A defective signal peptide in the maize high-lysine mutant floury 2

Dynamics of ER stress-induced gene regulation in plants

Endoplasmic reticulum (ER) stress is a potentially lethal condition that is induced by the abnormal accumulation of unfolded or misfolded secretory proteins in the ER. In eukaryotes, ER stress is managed by the unfolded protein response (UPR) through a tightly regulated, yet highly dynamic, reprogramming of gene transcription. Although the core principles of the UPR are similar across eukaryotes, unique features of the plant UPR reflect the adaptability of plants to their ever-changing environments and the need to balance the demands of growth and development with the response to environmental stressors. The past decades have seen notable progress in understanding the mechanisms underlying ER stress sensing and signalling transduction pathways, implicating the UPR in the effects of physiological and induced ER stress on plant growth and crop yield. Facilitated by sequencing technologies and advances in genetic and genomic resources, recent efforts have driven the discovery of transcriptional regulators and elucidated the mechanisms that mediate the dynamic and precise gene regulation in response to ER stress at the systems level.

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.

StresSeed: The Unfolded Protein Response During Seed Development

During seed development, the endoplasmic reticulum (ER) takes care of the synthesis and structural maturation of very high amounts of storage proteins in a relatively short time. The ER must thus adjust its extension and machinery to optimize this process. The major signaling mechanism to maintain ER homeostasis is the unfolded protein response (UPR). Both storage proteins that assemble into ER-connected protein bodies and those that are delivered to protein storage vacuoles stimulate the UPR, but its extent and features are specific for the different storage protein classes and even for individual members of each class. Furthermore, evidence exists for anticipatory UPR directly connected to the development of storage seed cells and for selective degradation of certain storage proteins soon after their synthesis, whose signaling details are however still largely unknown. All these events are discussed, also in the light of known features of mammalian UPR.

Monitoring calcium handling by the plant endoplasmic reticulum with a low-Ca2+ -affinity targeted aequorin reporter

Precise measurements of dynamic changes in free Ca²⁺ concentration in the lumen of the plant endoplasmic reticulum (ER) have been lacking so far, despite increasing evidence for the contribution of this intracellular compartment to Ca²⁺ homeostasis and signalling in the plant cell. In the present study, we targeted an aequorin chimera with reduced Ca²⁺ affinity to the ER membrane and facing the ER lumen. To this aim, the cDNA for a low-Ca²⁺ -affinity aequorin variant (AEQmut) was fused to the nucleotide sequence encoding a non-cleavable N-terminal ER signal peptide (fl2). The correct targeting of fl2-AEQmut was confirmed by immunocytochemical analyses in transgenic Arabidopsis thaliana (Arabidopsis) seedlings. An experimental protocol well-established in animal cells - consisting of ER Ca²⁺ depletion during photoprotein reconstitution followed by ER Ca²⁺ refilling - was applied to carry out ER Ca²⁺ measurements in planta. Rapid and transient increases of the ER luminal Ca²⁺ concentration ([Ca²⁺ ]_ER ) were recorded in response to different environmental stresses, displaying stimulus-specific Ca²⁺ signatures. The comparative analysis of ER and chloroplast Ca²⁺ dynamics indicates a complex interplay of these organelles in shaping cytosolic Ca²⁺ signals during signal transduction events. Our data highlight significant differences in basal [Ca²⁺ ]_ER and Ca²⁺ handling by plant ER compared to the animal counterpart. The set-up of an ER-targeted aequorin chimera extends and complements the currently available toolkit of organelle-targeted Ca²⁺ indicators by adding a reporter that improves our quantitative understanding of Ca²⁺ homeostasis in the plant endomembrane system.

Evolution of the unfolded protein response in plants

The unfolded protein response (UPR) in plants is elicited by endoplasmic reticulum stress, which can be brought about by adverse environmental conditions. The response is mediated by a conserved signalling network composed of two branches - one branch involving inositol requiring enzyme1- basic leucine zipper60 (IRE1-bZIP60) signalling pathway and another branch involving the membrane transcription factors, bZIP17 and -28. The UPR has been reported in Chlamydomonas reinhardtii, a unicellular green alga, which lacks some canonical UPR signalling components found in vascular plants, raising the question whether C. reinhardtii uses other means such as oxidative signalling or Regulated IRE1-Dependent Decay to activate the UPR. In vascular plants, IRE1 splices bZIP60 mRNA in response to endoplasmic reticulum stress by cutting at a site in the RNA that is highly conserved in structure and sequence. Monocots have a single IRE1 gene required for viability in rice, while dicots have two IRE1 genes, IRE1a and -b. Brassicas have a third IRE1 gene, IRE1c, which lacks a lumenal domain, but is required in combination with IRE1b for gametogenesis. Vascular and non-vascular plants upregulate a similar set of genes in response to endoplasmic reticulum stress despite differences in the complexity of their UPR signalling networks.

Genetic engineering to improve essential and conditionally essential amino acids in maize: transporter engineering as a reference

Ruminants and humans are unable to synthesize essential amino acids (EAAs) and conditionally essential amino acids (CEAAs) under normal conditions and need to acquire them from plant sources. Maize plays, as a major crop, a central role in global food security. However, maize is deficient in several EAAs and CEAAs. Genetic engineering has been successfully used to enrich the EAA content of maize to some extent, including the content of Lys, Trp, and Met. However, research on other EAAs is lacking. Genetic engineering provides several viable approaches for increasing the EAA content in maize, including transformation of a single gene, transformation of multiple genes in a single cassette, overexpression of putative amino acid transporters, engineering the amino acid biosynthesis pathway including silencing of feedback inhibition enzymes, and overexpression of major enzymes in this pathway. These challenging processes require a deep understanding of the biosynthetic and metabolic pathways of individual amino acids, and the interaction of individual amino acids with other metabolic pathways.

Maize kernel development

Maize (Zea mays) is a leading cereal crop in the world. The maize kernel is the storage organ and the harvest portion of this crop and is closely related to its yield and quality. The development of maize kernel is initiated by the double fertilization event, leading to the formation of a diploid embryo and a triploid endosperm. The embryo and endosperm are then undergone independent developmental programs, resulting in a mature maize kernel which is comprised of a persistent endosperm, a large embryo, and a maternal pericarp. Due to the well-characterized morphogenesis and powerful genetics, maize kernel has long been an excellent model for the study of cereal kernel development. In recent years, with the release of the maize reference genome and the development of new genomic technologies, there has been an explosive expansion of new knowledge for maize kernel development. In this review, we overviewed recent progress in the study of maize kernel development, with an emphasis on genetic mapping of kernel traits, transcriptome analysis during kernel development, functional gene cloning of kernel mutants, and genetic engineering of kernel traits.

