Comparative Transcriptome Analysis Revealing the Potential Salt Tolerance Mechanism of Exogenous Abscisic Acid Application in Melilotus albus

<i>Melilotus albus</i>, which contains abundant pharmacologically active coumarins, is usually used as a rotation crop and green manure worldwide. Abscisic acid (ABA) is a crucial plant hormone that plays an important role in plant stress responses. There is a paucity of information about the ABA signaling pathway and its regulatory network in <i>M. albus</i>. Here, we performed a comparative physiological and transcriptomic analysis to assess the response of <i>M. albus</i> to exogenous ABA. Physiological analysis revealed that proline (Pro), soluble protein and H₂O₂ content after ABA treatment 3 h significantly increased by 14.0%, 12.0% and 32.4% compared with 0 h in <i>M. albus</i>. A total of 19,855 differentially expressed genes (DEGs) were identified under ABA treatment, including 13,392 in shoots and 15,471 in roots. We obtained two modules that were significantly correlated with the ABA treatment (the darkorange module was positively correlated at 24 h in the shoot, brown2 module positively correlated at 3 h in the root) by weighted correlation network analysis (WGCNA). KEGG enrichment analysis showed that genes within two modules were primarily enriched in protein synthesis and metabolism, secondary metabolites, purine and pyrimidine metabolism, and phenylalanine, tyrosine and tryptophan biosynthesis. GO enrichment analysis indicated that genes within two modules were primarily enriched in energy substance metabolism. These pathways were mainly associated with abiothic stress, which indicated that exogenous application of ABA activated the stress resistance system of <i>M. albus</i>. The hub gene <i>4CL1</i> (4-Coumarate: CoA ligase 1) was translated and expressed in yeast, resulting in enhanced salt and ABA tolerance in the transgenic yeast. Overexpression of <i>Ma4CL1</i> in <i>M. albus</i> improved the salt resistance of the transgenic plants. Profiling ABA-responsive genes offers valuable insights into the molecular functions of regulatory genes and will facilitate future molecular breeding efforts in <i>M. albus</i>.