The Difference in the Mechanisms of the TCA Cycle, Organic Acid Metabolism and Secretion of Rapeseed Roots Responding to Saline and Alkaline Stresses

Currently, the differences in the responses of the organic acid metabolism in rapeseed (Brassica napus L.) roots to saline and alkaline stresses are still unknown. To clarify the differences, different saline (100 (LS) and 200 (HS) mmol/L NaCl) and alkaline (20 (LA) and 40 (HA) mmol/L Na2CO3) treatments were applied to rapeseed. Then, targeted metabolomics was used to quantitatively analyze the changes in organic acid metabolism in the root system. The results showed that compared with the control group without stress (CK), 21, 18, 27, and 20 differentially accumulated organic acid metabolites were detected in the rapeseed roots under LS, HS, LA, and HA, respectively. In addition, 26, 6, 34, and 14 differentially accumulated organic acids were detected in the rapeseed root exudates under LS, HS, LA, and HA, respectively. Based on the activities of key enzymes related to the tricarboxylic acid cycle (TCA), antioxidant enzyme activities, organic acid metabolism, and KEGG (Kyoto Encyclopedia of Genes and Genomes) enrichment analysis in rapeseed roots, rapeseed mainly resisted saline and alkaline stresses by increasing organic acid synthesis and scavenging reactive oxygen species. Specifically, rapeseed resisted saline stress mainly by increasing the secretion of TCA cycle-related organic acids such as succinic acid, L-malic acid, fumaric acid, and cis-aconitic acid. In addition to secreting organic acids, rapeseed also resisted alkaline stress by increasing the secretion of phenolic acids such as 4-hydroxybenzoic acid, ferulic acid, and 4-coumaric acid. Notably, the number of secreted organic acid types and the increase in organic acid content under alkaline stress were higher than those under saline stress. The results of this study provide an important basis for the breeding of saline and alkaline stress-tolerant rapeseed varieties.