Functions of maize genes encoding pyruvate phosphate dikinase in developing endosperm

Genetic variation in ZmKW1 contributes to kernel weight and size in dent corn and popcorn

Kernel weight is a critical factor that essentially affects maize (Zea mays) yield. In natural inbred lines, popcorn kernels exhibit overtly smaller sizes compared to dent corn kernels, and kernel weight, which is controlled by multiple genetic loci, varies widely. Here, we characterized a major quantitative trait locus on chromosome 1, responsible for controlling kernel weight (qKW1) and size. The qKW1 locus encodes a protein containing a seven in absentia domain with E3 ubiquitin ligase activity, expressed prominently from the top to the middle region of the endosperm. The presence and function of qKW1 were confirmed through ZmKW1 gene editing, where the mutations in ZmKW1 within dent corn significantly increased kernel weight, consistent with alterations in kernel size, while overexpression of ZmKW1 had the opposite effect. ZmKW1 acts as a negative regulator of kernel weight and size by reducing both the number and size of the endosperm cells and impacting endosperm filling. Notably, the popcorn allele qKW1N and the dent corn allele qKW1D encode identical proteins; however, the differences in promoter activity arise due to the insertion of an Indel-1346 sequence in the qKW1N promoter, resulting in higher expression levels compared to qKW1D, thus contributing to the variation in kernel weight and size between popcorn and dent corn kernels. Linkage disequilibrium analysis of the 2.8 kb promoter region of ZmKW1 in a dataset comprising 111 maize association panels identified two distinct haplotypes. Our results provide insight into the mechanisms underlying kernel development and yield regulation in dent corn and popcorn, with a specific focus on the role of the ubiquitination system.

Orchestrating seed storage protein and starch accumulation toward overcoming yield-quality trade-off in cereal crops

Achieving high yield and good quality in crops is essential for human food security and health. However, there is usually disharmony between yield and quality. Seed storage protein (SSP) and starch, the predominant components in cereal grains, determine yield and quality, and their coupled synthesis causes a yield-quality trade-off. Therefore, dissection of the underlying regulatory mechanism facilitates simultaneous improvement of yield and quality. Here, we summarize current findings about the synergistic molecular machinery underpinning SSP and starch synthesis in the leading staple cereal crops, including maize, rice and wheat. We further evaluate the functional conservation and differentiation of key regulators and specify feasible research approaches to identify additional regulators and expand insights. We also present major strategies to leverage resultant information for simultaneous improvement of yield and quality by molecular breeding. Finally, future perspectives on major challenges are proposed.

Dry-milled flour rice 'Seolgaeng' harbors a mutated fructose-6-phosphate 2-kinase/fructose-2,6-bisphosphatase2

'Seolgaeng', an opaque-endosperm rice (Oryza sativa) mutant, is used to prepare high-quality dry-milled rice flour. The mutation causing its opaque-endosperm phenotype was unknown. Map-based cloning identified a missense mutation in the gene FRUCTOSE-6-PHOSPHATE 2-KINASE/FRUCTOSE-2,6-BISPHOSPHATASE 2 (OsF2KP2) in Seolgaeng. Transfer DNA insertion and clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated nuclease 9 (Cas9)-induced f2kp2 mutants exhibited opaque endosperm. Rice harbors another F2KP gene, OsF2KP1. CRISPR/Cas9-induced double mutants of OsF2KP1 and OsF2KP2 (f2kp-d) possessed more opaque endosperm compared to f2kp2 single mutants, whereas the endosperm of the f2kp1 single mutant was normal. Grain hardness and damaged starch content were significantly reduced in f2kp2 mutants compared to the wild type and f2kp1. Amylose content was lower than normal in f2kp2 mutants but not f2kp1. Grain hardness and amylose content were much lower in f2kp-d than in f2kp2. Starch polymerization analysis revealed altered amylopectin structure in f2kp2 and f2kp-d mutants. F2KP activity was lower in f2kp2 and much lower in the double mutants when compared to the wild types, but f2kp1 showed no significant difference. In coleoptiles, hypoxia induced OsF2KP2 expression but downregulated OsF2KP1. These results suggest that OsF2KP2 functions as the main F2KP isoform in endosperm experiencing hypoxia, but OsF2KP1 may partially compensate for the absence of OsF2KP2. We propose that F2KP has a crucial role in inorganic pyrophosphate-utilizing energy metabolism for starch biosynthesis in rice endosperm.

