Maize is an important crop with a high level of genome diversity and heterosis. The genome sequence of a typical female line, B73, was previously released. Here, we report a de novo genome assembly of a corresponding male representative line, Mo17. More than 96.4% of the 2,183 Mb assembled genome can be accounted for by 362 scaffolds in ten pseudochromosomes with 38,620 annotated protein-coding genes. Comparative analysis revealed large gene-order and gene structural variations: approximately 10% of the annotated genes were mutually nonsyntenic, and more than 20% of the predicted genes had either large-effect mutations or large structural variations, which might cause considerable protein divergence between the two inbred lines. Our study provides a high-quality reference-genome sequence of an important maize germplasm, and the intraspecific gene order and gene structural variations identified should have implications for heterosis and genome evolution.

These authors contributed equally to this work. SUMMARYPlants can respond to environmental changes with various mechanisms occurred at transcriptional and translational levels. Thus far, there have been relatively extensive understandings of stress responses of plants on transcriptional level, while little information is known about that on translational level. To uncover the landscape of translation in plants in response to drought stress, we performed the recently developed ribosome profiling assay with maize seedlings growing under normal and drought conditions. Comparative analysis of the ribosome profiling data and the RNA-seq data showed that the fold changes of gene expression at transcriptional level were moderately correlated with that of translational level globally (R 2 = 0.69). However, less than half of the responsive genes were shared by transcription and translation under drought condition, suggesting that drought stress can introduce transcriptional and translational responses independently. We found that the translational efficiencies of 931 genes were changed significantly in response to drought stress. Further analysis revealed that the translational efficiencies of genes were highly influenced by their sequence features including GC content, length of coding sequences and normalized minimal free energy. In addition, we detected potential translation of 3063 upstream open reading frames (uORFs) on 2558 genes and these uORFs may affect the translational efficiency of downstream main open reading frames (ORFs). Our study indicates that plant can respond to drought stress with highly dynamic translational mechanism, that acting synergistically with that of transcription.

Soil salinity is one of several major abiotic stresses that constrain maize productivity worldwide. An improved understanding of salt-tolerance mechanisms will thus enhance the breeding of salt-tolerant maize and boost productivity. Previous studies have indicated that the maintenance of leaf Na concentration is essential for maize salt tolerance, and the difference in leaf Na exclusion has previously been associated with variation in salt tolerance between maize varieties. Here, we report the identification and functional characterization of a maize salt-tolerance quantitative trait locus (QTL), Zea mays Na Content1 (ZmNC1), which encodes an HKT-type transporter (designated as ZmHKT1). We show that a natural ZmHKT1 loss-of-function allele containing a retrotransposon insertion confers increased accumulation of Na in leaves, and salt hypersensitivity. We next show that ZmHKT1 encodes a plasma membrane-localized Na -selective transporter, and is preferentially expressed in root stele (including the parenchyma cells surrounding the xylem vessels). We also show that loss of ZmHKT1 function increases xylem sap Na concentration and causes increased root-to-shoot Na delivery, indicating that ZmHKT1 promotes leaf Na exclusion and salt tolerance by withdrawing Na from the xylem sap. We conclude that ZmHKT1 is a major salt-tolerance QTL and identifies an important new gene target in breeding for improved maize salt tolerance.

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Broomcorn millet (Panicum miliaceum L.) has strong tolerance to abiotic stresses, and is probably one of the oldest crops, with its earliest cultivation that dated back to ca. ~10,000 years. We report here its genome assembly through a combination of PacBio sequencing, BioNano, and Hi-C (in vivo) mapping. The 18 super scaffolds cover ~95.6% of the estimated genome (~887.8 Mb). There are 63,671 protein-coding genes annotated in this tetraploid genome. About ~86.2% of the syntenic genes in foxtail millet have two homologous copies in broomcorn millet, indicating rare gene loss after tetraploidization in broomcorn millet. Phylogenetic analysis reveals that broomcorn millet and foxtail millet diverged around ~13.1 Million years ago (Mya), while the lineage specific tetraploidization of broomcorn millet may be happened within ~5.91 million years. The genome is not only beneficial for the genome assisted breeding of broomcorn millet, but also an important resource for other Panicum species.

