Decoding the Litsea Genome and Evolution and Scents’ Synthesis of the Laurel Plant Family

A recent Nature Communications article co-authored by Tsinghua SIGS researchers sheds light on the evolution of the laurel family, its floral structures and scent-producing mechanisms.

Tsinghua SIGS Prof. Laiqiang Huang’s group took part in a collaborative research on Litsea genome sequencing and analysis, spearheaded by Prof. Yang-Dong Wang at the Research Institute of Subtropical Forestry, Chinese Academy of Forestry, and Prof. Zhong-Jian Liu at Fujian Agriculture and Forestry University.

In the article, entitled “The Litsea Genome and the Evolution of the Laurel Family”, researchers released the first chromosome-level assembly of the Litsea cubeba genome, additional genomic and transcriptomic data for many other Lauraceae species, and elucidated the evolution of the genome and flowers, and the genetic basis for the scents’ biosynthesis of the laurel family. 

Litsea cubeba is a representative species of the Lauraceae, a family from the order Laurales in Magnoliids. With a total of 2500–3000 globally distributed species in 45 genera, they are highly valuable both ecologically and economically, because they produce a high yield of specific scents --- various terpenoids mainly monoterpenes and sesquiterpenes, for spices, perfumes, and medicine. However, important scientific questions remain regarding the relative phylogenetic position of the Laurales and Magnoliales, the evolutionary relationships within the laurel family, the evolution of floral structures and characters, and the scents’ biosynthetic pathways in Lauraceae species.

In the present study, the whole genome of Litsea cubeba was sequenced, assembled, annotated and analyzed, resulting in a high quality, chromosome-level assembly of the L. cubeba genome. L. cubeba has a diploid genome (2n=24) with an estimated haploid genome size of 1370.14 Mb, and with 31,329 protein-coding genes predicted. Meanwhile, the genomes of other 47 species of 20 genera in Lauraceae were sequenced at a low coverage. Furthermore, mixed-tissue and flower bud transcriptomes were analyzed for 23 species of 16 genera in the laurel family, followed by functional verifications of terpene synthase (TPS) genes using transient overexpression and enzyme activity assay.

Phylogenomic analyses show phylogenetic discordance at the position of Magnoliids, suggesting incomplete lineage sorting during the divergence of monocots, eudicots, and Magnoliids, while it seems more likely that Magnoliids were a sister group to eudicots after their common ancestor diverged from monocots. Two whole-genome duplication (WGD) events were identified, an ancient WGD event occurred just before the divergence of Laurales and Magnoliales, and another more recent WGD event occurred before the divergence in Lauraceae.

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            Fig. 1. Whole-genome duplications (WGD) in Laurales 


Based on analyses of the genomes, transcriptomes and key floral genes, phylogenetic relationships within Lauraceae were established, providing evidence for the evolution of inflorescences in Lauraceae from spikes, racemes, panicles to umbels, and the identification of key genes in controlling development of bisexual (dioecious) and unisexual (monoecious) flowers.

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Fig. 2. The evolution of floral structures in Lauraceae 


Further analyses reveal that the family of genes of monoterpene synthase (mono-TPS or TPS-b), the key enzyme for the biosynthesis of monoterpenes are significantly expanded in Lauraceae, and Lauraceae species with a high percentage content of essential oil (terpenoids) have larger numbers of TPS-b members.These indicate that TPS-b enzymes are responsible for the diversity and yield of terpenoids. Expression and functional studies of TPS-b genes conducted in L. cubeba confirmed the role of the TPSs.

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 Fig. 3. Phylogeny and functional verification of L. cubeba TPSs 


The findings from this study offer insight into the genetic diversity and evolution of Laurales and how they produce specific scents, and advance the understanding of the evolution and diversification of Lauraceae, which could also be valuable for improving genetic breeding and industrial application of the plants.

For the article, Prof. Yang-Dong Wang and Prof. Zhong-Jian Liu are corresponding authors, Prof. Laiqiang Huang is co-author and his postdoc Ke-Wei Liu is co-first author. 

Link to the article: https://www.nature.com/articles/s41467-020-15493-5

 



Edited by Karen Lee

 







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