The rubber tree, Hevea brasiliensis, produces natural rubber that serves as an essential industrial raw material. Here, we present a high-quality reference genome for a rubber tree cultivar GT1 using single-molecule real-time sequencing (SMRT) and Hi-C technologies to anchor the $1.47-Gb genome assembly into 18 pseudochromosomes. The chromosome-based genome analysis enabled us to establish a model of spurge chromosome evolution, since the common paleopolyploid event occurred before the split of Hevea and Manihot. We show recent and rapid bursts of the three Hevea-specific LTR-retrotransposon families during the last 10 million years, leading to the massive expansion by $65.88% ($970 Mbp) of the whole rubber tree genome since the divergence from Manihot. We identify large-scale expansion of genes associated with whole rubber biosynthesis processes, such as basal metabolic processes, ethylene biosynthesis, and the activation of polysaccharide and glycoprotein lectin, which are important properties for latex production. A map of genomic variation between the cultivated and wild rubber trees was obtained, which contains $15.7 million high-quality single-nucleotide polymorphisms. We identified hundreds of candidate domestication genes with drastically lowered genomic diversity in the cultivated but not wild rubber trees despite a relatively short domestication history of rubber tree, some of which are involved in rubber biosynthesis. This genome assembly represents key resources for future rubber tree research and breeding, providing novel targets for improving plant biotic and abiotic tolerance and rubber production.

Macadamia tetraphylla (Proteaceae) is one of the two macadamia plants that are edible and of cultivated value. Only two chloroplast genomes were reported in Proteaceae so far. In this study, we report the complete chloroplast genome sequence of M. tetraphylla, which is the third reported chloroplast genome in Proteaceae. The chloroplast genome is 159,195 bp long and includes 113 genes. Its LSC, SSC and IR regions are 87,951, 18,748 and 26,248 bp long, respectively. Phylogenetic analysis indicates that M. tetraphylla was clustered with other two species of Proteaceae, the M. integrifoia and M. ternifolia.

Macadamia is a kind of evergreen nut trees which belong to the Proteaceae family. The two commercial macadamia species, Macadamia integrifolia and M. tetraphylla, are highly prized for their edible kernels. Catherine et al. reported M. integrifolia genome using NGS sequencing technology. However, the lack of a high-quality assembly for M. tetraphylla hinders the progress in biological research and breeding program. In this study, we report a high-quality genome sequence of M. tetraphylla using the Oxford Nanopore Technologies (ONT) technology. We generated an assembly of 750.54 Mb with a contig N50 length of 1.18 Mb, which is close to the size estimated by flow cytometry and k-mer analysis. Repetitive sequence represent 58.57% of the genome sequence, which is strikingly higher compared with M. integrifolia. A total of 31,571 protein-coding genes were annotated with an average length of 6,055 bp, of which 92.59% were functionally annotated. The genome sequence of M. tetraphylla will provide novel insights into the breeding of novel strains and genetic improvement of agronomic traits.

Macadamia ternifolia is a subtropical fruit tree of the family Proteaceae. Chloroplast genome sequences play a significant role in the development of molecular markers in plant phylogenetic and population genetic studies. In this study, we report the complete chloroplast genome sequence of M. ternifolia. The chloroplast genome is 159,669 bp long and includes 113 genes. Its LSC, SSC, and IR regions are 88,072, 18,801, and 26,408 bp long, respectively. Sequence comparison of M. ternifolia and M. integrifolia indicates large sequence conservation between these two species, only few sequence variations including single nucleotide polymorphisms (SNPs) and inserts/deletes (InDels) were detected. ARTICLE HISTORY

Macadamia is an evergreen tree belonging to the Proteaceae family. The two commercial macadamia species, Macadamia integrifolia and M. tetraphylla, are highly prized for their edible kernels. The M. integrifolia genome was recently sequenced, but the genome of M. tetraphylla has to date not been published, which limits the study of biological research and breeding in this species. This study reports a high-quality genome sequence of M. tetraphylla based on the Oxford Nanopore Technologies technology and high-throughput chromosome conformation capture techniques (Hi-C). An assembly of 750.87 Mb with 51.11 Mb N50 length was generated, close to the 740 and 758 Mb size estimates by flow cytometry and k-mer analysis, respectively. Genome annotation indicated that 61.42% of the genome is composed of repetitive sequences and 34.95% is composed of long terminal repeat retrotransposons. Up to 31,571 protein-coding genes were predicted, of which 92.59% were functionally annotated. The average gene length was 6,055 bp. Comparative genome analysis revealed that the gene families associated with defense response, lipid transport, steroid biosynthesis, triglyceride lipase activity, and fatty acid metabolism are expanded in the M. tetraphylla genome. The distribution of fourfold synonymous third-codon transversion showed a recent whole-genome duplication event in M. tetraphylla. Genomic and transcriptomic analysis identified 187 genes encoding 33 crucial oil biosynthesis enzymes, depicting a comprehensive map of macadamia lipid biosynthesis. Besides, the 55 identified WRKY genes exhibited preferential expression in root as compared to that in other tissues. The genome sequence of M. tetraphylla provides novel insights for breeding novel varieties and genetic improvement of agronomic traits.

Macadamia (Macadamia spp.) has been widely planted in southern China and has been now developed into an important industry. China has the largest area of macadamia plantation in the world but provides only 3% production of the world. Current farming systems have a fertilizer surplus of about 73 g of nitrogen (N), 103 g of phosphorus (P), and 24 g of potassium (K) per macadamia plant per year in southern China. Optimizing fertilization recommended for macadamia improves production by about 5 kg per plant. Macadamia develops cluster roots (i.e., proteoid roots) in a Pstarvation environment. Overuse of P fertilizers restrains the development of cluster roots as well as rhizosphere processes, thus decreasing the P-use efficiency. Excessive fertilization, especially P fertilization, is one of the major limiting factors in China macadamia production. This study is the first to analyze current management practices and then discuss approaches of improving nutrient management based on the specific root biology of macadamia. For a sustainable macadamia industry, it is imperative to develop appropriate nutrient management by integrating root-zone soil nutrient supply, fertilizer application, and rhizosphere processes.

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Theobroma grandiflorum (Willd. ex Spreng.) K. Schum., an economically important tree in the family Malvaceae, is native to the Amazonian region of South America and it is widely cultivated as a tropical crop. Herein, we report the complete chloroplast genome of T. grandiflorum. The size of the complete chloroplast genome of T. grandiflorum is 160,606 bp with 36.88% GC content, which includes a pair of inverted repeat regions (IRs) of 25,496 bp separated by a large single-copy region (LSC) of 89,429 bp and a small single copy (SSC) region of 20,185 bp. A total of 126 genes were annotated, of which 82 were protein-coding genes, 36 were transfer RNA (tRNA) genes, and 8 were ribosomal RNA (rRNA) genes. A maximum-likelihood (ML) analysis supported a close relationship between T. grandiflorum and T. cacao. This study will provide useful information for further phylogenetic and evolutionary analysis of Malvaceae.