holds an elongated conformation (i.e., prolate ellipsoid) in all solutions, as revealed from SAXS data. There is an "aging effect" for zein in aqueous ethanol solutions, as evidenced by the transition of Newtonian rheological profiles for fresh zein solutions to the non-Newtonian shear thinning behavior for zein solutions after storage at room temperature for 24 h. Such shear thinning behavior becomes more pronounced for zein solutions at higher concentrations. The SLS results clearly show that acetic acid is a better solvent to dissolve zein than aqueous ethanol solution, as supported by a more negative second virial coefficient. This is majorly caused by the protonation of the protein, which was further verified by the dissolution of zein in water (a nonsolvent for zein) with the addition of acids.

The maize high-lysine mutant opaque7 is defective in an acyl-CoA synthetase-like protein

Maize (Zea mays) has a large class of seed mutants with opaque or nonvitreous endosperms that could improve the nutritional quality of our food supply. The phenotype of some of them appears to be linked to the improper formation of protein bodies (PBs) where zein storage proteins are deposited. Although a number of genes affecting endosperm vitreousness have been isolated, it has been difficult to clone opaque7 (o7), mainly because of its low penetrance in many genetic backgrounds. The o7-reference (o7-ref) mutant arose spontaneously in a W22 inbred, but is poorly expressed in other lines. We report here the isolation of o7 with a combination of map-based cloning and transposon tagging. We first identified an o7 candidate gene by map-based cloning. The putative o7-ref allele has a 12-bp in-frame deletion of codons 350-353 in a 528-codon-long acyl-CoA synthetase-like gene (ACS). We then confirmed this candidate gene by generating another mutant allele from a transposon-tagging experiment using the Activator/Dissociation (Ac/Ds) system in a W22 background. The second allele, isolated from ∼1 million gametes, presented a 2-kb Ds insertion that resembles the single Ds component of double-Ds, McClintock's original Dissociation element, at codon 496 of the ACS gene. PBs exhibited striking membrane invaginations in the o7-ref allele and a severe number reduction in the Ds-insertion mutant, respectively. We propose a model in which the ACS enzyme plays a key role in membrane biogenesis, by taking part in protein acylation, and that altered PBs render the seed nonvitreous.

Gamma-zeins are essential for endosperm modification in quality protein maize

Essential amino acids like lysine and tryptophan are deficient in corn meal because of the abundance of zein storage proteins that lack these amino acids. A natural mutant, opaque 2 (o2) causes reduction of zeins, an increase of nonzein proteins, and as a consequence, a doubling of lysine levels. However, o2's soft inferior kernels precluded its commercial use. Breeders subsequently overcame kernel softness, selecting several quantitative loci (QTLs), called o2 modifiers, without losing the high-lysine trait. These maize lines are known as "quality protein maize" (QPM). One of the QTLs is linked to the 27-kDa gamma-zein locus on chromosome 7S. Moreover, QPM lines have 2- to 3-fold higher levels of the 27-kDa gamma-zein, but the physiological significance of this increase is not known. Because the 27- and 16-kDa gamma-zein genes are highly conserved in DNA sequence, we introduced a dominant RNAi transgene into a QPM line (CM105Mo2) to eliminate expression of them both. Elimination of gamma-zeins disrupts endosperm modification by o2 modifiers, indicating their hypostatic action to gamma-zeins. Abnormalities in protein body structure and their interaction with starch granules in the F1 with Mo2/+; o2/o2; gammaRNAi/+ genotype suggests that gamma-zeins are essential for restoring protein body density and starch grain interaction in QPM. To eliminate pleiotropic effects caused by o2, the 22-kDa alpha-zein, gamma-zein, and beta-zein RNAis were stacked, resulting in protein bodies forming as honeycomb-like structures. We are unique in presenting clear demonstration that gamma-zeins play a mechanistic role in QPM, providing a previously unexplored rationale for molecular breeding.