The dynamic transcriptome of waxy maize (Zea mays L. sinensis Kulesh) during seed development

BACKGROUND

Waxy maize (Zea mays L. sinensis Kulesh) is a mutant of maize (Zea mays L.) with a mutation at Waxy1 (Wx1) gene locus. The seed of waxy maize has higher viscosity compared to regular maize. By now, we know little about the expression patterns of genes that involved in the seed development of waxy maize.

OBJECTIVE

By analyzing the transcriptome data during waxy maize seed development, we attempt to dig out the genes that may influence the seed development of waxy maize.

METHODS

The seeds of waxy maize inbred line SWL01 from six phases after pollination were used to do RNA-seq. Bioinformatics methods were used to analyze the expression patterns of the expressed genes, to identify the genes involved in waxy maize seed development.

RESULTS

A total of 24,546 genes including 1611 transcription factors (TFs) were detected during waxy maize seed development. Coexpression analysis of expressed genes revealed the dynamic processes of waxy maize seed development. Particularly, 2457 genes including 177 TFs were specially expressed in waxy maize seed, some of which mainly involved in the process of seed dormancy and maturation. In addition, 2681, 5686, 4491, 4386, 3669 and 4624 genes were identified to be differential expressed genes (DEGs) at six phases compared to regular maize B73, and 113 DEGs among them may be key genes that lead the difference of seed development between waxy and regular maizes in milk stage.

CONCLUSION

In summary, we elucidated the expression patterns of expressed genes during waxy maize seed development globally. A series of genes that associated with seed development were identified in our research, which may provide an important resource for functional study of waxy maize seed development to help molecular assisted breeding.

Eukaryotic peptide chain release factor 1 participates in translation termination of specific cysteine-poor prolamines in rice endosperm

Prolamines are alcohol-soluble proteins classified as either cysteine-poor (CysP) or cysteine-rich (CysR) based on whether they can be alcohol-extracted without or with reducing agents, respectively. In rice esp1 mutants, various CysP prolamines exhibit both reduced and normal amounts of isoelectric focusing bands, indicating that the mutation affects only certain prolamine classes. To examine the genetic regulation of CysP prolamine synthesis and accumulation, we constructed a high-resolution genetic linkage map of ESP1. The ESP1 gene was mapped to within a 20 kb region on rice chromosome 7. Sequencing analysis of annotated genes in this region revealed a single-nucleotide polymorphism within eukaryotic peptide chain release factor (eRF1), which participates in stop-codon recognition and nascent-polypeptide release from ribosomes during translation. A subsequent complementation test revealed that ESP1 encodes eRF1. We also identified UAA as the stop codon of CysP prolamines with reduced concentration in esp1 mutants. Recognition assays and microarray analysis confirmed that ESP1/eRF1 recognizes UAA/UAG, but not UGA. Our results provide convincing evidence that ESP1/eRF1 participates in the translation termination of CysP prolamines during seed development.

Response to Persistent ER Stress in Plants: A Multiphasic Process That Transitions Cells from Prosurvival Activities to Cell Death

The unfolded protein response (UPR) is a highly conserved response that protects plants from adverse environmental conditions. The UPR is elicited by endoplasmic reticulum (ER) stress, in which unfolded and misfolded proteins accumulate within the ER. Here, we induced the UPR in maize (Zea mays) seedlings to characterize the molecular events that occur over time during persistent ER stress. We found that a multiphasic program of gene expression was interwoven among other cellular events, including the induction of autophagy. One of the earliest phases involved the degradation by regulated IRE1-dependent RNA degradation (RIDD) of RNA transcripts derived from a family of peroxidase genes. RIDD resulted from the activation of the promiscuous ribonuclease activity of ZmIRE1 that attacks the mRNAs of secreted proteins. This was followed by an upsurge in expression of the canonical UPR genes indirectly driven by ZmIRE1 due to its splicing of Zmbzip60 mRNA to make an active transcription factor that directly upregulates many of the UPR genes. At the peak of UPR gene expression, a global wave of RNA processing led to the production of many aberrant UPR gene transcripts, likely tempering the ER stress response. During later stages of ER stress, ZmIRE1's activity declined, as did the expression of survival modulating genes, Bax inhibitor1 and Bcl-2-associated athanogene7, amid a rising tide of cell death. Thus, in response to persistent ER stress, maize seedlings embark on a course of gene expression and cellular events progressing from adaptive responses to cell death.

Functions of maize genes encoding pyruvate phosphate dikinase in developing endosperm

Maize opaque2 (o2) mutations are beneficial for endosperm nutritional quality but cause negative pleiotropic effects for reasons that are not fully understood. Direct targets of the bZIP transcriptional regulator encoded by o2 include pdk1 and pdk2 that specify pyruvate phosphate dikinase (PPDK). This enzyme reversibly converts AMP, pyrophosphate, and phosphoenolpyruvate to ATP, orthophosphate, and pyruvate and provides diverse functions in plants. This study addressed PPDK function in maize starchy endosperm where it is highly abundant during grain fill. pdk1 and pdk2 were inactivated individually by transposon insertions, and both genes were simultaneously targeted by endosperm-specific RNAi. pdk2 accounts for the large majority of endosperm PPDK, whereas pdk1 specifies the abundant mesophyll form. The pdk1- mutation is seedling-lethal, indicating that C4 photosynthesis is essential in maize. RNAi expression in transgenic endosperm eliminated detectable PPDK protein and enzyme activity. Transgenic kernels weighed the same on average as nontransgenic siblings, with normal endosperm starch and total N contents, indicating that PPDK is not required for net storage compound synthesis. An opaque phenotype resulted from complete PPDK knockout, including loss of vitreous endosperm character similar to the phenotype conditioned by o2-. Concentrations of multiple glycolytic intermediates were elevated in transgenic endosperm, energy charge was altered, and starch granules were more numerous but smaller on average than normal. The data indicate that PPDK modulates endosperm metabolism, potentially through reversible adjustments to energy charge, and reveal that o2- mutations can affect the opaque phenotype through regulation of PPDK in addition to their previously demonstrated effects on storage protein gene expression.

Anti-Cancer Activity of Maize Bioactive Peptides

Cancer is one of the main chronic degenerative diseases worldwide. In recent years, consumption of whole-grain cereals and their derivative food products has been associated with a reduced risk of various types of cancer. The main biomolecules in cereals include proteins, peptides, and amino acids, all of which are present in different quantities within the grain. Some of these peptides possess nutraceutical properties and exert biological effects that promote health and prevent cancer. In this review, we report the current status and advances in knowledge regarding the bioactive properties of maize peptides, such as antioxidant, antihypertensive, hepatoprotective, and anti-tumor activities. We also highlight the potential biological mechanisms through which maize bioactive peptides exert anti-cancer activity. Finally, we analyze and emphasize the potential applications of maize peptides.