A Systemic Investigation of Genetic Architecture and Gene Resources Controlling Kernel Size-Related Traits in Maize

Grain yield is the most critical and complex quantitative trait in maize. Kernel length (KL), kernel width (KW), kernel thickness (KT) and hundred-kernel weight (HKW) associated with kernel size are essential components of yield-related traits in maize. With the extensive use of quantitative trait locus (QTL) mapping and genome-wide association study (GWAS) analyses, thousands of QTLs and quantitative trait nucleotides (QTNs) have been discovered for controlling these traits. However, only some of them have been cloned and successfully utilized in breeding programs. In this study, we exhaustively collected reported genes, QTLs and QTNs associated with the four traits, performed cluster identification of QTLs and QTNs, then combined QTL and QTN clusters to detect consensus hotspot regions. In total, 31 hotspots were identified for kernel size-related traits. Their candidate genes were predicted to be related to well-known pathways regulating the kernel developmental process. The identified hotspots can be further explored for fine mapping and candidate gene validation. Finally, we provided a strategy for high yield and quality maize. This study will not only facilitate causal genes cloning, but also guide the breeding practice for maize.

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.

Fructose-6-phosphate-2-kinase/fructose-2,6-bisphosphatase regulates energy metabolism and synthesis of storage products in developing rice endosperm

Starch accounts for about 80-85 % of the dry weight of grains and determines yield by impact on grain weight. And, the content and composition of starch also determine appearance, eating, cooking and nutritional quality of rice. By coordinating crucial reactions of the primary carbohydrate metabolism in all eukaryotes, fructose-2,6-bisphosphate (Fru-2,6-P₂) is a traffic signal in metabolism. However, the metabolic regulation of starch in plant sink tissues by Fru-2,6-P₂ remains unclear. Here we isolated rice mutant floury endosperm23 (flo23) which has opaque endosperm and anomalous compound starch grains (SGs). flo23 mutant grains had reduced contents of starch, lipids and proteins. Map-based cloning and genetic complementation experiments showed that FLO23 encodes a cytoplasmic Fructose-6-phosphate-2-kinase/Fructose-2,6-bisphosphatase (F2KP). Mutation of OsF2KP2 decreased Fru-2,6-P₂ content in endosperm cells, leading to drastically reduced phosphoenolpyruvate (PEP) and pyruvate contents and disordered glycolysis and energy metabolism. The results imply that OsF2KP2 participates in the glycolytic pathway by providing precursors and energy for synthesis of grain storage compounds.

Encoded C4 homologue enzymes genes function under abiotic stresses in C3 plant

Plant organisms assimilate CO₂ through the photosynthetic pathway, which facilitates in the synthesis of sugar for plant development. As environmental elements including water level, CO₂ concentration, temperature and soil characteristics change, the plants may recruit series of genes to help adapt the hostile environments and challenges. C4 photosynthesis plants are an excellent example of plant evolutionary adaptation to diverse condition. Compared with C3 photosynthesis plants, C4 photosynthesis plants have altered leaf anatomy and new metabolism for CO₂ capture, with multiple related enzymes such as phosphoenolpyruvate carboxylase (PEPCase), pyruvate orthophosphate dikinase (PPDK), NAD(P)-malic enzyme (NAD(P)-ME), NAD(P) - malate dehydrogenase (NAD(P)-MDH) and carbonic anhydrases (CA), identified to participate in the carbon concentrating mechanism (CCM) pathway. Recently, great achievements about C4 CCM-related genes have been made in the dissection of C3 plant development processes involving various stresses. In this review, we describe the functions of C4 CCM-related homologous genes in carbon and nitrogen metabolism in C3 plants. We further summarize C4 CCM-related homologous genes' functions in response to stresses in C3 plants. The understanding of C4 CCM-related genes' function in response to abiotic stress in plant is important to modify the crop plants for climate diversification.

Role of C4 photosynthetic enzyme isoforms in C3 plants and their potential applications in improving agronomic traits in crops

As compared to C3, C4 plants have higher photosynthetic rates and better tolerance to high temperature and drought. These traits are highly beneficial in the current scenario of global warming. Interestingly, all the genes of the C4 photosynthetic pathway are present in C3 plants, although they are involved in diverse non-photosynthetic functions. Non-photosynthetic isoforms of carbonic anhydrase (CA), phosphoenolpyruvate carboxylase (PEPC), malate dehydrogenase (MDH), the decarboxylating enzymes NAD/NADP-malic enzyme (NAD/NADP-ME), and phosphoenolpyruvate carboxykinase (PEPCK), and finally pyruvate orthophosphate dikinase (PPDK) catalyze reactions that are essential for major plant metabolism pathways, such as the tricarboxylic acid (TCA) cycle, maintenance of cellular pH, uptake of nutrients and their assimilation. Consistent with this view differential expression pattern of these non-photosynthetic C3 isoforms has been observed in different tissues across the plant developmental stages, such as germination, grain filling, and leaf senescence. Also abundance of these C3 isoforms is increased considerably in response to environmental fluctuations particularly during abiotic stress. Here we review the vital roles played by C3 isoforms of C4 enzymes and the probable mechanisms by which they help plants in acclimation to adverse growth conditions. Further, their potential applications to increase the agronomic trait value of C3 crops is discussed.

