Deposition of storage proteins

Selectively advantageous instability in biotic and pre-biotic systems and implications for evolution and aging

Rules of biology typically involve conservation of resources. For example, common patterns such as hexagons and logarithmic spirals require minimal materials, and scaling laws involve conservation of energy. Here a relationship with the opposite theme is discussed, which is the selectively advantageous instability (SAI) of one or more components of a replicating system, such as the cell. By increasing the complexity of the system, SAI can have benefits in addition to the generation of energy or the mobilization of building blocks. SAI involves a potential cost to the replicating system for the materials and/or energy required to create the unstable component, and in some cases, the energy required for its active degradation. SAI is well-studied in cells. Short-lived transcription and signaling factors enable a rapid response to a changing environment, and turnover is critical for replacement of damaged macromolecules. The minimal gene set for a viable cell includes proteases and a nuclease, suggesting SAI is essential for life. SAI promotes genetic diversity in several ways. Toxin/antitoxin systems promote maintenance of genes, and SAI of mitochondria facilitates uniparental transmission. By creating two distinct states, subject to different selective pressures, SAI can maintain genetic diversity. SAI of components of synthetic replicators favors replicator cycling, promoting emergence of replicators with increased complexity. Both classical and recent computer modeling of replicators reveals SAI. SAI may be involved at additional levels of biological organization. In summary, SAI promotes replicator genetic diversity and reproductive fitness, and may promote aging through loss of resources and maintenance of deleterious alleles.

Validation of low-cost reflectometer to identify phytochemical accumulation in food crops

Diets consisting of greater quantity/diversity of phytochemicals are correlated with reduced risk of disease. This understanding guides policy development increasing awareness of the importance of consuming fruits, grains, and vegetables. Enacted policies presume uniform concentrations of phytochemicals across crop varieties regardless of production/harvesting methods. A growing body of research suggests that concentrations of phytochemicals can fluctuate within crop varieties. Improved awareness of how cropping practices influence phytochemical concentrations are required, guiding policy development improving human health. Reliable, inexpensive laboratory equipment represents one of several barriers limiting further study of the complex interactions influencing crop phytochemical accumulation. Addressing this limitation our study validated the capacity of a low-cost Reflectometer ($500) to measure phytochemical content in selected crops, against a commercial grade laboratory spectrophotometer. Our correlation results ranged from r2 = 0.81 for protein in wheat and oats to r2 = 0.99 for polyphenol content in lettuce in both the Reflectometer and laboratory spectrophotometer assessment, suggesting the Reflectometer provides an accurate accounting of phytochemical content within evaluated crops. Repeatability evaluation demonstrated good reproducibility of the Reflectometer to assess crop phytochemical content. Additionally, we confirmed large variation in phytochemical content within specific crop varieties, suggesting that cultivar is but one of multiple drivers of phytochemical accumulation. Our findings indicate dramatic nutrient variations could exist across the food supply, a point whose implications are not well understood. Future studies should investigate the interactions between crop phytochemical accumulation and farm management practices that influence specific soil characteristics.

Post-translational cleavage of HMW-GS Dy10 allele improves the cookie-making quality in common wheat (Triticum aestivum)

Wheat is a major staple food crop worldwide because of the unique properties of wheat flour. High molecular weight glutenin subunits (HMW-GSs), which are among the most critical determinants of wheat flour quality, are responsible for the formation of glutenin polymeric structures via interchain disulfide bonds. We herein describe the identification of a new HMW-GS Dy10 allele (Dy10-m619SN). The amino acid substitution (serine-to-asparagine) encoded in this allele resulted in a partial post-translational cleavage that produced two new peptides. These new peptides disrupted the interactions among gluten proteins because of the associated changes to the number of available cysteine residues for interchain disulfide bonds. Consequently, Dy10-m619SN expression decreased the size of glutenin polymers and weakened glutens, which resulted in wheat dough with improved cookie-making quality, without changes to the glutenin-to-gliadin ratio. In this study, we clarified the post-translational processing of HMW-GSs and revealed a new genetic resource useful for wheat breeding.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-021-01238-9.

Structural aspects of cypsela and seed development of Trichocline catharinensis (Cabrera): a Brazilian endemic species

This is the first study to describe in a timescale morphohistological and ultrastructural characteristics of fruit (cypsela) and seed development in Trichocline catharinensis, which was completed 21 days after anthesis (DAA). At anthesis, we identified an ovary with three differentiated regions, including the inner epidermis, inner part, and outer epidermis. The mature ovule showed an integument with the outer epidermis, integumentary parenchyma, and endothelium. Cells around the endothelium form the periendothelial zone with thick cell walls that showed Periodic acid-Schiff (PAS)-positive reaction. The periendothelial zone and endothelium showed degradation of the cells during embryogenesis. The main stages of embryo development from fecundation through mature seed were identified. The ripe cypsela showed the pericarp (exocarp), seed coat (exotesta), and remaining endosperm surrounding the embryo. Mature embryos were straight with shoot apical meristem (SAM), and root apical meristem (RAM) was separated by the hypocotyl. Light microscopy (LM) and transmission electron microscopy (TEM) analyses indicate cells with characteristics of meristem cells, as well as proteins and lipid bodies and mitochondria with few cristae in cotyledon cells. Our findings provide insight into taxonomic and physiological studies by detailing cypsela and seed ontogenesis from an endemic and vulnerable Asteraceae from southern Brazil. This study is also a starting point for establishing the biological criteria for seed harvesting and future studies of seed physiology and conservation of plant genetic resource.

