Integrated morpho-physiological, transcriptomic and metabolomic data to reveal the differential chilling defense mechanisms of two ecologically diverged species of Forsythia

Long-term niche differentiation will lead to the evolution of diverse adaptive strategies for species in diverse environments. The present study selected two Forsythia species, Forsythia mandshurica (Fm)—which naturally occurs in a cold temperate zone and Forsythia suspensa (Fs)—which thrives in a warm temperate zone—to reveal their differential chilling defense mechanisms by integrating morpho-physiological, transcriptomic, and metabolomic data. Transcriptome results show that Fm has evolved in a series of adaptive mechanisms designed to help the plants to cope with chilling stress by enhancing sugar, amino acid, hormone, polyamine, and phenol content to improve cell osmotic potential and to mitigate petal browning. Metabolomic data suggested the increased chilling resistance of Fm relies on in the plant being rich in α-linolenic acid, linoleic acid, as well as two amino acids, Phe and Trp, and has low levels of cinnamic acid and gramine in flowers compared to Fs. A higher abundance of glutathione disulfide and NADPH regulated by glutathione peroxidases and NADPH improved the ability of the cellular antioxidant and reduction-oxidation system stability in Fm; Additionally, the elevated levels of pyruvate, α-ketoglutaric acid, and oxaloacetic acid in Fm contributed to a significantly enhanced ATP production in mitochondria. Through Ka/Ks and gene expression analysis, four transcription factors, EVM0025036 (bHLH), EVM0010639 and EVM0007275 (AP2), and EVM0025908 (bZIP) were identified that may contribute to the high cold tolerance of Fm. These adaptations highlight the intricate interplay between genetic and physiological processes that shape the survival strategies of plants in response to their specific ecological niches.