Heterotic patterns of sugar and amino acid components in developing maize kernels

Carbohydrate Repartitioning in the Rice Starch Branching Enzyme IIb Mutant Stimulates Higher Resistant Starch Content and Lower Seed Weight Revealed by Multiomics Analysis

The starch branching enzyme IIb mutant (be2b) in rice significantly increases the resistant starch (RS) content and leads to reduced seed weight. However, the underlying metabolic mechanisms remain unclear. Proteomic analysis indicated that upregulation of starch synthase IIa (SSIIa) and SSIIIa and downregulation of BEI and SSI were possibly responsible for the decreased short amylopectin chains (DP 6-15) and increased longer chains (DP > 16) of be2b starch. The upregulation of granule-bound starch synthase led to increased amylose content (AC). These changes in the amylopectin structure and AC accounted for the increased RS content. α-Amylase 2A showed the strongest upregulation (up to 8.45-fold), indicating that the loss of BEIIb activity enhanced starch degradation. Upregulation of glycolysis-related proteins stimulated carbohydrate repartitioning through glycerate-3-phosphate and promoted the accumulation of tricarboxylic acid cycle intermediates, amino acids, and fatty acids. The unexpected carbohydrate partitioning and enhanced starch degradation resulted in the reduced seed weight in the be2b mutant.

The multi-omics basis of potato heterosis

Heterosis is a fundamental biological phenomenon characterized by the superior performance of hybrids over their parents. Although tremendous progress has been reported in seed crops, the molecular mechanisms underlying heterosis in clonally propagated crops are largely unknown. Potato (Solanum tuberosum L.) is the most important tuber crop and an ongoing revolution is transforming potato from a clonally propagated tetraploid crop into a seed-propagated diploid hybrid potato. In our previous study, we developed the first generation of highly homozygous inbred lines of potato and hybrids with strong heterosis. Here, we integrated transcriptome, metabolome, and DNA methylation data to explore the genetic and molecular basis of potato heterosis at three developmental stages. We found that the initial establishment of heterosis in diploid potato was mainly due to dominant complementation. Flower color, male fertility, and starch and sucrose metabolism showed obvious gene dominant complementation in hybrids, and hybrids devoted more energy to primary metabolism for rapid growth. In addition, we identified ~2 700 allele-specific expression genes at each stage, which likely function in potato heterosis and might be regulated by CHH allele-specific methylation level. Our multi-omics analysis provides insight into heterosis in potato and facilitates the exploitation of heterosis in potato breeding.

Transcriptome-wide analysis of epitranscriptome and translational efficiency associated with heterosis in maize

Heterosis has been extensively utilized to increase productivity in crops, yet the underlying molecular mechanisms remain largely elusive. Here, we generated transcriptome-wide profiles of mRNA abundance, m6A methylation, and translational efficiency from the maize F1 hybrid B73×Mo17 and its two parental lines to ascertain the contribution of each regulatory layer to heterosis at the seedling stage. We documented that although the global abundance and distribution of m6A remained unchanged, a greater number of genes had gained an m6A modification in the hybrid. Superior variations were observed at the m6A modification and translational efficiency levels when compared with mRNA abundance between the hybrid and parents. In the hybrid, the vast majority of genes with m6A modification exhibited a non-additive expression pattern, the percentage of which was much higher than that at levels of mRNA abundance and translational efficiency. Non-additive genes involved in different biological processes were hierarchically coordinated by discrete combinations of three regulatory layers. These findings suggest that transcriptional and post-transcriptional regulation of gene expression make distinct contributions to heterosis in hybrid maize. Overall, this integrated multi-omics analysis provides a valuable portfolio for interpreting transcriptional and post-transcriptional regulation of gene expression in hybrid maize, and paves the way for exploring molecular mechanisms underlying hybrid vigor.

Temporal Regulation of the Metabolome and Proteome in Photosynthetic and Photorespiratory Pathways Contributes to Maize Heterosis

Heterosis or hybrid vigor is widespread in plants and animals. Although the molecular basis for heterosis has been extensively studied, metabolic and proteomic contributions to heterosis remain elusive. Here we report an integrative analysis of time-series metabolome and proteome data in maize (Zea mays) hybrids and their inbred parents. Many maize metabolites and proteins are diurnally regulated, and many of these show nonadditive abundance in the hybrids, including key enzymes and metabolites involved in carbon assimilation. Compared with robust trait heterosis, metabolic heterosis is relatively mild. Interestingly, most amino acids display negative mid-parent heterosis (MPH), i.e., having lower values than the average of the parents, while sugars, alcohols, and nucleoside metabolites show positive MPH. From the network perspective, metabolites in the photosynthetic pathway show positive MPH, whereas metabolites in the photorespiratory pathway show negative MPH, which corresponds to nonadditive protein abundance and enzyme activities of key enzymes in the respective pathways in the hybrids. Moreover, diurnally expressed proteins that are upregulated in the hybrids are enriched in photosynthesis-related gene-ontology terms. Hybrids may more effectively remove toxic metabolites generated during photorespiration, and thus maintain higher photosynthetic efficiency. These metabolic and proteomic resources provide unique insight into heterosis and its utilization for high yielding maize and other crop plants.