Transcriptome and Metabolome Analyses Reveals the Pathway and Metabolites of Grain Quality Under Phytochrome B in Rice (Oryza sativa L.)

BACKGROUND

Grain size and chalkiness is a critical agronomic trait affecting rice yield and quality. The application of transcriptomics to rice has widened the understanding of complex molecular responsive mechanisms, differential gene expression, and regulatory pathways under varying conditions. Similarly, metabolomics has also contributed drastically for rice trait improvements. As master regulators of plant growth and development, phys influence seed germination, vegetative growth, photoperiodic flowering, shade avoidance responses. OsPHYB can regulate a variety of plant growth and development processes, but little is known about the roles of rice gene OsPHYB in modulating grain development.

RESULTS

In this study, rice phytochrome B (OsPHYB) was edited using CRISPR/Cas9 technology. We found that OsPHYB knockout increased rice grain size and chalkiness, and increased the contents of amylose, free fatty acids and soluble sugar, while the gel consistency and contents of proteins were reduced in mutant grains. Furthermore, OsPHYB is involved in the regulation of grain size and chalk formation by controlling cell division and complex starch grain morphology. Transcriptomic analysis revealed that loss of OsPHYB function affects multiple metabolic pathways, especially enhancement of glycolysis, fatty acid, oxidative phosphorylation, and antioxidant pathways, as well as differential expression of starch and phytohormone pathways. An analysis of grain metabolites showed an increase in the free fatty acids and lysophosphatidylcholine, whereas the amounts of sugars, alcohols, amino acids and derivatives, organic acids, phenolic acids, alkaloids, nucleotides and derivatives, and flavonoids decreased, which were significantly associated with grain size and chalk formation.

CONCLUSIONS

Our study reveals that, OsPHYB plays an important regulatory role in the growth and development of rice grains, especially grain size and chalkiness. Furthermore, OsPHYB regulates grain size and chalkiness formation by affecting gene metabolism interaction network. Thus, this study not only revealed that OsPHYB plays a vital role in regulating grain size and chalkiness of rice but reveal new functions and highlighted the importance and value of OsPHYB in rice grain development and provide a new strategy for yield and quality improvement in rice breeding.

Energy status-promoted growth and development of Arabidopsis require copper deficiency response transcriptional regulator SPL7

Copper (Cu) is a cofactor of around 300 Arabidopsis proteins, including photosynthetic and mitochondrial electron transfer chain enzymes critical for adenosine triphosphate (ATP) production and carbon fixation. Plant acclimation to Cu deficiency requires the transcription factor SQUAMOSA PROMOTER-BINDING PROTEIN-LIKE7 (SPL7). We report that in the wild type (WT) and in the spl7-1 mutant, respiratory electron flux via Cu-dependent cytochrome c oxidase is unaffected under both normal and low-Cu cultivation conditions. Supplementing Cu-deficient medium with exogenous sugar stimulated growth of the WT, but not of spl7 mutants. Instead, these mutants accumulated carbohydrates, including the signaling sugar trehalose 6-phosphate, as well as ATP and NADH, even under normal Cu supply and without sugar supplementation. Delayed spl7-1 development was in agreement with its attenuated sugar responsiveness. Functional TARGET OF RAPAMYCIN and SNF1-RELATED KINASE1 signaling in spl7-1 argued against fundamental defects in these energy-signaling hubs. Sequencing of chromatin immunoprecipitates combined with transcriptome profiling identified direct targets of SPL7-mediated positive regulation, including Fe SUPEROXIDE DISMUTASE1 (FSD1), COPPER-DEFICIENCY-INDUCED TRANSCRIPTION FACTOR1 (CITF1), and the uncharacterized bHLH23 (CITF2), as well as an enriched upstream GTACTRC motif. In summary, transducing energy availability into growth and reproductive development requires the function of SPL7. Our results could help increase crop yields, especially on Cu-deficient soils.

