Physiological responses and screening of key genes involved in the cold tolerance of herbaceous peony cultivated over-winter in Harbin, the coldest metropolis of China

Herbaceous peony is a globally-renowned ornamental plant. Extreme winter conditions can severely damage the underground buds and crowns, and hinder subsequent bud break and growth of herbaceous peony, although it has a certain degree of cold tolerance. This study offers new insights into the cold tolerance of herbaceous peony by comparing two cultivars of contrasting provenances and tolerances to cold. Physiological observation, transcriptome sequencing, and gene expression analysis were performed on overwintering crown buds exposed to freezing conditions in Harbin. The two cultivars demonstrated relatively significant morphological and physiological differences in response to cold stress. Specifically, HL exhibited superior morphological traits, including more stems (4.30 vs 3.47) and flowers (12.67 vs 5.87) per plant, as well as significantly higher soluble sugar content (e.g., 280 mg/g FW vs 210 mg/g FW in January). Transcriptome results show that various expressed genes were in connection with starch and sucrose metabolism, plant-pathogen interactions, and phytohormone signaling pathways. The co-expression analysis identified and visualized the key genes in the two modules, ME black and ME yellowgreen. Integrating physiological changes with expression of genes, several key genes were identified, such as XERO1, LTI65, NADP-ME4, WRKY39, and ASA2, which are probably involved in the cold-stress response, and thus deserve further exploration in gene function and interaction. This study provides new insight into the cold-tolerance of herbaceous peony and paves the way for its future studies, and also contributes to breeding strong cold-resistant cultivars of herbaceous peony in the high-latitude regions with harsh winters.