Hordein Accumulation in Developing Barley Grains

The temporal pattern of accumulation of hordein storage proteins in developing barley grains was studied by enzyme-linked immunosorbent assay (ELISA), western blot and liquid chromatography tandem mass spectrometry (LC-MS/MS). Hordein accumulation was compared to the pattern seen for two abundant control proteins, serpin Z4 (an early accumulator) and lipid transferase protein (LTP1, a late accumulator). Hordeins were detected from 6 days post-anthesis (DPA) and peaked at 30 DPA. Changes in fresh weight indicate that desiccation begins at 20 DPA and by 37 DPA fresh weight had decreased by 35%. ELISA analysis of hordein content, expressed on a protein basis, increased to a maximum at 30 DPA followed by a 17% decrease by 37 DPA. The accumulation of 39 tryptic and 29 chymotryptic hordein peptides representing all classes of hordein was studied by LC-MS/MS. Most peptides increased to a maximum at 30 DPA, and either remained at the maximum or did not decrease significantly. Only five tryptic peptides, members of the related B1- and γ1-hordeins decreased significantly by 21-51% at 37 DPA. Thus, the concentration of some specific peptides was reduced while remaining members of the same family were not affected. The N-terminal signal region was removed by proteolysis during co-translation. In addition to a suite of previously characterized hordeins, two novel barley B-hordein isoforms mapping to wheat low molecular weight glutenins (LMW-GS-like B-hordeins), and two avenin-like proteins (ALPs) sharing homology with wheat ALPs, were identified. These identified isoforms have not previously been mapped in the barley genome. Cereal storage proteins provide significant nutritional content for human consumption and seed germination. In barley, the bulk of the storage proteins comprise the hordein family and the final hordein concentration affects the quality of baked and brewed products. It is therefore important to study the accumulation of hordeins as this knowledge may assist plant breeding for improved health outcomes (by minimizing triggering of detrimental immune responses), nutrition and food processing properties.

Development of plant-produced protein body vaccine candidates for bluetongue virus

BACKGROUND

Bluetongue is a disease of domestic and wild ruminants caused by bluetongue virus serotypes (BTV), which have caused serious outbreaks worldwide. Commercially available vaccines are live-attenuated or inactivated virus strains: these are effective, but there is the risk of reversion to virulence or reassortment with circulating strains for live virus, and residual live virus for the inactivated vaccines. The live-attenuated virus vaccines are not able to distinguish naturally infected animals from vaccinated animals (DIVA compliant). Recombinant vaccines are preferable to minimize the risks associated with these vaccines, and would also enable the development of candidate vaccines that are DIVA-compliant.

RESULTS

In this study, two novel protein body (PB) plant-produced vaccines were developed, Zera®-VP2ep and Zera®-VP2. Zera®-VP2ep contained B-cell epitope sequences of multiple BTV serotypes and Zera®-VP2 contained the full-length BTV-8 VP2 codon-optimised sequence. In addition to fulfilling the DIVA requirement, Zera®-VP2ep was aimed at being multivalent with the ability to stimulate an immune response to several BTV serotypes. Both these candidate vaccines were successfully made in N. benthamiana via transient Agrobacterium-mediated expression, and in situ TEM analysis showed that the expressed proteins accumulated within the cytoplasm of plant cells in dense membrane-defined PBs. The peptide sequences included in Zera®-VP2ep contained epitopes that bound antibodies produced against native VP2. Preliminary murine immunogenicity studies showed that the PB vaccine candidates elicited anti-VP2 immune responses in mice without the use of adjuvant.

CONCLUSIONS

These proof of concept results demonstrate that Zera®-VP2ep and Zera®-VP2 have potential as BTV vaccines and their development should be further investigated.

The intracellular fate of a recombinant protein is tissue dependent

Recombinant proteins directed to the secretory pathway in plants require a signal peptide for entry into the endoplasmic reticulum. In the absence of further targeting information, such proteins are generally secreted via the default pathway to the apoplast. This has been well documented in protoplasts and leaf tissue, but the trafficking of recombinant proteins in seeds and other storage tissues has rarely been investigated. We used Aspergillus niger phytase as a model glycoprotein to compare the intracellular fate of a recombinant protein in the leaves and seeds of rice (Oryza sativa). Using fluorescence and electron microscopy we showed that the recombinant protein was efficiently secreted from leaf cells as expected. In contrast, within endosperm cells it was retained in endoplasmic reticulum-derived prolamin bodies and protein storage vacuoles. Consistent with our immunolocalization data, the phytase produced in endosperm cells possessed oligomannose and vacuolar-type N-glycans [Man(3)(Xyl)(Fuc)GlcNAc(2)], whereas the phytase produced in leaves contained predominantly secretion-type N-glycans [GlcNAc(2)Man(3)(Xyl)(Fuc)GlcNAc(2)]. The latter could not be detected in preparations of the endosperm-derived phytase. Our results show that the intracellular deposition and modification of a recombinant protein is tissue dependent.