Dynamic Transcriptome-Based Weighted Gene Co-expression Network Analysis Reveals Key Modules and Hub Genes Associated With the Structure and Nutrient Formation of Endosperm for Wax Corn

The endosperm of corn kernel consists of two components, a horny endosperm, and a floury endosperm. In the experiment, a kind of floury endosperm corn was identified. The result of phenotypic trait analysis and determination of amino acid content showed that the floury endosperm filled with the small, loose, and scattered irregular spherical shape starch granules and contained higher content of amino acid. The starch biochemical properties are similar between floury corns and regular flint corn. By using dynamically comparative transcriptome analysis of endosperm at 20, 25, and 30 DAP, a total of 113.42 million raw reads and 50.508 thousand genes were obtained. By using the weighted gene co-expression network analysis, 806 genes and six modules were identified. And the turquoise module with 459 genes was proved to be the key module closely related to the floury endosperm formation. Nine zein genes in turquoise module, including two zein-alpha A20 (Zm00001d019155 and Zm00001d019156), two zein-alpha A30 (Zm00001d048849 and Zm00001d048850), one 50 kDa gamma-zein (Zm00001d020591), one 22 kDa alpha-zein 14 (Zm00001d048817), one zein-alpha 19D1 (Zm00001d030855), one zein-alpha 19B1 (Zm00001d048848), and one FLOURY 2 (Zm00001d048808) were identified closely related the floury endosperm formation. Both zein-alpha 19B1 (Zm00001d048848) and zein-alpha A30 (Zm00001d048850) function as source genes with the highest expression level in floury endosperm. These results may provide the supplementary molecular mechanism of structure and nutrient formation for the floury endosperm of maize.

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.

Cytosolic malate dehydrogenase 4 modulates cellular energetics and storage reserve accumulation in maize endosperm

Cytosolic malate dehydrogenase (MDH) is a key enzyme that regulates the interconversion between malate and oxaloacetate (OAA). However, its role in modulating storage compound accumulation in maize endosperm is largely unknown. Here, we characterized a novel naturally occurring maize mdh4-1 mutant, which produces small, opaque kernels and exhibits reduced starch but enhanced lysine content. Map-based cloning, functional complementation and allelism analyses identified ZmMdh4 as the causal gene. Enzymatic assays demonstrated that ZmMDH4 predominantly catalyses the conversion from OAA to malate. In comparison, the activity of the mutant enzyme, which lacks one glutamic acid (Glu), was completed abolished, demonstrating that the Glu residue was essential for ZmMDH4 function. Knocking down ZmMdh4 in vivo led to a substantial metabolic shift towards glycolysis and a dramatic disruption in the activity of the mitochondrial complex I, which was correlated with transcriptomic alterations. Taken together, these results demonstrate that ZmMdh4 regulates the balance between mitochondrial respiration and glycolysis, ATP production and endosperm development, through a yet unknown feedback regulatory mechanism in mitochondria.

A SnRK1-ZmRFWD3-Opaque2 Signaling Axis Regulates Diurnal Nitrogen Accumulation in Maize Seeds

Zeins are the predominant storage proteins in maize (Zea mays) seeds, while Opaque2 (O2) is a master transcription factor for zein-encoding genes. How the activity of O2 is regulated and responds to external signals is yet largely unknown. Here, we show that the E3 ubiquitin ligase ZmRFWD3 interacts with O2 and positively regulates its activity by enhancing its nuclear localization. Ubiquitination of O2 enhances its interaction with maize importin1, the α-subunit of Importin-1 in maize, thus enhancing its nuclear localization ability. We further show that ZmRFWD3 can be phosphorylated by a Suc-responsive protein kinase, ZmSnRK1, which leads to its degradation. We demonstrated that the activity of O2 responds to Suc levels through the ZmSnRK1-ZmRFWD3-O2 signaling axis. Intriguingly, we found that Suc levels, as well as ZmRFWD3 levels and the cytonuclear distribution of O2, exhibit diurnal patterns in developing endosperm, leading to the diurnal transcription of O2-regulated zein genes. Loss of function in ZmRFWD3 disrupts the diurnal patterns of O2 cytonuclear distribution and zein biosynthesis, and consequently changes the C/N ratio in mature seeds. We therefore identify a SnRK1-ZmRFWD3-O2 signaling axis that transduces source-to-sink signals and coordinates C and N assimilation in developing maize seeds.

Review: Crucial role of inorganic pyrophosphate in integrating carbon metabolism from sucrose breakdown to starch synthesis in rice endosperm

The endosperm is a primary constituent of mature seeds in rice as well as in other cereal crops, serving as the major storage reserve of starch. Observations indicate that the central part of the endosperm is subject to hypoxic conditions, which require a switch of energy metabolism owing to limited mitochondrial respiration. Uniquely, this endosperm generates a large source of inorganic pyrophosphate (PPi) as a byproduct of the reaction of ADP glucose pyrophosphorylase in the cytosol. Recent results derived from examination of the mutants of cereal crops, especially rice, for PPi-utilizing enzymes clearly suggest an important role of PPi as an alternative energy currency for integrating carbon metabolism from sucrose breakdown to starch synthesis in the endosperm. Thus, the present review provides an outline of the interlaced PPi-dependent metabolic pathways, which are critical for starch synthesis in the endosperm in terms of energy metabolism, along with its application to enhance yield potential.