Comparative study on cytogenetics and transcriptome between diploid and autotetraploid rice hybrids harboring double neutral genes

Double pollen fertility neutral genes, San and Sbn, can control pollen sterility in intersubspecific (indica × japonica) rice hybrids, which has excellent potential to increase rice yield. Previous studies showed that polyploidy could increase the interaction of three pollen sterility loci, i.e. Sa, Sb and Sc, which cause pollen sterility in autotetraploid rice hybrids, and hybrid fertility could be improved by double neutral genes, San and Sbn, in autotetraploid rice hybrids. We compared cytological and transcriptome data between autotetraploid and diploid rice hybrid during meiosis and single microspore stages to understand the molecular mechanism of neutral genes for overcoming pollen sterility in autotetraploid rice hybrids, which harbored double neutral genes. Cytological results revealed that the double neutral genes resulted in higher pollen fertility (76.74%) and lower chromosomal abnormalities in autotetraploid hybrid than in parents during metaphase I, metaphase II, anaphase I and anaphase II. Moreover, autotetraploid rice hybrid displayed stronger heterosis than a diploid hybrid. Compared with diploid rice hybrid, a total of 904 and 68 differently expressed genes (DEGs) were identified explicitly in autotetraploid hybrid at meiosis and single microspore stages, respectively. Of these, 133 and 41 genes were detected in higher-parent dominance and transgressive up-regulation dominance, respectively, which were considered autotetraploid potential heterosis genes, including a meiosis-related gene (Os01g0917500, MSP1) and two meiosis specific-genes (Os07g0624900 and Os04g0208600). Gene Ontology (GO) and Kyoto Encyclopedia of Gene and Genomes pathway (KEGG) analysis revealed that DEGs significantly enriched in amino acid metabolism and photosynthesis metabolism. These results indicated that meiosis-specific and meiosis-related genes, and amino acids and photosynthesis metabolism-related genes contribute to higher yield and pollen fertility in autotetraploid rice hybrid. This study provides a theoretical basis for molecular mechanisms of heterosis in autotetraploid rice harboring double neutral genes for pollen fertility.

Toward Mass Spectrometry Imaging in the Metabolomics Scale: Increasing Metabolic Coverage Through Multiple On-Tissue Chemical Modifications

Exploring the metabolic differences directly on tissues is essential for the comprehensive understanding of how multicellular organisms function. Mass spectrometry imaging (MSI) is an attractive technique toward this goal; however, MSI in metabolomics scale has been hindered by multiple limitations. This is most notable for single cell level high-spatial resolution imaging because of the limited number of molecules in small sampling size and the low ionization yields of many metabolites. Several on-tissue chemical derivatization approaches have been reported to increase MSI signals of targeted compounds, especially in matrix-assisted laser desorption/ionization (MALDI)-MSI. Herein, we adopt a combination of chemical derivatization reactions, to selectively enhance metabolite signals of a specific functional group for each consecutive tissue section. Three well-known on-tissue derivatization methods were used as a proof of concept experiment: coniferyl aldehyde for primary amines, Girard's reagent T for carbonyl groups, and 2-picolylamine for carboxylic acids. This strategy was applied to the cross-sections of leaves and roots from two different maize genotypes (B73 and Mo17), and enabled the detection of over six hundred new unique metabolite features compared to without modification. Statistical analysis indicated quantitative variation between metabolites in the tissue sections, while MS images revealed differences in localization of these metabolites. Combined, this untargeted approach facilitated the visualization of various classes of compounds, demonstrating the potential for untargeted MSI in the metabolomics scale.

Heterotic patterns of primary and secondary metabolites in the oilseed crop Brassica juncea

Heterosis refers to the superior performance of F1 hybrids over their respective parental inbred lines. Although the genetic and expression basis of heterosis have been previously investigated, the metabolic basis for this phenomenon is poorly understood. In a preliminary morphological study in Brassica juncea, we observed significant heterosis at the 50% flowering stage, wherein both the growth and reproduction of F1 reciprocal hybrids were greater than that of their parents. To identify the possible metabolic causes or consequences of this heterosis, we carried out targeted LC-MS analysis of 48 primary (amino acids and sugars) and secondary metabolites (phytohormones, glucosinolates, flavonoids, and phenolic esters) in five developmental tissues at 50% flowering in hybrids and inbred parents. Principal component analysis (PCA) of metabolites clearly separated inbred lines from their hybrids, particularly in the bud tissues. In general, secondary metabolites displayed more negative heterosis values in comparison to primary metabolites. The tested primary and secondary metabolites displayed both additive and non-additive modes of inheritance in F1 hybrids, wherein the number of metabolites showing an additive mode of inheritance were higher in buds and siliques (52.77-97.14%) compared to leaf tissues (47.37-80%). Partial least regression (PLS) analysis further showed that primary metabolites, in general, displayed higher association with morphological parameters in F1 hybrids. Overall, our results are consistent with a resource-cost model for heterosis in B. juncea, where metabolite allocation in hybrids appears to favor growth, at the expense of secondary metabolism.