Unexpected deposition patterns of recombinant proteins in post-endoplasmic reticulum compartments of wheat endosperm

Protein transport within cereal endosperm cells is complicated by the abundance of endoplasmic reticulum (ER)-derived and vacuolar protein bodies. For wheat storage proteins, two major transport routes run from the ER to the vacuole, one bypassing and one passing through the Golgi. Proteins traveling along each route converge at the vacuole and form aggregates. To determine the impact of this trafficking system on the fate of recombinant proteins expressed in wheat endosperm, we used confocal and electron microscopy to investigate the fate of three recombinant proteins containing different targeting information. KDEL-tagged recombinant human serum albumin, which is retrieved to the ER lumen in leaf cells, was deposited in prolamin aggregates within the vacuole of endosperm cells, most likely following the bulk of endogenous glutenins. Recombinant fungal phytase, a glycoprotein designed for secretion, was delivered to the same compartment, with no trace of the molecule in the apoplast. Glycan analysis revealed that this protein had passed through the Golgi. The localization of human serum albumin and phytase was compared to that of recombinant legumin, which contains structural targeting information directing it to the vacuole. Uniquely, legumin accumulated in the globulin inclusion bodies at the periphery of the prolamin bodies, suggesting a different mode of transport and/or aggregation. Our results demonstrate that recombinant proteins are deposited in an unexpected pattern within wheat endosperm cells, probably because of the unique storage properties of this tissue. Our data also confirm that recombinant proteins are invaluable tools for the analysis of protein trafficking in cereals.

Zeolin. A new recombinant storage protein constructed using maize gamma-zein and bean phaseolin

The major seed storage proteins of maize (Zea mays) and bean (Phaseolus vulgaris), zein and phaseolin, accumulate in the endoplasmic reticulum (ER) and in storage vacuoles, respectively. We show here that a chimeric protein composed of phaseolin and 89 amino acids of gamma-zein, including the repeated and the Pro-rich domains, maintains the main characteristics of wild-type gamma-zein: It is insoluble unless its disulfide bonds are reduced and forms ER-located protein bodies. Unlike wild-type phaseolin, the protein, which we called zeolin, accumulates to very high amounts in leaves of transgenic tobacco (Nicotiana tabacum). A relevant proportion of the ER chaperone BiP is associated with zeolin protein bodies in an ATP-sensitive fashion. Pulse-chase labeling confirms the high affinity of BiP to insoluble zeolin but indicates that, unlike structurally defective proteins that also extensively interact with BiP, zeolin is highly stable. We conclude that the gamma-zein portion is sufficient to induce the formation of protein bodies also when fused to another protein. Because the storage proteins of cereals and legumes nutritionally complement each other, zeolin can be used as a starting point to produce nutritionally balanced and highly stable chimeric storage proteins.

Identification of the protein storage vacuole and protein targeting to the vacuole in leaf cells of three plant species

Protein storage vacuoles (PSVs) are specialized vacuoles devoted to the accumulation of large amounts of protein in the storage tissues of plants. In this study, we investigated the presence of the storage vacuole and protein trafficking to the compartment in cells of tobacco (Nicotiana tabacum), common bean (Phaseolus vulgaris), and Arabidopsis leaf tissue. When we expressed phaseolin, the major storage protein of common bean, or an epitope-tagged version of alpha-tonoplast intrinsic protein (alpha-TIP, a tonoplast aquaporin of PSV), in protoplasts derived from leaf tissues, these proteins were targeted to a compartment ranging in size from 2 to 5 microm in all three plant species. Most Arabidopsis leaf cells have one of these organelles. In contrast, from one to five these organelles occurred in bean and tobacco leaf cells. Also, endogenous alpha-TIP is localized in a similar compartment in untransformed leaf cells of common bean and is colocalized with transiently expressed epitope-tagged alpha-TIP. In Arabidopsis, phaseolin contained N-glycans modified by Golgi enzymes and its traffic was sensitive to brefeldin A. However, trafficking of alpha-TIP was insensitive to brefeldin A treatment and was not affected by the dominant-negative mutant of AtRab1. In addition, a modified alpha-TIP with an insertion of an N-glycosylation site has the endoplasmic reticulum-type glycans. Finally, the early step of phaseolin traffic, from the endoplasmic reticulum to the Golgi complex, required the activity of the small GTPase Sar1p, a key component of coat protein complex II-coated vesicles, independent of the presence of the vacuolar sorting signal in phaseolin. Based on these results, we propose that the proteins we analyzed are targeted to the PSV or equivalent organelle in leaf cells and that proteins can be transported to the PSV by two different pathways, the Golgi-dependent and Golgi-independent pathways, depending on the individual cargo proteins.