A comparison of proteomic, genomic, and osteological methods of archaeological sex estimation

Sex estimation of skeletons is fundamental to many archaeological studies. Currently, three approaches are available to estimate sex-osteology, genomics, or proteomics, but little is known about the relative reliability of these methods in applied settings. We present matching osteological, shotgun-genomic, and proteomic data to estimate the sex of 55 individuals, each with an independent radiocarbon date between 2,440 and 100 cal BP, from two ancestral Ohlone sites in Central California. Sex estimation was possible in 100% of this burial sample using proteomics, in 91% using genomics, and in 51% using osteology. Agreement between the methods was high, however conflicts did occur. Genomic sex estimates were 100% consistent with proteomic and osteological estimates when DNA reads were above 100,000 total sequences. However, more than half the samples had DNA read numbers below this threshold, producing high rates of conflict with osteological and proteomic data where nine out of twenty conditional DNA sex estimates conflicted with proteomics. While the DNA signal decreased by an order of magnitude in the older burial samples, there was no decrease in proteomic signal. We conclude that proteomics provides an important complement to osteological and shotgun-genomic sex estimation.

The defective effect of starch branching enzyme IIb from weak to strong induces the formation of biphasic starch granules in amylose-extender maize endosperm

Biphasic starch granules in maize ae mutant underwent the weak to strong SBEIIb-defective effect during endosperm development, leading to no birefringence in their exterior due to extended long branch-chains of amylopectin. Biphasic starch granules are usually detected regionally in cereal endosperm lacking starch branching enzyme (SBE). However, their molecular structure, formation mechanism, and regional distribution are unclear. In this research, biphasic starch granules were observed in the inner region of crown endosperm of maize ae mutant, and had poorly oriented structure with comb-like profiles in their exterior. The inner endosperm (IE) rich in biphasic starch granules and outer endosperm (OE) without biphasic starch granules were investigated. The starch had lower amylose content and higher proportion of long branch-chains of amylopectin in IE than in OE, and the exterior of biphasic starch granules had less amylose and more long branch-chains of amylopectin than the interior. Compared with OE, the expression pattern of starch synthesis related enzymes changed significantly in IE. The granule-bound starch synthase I activity within biphasic starch granules decreased slightly. The IE experienced more severe hypoxic stress than OE, and the up-regulated anaerobic respiration pathway indicated an increase in carbon consumption. The starch in IE underwent the SBEIIb-defective effect from weak to strong due to the lack of sufficient carbon inflow, leading to the formation of biphasic starch granules and their regional distribution in endosperm. The results provided information for understanding the biphasic starch granules.

A New SNP in Rice Gene Encoding Pyruvate Phosphate Dikinase (PPDK) Associated with Floury Endosperm

Rice varieties with suitable flour-making qualities are required to promote the rice processed-food industry and to boost rice consumption. A rice mutation, Namil(SA)-flo1, produces grains with floury endosperm. Overall, grains with low grain hardness, low starch damage, and fine particle size are more suitable for use in flour processing grains with waxy, dull endosperm with normal grain hardness and a high amylose content. In this study, fine mapping found a C to T single nucleotide polymorphism (SNP) in exon 2 of the gene encoding cytosolic pyruvate phosphate dikinase (cyOsPPDK). The SNP resulted in a change of serine to phenylalanine acid at amino acid position 101. The gene was named FLOURY ENDOSPERM 4-5 (FLO4-5). Co-segregation analysis with the developed cleaved amplified polymorphic sequence (CAPS) markers revealed co-segregation between the floury phenotype and the flo4-5. This CAPS marker could be applied directly for marker-assisted selection. Real-time RT-PCR experiments revealed that PPDK was expressed at considerably higher levels in the flo4-5 mutant than in the wild type during the grain filling stage. Plastid ADP-glucose pyrophosphorylase small subunit (AGPS2a and AGPS2b) and soluble starch synthase (SSIIb and SSIIc) also exhibited enhanced expression in the flo4-5 mutant.