Spatial and temporal expression modes of MicroRNAs in an elite rice hybrid and its parental lines

Heterosis is a commonly observed phenomenon in nature and refers to the superior performance of hybrids relative to both parents. The molecular mechanisms of heterosis are mostly unknown. Quantitative trait locus (QTL) mapping has been used to explain the genetic basis of heterosis, and large amounts of QTLs have been mapped for various agronomic traits, but the nature of QTL contributing to heterosis is still enigmatic. MicroRNAs (miRNAs) are master regulators in the processes of plant development and trait performance, and many miRNAs are predicted to reside in QTL intervals. We analyzed the expression modes of miRNAs, which were picked up from miRNA databases and chosen from those predicted from QTL intervals by bioinformatic approaches, in different organs at developmental stages of an elite rice hybrid and its parents. All possible modes of action for miRNA expression were detected, but most miRNAs showed nonadditive mode, and different stages and distinct organs displayed different patterns of miRNA expression. A large proportion of miRNAs were not detected for expression in leaves but expressed in the culms and roots of the hybrid at tillering stage. MiRNAs from grain-weight QTL intervals have multiple effects on grain development. Together, our results reveal that miRNAs, especially those from QTL intervals, play roles in heterotic performance in this elite rice hybrid, our results also shade new light on understanding the molecular mechanisms of heterosis.

Complementation contributes to transcriptome complexity in maize (Zea mays L.) hybrids relative to their inbred parents

Typically, F₁-hybrids are more vigorous than their homozygous, genetically distinct parents, a phenomenon known as heterosis. In the present study, the transcriptomes of the reciprocal maize (Zea mays L.) hybrids B73×Mo17 and Mo17×B73 and their parental inbred lines B73 and Mo17 were surveyed in primary roots, early in the developmental manifestation of heterotic root traits. The application of statistical methods and a suitable experimental design established that 34,233 (i.e., 86%) of all high-confidence maize genes were expressed in at least one genotype. Nearly 70% of all expressed genes were differentially expressed between the two parents and 42%–55% of expressed genes were differentially expressed between one of the parents and one of the hybrids. In both hybrids, ∼10% of expressed genes exhibited nonadditive gene expression. Consistent with the dominance model (i.e., complementation) for heterosis, 1124 genes that were expressed in the hybrids were expressed in only one of the two parents. For 65 genes, it could be shown that this was a consequence of complementation of genomic presence/absence variation. For dozens of other genes, alleles from the inactive inbred were activated in the hybrid, presumably via interactions with regulatory factors from the active inbred. As a consequence of these types of complementation, both hybrids expressed more genes than did either parental inbred. Finally, in hybrids, ∼14% of expressed genes exhibited allele-specific expression (ASE) levels that differed significantly from the parental-inbred expression ratios, providing further evidence for interactions of regulatory factors from one parental genome with target genes from the other parental genome.

MicroRNA transcriptomic analysis of heterosis during maize seed germination

Heterosis has been utilized widely in the breeding of maize and other crops, and plays an important role in increasing yield, improving quality and enhancing stresses resistance, but the molecular mechanism responsible for heterosis is far from clear. To illustrate whether miRNA-dependent gene regulation is responsible for heterosis during maize germination, a deep-sequencing technique was applied to germinating embryos of a maize hybrid, Yuyu22, which is cultivated widely in China and its parental inbred lines, Yu87-1 and Zong3. The target genes of several miRNAs showing significant expression in the hybrid and parental lines were predicted and tested using real-time PCR. A total of 107 conserved maize miRNAs were co-detected in the hybrid and parental lines. Most of these miRNAs were expressed non-additively in the hybrid compared to its parental lines. These results indicated that miRNAs might participate in heterosis during maize germination and exert an influence via the decay of their target genes. Novel miRNAs were predicted follow a rigorous criterion and only the miRNAs detected in all three samples were treated as a novel maize miRNA. In total, 34 miRNAs belonged to 20 miRNA families were predicted in germinating maize seeds. Global repression of miRNAs in the hybrid, which might result in enhanced gene expression, might be one reason why the hybrid showed higher embryo germination vigor compared to its parental lines.

