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Abstract Background NAC domain transcription factors initiate secondary cell wall biosynthesis in Arabidopsis fibres and vessels by activating numerous transcriptional regulators and biosynthetic genes. NAC family member SND2 is an indirect target of a principal regulator of fibre secondary cell wall formation, SND1. A previous study showed that overexpression of SND2 produced a fibre cell-specific increase in secondary cell wall thickness in Arabidopsis stems, and that the protein was able to transactivate the cellulose synthase8 (CesA8) promoter. However, the full repertoire of genes regulated by SND2 is unknown, and the effect of its overexpression on cell wall chemistry remains unexplored. Results We overexpressed SND2 in Arabidopsis and analyzed homozygous lines with regards to stem chemistry, biomass and fibre secondary cell wall thickness. A line showing upregulation of CesA8 was selected for transcriptome-wide gene expression profiling. We found evidence for upregulation of biosynthetic genes associated with cellulose, xylan, mannan and lignin polymerization in this line, in agreement with significant co-expression of these genes with native SND2 transcripts according to public microarray repositories. Only minor alterations in cell wall chemistry were detected. Transcription factor MYB103, in addition to SND1, was upregulated in SND2-overexpressing plants, and we detected upregulation of genes encoding components of a signal transduction machinery recently proposed to initiate secondary cell wall formation. Several homozygous T4 and hemizygous T1 transgenic lines with pronounced SND2 overexpression levels revealed a negative impact on fibre wall deposition, which may be indirectly attributable to excessive overexpression rather than co-suppression. Conversely, overexpression of SND2 in Eucalyptus stems led to increased fibre cross-sectional cell area. Conclusions This study supports a function for SND2 in the regulation of cellulose and hemicellulose biosynthetic genes in addition of those involved in lignin polymerization and signalling. SND2 seems to occupy a subordinate but central tier in the secondary cell wall transcriptional network. Our results reveal phenotypic differences in the effect of SND2 overexpression between woody and herbaceous stems and emphasize the importance of expression thresholds in transcription factor studies.

Abstract Background De novo assembly of transcript sequences produced by short-read DNA sequencing technologies offers a rapid approach to obtain expressed gene catalogs for non-model organisms. A draft genome sequence will be produced in 2010 for a Eucalyptus tree species (E. grandis) representing the most important hardwood fibre crop in the world. Genome annotation of this valuable woody plant and genetic dissection of its superior growth and productivity will be greatly facilitated by the availability of a comprehensive collection of expressed gene sequences from multiple tissues and organs. Results We present an extensive expressed gene catalog for a commercially grown E. grandis × E. urophylla hybrid clone constructed using only Illumina mRNA-Seq technology and de novo assembly. A total of 18,894 transcript-derived contigs, a large proportion of which represent full-length protein coding genes were assembled and annotated. Analysis of assembly quality, length and diversity show that this dataset represent the most comprehensive expressed gene catalog for any Eucalyptus tree. mRNA-Seq analysis furthermore allowed digital expression profiling of all of the assembled transcripts across diverse xylogenic and non-xylogenic tissues, which is invaluable for ascribing putative gene functions. Conclusions De novo assembly of Illumina mRNA-Seq reads is an efficient approach for transcriptome sequencing and profiling in Eucalyptus and other non-model organisms. The transcriptome resource (Eucspresso, http://eucspresso.bi.up.ac.za/) generated by this study will be of value for genomic analysis of woody biomass production in Eucalyptus and for comparative genomic analysis of growth and development in woody and herbaceous plants.

The genus Asparagus arose ∼9 to 15 million years ago (Ma), and transitions from hermaphroditism to dioecy (separate sexes) occurred ∼3 to 4 Ma. Roughly 27% of extant Asparagus species are dioecious, while the remaining are bisexual with monoclinous flowers. As such, Asparagus is an ideal model taxon for studying the early stages of dioecy and sex chromosome evolution in plants. Until now, however, understanding of diversification and shifts from hermaphroditism to dioecy in Asparagus has been hampered by the lack of robust species tree estimates for the genus. In this study, a genus-wide phylogenomic analysis including 1,726 nuclear loci and comprehensive species sampling supports two independent origins of dioecy in Asparagus —first in a widely distributed Eurasian clade and then in a clade restricted to the Mediterranean Basin. Modeling of ancestral biogeography indicates that both dioecy origins were associated with range expansion out of southern Africa. Our findings also reveal several bursts of diversification across the phylogeny, including an initial radiation in southern Africa that gave rise to 12 major clades in the genus, and more recent radiations that have resulted in paraphyly and polyphyly among closely related species, as expected given active speciation processes. Lastly, we report that the geographic origin of domesticated garden asparagus (Asparagus officinalis L.) was likely in western Asia near the Mediterranean Sea. The presented phylogenomic framework for Asparagus is foundational for ongoing genomic investigations of diversification and functional trait evolution in the genus and contributes to its utility for understanding the origin and early evolution of dioecy and sex chromosomes. [ABSTRACT FROM AUTHOR]

DUF1218 is a land plant-specific innovation and has previously been shown to be associated with cell wall biology, vasculature patterning and abiotic/biotic stress response. The Arabidopsis genome encodes 15 members, two of which (At1g31720 and At4g27435) are preferentially expressed in the secondary cell wall depositing inflorescence stems. To further our understanding of the roles of DUF1218-containing proteins in secondary cell wall biology, we functionally characterized At1g31720 (herein referred to as MODIFYING WALL LIGNIN-1 or MWL-1). Since related gene family members may contribute to functional redundancy, we also characterized At4g19370 (MWL-2), the most closely related gene to MWL-1 in the protein family. Subcellular localization revealed that both Arabidopsis proteins are targeted to the cell periphery. The single T-DNA knockout lines, mwl-1 and mwl-2, and independent overexpression lines showed no significant differences in plant growth or changes in total lignin content relative to wild-type (WT) control plants. However, the double homozygous mutant, mwl-1/mwl-2, had smaller rosettes with a significant decrease in rosette fresh weight and stem height relative to the WT control at four weeks and six weeks, respectively. Moreover, mwl-1/mwl-2 showed a significant reduction in total lignin content (by ca. 11% relative to WT) and an increase in syringyl/guaiacyl (S/G) monomer ratio relative to the control plants. Our study has identified two additional members of the DUF1218 family in Arabidopsis as novel contributors to secondary cell wall biology, specifically lignin biosynthesis, and these proteins appear to function redundantly.