Proteogenomic analysis of pitaya reveals cold stress-related molecular signature

Pitayas (Hylocereus spp.) is an attractive, highly nutritious and commercially valuable tropical fruit. However, low-temperature damage limits crop production. Genome of pitaya has not been sequenced yet. In this study, we sequenced the transcriptome of pitaya as the reference and further investigated the proteome under low temperature. By RNAseq technique, approximately 25.3 million reads were obtained, and further trimmed and assembled into 81,252 unigene sequences. The unigenes were searched against UniProt, NR and COGs at NCBI, Pfam, InterPro and Kyoto Encyclopedia of Genes and Genomes (KEGG) database, and 57,905 unigenes were retrieved annotations. Among them, 44,337 coding sequences were predicted by Trandecoder (v2.0.1), which served as the reference database for label-free proteomic analysis study of pitaya. Here, we identified 116 Differentially Abundant Proteins (DAPs) associated with the cold stress in pitaya, of which 18 proteins were up-regulated and 98 proteins were down-regulated. KEGG analysis and other results showed that these DAPs mainly related to chloroplasts and mitochondria metabolism. In summary, chloroplasts and mitochondria metabolism-related proteins may play an important role in response to cold stress in pitayas.

Conversion of phosphoenolpyruvate to pyruvate in Thermoanaerobacterium saccharolyticum

Thermoanaerobacterium saccharolyticum is an anaerobic thermophile that can ferment hemicellulose to produce biofuels, such as ethanol. It has been engineered to produce ethanol at high yield and titer. T. saccharolyticum uses the Embden-Meyerhof-Parnas (EMP) pathway for glycolysis. However, the genes and enzymes used in each step of the EMP pathway in T. saccharolyticum are not completely known. In T. saccharolyticum, both pyruvate kinase (PYK) and pyruvate phosphate dikinase (PPDK) are highly expressed based on transcriptomic and proteomic data. Both enzymes catalyze the formation of pyruvate from phosphoenolpyruvate (PEP). PYK is typically the last step of EMP glycolysis pathway while PPDK is reversible and is found mostly in C4 plants and some microorganisms. It is not clear what role PYK and PPDK play in T. saccharolyticum metabolism and fermentation pathways and whether both are necessary. In this study we deleted the ppdk gene in wild type and homoethanologen strains of T. saccharolyticum and showed that it is not essential for growth or ethanol production.

Pyrophosphate-fructose 6-phosphate 1-phosphotransferase (PFP1) regulates starch biosynthesis and seed development via heterotetramer formation in rice (Oryza sativa L.)

Pyrophosphate-fructose 6-phosphate 1-phosphotransferase (PFP1) reversibly converts fructose 6-phosphate and pyrophosphate to fructose 1, 6-bisphosphate and orthophosphate during glycolysis, and has diverse functions in plants. However, mechanisms underlying the regulation of starch metabolism by PFP1 remain elusive. This study addressed the function of PFP1 in rice floury endosperm and defective grain filling. Compared with the wild type, pfp1-3 exhibited remarkably low grain weight and starch content, significantly increased protein and lipid content, and altered starch physicochemical properties and changes in embryo development. Map-based cloning revealed that pfp1-3 is a novel allele and encodes the regulatory β-subunit of PFP1 (PFP1β). Measurement of nicotinamide adenine dinucleotide (NAD+) showed that mutation of PFP1β markedly decreased its enzyme activity. PFP1β and three of four putative catalytic α-subunits of PFP1, PFP1α1, PFP1α2, and PFP1α4, interacted with each other to form a heterotetramer. Additionally, PFP1β, PFP1α1 and PFP1α2 also formed homodimers. Furthermore, transcriptome analysis revealed that mutation of PFP1β significantly altered expression of many essential enzymes in starch biosynthesis pathways. Concentrations of multiple lipid and glycolytic intermediates and trehalose metabolites were elevated in pfp1-3 endosperm, indicating that PFP1 modulates endosperm metabolism, potentially through reversible adjustments to metabolic fluxes. Taken together, these findings provide new insights into seed endosperm development and starch biosynthesis and will help in the breeding of rice cultivars with higher grain yield and quality.

NegFluo, a Fast and Efficient Method to Determine Starch Granule Size and Morphology In Situ in Plant Chloroplasts

Starch granules that accumulate in the plastids of plants vary in size, shape, phosphate, or protein content according to their botanical origin. Depending on their size, the applications in food and nonfood industries differ. Being able to master starch granule size for a specific plant, without alteration of other characteristics (phosphate content, protein content, etc.), is challenging. The development of a simple and effective screening method to determine the size and shape of starch granules in a plant population is therefore of prime interest. In this study, we propose a new method, NegFluo, that combines negative confocal autofluorescence imaging in leaf and machine learning (ML)-based image analysis. It provides a fast, automated, and easy-to-use pipeline for both in situ starch granule imaging and its morphological analysis. NegFluo was applied to Arabidopsis leaves of wild-type and ss4 mutant plants. We validated its accuracy by comparing morphological quantifications using NegFluo and state-of-the-art methods relying either on starch granule purification or on preparation-intensive electron microscopy combined with manual image analysis. NegFluo thus opens the way to fast in situ analysis of starch granules.

