Summary: Kenaf is an annual crop that is widely cultivated as a source of bast (phloem) fibres, the phytoremediation of heavy metal‐contaminated farmlands and textile‐relevant compounds. Leaf shape played a unique role in kenaf improvement, due to the inheritance as a single locus and the association with fibre development in typical lobed‐leaf varieties. Here we report a high‐quality genome assembly and annotation for var. ‘Fuhong 952’ with 1078 Mbp genome and 66 004 protein‐coding genes integrating single‐molecule real‐time sequencing, a high‐density genetic map and high‐throughput chromosome conformation capture techniques. Gene mapping assists the identification of a homeobox transcription factor LATE MERISTEM IDENTITY 1 (HcLMI1) gene controlling lobed‐leaf. Virus‐induced gene silencing (VIGS) of HcLMI1 in a lobed‐leaf variety was critical to induce round (entire)‐like leaf formation. Candidate genes involved in cell wall formation were found in quantitative trait loci (QTL) for fibre yield and quality‐related traits. Comparative genomic and transcriptome analyses revealed key genes involved in bast fibre formation, among which there are twice as many cellulose synthase A (CesA) genes due to a recent whole‐genome duplication after divergence from Gossypium. Population genomic analysis showed two recent population bottlenecks in kenaf, suggesting domestication and improvement process have led to an increase in fibre biogenesis and yield. This chromosome‐scale genome provides an important framework and toolkit for sequence‐directed genetic improvement of fibre crops.

Number of seeds per silique (NSS) is an important determinant of seed yield potential in Brassicaceae crops and is controlled by naturally occurring quantitative trait loci (QTLs). We previously mapped a major QTL, qSS.C9, on the C9 chromosome that controls NSS in rapeseed (Brassica napus L.). To gain a better understanding of how qSS.C9 controls NSS in rapeseed, we isolated this locus through a map-based cloning strategy. qSS.C9 encodes a predicted small protein with 119 amino acids, designated as BnaC9.SMG7b, that shows homology with the Ever Shorter Telomere 1-tertratricopeptide repeats (EST-TPR) and EST-central domains (CD) of Arabidopsis SUPPRESSOR WITH MORPHOGENETIC EFFECTS ON GENITALIA7 (SMG7). BnaC9.SMG7b plays a role in regulating the formation of functional female gametophyte, thus determining the formation of functional megaspores and then mature ovules. Natural loss or artificial knockdown of BnaC9.SMG7b significantly reduces the number of functional ovules per silique and thus results in decreased seed number, indicating that qSS.C9 is a positive regulator of NSS in B. napus. Sequence and function analyses show that BnaC9.SMG7b experiences a subfunctionalization process that causes loss of function in nonsense-mediated mRNA decay (NMD), as in Arabidopsis SMG7. Haplotype analysis in 84 accessions showed that the favorable BnaC9.SMG7b alleles are prevalent in modern B. napus germplasms, suggesting that this locus has been a major selection target of B. napus improvement. Our results represent the first step toward unraveling the molecular mechanism that controls the natural variation of NSS in rapeseed.

Summary: Cultivated jute, which comprises the two species Corchorus capsularis and C. olitorius, is the second most important natural fibre source after cotton. Here we describe chromosome‐level assemblies of the genomes of both cultivated species. The C. capsularis and C. olitorius assemblies are each comprised of seven pseudo‐chromosomes, with the C. capsularis assembly consisting of 336 Mb with 25,874 genes and the C. olitorius assembly containing 361 Mb with 28 479 genes. Although the two Corchorus genomes exhibit collinearity, the genome of C. olitorius contains 25 Mb of additional sequences than that of C. capsularis with 13 putative inversions, which might give a hint to the difference of phenotypic variants between the two cultivated jute species. Analysis of gene expression in isolated fibre tissues reveals candidate genes involved in fibre development. Our analysis of the population structures of 242 cultivars from C. capsularis and 57 cultivars from C. olitorius by whole‐genome resequencing resulted in post‐domestication bottlenecks occurred ~2000 years ago in these species. We identified hundreds of putative significant marker‐trait associations (MTAs) controlling fibre fineness, cellulose content and lignin content of fibre by integrating data from genome‐wide association studies (GWAS) with data from analyses of selective sweeps due to natural and artificial selection in these two jute species. Among them, we further validated that CcCOBRA1 and CcC4H1 regulate fibre quality in transgenic plants via improving the biosynthesis of the secondary cell wall. Our results yielded important new resources for functional genomics research and genetic improvement in jute and allied fibre crops.

Kenaf (Hibiscus cannabinus) is one of the most fast-growing bast in the world and belongs to the family Malvaceae. However, the systematic classification and chloroplast (cp) genome of kenaf has not been reported to date. In this study, we sequenced the cp genome of kenaf and conducted phylogenetic and comparative analyses in the family of Malvaceae. The sizes of H. cannabinus cp genomes were 162,903 bp in length, containing 113 unique genes (79 protein-coding genes, four rRNA genes, and 30 tRNA genes). Phylogenetic analysis indicated that the cp genome sequence of H. cannabinus has closer relationships with Talipariti hamabo and Abelmoschus esculentus than with Hibiscus syriacus, which disagrees with the taxonomical relationship. Further analysis obtained a new version of the cp genome annotation of H. syriacus and found that the orientation variation of small single copy (SSC) region exists widely in the family of Malvaceae. The highly variable ycf1 and the highly conserved gene rrn32 were identified among the family of Malvaceae. In particular, the explanation for two different SSC orientations in the cp genomes associated with phylogenetic analysis is discussed. These results provide insights into the systematic classification of the Hibiscus genus in the Malvaceae family.