Haplotype-resolved genome assembly of upland switchgrass provides insights into cold tolerance

Switchgrass (Panicum virgatum L.) is a bioenergy and forage crop. Upland switchgrass exhibits superior cold tolerance than lowland ecotype, but the underlying molecular mechanisms remain unclear. Here, we presented a high-quality haplotype-resolved genome of the upland ecotype Jingji31 and conducted multi-omics analysis to understand its cold tolerance. The divergence between upland and lowland ecotypes of switchgrass occurred after the differentiation of the two subgenomes (K and N). Under cold stress, the K subgenome has more differentially expressed genes (DEGs). Transcriptome analysis revealed ecotype-specific differential expressions among members of the cold-responsive (COR) gene families. Specifically, certain members of the AFB1, ATL80, HOS10, and STRS2 gene families exhibited opposite expression changes between the two ecotypes, potentially contributing to their differential cold tolerance. By using haplotype-resolved genome, we identified more cold-induced allele-specific expressions (ASEs) in the upland ecotype, and these ASEs were significantly enriched in the COR gene families. Genome-wide association study detected an association signal on Chr3K related to overwintering rate, which overlapped with a selective sweep region and contained a cytochrome P450 (CYP450) gene highly expressed under cold stress. Heterologous overexpression of CYP450 in rice alleviated leaf wilting and improved cold tolerance. Our study provides a high-quality haplotype-resolved genome of upland switchgrass, to advance conceptual understanding of plant cold tolerance for breeding crops with enhanced cold adaptation.