Metabolomics analysis reveals the effect of removing cotyledons on salt tolerance in castor plant roots during early seedling establishment

Castor is an important oil crop with moderate salt tolerance that is grown worldwide. The early seedling stage is the most vulnerable phase during a plant's lifecycle in a saline environment. Cotyledons play an important role in seedling establishment as the storage and first photosynthetic organ, and in the resistance of the seedling to salt stress. However, how cotyledons affect the metabolic response of castor roots in saline environments is unclear. Here, liquid chromatography-mass spectrometry analytical methods were used to examine the effects of removing cotyledons (no cotyledons removed, NR; one cotyledon removed, OR; both cotyledons removed, TR) on the metabolite changes in the roots under salt stress. The results showed that removing cotyledons inhibited the growth of TR roots much greater than NR or OR roots. Metabolomics analysis showed that the roots of TR seedlings had fewer metabolites involved in the salt stress response than NR seedlings. Moreover, the KEGG pathway enrichment analysis demonstrated that salt-induced metabolites were associated with major biological pathways related to tropane, piperidine, and pyridine alkaloid biosynthesis; amino acid biosynthesis; phenylpropanoid biosynthesis; amino sugar and nucleotide sugar metabolism, as well as arginine and proline metabolism. However, the contents of these metabolites decreased or did not change significantly in the TR roots compared to the NR roots under salt stress. Additionally, the contents of caffeyl alcohol, scopoletin, and syringin in the phenylpropanoid biosynthetic pathway increased significantly in the roots of TR seedlings under salt stress compared to the control treatment. Therefore, we concluded that castor plants with cotyledons enhanced particular metabolic pathways in the roots that protected cell membrane structure, maintained cell osmotic potential, resisted reactive oxygen species and provided energy under salt stress. In addition, the roots of castor plants without cotyledons may be resistant to salt stress by enhancing phenylpropanoid biosynthesis to maintain cell differentiation and enhance root lignification. Taken together, these findings provide new insight into the physiological functions of cotyledons in the aboveground-belowground interactions of castor plants under salt stress.