Effects of long-term exposure to elevated temperature on Zea mays endosperm development during grain fill

Cereal yields decrease when grain fill proceeds under conditions of prolonged, moderately elevated temperatures. Endosperm-endogenous processes alter both rate and duration of dry weight gain, but underlying mechanisms remain unclear. Heat effects could be mediated by either abnormal, premature cessation of storage compound deposition or accelerated implementation of normal development. This study used controlled environments to isolate temperature as the sole environmental variable during Zea mays kernel-fill, from 12 days after pollination to maturity. Plants subjected to elevated day, elevated night temperatures (38°C day, 28°C night (38/28°C])) or elevated day, normal night (38/17°C), were compared with those from controls grown under normal day and night conditions (28/17°C). Progression of change over time in endosperm tissue was followed to dissect contributions at multiple levels, including transcriptome, metabolome, enzyme activities, product accumulation, and tissue ultrastructure. Integrated analyses indicated that the normal developmental program of endosperm is fully executed under prolonged high-temperature conditions, but at a faster rate. Accelerated development was observed when both day and night temperatures were elevated, but not when daytime temperature alone was increased. Although transcripts for most components of glycolysis and respiration were either upregulated or minimally affected, elevated temperatures decreased abundance of mRNAs related to biosynthesis of starch and storage proteins. Further analysis of 20 central-metabolic enzymes revealed six activities that were reduced under high-temperature conditions, indicating candidate roles in the observed reduction of grain dry weight. Nonetheless, a striking overall resilience of grain filling in the face of elevated temperatures can be attributed to acceleration of normal endosperm development.

Precise tuning of the glyoxylate cycle in Escherichia coli for efficient tyrosine production from acetate

BACKGROUND

Acetate is one of promising feedstocks owing to its cheap price and great abundance. Considering that tyrosine production is gradually shifting to microbial production method, its production from acetate can be attempted to further improve the economic feasibility of its production.

RESULTS

Here, we engineered a previously reported strain, SCK1, for efficient production of tyrosine from acetate. Initially, the acetate uptake and gluconeogenic pathway were amplified to maximize the flux toward tyrosine. As flux distribution between glyoxylate and TCA cycles is critical for efficient precursor supplementation, the activity of the glyoxylate cycle was precisely controlled by expression of isocitrate lyase gene under different-strength promoters. Consequently, the engineered strain with optimal flux distribution produced 0.70 g/L tyrosine with 20% of the theoretical maximum yield which are 1.6-fold and 1.9-fold increased values of the parental strain.

CONCLUSIONS

Tyrosine production from acetate requires precise tuning of the glyoxylate cycle and we obtained substantial improvements in production titer and yield by synthetic promoters and 5' untranslated regions (UTRs). This is the first demonstration of tyrosine production from acetate. Our strategies would be widely applicable to the production of various chemicals from acetate in future.

Starch formation inside plastids of higher plants

Starch is a water-insoluble polyglucan synthesized inside the plastid stroma within plant cells, serving a crucial role in the carbon budget of the whole plant by acting as a short-term and long-term store of energy. The highly complex, hierarchical structure of the starch granule arises from the actions of a large suite of enzyme activities, in addition to physicochemical self-assembly mechanisms. This review outlines current knowledge of the starch biosynthetic pathway operating in plant cells in relation to the micro- and macro-structures of the starch granule. We highlight the gaps in our knowledge, in particular, the relationship between enzyme function and operation at the molecular level and the formation of the final, macroscopic architecture of the granule.

Maize Oxalyl-CoA Decarboxylase1 Degrades Oxalate and Affects the Seed Metabolome and Nutritional Quality

The organic acid oxalate occurs in microbes, animals, and plants; however, excessive oxalate accumulation in vivo is toxic to cell growth and decreases the nutritional quality of certain vegetables. However, the enzymes and functions required for oxalate degradation in plants remain largely unknown. Here, we report the cloning of a maize (Zea mays) opaque endosperm mutant that encodes oxalyl-CoA decarboxylase1 (EC4.1.1.8; OCD1). Ocd1 is generally expressed and is specifically induced by oxalate. The ocd1 mutant seeds contain a significantly higher level of oxalate than the wild type, indicating that the ocd1 mutants have a defect in oxalate catabolism. The maize classic mutant opaque7 (o7) was initially cloned for its high lysine trait, although the gene function was not understood until its homolog in Arabidopsis thaliana was found to encode an oxalyl-CoA synthetase (EC 6.2.1.8), which ligates oxalate and CoA to form oxalyl-CoA. Our enzymatic analysis showed that ZmOCD1 catalyzes oxalyl-CoA, the product of O7, into formyl-CoA and CO₂ for degradation. Mutations in ocd1 caused dramatic alterations in the metabolome in the endosperm. Our findings demonstrate that ZmOCD1 acts downstream of O7 in oxalate degradation and affects endosperm development, the metabolome, and nutritional quality in maize seeds.