Genetic architecture of kernel composition in global sorghum germplasm

Background

Sorghum [Sorghum bicolor (L.) Moench] is an important cereal crop for dryland areas in the United States and for small-holder farmers in Africa. Natural variation of sorghum grain composition (protein, fat, and starch) between accessions can be used for crop improvement, but the genetic controls are still unresolved. The goals of this study were to quantify natural variation of sorghum grain composition and to identify single-nucleotide polymorphisms (SNPs) associated with variation in grain composition concentrations.

Results

In this study, we quantified protein, fat, and starch in a global sorghum diversity panel using near-infrared spectroscopy (NIRS). Protein content ranged from 8.1 to 18.8%, fat content ranged from 1.0 to 4.3%, and starch content ranged from 61.7 to 71.1%. Durra and bicolor-durra sorghum from Ethiopia and India had the highest protein and fat and the lowest starch content, while kafir sorghum from USA, India, and South Africa had the lowest protein and the highest starch content. Genome-wide association studies (GWAS) identified quantitative trait loci (QTL) for sorghum protein, fat, and starch. Previously published RNAseq data was used to identify candidate genes within a GWAS QTL region. A putative alpha-amylase 3 gene, which has previously been shown to be associated with grain composition traits, was identified as a strong candidate for protein and fat variation.

Conclusions

We identified promising sources of genetic material for manipulation of grain composition traits, and several loci and candidate genes that may control sorghum grain composition. This survey of grain composition in sorghum germplasm and identification of protein, fat, and starch QTL contributes to our understanding of the genetic basis of natural variation in sorghum grain nutritional traits.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3403-x) contains supplementary material, which is available to authorized users.

Pigeonetics takes flight: Evolution, development, and genetics of intraspecific variation

Intensive artificial selection over thousands of years has produced hundreds of varieties of domestic pigeon. As Charles Darwin observed, the morphological differences among breeds can rise to the magnitude of variation typically observed among different species. Nevertheless, different pigeon varieties are interfertile, thereby enabling forward genetic and genomic approaches to identify genes that underlie derived traits. Building on classical genetic studies of pigeon variation, recent molecular investigations find a spectrum of coding and regulatory alleles controlling derived traits, including plumage color, feather growth polarity, and limb identity. Developmental and genetic analyses of pigeons are revealing the molecular basis of variation in a classic example of extreme intraspecific diversity, and have the potential to nominate genes that control variation among other birds and vertebrates in general.

Maize opaque10 Encodes a Cereal-Specific Protein That Is Essential for the Proper Distribution of Zeins in Endosperm Protein Bodies

Cereal storage proteins are major nitrogen sources for humans and livestock. Prolamins are the most abundant storage protein in most cereals. They are deposited into protein bodies (PBs) in seed endosperm. The inner structure and the storage mechanism for prolamin PBs is poorly understood. Maize opaque10 (o10) is a classic opaque endosperm mutant with misshapen PBs. Through positional cloning, we found that O10 encodes a novel cereal-specific PB protein. Its middle domain contains a seven-repeat sequence that is responsible for its dimerization. Its C terminus contains a transmembrane motif that is required for its ER localization and PB deposition. A cellular fractionation assay indicated that O10 is initially synthesized in the cytoplasm and then anchored to the ER and eventually deposited in the PB. O10 can interact with 19-kD and 22-kD α-zeins and 16-kD and 50-kD γ-zeins through its N-terminal domain. An immunolocalization assay indicated that O10 co-localizes with 16-kD γ-zein and 22-kD α-zein in PBs, forming a ring-shaped structure at the interface between the α-zein-rich core and the γ-zein-rich peripheral region. The loss of O10 function disrupts this ring-shaped distribution of 22-kD and 16-kD zeins, resulting in misshapen PBs. These results showed that O10, as a newly evolved PB protein, is essential for the ring-shaped distribution of 22-kD and 16-kD zeins and controls PB morphology in maize endosperm.

Through the positional cloning of the maize classic endosperm mutant opaque10 (o10), we identified a novel protein critical for PB morphology. O10 is a fast-evolving cereal-specific gene with recent origin. A thorough characterization of its three functional domains revealed its important functions for storage protein deposition and organization in PBs. O10 determines a ring-shaped layer in PBs through direct interaction with two major storage proteins (22-kD and 16-kD zeins). This newly characterized PB layer maintains a stable spherical morphology for PB. This study advanced our understanding of PB structure and function. Furthermore, this study demonstrated the origin of a new functional gene and the functional evolution of a storage organelle that is highly valuable to humans.

Cellular Response to the high protein digestibility/high-Lysine (hdhl) sorghum mutation

A high protein digestibility/high-lysine mutant P721Q (hdhl) with a multi-folded protein body morphology has been developed, with a 22kDa α-kafirin single point mutation having also been recently identified. Relatively little is known regarding the resulting cellular response in hdhl endosperm. The aim is to elucidate these biochemical changes. Two-dimentional gel electrophoresis showed an apparent increase of non-kafirin and a decrease in kafirins content in hdhl endosperm. Mass spectrometry data yielded the identity of differentially expressed non-kafirin proteins in hdhl, wild-type lines such as cytoskeleton and chaperones proteins, and also others involved in amino acids and carbohydrates biochemical synthesis pathways. Western blot analysis showed that chaperone proteins were more highly expressed in the hdhl than the wild-type sorghum and confirmed the non-kafirin proteins proteomic results. Two-dimentional gel electrophoresis showed that the γ-kafirin subunits content had decreased, and the 22kDa α-kafirin subunit was increased in hdhl without any apparent molecular mass change. The observed differential expression most likely led to proteins interactions between γ- and α-kafirin subunits in particular, which resulted in a kafirins packing differently to form the protein body's multi-folded morphology, while also improving its digestibility.

Root iTRAQ protein profile analysis of two Citrus species differing in aluminum-tolerance in response to long-term aluminum-toxicity

BACKGROUND

Limited information is available on aluminum (Al)-toxicity-responsive proteins in woody plant roots. Seedlings of 'Xuegan' (Citrus sinensis) and 'Sour pummelo' (Citrus grandis) were treated for 18 weeks with nutrient solution containing 0 (control) or 1.2 mM AlCl3 · 6H2O (+Al). Thereafter, we investigated Citrus root protein profiles using isobaric tags for relative and absolute quantification (iTRAQ). The aims of this work were to determine the molecular mechanisms of plants to deal with Al-toxicity and to identify differentially expressed proteins involved in Al-tolerance.