Metabolic and phenotypic responses of greenhouse-grown maize hybrids to experimentally controlled drought stress

Adaptation to abiotic stresses like drought is an important acquirement of agriculturally relevant crops like maize. Development of enhanced drought tolerance in crops grown in climatic zones where drought is a very dominant stress factor therefore plays an essential role in plant breeding. Previous studies demonstrated that corn yield potential and enhanced stress tolerance are associated traits. In this study, we analyzed six different maize hybrids for their ability to deal with drought stress in a greenhouse experiment. We were able to combine data from morphophysiological parameters measured under well-watered conditions and under water restriction with metabolic data from different organs. These different organs possessed distinct metabolite compositions, with the leaf blade displaying the most considerable metabolome changes following water deficiency. Whilst we could show a general increase in metabolite levels under drought stress, including changes in amino acids, sugars, sugar alcohols, and intermediates of the TCA cycle, these changes were not differential between maize hybrids that had previously been designated based on field trial data as either drought-tolerant or susceptible. The fact that data described here resulted from a greenhouse experiment with rather different growth conditions compared to natural ones in the field may explain why tolerance groups could not be confirmed in this study. We were, however, able to highlight several metabolites that displayed conserved responses to drought as well as metabolites whose levels correlated well with certain physiological traits.

Nonadditive protein accumulation patterns in Maize (Zea mays L.) hybrids during embryo development

Heterosis describes the superior performance of heterozygous F(1)-hybrid plants compared to their homozygous parental inbred lines. In the present study, heterosis was detected for length, weight, and the time point of seminal root primordia initiation in maize (Zea mays L.) embryos of the reciprocal F(1)-hybrids UH005xUH250 and UH250xUH005. A two-dimensional gel electrophoresis (2-DE) proteome survey of the most abundant proteins of the reciprocal hybrids and their parental inbred lines 25 and 35 days after pollination revealed that 141 of 597 detected proteins (24%) exhibited nonadditive accumulation in at least one hybrid. Approximately 44% of all nonadditively accumulated proteins displayed an expression pattern that was not distinguishable from the low parent value. Electrospray ionization-tandem mass spectrometry (ESI-MS/MS) analyses and subsequent functional classification of the 141 proteins revealed that development, protein metabolism, redox-regulation, glycolysis, and amino acid metabolism were the most prominent functional classes among nonadditively accumulated proteins. In 35-day-old embryos of the hybrid UH250xUH005, a significant up-regulation of enzymes related to glucose metabolism which often exceeded the best parent values was observed. A comparison of nonadditive protein accumulation between rice and maize embryo data sets revealed a significant overlap of nonadditively accumulated proteins suggesting conserved organ- or tissue-specific regulatory mechanisms in monocots related to heterosis.

Heterosis in early seed development: a comparative study of F1 embryo and endosperm tissues 6 days after fertilization

Heterosis specifies the superior performance of heterozygous individuals and although used in plant breeding the underlying molecular mechanisms still remain largely elusive. In this study, we demonstrate the manifestation of heterosis in hybrid maize embryo and endosperm tissue 6 days after fertilization in crosses of several inbred lines. We provide a comparative analysis of heterosis-associated gene expression in these tissues by a combined approach of suppression subtractive hybridization and microarray hybridizations. Non-additive expression pattern indicated a trans-regulatory mechanism to act early after fertilization in hybrid embryo and endosperm although the majority of genes showed mid-parental expression levels in embryo and dosage dependent expression levels in endosperm. The consistent expression pattern within both tissues and both inbred line genotype combinations of genes coding for chromatin related proteins pointed to heterosis-related epigenetic processes. These and genes involved in other biological processes, identified in this study, might provide entry points for the investigation of regulatory networks associated with the specification of heterosis.

Correlation between parental transcriptome and field data for the characterization of heterosis in Zea mays L

Heterosis is widely exploited in plant breeding, although its molecular basis is still not fully understood. For the characterization of this phenomenon and the development of transcriptome-based methods to predict hybrid performance (HP), we applied a microarray (46k) analysis of 21 European maize (Zea mays L.), 14 dent and 7 flint parental inbred lines. Expression profiles of the parental inbreds at the seedling stage were correlated with grain yield (GY) and grain dry matter content (GDMC) of 98 flint x dent factorial crosses at six locations. We observed highly significant correlations of the parental expression levels of certain differentially expressed genes with heterosis and HP for GY and also with HP for GDMC. This strong correlation provided first evidence toward a prediction potential of the genes and their expression levels. The identified gene set based on the parental transcriptome data revealed functional characteristics of HP and heterosis. Gene ontology (GO) analyses were performed to compare genes correlated with their expression pattern to HP for GY and GDMC, respectively. Between these gene groups, mostly different functional classes of genes were found to be enriched or underrepresented. The phenomenon of heterosis was characterized by the over- and underrepresentation of specific GO terms among heterosis-correlated genes.