Gluconeogenesis and nitrogen metabolism in maize

Two pathways can be used by gluconeogenesis in plants: one employs phosphoenolpyruvate carboxykinase (PEPCK) and the other pyruvate orthophosphate dikinase (PPDK). The occurrence-location of these enzymes was determined in developing kernels of maize. PPDK was much more abundant than PEPCK in extracts of whole kernels. However, their location within the kernel was different. PPDK was particularly abundant in the peripheral endosperm (in which alanine is abundant), whereas PEPCK was localised in the pedicel and basal endosperm transfer cells (where asparagine is metabolised). The abundance of these enzymes was also determined in maize roots where there was a massive increase in abundance of PEPCK and a small increase in abundance of PPDK when they were fed ammonium; PEPCK was located in the pericycle and various cell types associated with the vasculature. On the other hand, there was a large increase in abundance of PPDK in roots subjected to anoxia (which induces an accumulation of alanine), whereas the abundance of PEPCK was decreased. These results show: firstly, that gluconeogenesis can potentially occur in many different tissues of maize. Secondly, within one organ PPDK can be abundant in some tissues and PEPCK in others. Thirdly, the abundance of PPDK and PEPCK is often associated with the metabolism of certain nitrogenous compounds and can be dramatically altered by factors related to nitrogen metabolism. In maize roots and developing kernels PPDK was associated with alanine metabolism. By contrast, the presence of PEPCK in maize roots and kernels was associated with either ammonium or asparagine metabolism. We propose that gluconeogenesis is often a component of a widespread mechanism that is used in coordinating the import/mobilisation of nitrogenous compounds with their utilisation. Further, potentially component of this mechanism may have provided building blocks that were used in the evolution of processes such as C₄ photosynthesis, Crassulacean acid metabolism, stomatal metabolism and the biochemical pH stat.

A Novel Mutation of OsPPDKB, Encoding Pyruvate Orthophosphate Dikinase, Affects Metabolism and Structure of Starch in the Rice Endosperm

Starch, as a main energy storage substance, plays an important role in plant growth and human life. Despite the fact that several enzymes and regulators involved in starch biosynthesis have been identified, the regulating mechanism of starch synthesis is still unclear. In this study, we isolated a rice floury endosperm mutant M14 from a mutant pool induced by ⁶⁰Co. Both total starch content and amylose content in M14 seeds significantly decreased, and starch thermal and pasting properties changed. Compound starch granules were defected in the floury endosperm of M14 seeds. Map-based cloning and a complementation test showed that the floury endosperm phenotype was determined by a gene of OsPPDKB, which encodes pyruvate orthophosphate dikinase (PPDK, EC 2.7.9.1). Subcellular localization analysis demonstrated that PPDK was localized in chloroplast and cytoplasm, the chOsPPDKB highly expressed in leaf and leaf sheath, and the cyOsPPDKB constitutively expressed with a high expression in developing endosperm. Moreover, the expression of starch synthesis-related genes was also obviously altered in M14 developing endosperm. The above results indicated that PPDK played an important role in starch metabolism and structure in rice endosperm.

Characterization of maize leaf pyruvate orthophosphate dikinase using high throughput sequencing

In C₄ photosynthesis, pyruvate orthophosphate dikinase (PPDK) catalyzes the regeneration of phosphoenolpyruvate in the carbon shuttle pathway. Although the biochemical function of PPDK in maize is well characterized, a genetic analysis of PPDK has not been reported. In this study, we use the maize transposable elements Mutator and Ds to generate multiple mutant alleles of PPDK. Loss-of-function mutants are seedling lethal, even when plants were grown under 2% CO₂ , and they show very low capacity for CO₂ assimilation, indicating C₄ photosynthesis is essential in maize. Using RNA-seq and GC-MS technologies, we examined the transcriptional and metabolic responses to a deficiency in PPDK activity. These results indicate loss of PPDK results in downregulation of gene expression of enzymes of the C₄ cycle, the Calvin cycle, and components of photochemistry. Furthermore, the loss of PPDK did not change Kranz anatomy, indicating that this metabolic defect in the C₄ cycle did not impinge on the morphological differentiation of C₄ characters. However, sugar metabolism and nitrogen utilization were altered in the mutants. An interaction between light intensity and genotype was also detected from transcriptome profiling, suggesting altered transcriptional and metabolic responses to environmental and endogenous signals in the PPDK mutants.