RESULTS

C. sinensis was more tolerant to Al-toxicity than C. grandis. We isolated 347 differentially expressed proteins from + Al Citrus roots. Among these proteins, 202 (96) proteins only presented in C. sinensis (C. grandis), and 49 proteins were shared by the two species. Of the 49 overlapping proteins, 45 proteins were regulated in the same direction upon Al exposure in the both species. These proteins were classified into following categories: sulfur metabolism, stress and defense response, carbohydrate and energy metabolism, nucleic acid metabolism, protein metabolism, cell transport, biological regulation and signal transduction, cell wall and cytoskeleton metabolism, and jasmonic acid (JA) biosynthesis. The higher Al-tolerance of C. sinensis may be related to several factors, including: (a) activation of sulfur metabolism; (b) greatly improving the total ability of antioxidation and detoxification; (c) up-regulation of carbohydrate and energy metabolism; (d) enhancing cell transport; (e) decreased (increased) abundances of proteins involved in protein synthesis (proteiolysis); (f) keeping a better balance between protein phosphorylation and dephosphorylation; and (g) increasing JA biosynthesis.

CONCLUSIONS

Our results demonstrated that metabolic flexibility was more remarkable in C. sinenis than in C. grandis roots, thus improving the Al-tolerance of C. sinensis. This provided the most integrated view of the adaptive responses occurring in Al-toxicity roots.

The contribution of wheat to human diet and health

Wheat is the most important staple crop in temperate zones and is in increasing demand in countries undergoing urbanization and industrialization. In addition to being a major source of starch and energy, wheat also provides substantial amounts of a number of components which are essential or beneficial for health, notably protein, vitamins (notably B vitamins), dietary fiber, and phytochemicals. Of these, wheat is a particularly important source of dietary fiber, with bread alone providing 20% of the daily intake in the UK, and well-established relationships between the consumption of cereal dietary fiber and reduced risk of cardio-vascular disease, type 2 diabetes, and forms of cancer (notably colo-rectal cancer). Wheat shows high variability in the contents and compositions of beneficial components, with some (including dietary fiber) showing high heritability. Hence, plant breeders should be able to select for enhanced health benefits in addition to increased crop yield.

Role of proline in cell wall synthesis and plant development and its implications in plant ontogeny

Proline is a proteogenic amino acid and accumulates both under stress and non-stress conditions as a beneficial solute in plants. Recent discoveries point out that proline plays an important role in plant growth and differentiation across life cycle. It is a key determinant of many cell wall proteins that plays important roles in plant development. The role of extensins, arabinogalactan proteins and hydroxyproline- and proline-rich proteins as important components of cell wall proteins that play pivotal roles in cell wall signal transduction cascades, plant development and stress tolerance is discussed in this review. Molecular insights are also provided here into the plausible roles of proline transporters modulating key events in plant development. In addition, the roles of proline during seed developmental transitions including storage protein synthesis are discussed.

Proline responding1 Plays a Critical Role in Regulating General Protein Synthesis and the Cell Cycle in Maize

Proline, an important amino acid, accumulates in many plant species. Besides its role in plant cell responses to environmental stresses, the potential biological functions of proline in growth and development are unclear. Here, we report cloning and functional characterization of the maize (Zea mays) classic mutant proline responding1 (pro1) gene. This gene encodes a Δ1-pyrroline-5- carboxylate synthetase that catalyzes the biosynthesis of proline from glutamic acid. Loss of function of Pro1 significantly inhibits proline biosynthesis and decreases its accumulation in the pro1 mutant. Proline deficiency results in an increased level of uncharged tRNApro AGG accumulation and triggers the phosphorylation of eukaryotic initiation factor 2α (eIF2α) in the pro1 mutant, leading to a general reduction in protein synthesis in this mutant. Proline deficiency also downregulates major cyclin genes at the transcriptional level, causing cell cycle arrest and suppression of cell proliferation. These processes are reversible when external proline is supplied to the mutant, suggesting that proline plays a regulatory role in the cell cycle transition. Together, the results demonstrate that proline plays an important role in the regulation of general protein synthesis and the cell cycle transition in plants.

Mutation in the seed storage protein kafirin creates a high-value food trait in sorghum

Sustainable food production for the earth's fast-growing population is a major challenge for breeding new high-yielding crops, but enhancing the nutritional quality of staple crops can potentially offset limitations associated with yield increases. Sorghum has immense value as a staple food item for humans in Africa, but it is poorly digested. Although a mutant exhibiting high-protein digestibility and lysine content has market potential, the molecular nature of the mutation is previously unknown. Here, building on knowledge from maize mutants, we take a direct approach and find that the high-digestible sorghum phenotype is tightly linked to a single-point mutation, rendering the signal peptide of a seed storage protein kafirin resistant to processing, indirectly reducing lysine-poor kafirins and thereby increasing lysine-rich proteins in the seeds. These findings indicate that a molecular marker can be used to accelerate introduction of this high nutrition and digestibility trait into different sorghum varieties.

Endoplasmic reticulum (ER) stress response and its physiological roles in plants

The endoplasmic reticulum (ER) stress response is a highly conserved mechanism that results from the accumulation of unfolded or misfolded proteins in the ER. The response plays an important role in allowing plants to sense and respond to adverse environmental conditions, such as heat stress, salt stress and pathogen infection. Since the ER is a well-controlled microenvironment for proper protein synthesis and folding, it is highly susceptible to stress conditions. Accumulation of unfolded or misfolded proteins activates a signaling pathway, called the unfolded protein response (UPR), which acts to relieve ER stress and, if unsuccessful, leads to cell death. Plants have two arms of the UPR signaling pathway, an arm involving the proteolytic processing of membrane-associated basic leucine zipper domain (bZIP) transcription factors and an arm involving RNA splicing factor, IRE1, and its mRNA target. These signaling pathways play an important role in determining the cell's fate in response to stress conditions.

Evolution of the unfolded protein response

The unfolded protein response (UPR) is a network of signaling pathways that responds to stress in the endoplasmic reticulum (ER). The general output of the UPR is to upregulate genes involved in ER function, thus restoring and/or increasing the capacity of the ER to fold and process proteins. In parallel, many organisms have mechanisms for limiting the load on the ER by attenuating translation or degrading ER-targeted mRNAs. Despite broad conservation of these signaling pathways across eukaryotes, interesting variations demonstrate a variety of mechanisms for managing ER stress. How do early-diverging protozoa respond to stress when they lack traditional transcriptional regulation? What is the role of the ER stress sensor Ire1 in fungal species that are missing its main target? Here I describe how diverse species have optimized the UPR to fit their needs. This article is part of a Special Issue entitled: Functional and structural diversity of endoplasmic reticulum.

