Transcriptomic and metabolomic analyses reveal that exogenous strigolactones alleviate the response of melon root to cadmium stress

Strigolactones (SLs) are classified into plant hormones, playing a key role as a mediator of plant growth in response to several abiotic stresses. Cadmium (Cd), a common heavy metal and soil pollutant, can suppress plant growth and development. In this work, we explored the effects of exogenous SLs on root formation in response to Cd stress using melon seeds subjected to seven germination treatments: CK (control), Cd (300 μmol·L-1 CdCl2), and SL1–SL5 (CdCl2-stressed seeds pretreated with 0.1, 0.5, 1, 2, and 3 μmol·L-1 GR24 solutions). The results indicated that SLs increased the antioxidant enzyme activities and root vigor and decreased the malondialdehyde (MDA) contents in the roots of Cd-stressed melon seedlings. Then we used transcriptomic and metabolomic analyses to explore the mechanisms by which exogenous SLs protect against Cd stress. There were 242 significant differentially expressed genes (DEGs) (78 upregulated, 164 downregulated) and 247 significantly differentially expressed metabolites (DEMs) (222 upregulated, 25 downregulated) between the Cd and SL3 treatments. SLs altered the expression of genes related to redox formation processes, including peroxidase (POD), lipoxygenase (LOX), glutamate dehydrogenase (GDH), and glutathione S-transferase (GST). In addition, we found that SLs regulated the expression of the MYB, AP2/ERF, bHLH, and WRKY transcription factor families. The combined transcriptomic and metabolomic analyses revealed that the DEGs and DEMs involved in Cd stress alleviation were mainly related to the gene expression of jasmonic acid (JA) and flavonoid biosynthesis. SLs might induce LOX-related genes to regulate JA biosynthesis. Moreover, SLs might promote flavonoid biosynthesis by regulating eleven flavonoid-related genes and eight metabolites. The results provide a new perspective for studying the adaptation of plants to Cd stress.