The maize opaque2 (o2) mutant has a high nutritional value but it develops a chalky endosperm that limits its practical use. Genetic selection for o2 modifiers can convert the normally chalky endosperm of the mutant into a hard, vitreous phenotype, yielding what is known as quality protein maize (QPM). Previous studies have shown that enhanced expression of 27-kDa γ-zein in QPM is essential for endosperm modification. Taking advantage of genome-wide association study analysis of a natural population, linkage mapping analysis of a recombinant inbred line population, and map-based cloning, we identified a quantitative trait locus (qγ27) affecting expression of 27-kDa γ-zein. qγ27 was mapped to the same region as the major o2 modifier (o2 modifier1) on chromosome 7 near the 27-kDa γ-zein locus. qγ27 resulted from a 15.26-kb duplication at the 27-kDa γ-zein locus, which increases the level of gene expression. This duplication occurred before maize domestication; however, the gene structure of qγ27 appears to be unstable and the DNA rearrangement frequently occurs at this locus. Because enhanced expression of 27-kDa γ-zein is critical for endosperm modification in QPM, qγ27 is expected to be under artificial selection. This discovery provides a useful molecular marker that can be used to accelerate QPM breeding.B y 2030, the world population is predicted to reach 8.5 billion people. As a consequence, food production will need to be increased by more than 50% (1). However, the rate of food production has not kept pace with explosive population growth. Enhancing the nutritional quality of staple crops is one strategy for addressing the emerging food crisis (2, 3).Maize (Zea mays) is the highest-yielding crop in the world, but it cannot be used as the sole protein source for humans and monogastric livestock because its main storage proteins, zeins, are deficient in the essential amino acids, lysine and tryptophan (4). The poor protein quality of maize can be improved by the opaque2 (o2) mutation, which increases the lysine and tryptophan levels by decreasing the synthesis of zeins and compensatorily increasing other (nonzein) seed proteins. However, unfortunately, the chalky and soft texture of o2 kernels limits utilization of this mutant (5). The creation of quality protein maize (QPM) was based on modification of o2 by accumulating quantitative trait loci (QTLs), called o2 modifiers, that lead to a hard, vitreous endosperm (5, 6). The development of QPM has greatly improved the lives of people who suffer from malnutrition in the developing countries (7).Although QPM breeding has gone on more than 50 y, neither the mechanism nor the genetic components controlling endosperm modification are well understood. Seven o2 modifiers have been located on six chromosomes (8), including one, designated o2 modifier1 in bin 7.02 near the 27-kDa γ-zein locus, which has a major effect on endosperm modification; the other six loci contribute smaller effects (8). Coincidently, the RNA transcript and protein levels of 27-kDa γ-zein accu...

Crop domestication and further breeding improvement have long been important areas of genetics and genomics studies. With the rapid advancing of next-generation sequencing (NGS) technologies, the amount of population genomics data has surged rapidly. Analyses of the mega genomics data have started to uncover a previously unknown pattern of genome-wide changes with crop domestication and breeding. Selection during domestication and breeding drastically reshaped crop genomes, which have ended up with regions of greatly reduced genetic diversity and apparent enrichment of potentially beneficial alleles located in both genic and non-genic regions. Increasing evidences suggest that epigenetic modifications also played an important role during domestication and breeding.

A complete telomere-to-telomere (T2T) finished genome has been the long pursuit of genomic research. Through generating deep coverage ultralong Oxford Nanopore Technology (ONT) and PacBio HiFi reads, we report here a complete genome assembly of maize with each chromosome entirely traversed in a single contig. The 2,178.6 Mb T2T Mo17 genome with a base accuracy of over 99.99% unveiled the structural features of all repetitive regions of the genome. There were several super-long simple-sequence-repeat arrays having consecutive thymine–adenine–guanine (TAG) tri-nucleotide repeats up to 235 kb. The assembly of the entire nucleolar organizer region of the 26.8 Mb array with 2,974 45S rDNA copies revealed the enormously complex patterns of rDNA duplications and transposon insertions. Additionally, complete assemblies of all ten centromeres enabled us to precisely dissect the repeat compositions of both CentC-rich and CentC-poor centromeres. The complete Mo17 genome represents a major step forward in understanding the complexity of the highly recalcitrant repetitive regions of higher plant genomes.