Changes in endogenous gene transcript and protein levels in maize plants expressing the soybean ferritin transgene

Transgenic agricultural crops with increased nutritive value present prospects for contributing to public health. However, their acceptance is poor in many countries due to the perception that genetic modification may cause unintended effects on expression of native genes in the host plant. Here, we tested effects of soybean ferritin transgene (SoyFer1, M64337) on transcript and protein levels of endogenous genes in maize. Results showed that the transgene was successfully introduced and expressed in the maize seed endosperm. mRNA abundance of seven tested iron homeostasis genes and seed storage protein genes differed significantly between seed samples positive and negative for the transgene. The PCR negative samples had higher zein and total protein content compared to the positive samples. However, PCR positive samples had significantly higher concentrations of calcium, magnesium, and iron. We have shown that the soybean ferritin transgene affected the expression of native iron homeostasis genes in the maize plant. These results underscore the importance of taking a holistic approach to the evaluation of transgenic events in target plants, comparing the transgenic plant to the untransformed controls.

Plant transducers of the endoplasmic reticulum unfolded protein response

The unfolded protein response (UPR) activates a set of genes to overcome accumulation of unfolded proteins in the endoplasmic reticulum (ER), a condition termed ER stress, and constitutes an essential part of ER protein quality control that ensures efficient maturation of secretory and membrane proteins in eukaryotes. Recent studies on Arabidopsis and rice identified the signaling pathway in which the ER membrane-localized ribonuclease IRE1 (inositol-requiring enzyme 1) catalyzes unconventional cytoplasmic splicing of mRNA, thereby producing the active transcription factor Arabidopsis bZIP60 (basic leucine zipper 60) and its ortholog in rice. Here we review recent findings identifying the molecular components of the plant UPR, including IRE1/bZIP60 and the membrane-bound transcription factors bZIP17 and bZIP28, and implicating its importance in several physiological phenomena such as pathogen response.

Plant transducers of the endoplasmic reticulum unfolded protein response

The unfolded protein response (UPR) activates a set of genes to overcome accumulation of unfolded proteins in the endoplasmic reticulum (ER), a condition termed ER stress, and constitutes an essential part of ER protein quality control that ensures efficient maturation of secretory and membrane proteins in eukaryotes. Recent studies on Arabidopsis and rice identified the signaling pathway in which the ER membrane-localized ribonuclease IRE1 (inositol-requiring enzyme 1) catalyzes unconventional cytoplasmic splicing of mRNA, thereby producing the active transcription factor Arabidopsis bZIP60 (basic leucine zipper 60) and its ortholog in rice. Here we review recent findings identifying the molecular components of the plant UPR, including IRE1/bZIP60 and the membrane-bound transcription factors bZIP17 and bZIP28, and implicating its importance in several physiological phenomena such as pathogen response.

Opaque7 encodes an acyl-activating enzyme-like protein that affects storage protein synthesis in maize endosperm

In maize, a series of seed mutants with starchy endosperm could increase the lysine content by decreased amount of zeins, the main storage proteins in endosperm. Cloning and characterization of these mutants could reveal regulatory mechanisms for zeins accumulation in maize endosperm. Opaque7 (o7) is a classic maize starchy endosperm mutant with large effects on zeins accumulation and high lysine content. In this study, the O7 gene was cloned by map-based cloning and confirmed by transgenic functional complementation and RNAi. The o7-ref allele has a 12-bp in-frame deletion. The four-amino-acid deletion caused low accumulation of o7 protein in vivo. The O7 gene encodes an acyl-activating enzyme with high similarity to AAE3. The opaque phenotype of the o7 mutant was produced by the reduction of protein body size and number caused by a decrease in the α-zeins concentrations. Analysis of amino acids and metabolites suggested that the O7 gene might affect amino acid biosynthesis by affecting α-ketoglutaric acid and oxaloacetic acid. Transgenic rice seeds containing RNAi constructs targeting the rice ortholog of maize O7 also produced lower amounts of seed proteins and displayed an opaque endosperm phenotype, indicating a conserved biological function of O7 in cereal crops. The cloning of O7 revealed a novel regulatory mechanism for storage protein synthesis and highlighted an effective target for the genetic manipulation of storage protein contents in cereal seeds.

Genetic analysis of cysteine-poor prolamin polypeptides reduced in the endosperm of the rice esp1 mutant

The esp1 mutant CM21 specifically exhibits reduced levels of cysteine-poor (CysP) prolamin bands with pIs of 6.65, 6.95, 7.10, and 7.35 in rice seed. Matrix-assisted laser desorption-ionization time-of-flight mass spectrometry (MALDI-TOF MS) analysis demonstrated that the bands with pIs 6.65, 6.95, and 7.35 are encoded by different structural genes. These results suggest that the Esp1 locus encodes a regulatory factor involved in the synthesis and/or accumulation of CysP prolamin molecules. Isoelectric focusing (IEF) analysis of CysP prolamins in chromosome substitution lines showed that structural genes for bands with pI values of 6.95, 7.10, and 7.35, which are reduced in esp1 mutant lines, are located as a gene cluster in the 44.2 cM region on chromosome 5.

Cereal seed storage protein synthesis: fundamental processes for recombinant protein production in cereal grains

Cereal seeds provide an ideal production platform for high-value products such as pharmaceuticals and industrial materials because seeds have ample and stable space for the deposition of recombinant products without loss of activity at room. Seed storage proteins (SSPs) are predominantly synthesized and stably accumulated in maturing endosperm tissue. Therefore, understanding the molecular mechanisms regulating SSP expression and accumulation is expected to provide valuable information for producing higher amounts of recombinant products. SSP levels are regulated by several steps at the transcriptional (promoters, transcription factors), translational and post-translational levels (modification, processing trafficking, and deposition). Our objective is to develop a seed production platform capable of producing very high yields of recombinant product. Towards this goal, we review here the individual regulatory steps controlling SSP synthesis and accumulation.