SUMMARY: The king protea (Protea cynaroides), an early‐diverging eudicot, is the most iconic species from the Megadiverse Cape Floristic Region, and the national flower of South Africa. Perhaps best known for its iconic flower head, Protea is a key genus for the South African horticulture industry and cut‐flower market. Ecologically, the genus and the family Proteaceae are important models for radiation and adaptation, particularly to soils with limited phosphorus bio‐availability. Here, we present a high‐quality chromosome‐scale assembly of the P. cynaroides genome as the first representative of the fynbos biome. We reveal an ancestral whole‐genome duplication event that occurred in the Proteaceae around the late Cretaceous that preceded the divergence of all crown groups within the family and its extant diversity in all Southern continents. The relatively stable genome structure of P. cynaroides is invaluable for comparative studies and for unveiling paleopolyploidy in other groups, such as the distantly related sister group Ranunculales. Comparative genomics in sequenced genomes of the Proteales shows loss of key arbuscular mycorrhizal symbiosis genes likely ancestral to the family, and possibly the order. The P. cynaroides genome empowers new research in plant diversification, horticulture and adaptation, particularly to nutrient‐poor soils. Significance Statement: Our chromosome‐level Protea cynaroides genome assembly is the first endemic South African plant genome to be published, and can signify a new wave of research focusing on indigenous fynbos and endemic African plant species. We foresee this serving as increasing visibility of plant genomics research from the African continent and the Global South, and enriching global biodiversity, sustainability, evolution and conservation research. [ABSTRACT FROM AUTHOR]

Background De novo phased (haplo)genome assembly using long-read DNA sequencing data has improved the detection and characterization of structural variants (SVs) in plant and animal genomes. Able to span across haplotypes, long reads allow phased, haplogenome assembly in highly outbred organisms such as forest trees. Eucalyptus tree species and interspecific hybrids are the most widely planted hardwood trees with F1 hybrids of Eucalyptus grandis and E. urophylla forming the bulk of fast-growing pulpwood plantations in subtropical regions. The extent of structural variation and its effect on interspecific hybridization is unknown in these trees. As a first step towards elucidating the extent of structural variation between the genomes of E. grandis and E. urophylla , we sequenced and assembled the haplogenomes contained in an F1 hybrid of the two species. Findings Using Nanopore sequencing and a trio-binning approach, we assembled the separate haplogenomes (566.7 Mb and 544.5 Mb) to 98.0% BUSCO completion. High-density SNP genetic linkage maps of both parents allowed scaffolding of 88.0% of the haplogenome contigs into 11 pseudo-chromosomes (scaffold N50 of 43.8 Mb and 42.5 Mb for the E. grandis and E. urophylla haplogenomes, respectively). We identify 48,729 SVs between the two haplogenomes providing the first detailed insight into genome structural rearrangement in these species. The two haplogenomes have similar gene content, 35,572 and 33,915 functionally annotated genes, of which 34.7% are contained in genome rearrangements. Conclusions Knowledge of SV and haplotype diversity in the two species will form the basis for understanding the genetic basis of hybrid superiority in these trees. [ABSTRACT FROM AUTHOR]

SUMMARY: To improve our understanding of genetic mechanisms underlying complex traits in plants, a comprehensive analysis of gene variants is required. Eucalyptus is an important forest plantation genus that is highly outbred. Trait dissection and molecular breeding in eucalypts currently relies on biallelic single‐nucleotide polymorphism (SNP) markers. These markers fail to capture the large amount of haplotype diversity in these species, and thus multi‐allelic markers are required. We aimed to develop a gene‐based haplotype mining panel for Eucalyptus species. We generated 17 999 oligonucleotide probe sets for targeted sequencing of selected regions of 6293 genes implicated in growth and wood properties, pest and disease resistance, and abiotic stress responses. We identified and phased 195 834 SNPs using a read‐based phasing approach to reveal SNP‐based haplotypes. A total of 8915 target regions (at 4637 gene loci) passed tests for Mendelian inheritance. We evaluated the haplotype panel in four Eucalyptus species (E. grandis, E. urophylla, E. dunnii and E. nitens) to determine its ability to capture diversity across eucalypt species. This revealed an average of 3.13–4.52 haplotypes per target region in each species, and 33.36% of the identified haplotypes were shared by at least two species. This haplotype mining panel will enable the analysis of haplotype diversity within and between species, and provide multi‐allelic markers that can be used for genome‐wide association studies and gene‐based breeding approaches. Significance Statement: We developed a haplotype sequencing panel for Eucalyptus targeting 8915 regions at 4637 gene loci associated with growth and wood properties, pest and disease resistance, and abiotic stress response providing a genome‐wide, multi‐allelic, gene‐centric genotyping resource for eucalypts. We tested the panel in four Eucalyptus species (E. grandis, E. dunnii, E. nitens and E. urophylla), and found an average of 3.65 haplotypes per target region per species, and 9.98 across all four species. [ABSTRACT FROM AUTHOR]