The chromosome-level genome of Elsholtzia splendens provides insights into genome evolution and environmental adaptation

Technical Summary: Chromosome-Level Genome of Elsholtzia splendens and Insights into Copper Adaptation

Problem Statement
Soil copper (Cu) contamination, driven by industrialization and agriculture, poses a severe threat to ecosystem stability and agricultural productivity. While Elsholtzia splendens (copper grass) is recognized as a highly efficient Cu-hyperaccumulating plant capable of phytoremediation, the genomic architecture underpinning its extreme tolerance and adaptive evolution has remained largely unresolved. Previous genomic resources for this species were limited to basic structural assemblies lacking systematic exploration of repeat dynamics, gene family evolution, and the regulatory networks governing copper stress responses.

Methodology
The study employed an integrated multi-omics approach to construct a high-quality, chromosome-scale genome and analyze its functional implications:

• Genome Assembly: A chromosome-level assembly was generated by combining PacBio HiFi long-read sequencing with Hi-C chromatin conformation capture data. This approach utilized de novo assembly (Hifiasm) followed by scaffolding and orientation (3D-DNA, Juicebox) to anchor contigs to eight pseudochromosomes.
• Annotation and Analysis: The genome underwent comprehensive annotation for repeats (including LTR-RTs), non-coding RNAs, and protein-coding genes using a combination of homology-based, ab initio, and transcriptome-supported methods.
• Evolutionary Genomics: Phylogenomic placement was established using 201 single-copy orthologs across 14 species. Gene family expansion/contraction was analyzed using CAFE, while positive selection was identified via branch-site models in PAML. Whole-genome duplication (WGD) and collinearity were assessed using MCScanX and Ks distribution analysis.
• Transcriptomics: RNA-seq was performed on roots, stems, leaves, and flowers, as well as seedlings subjected to control (CK), low copper (Cu100), and high copper (Cu500) treatments. Differential expression analysis and Weighted Gene Co-expression Network Analysis (WGCNA) were used to dissect tissue-specific regulatory logic.
• HMA Family Characterization: The Heavy Metal ATPase (HMA) gene family was identified via BLAST and HMMER, followed by phylogenetic reconstruction, haplotype network analysis, and duplication event classification.

Key Results

• Genome Assembly Quality: The final assembly spans 255.39 Mb, representing 85.78% of the estimated 297.74 Mb genome size. It comprises 92.01% of the sequence anchored to eight chromosomes with a contig N50 of 14.11 Mb. Completeness metrics (BUSCO) indicate 98.07% recovery of conserved land plant genes.
• Repeat Dynamics and Genome Evolution: Repetitive elements constitute 44.54% of the assembly, dominated by Long Terminal Repeat Retrotransposons (LTR-RTs). A striking finding is a recent, lineage-specific burst of LTR-RT proliferation (peaking at ~0.0241 Mya), which far exceeds activity levels in related Lamiaceae species. This burst is identified as a major driver of genomic restructuring.
• Phylogeny and Divergence: Phylogenomic analysis places E. splendens as the sister group to Salvia and Origanum, with a divergence time estimated at approximately 30.11 million years ago (Mya). Unlike some relatives that underwent recent Whole-Genome Duplication (WGD), E. splendens shows no recent WGD signature, relying instead on localized gene family remodeling.
• Positive Selection and Gene Families: Genome-wide scans identified 93 positively selected genes, including heavy metal transporters, redox regulators, and stress-responsive transcription factors. Gene family analysis revealed 129 expanded families (enriched in stress signaling, secondary metabolism, and RNA silencing) and 40 contracted families.
• Transcriptomic Response to Copper: Copper stress induced extensive, dose-dependent transcriptional reprogramming. A clear division of labor was observed: leaves prioritized oxidative detoxification and metal chelation, while stems specialized in metal sequestration, long-distance translocation, and cell wall modification (lignification).
• HMA Gene Family: Sixteen EsHMA genes were identified, with 15 belonging to the Cu/Ag subfamily. These genes display substantial haplotype diversity and are primarily expanded via proximal duplication. Expression profiling revealed tissue-specific and concentration-dependent regulation, with specific paralogs (e.g., EsHMA10–13) upregulated under high copper stress, suggesting roles in vacuolar sequestration and xylem loading.

Significance and Claims
The authors assert that this work moves beyond simple genome assembly to provide an integrated picture of the genomic innovations driving extreme copper tolerance in E. splendens. The study claims to:

1. Elucidate Mechanisms of Adaptation: Demonstrate how a combination of recent transposon bursts, targeted gene family expansion (particularly in stress resistance and secondary metabolism), and HMA-dominated regulatory networks jointly shape the species' hypertolerance.
2. Clarify Evolutionary History: Establish the phylogenetic position of E. splendens within Lamiaceae and highlight its unique evolutionary trajectory characterized by a recent LTR-RT burst rather than WGD.
3. Define Regulatory Logic: Reveal the organ-specific regulatory strategies (oxidative detoxification in leaves vs. sequestration in stems) that allow the plant to thrive in metalliferous soils.
4. Provide Genetic Targets: Offer concrete genetic targets, specifically the EsHMA paralogs and positively selected genes, for future phytoremediation efforts and the breeding of metal-tolerant crop varieties.

The paper concludes that these findings advance the mechanistic understanding of heavy metal adaptation in the Lamiaceae family and provide a foundational genomic resource for developing novel, efficient phytoremediation approaches to mitigate global soil copper pollution.