Rescue of a dominant mutant with RNA interference

Maize Mucronate1 is a dominant floury mutant based on a misfolded 16-kDa γ-zein protein. To prove its function, we applied RNA interference (RNAi) as a dominant suppressor of the mutant seed phenotype. A γ-zein RNAi transgene was able to rescue the mutation and restore normal seed phenotype. RNA interference prevents gene expression. In most cases, this is used to study gene function by creating a new phenotype. Here, we use it for the opposite purpose. We use it to reverse the creation of a mutant phenotype by restoring the normal phenotype. In the case of the maize Mucronate1 (Mc1) phenotype, interaction of a misfolded protein with other proteins is believed to be the basis for the Mc1 phenotype. If no misfolded protein is present, we can reverse the mutant to the normal phenotype. One can envision using this approach to study complex traits and in gene therapy.

Endoplasmic reticulum protein quality control and its relationship to environmental stress responses in plants

The endoplasmic reticulum (ER) has a sophisticated quality control (QC) system to eliminate improperly folded proteins from the secretory pathway. Given that protein folding is such a fastidious process and subject to adverse environmental conditions, the ER QC system appears to have been usurped to serve as an environmental sensor and responder in plants. Under stressful conditions, the ER protein folding machinery reaches a limit as the demands for protein folding exceed the capacity of the system. Under these conditions, misfolded or unfolded proteins accumulate in the ER, triggering an unfolded protein response (UPR). UPR mitigates ER stress by upregulating the expression of genes encoding components of the protein folding machinery or the ER-associated degradation system. In Arabidopsis thaliana, ER stress is sensed and stress signals are transduced by membrane-bound transcription factors, which are activated and mobilized under environmental stress conditions. Under acute or chronic stress conditions, UPR can also lead to apoptosis or programmed cell death. Despite recent progress in our understanding of plant protein QC, discovering how different environmental conditions are perceived is one of the major challenges in understanding this system. Since the ER QC system is one among many stress response systems in plants, another major challenge is determining the extent to which the ER QC system contributes to various stress responses in plants.

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.

Non-Mendelian regulation and allelic variation of methionine-rich delta-zein genes in maize

Sufficient methionine levels in the seed are critical for the supply of a balanced diet for feed and food. Currently, animal feed is supplemented with chemically synthesized methionine, which could be completely replaced with naturally synthesized methionine. However, insufficient levels of methionine are due to alleles of two genes in the maize genome that are expressed during seed development, which have a high percentage of methionine codons, ranging from 23 to 28%, while free methionine is very low. The two genes, dzs10 and dzs18, belong to the prolamin gene family that arose during the evolution of the grasses and were duplicated during a whole genome duplication event. We have found several dzs10 and dzs18 null alleles caused either by transposon insertion or frame shift mutations. Maize seeds with null mutations of both genes have a normal phenotype in contrast to other prolamin genes, explaining the accumulation of methionine deficiency in normal breeding efforts. Moreover, the trans-regulation of these genes deviates from Mendelian inheritance. One allele of the regulatory locus dzr1 is inherited in a parent-of-origin fashion, while another allele appears to prevent Mendelian segregation of the high-methionine phenotype in backcrosses.

Arabidopsis bZIP60 is a proteolysis-activated transcription factor involved in the endoplasmic reticulum stress response

Proteins synthesized in the endoplasmic reticulum (ER) of eukaryotic cells must be folded correctly before translocation out of the ER. Disruption of protein folding results in the induction of genes for ER-resident chaperones, for example, BiP. This phenomenon is known as the ER stress response. We report here that bZIP60, an Arabidopsis thaliana basic leucine zipper (bZIP) transcription factor with a transmembrane domain, is involved in the ER stress response. When compared with wild-type Arabidopsis plants, homozygous bzip60 mutant plants show a markedly weaker induction of many ER stress-responsive genes. The bZIP60 protein resides in the ER membrane under unstressed condition and is cleaved in response to ER stress caused by either tunicamycin or DTT. The N-terminal fragment containing the bZIP domain is then translocated into the nucleus. Cleavage of bZIP60 is independent of the function of Arabidopsis homologs of mammalian S1P and S2P proteases, which mediate the proteolytic cleavage of the mammalian transcription factor ATF6. In Arabidopsis, expression of the bZIP60 gene and cleavage of the bZIP60 protein are observed in anthers in the absence of stress treatment, suggesting that the ER stress response functions in the normal development of active secretory cells.

Endoplasmic reticulum quality control and the unfolded protein response: insights from plants

Protein quality control (QC) within the endoplasmic reticulum and the related unfolded protein response (UPR) pathway of signal transduction are major regulators of the secretory pathway, which is involved in virtually any aspect of development and reproduction. The study of plant-specific processes such as pathogen response, seed development and the synthesis of seed storage proteins and of particular toxins is providing novel insights, with potential implications for the general recognition events and mechanisms of action of QC and UPR.

Calreticulin mRNA and protein are localized to protein bodies in storage maize callus cells

Maize callus cells possess numerous protein bodies which develop as sub-compartments of the endoplasmic reticulum. We localized maize calreticulin mRNAs and protein in maize callus cells using in situ hybridization and immunocytochemistry. Calreticulin mRNAs were selectively targeted to the endoplasmic reticulum (ER) subdomains surrounding protein bodies. Profilin mRNAs, used as a positive control for in situ hybridization experiments, showed distinct and rather diffuse localization pattern. Using both, immunofluorescence and immunogold electron microscopy localization techniques, calreticulin was found to be enriched around and within protein bodies in maize callus storage cells. As a positive control for reticuloplasmins, HDEL antibody revealed labelling of protein bodies and of the nuclear envelope. The identity of protein bodies was confirmed by specific binding of an alpha zein antibody. These data suggest that calreticulin mRNA is targeted towards protein body forming subdomains of the ER, and that calreticulin is localized and enriched in these protein bodies. The possibility that calreticulin plays an important role in zein retention within the ER and/or its assembly and packaging into protein bodies during protein body biogenesis in maize callus is discussed.

Molecular mapping of the shrunken endosperm genes seg8 and sex1 in barley (Hordeum vulgare L.)

A number of mutations affecting seed development in barley (Hordeum vulgare L.) have been known for many years; however, to date, no research has been reported that elucidates the molecular structure of the causal genes. As a first step, we initiated the linkage mapping of the two shrunken endosperm genes seg8 and sex1 using microsatellite markers. The recessive gene seg8 was mapped in the centromeric region of chromosome 7H to a 4.6 cM interval flanked by markers GBM1516 and Bmag341. The recessive sex1 gene showed xenia effects and was located in the centromeric region of barley chromosome 6H, which is in accordance to the previously reported chromosomal location in the classical linkage map. It was flanked by markers GBM5012 and GBM1063 in a 4.2 cM interval. EST-derived microsatellite markers were used to establish the syntenic relationships to the genomic rice sequences. Two orthologous sites on rice chromosome 2 flanking a 4.1 Mb sequence had homology to the respective barley markers in the sex1 region. For the markers in the seg8 region orthologous sites on rice chromosome 6 were detected.

