Comparative responsive patterns of Opisthopappus taihangensis and Opisthopappus longilobus under salt stress by weighted gene co-expression network analysis (WGCNA) and genome-wide association studies (GWAS)

The unique and challenging environment of the cliffs gives Opisthopapus genus (Opisthopappus taihangensis and Opisthopappus longilobus) a strong tolerance to harsh climatic conditions. Under salt stress, the physiological indexes, superoxide dismutase (SOD), soluble protein (SP), chlorophyll (Chl), malondialdehyde (MDA), peroxidase (POD), and catalase (CAT), generally increased with the treatment levels. Through weighted gene co-expression network analysis (WGCNA), MEdarkmagenta module in O. taihangensis (182 genes) and MEdarkgreen module in O. longilobus (281 genes) were screened, which significantly related with SOD, CAT, SP, and POD. With genome-wide association studies (GWAS), it found that abundant SNPs was significantly correlated with the SOD index. Among WGCNA and GWAS, there was eleven common differentially expressed genes (c-DEGs) in O. taihangensis and seventeen c-DEGs in O. longilobus, respectively. For O. taihangensis, it responded to the stress primarily through the proteasome and lipid metabolism pathways involved in PSMB2, PSMA4, and PLD1_2 genes. In contrast, O. longilobus activated the oxidative phosphorylation pathway and then influenced the amino acid biosynthesis, terpenoid biosynthesis, and signaling transduction pathways (e.g., GA, JA, and MAPK) mainly involved MYC2, GID1, CYP82G1, TYRAAT, and MAPKKK17_18 genes. Compared with O. taihangensis, O. longilobus activated additional genes through more pathways to respond to salt stress. According to May-Wigner theory, the simple gene regulatory network was more stable than the complex. Thus, it suggested that O. taihangensis had superior tolerance and adaptation than O. longilobus. The results initially explored the potential responsive mechanism of two species under salt stress. These would establish the foundation for further investigations of the Opisthopapus tolerances, provide clues for the study of other cliff plants in the Taihang Mountains, and lay lights for the performing of Asteraceae cross-breeding.