The correlation between expression and localization of a foreign gene product in rice endosperm

Glucagon-like peptide 1 (GLP-1) is a 30 amino acid peptide hormone involved in insulin stimulation that is dependent upon blood glucose levels. We have previously reported that when this short peptide gene was directly expressed under the control of a glutelin promoter and its signal peptide, it was not accumulated in transgenic rice seed due to gene silencing. However, when the modified GLP-1 (mGLP-1) gene was enlarged to 5xmGLP-1 (mGLPx5) by tandem repeat, no silencing was observed. The mGLPx5 peptide could be accumulated in rice seed and its localization was mainly limited to the endoplasmic reticulum (ER). We also investigated alternative cellular localization sites that would increase accumulation. The relationship between the expression level and localization was examined by attaching the chitinase signal peptide to mGLPx5 to direct it into the intercellular space (apoplast), or by expression as a fusion protein with glutelin by insertion into a variable region of the acidic subunit, thus directing the peptide to protein body II (PB II). Attachment of the KDEL ER retention signal to the 6xmGLP-1 (mGLPx6) or its fusion to the C-terminus of the 13 kDa prolamin directed the peptide to the ER or PB I, respectively. Unexpectedly, these results indicated that mGLPx5 without any signal except for the glutelin signal peptide was accumulated to the greatest extent in rice endosperm. It can thus be concluded that the ER is a suitable intracellular organelle for accumulation of mGLPx5 peptide.

Effect of the opaque and floury mutations on the accumulation of dry matter and protein fractions in maize endosperm

Grains of nine opaque (o) and floury (fl) mutants of maize (Oh43o1, Oh43o2, B79o5, B37o7, W22o10, W22o11, W22o13, Oh43fl1 and Oh43fl2) were examined for the weight proportions of their component tissues and the content of eight nitrogen fractions in their endosperms. A linear regression was found connecting the amounts (mg per endosperm) of zeins and true proteins (crude proteins minus non-protein nitrogen) for the non-opaque2 mutants. The data points connecting zeins to true proteins present in the mature endosperms of six wild-type (+) inbred lines and their o2 versions were located outside (+) or within (o2) the 95% confidence range of the regression line. The data obtained from the developing and mature endosperms of the W22o7 inbred line (Di Fonzo et al., Plant Sci. Lett., 1979, 77) and the floury portion of mature endosperms of three other wild-type inbred lines fell practically on the regression line. The effects of genotype and environmental factors upon the relative accumulation rate of zeins were assessed from the present results and the data taken from the literature concerning the quantitative interdependence between zeins and true proteins in immature and mature endosperms.

Identification and characterization of endoplasmic reticulum-associated degradation proteins differentially affected by endoplasmic reticulum stress

The disposal of misfolded proteins from the lumen of the endoplasmic reticulum (ER) is one of the quality control mechanisms present in the protein secretory pathway. Through ER-associated degradation, misfolded substrates are targeted to the cytosol where they are degraded by the proteasome. We have identified four maize (Zea mays) Der1-like genes (Zm Derlins) that encode homologs of Der1p, a yeast (Saccharomyces cerevisiae) protein implicated in ER-associated degradation. Zm Derlins are capable of functionally complementing a yeast Der1 deletion mutant. Such complementation indicates that the Der1p function is conserved among species. Zm Derlin genes are expressed at low levels throughout the plant, but appear prevalent in tissues with high activity of secretory protein accumulation, including developing endosperm cells. Expression of three of the four Zm Derlin genes increases during ER stress, with Zm Derlin1-1 showing the strongest induction. Subcellular fractionation experiments localized Zm Derlin proteins to the membrane fraction of microsomes. In maize endosperm, Zm Derlin proteins were found primarily associated with ER-derived protein bodies regardless of the presence of an ER stress response.

Phylogenetic analyses identify 10 classes of the protein disulfide isomerase family in plants, including single-domain protein disulfide isomerase-related proteins

Protein disulfide isomerases (PDIs) are molecular chaperones that contain thioredoxin (TRX) domains and aid in the formation of proper disulfide bonds during protein folding. To identify plant PDI-like (PDIL) proteins, a genome-wide search of Arabidopsis (Arabidopsis thaliana) was carried out to produce a comprehensive list of 104 genes encoding proteins with TRX domains. Phylogenetic analysis was conducted for these sequences using Bayesian and maximum-likelihood methods. The resulting phylogenetic tree showed that evolutionary relationships of TRX domains alone were correlated with conserved enzymatic activities. From this tree, we identified a set of 22 PDIL proteins that constitute a well-supported clade containing orthologs of known PDIs. Using the Arabidopsis PDIL sequences in iterative BLAST searches of public and proprietary sequence databases, we further identified orthologous sets of 19 PDIL sequences in rice (Oryza sativa) and 22 PDIL sequences in maize (Zea mays), and resolved the PDIL phylogeny into 10 groups. Five groups (I-V) had two TRX domains and showed structural similarities to the PDIL proteins in other higher eukaryotes. The remaining five groups had a single TRX domain. Two of these (quiescin-sulfhydryl oxidase-like and adenosine 5'-phosphosulfate reductase-like) had putative nonisomerase enzymatic activities encoded by an additional domain. Two others (VI and VIII) resembled small single-domain PDIs from Giardia lamblia, a basal eukaryote, and from yeast. Mining of maize expressed sequence tag and RNA-profiling databases indicated that members of all of the single-domain PDIL groups were expressed throughout the plant. The group VI maize PDIL ZmPDIL5-1 accumulated during endoplasmic reticulum stress but was not found within the intracellular membrane fractions and may represent a new member of the molecular chaperone complement in the cell.

Expression of the sorghum 10-member kafirin gene cluster in maize endosperm

Functional analysis of chromosomal segments containing linked genes requires the insertion of contiguous genomic sequences from bacterial artificial chromosomes (BACs) into the genome. Therefore, we introduced a 90-kb large BAC clone carrying a 10-copy tandem array of kafirin storage protein genes from sorghum linkage group J, mixed with a selectable marker gene, directly into maize cells using the particle bombardment method. Transgenic plants were regenerated and seeds from eight different transgenic lines were produced. One such transgenic plant was selected that had the entire kafirin gene cluster on a single continuous DNA fragment spanning more than 45 kb integrated into its genome. When alcohol-soluble proteins from individual T2 and T3 seeds of this event were analyzed, significant levels of kafirin were found in addition to the endogenous zein storage proteins, demonstrating that the large exogenous DNA segment is stably integrated into the maize genome and expressed at high levels in subsequent generations. Therefore, we could provide a new utility of plant transformation by the particle bombardment method for functional genomics of multigene families and the modification of the nutritive quality of cereal grains. Despite a tandem array of highly homologous sequences at the transgenic locus, no gene silencing was observed, probably owing to the effects of co-transformed flanking sequences. The expression studies of the transgenic locus also revealed new features of storage protein gene promoters that differed from previous transient gene expression studies, thereby illustrating the significance of the concentration and configuration of DNA-protein interactions in the regulation of gene expression.

Induced mutations in cassava using somatic embryos and the identification of mutant plants with altered starch yield and composition

A cyclic somatic embryogenic system was used to induce mutations in cassava variety PRC 60a in vitro. Globular-stage somatic embryos were selected as suitable experimental materials, and 50 Gy of gamma-rays was determined to be the optimal dose for inducing mutations. During subsequent field trials, more than 50% of the regenerated mutant lines varied morphologically from wild-type plants. Consequently, we used this approach to induce genetic variability for obtaining novel cassava cultivars. Among the different mutant lines obtained, lines S14 and S15 showed large morphological variations. In 10-month-old S14 and S15 mutant lines, storage root yield was reduced 17-fold and 60-fold, respectively, compared to wild-type plants, while the storage roots of S15 mutant plants also exhibited an almost 50% decrease in starch content and a significant reduction (30%) in amylose content. These two features were observed throughout the different developmental stages of the storage roots in S15 plants.

A defective signal peptide in a 19-kD alpha-zein protein causes the unfolded protein response and an opaque endosperm phenotype in the maize De*-B30 mutant

Defective endosperm* (De*)-B30 is a dominant maize (Zea mays) mutation that depresses zein synthesis in the developing endosperm. The mutant kernels have an opaque, starchy phenotype, malformed zein protein bodies, and highly increased levels of binding protein and other chaperone proteins in the endosperm. Immunoblotting revealed a novel alpha-zein protein in De*-B30 that migrates between the 22- and 19-kD alpha-zein bands. Because the De*-B30 mutation maps in a cluster of 19-kD alpha-zein genes, we characterized cDNA clones encoding these proteins from a developing endosperm library. This led to the identification of a 19-kD alpha-zein cDNA in which proline replaces serine at the 15th position of the signal peptide. Although the corresponding gene does not appear to be highly expressed in De*-B30, it was found to be tightly linked with the mutant phenotype in a segregating F2 population. Furthermore, when the protein was synthesized in yeast cells, the signal peptide appeared to be less efficiently processed than when serine replaced proline. To test whether this gene is responsible for the De*-B30 mutation, transgenic maize plants expressing this sequence were created. T1 seeds originating from the transformants manifested an opaque kernel phenotype with enhanced levels of binding protein in the endosperm, similar to De*-B30. These results are consistent with the hypothesis that the De*-B30 mutation causes a defective signal peptide in a 19-kD alpha-zein protein.

Plant BiP gene family: differential expression, stress induction and protective role against physiological stresses

In contrast to yeast or mammalian counterpart, BiP (Binding Protein) from several plant species, such as maize, tobacco, Arabidopsis and soybean, is encoded by a multigene family. A systematic characterization and analysis of soybean BiP expression have provided evidence for the existence of multiple, complex regulatory mechanisms controlling plant BiP gene expression. In support of this observation, the soybean BiP gene family has been shown to exhibit organ-specific expression and differential regulation in response to abiotic stresses through distinct signaling pathways. As a member of the stress-regulated HSP70 family of protein, the elucidation of plant BiP function and regulation is likely to lead do new strategies to enhance crop tolerance to environmental stress. Consistent with this observation, transgenic plants overexpressing soybean BiP have demonstrated to exhibit increased tolerance to ER (endoplasmic reticulum) stressors during seed germination and enhanced tolerance to water deficit during plant growth.

Protein sorting and expression of a unique soybean cotyledon protein, GmSBP, destined for the protein storage vacuole

The initial biochemical characterization of the soybean sucrose-binding protein, GmSBP, within our lab and others produced several incongruous characteristics that required a re-characterization of GmSBP via sequence homology, cell biology, immunolocalization, and semi-quantitative analysis. The GmSBP proteins share amino acid sequence homology as well as putative structural homology with globulin-like seed storage proteins. A comparison to the major soybean seed storage proteins, glycinin and beta-conglycinin established several storage protein-like characteristics for GmSBP. All three proteins were present in a prevacuolar compartment and protein storage vacuole. All three proteins increased in expression during seed development and are remobilized during germination. Quantitatively, the relative concentrations of GmSBP, beta-conglycinin (alpha/alpha' subunits), and glycinin (acidic subunits) indicated that GmSBP contributes 19-fold less to the stored nitrogen. The quantitative differences between GmSBP and glycinin may be attributed to the unconserved order and spacing of cis-acting regulatory elements present within the promoter regions. Ultimately, GmSBP is transported to the mature protein storage vacuole. The biological function of GmSBP within the protein storage vacuole remains uncertain, but its localization is a remnant of its evolutionary link to a globulin-like or vicilin-like ancestor that gave rise to the 7S family of storage proteins.

Contiguous genomic DNA sequence comprising the 19-kD zein gene family from maize

A new approach has been undertaken to analyze the sequences and linear organization of the 19-kD zein genes in maize (Zea mays). A high-coverage, large-insert genomic library of the inbred line B73 based on bacterial artificial chromosomes was used to isolate a redundant set of clones containing members of the 19-kD zein gene family, which previously had been estimated to consist of 50 members. The redundant set of clones was used to create bins of overlapping clones that represented five distinct genomic regions. Representative clones containing the entire set of 19-kD zein genes were chosen from each region and sequenced. Seven bacterial artificial chromosome clones yielded 1,160 kb of genomic DNA. Three of them formed a contiguous sequence of 478 kb, the longest contiguous sequenced region of the maize genome. Altogether, these DNA sequences provide the linear organization of 25 19-kD zein genes, one-half the number previously estimated. It is suggested that the difference is because of haplotypes exhibiting different degrees of gene amplification in the zein multigene family. About one-half the genes present in B73 appear to be expressed. Because some active genes have only been duplicated recently, they are so conserved in their sequence that previous cDNA sequence analysis resulted in "unigenes" that were actually derived from different gene copies. This analysis also shows that the 22- and 19-kD zein gene families shared a common ancestor. Although both ancestral genes had the same incremental gene amplification, the 19-kD zein branch exhibited a greater degree of far-distance gene translocations than the 22-kD zein gene family.