Integrated mRNA-miRNA Analysis Reveals the Regulatory Network Under Salt–Alkali Stress in Alfalfa (Medicago sativa L.)

Soil salinization and alkalinization critically constrain alfalfa (Medicago sativa L.) productivity, yet the regulatory mechanisms underlying its responses to salt–alkali stress are not fully understood. In this study, the alfalfa variety “Zhongmu No. 1” was used as experimental material. The seeds were subjected to salt stress (75 mM NaCl), alkali stress (15 mM NaHCO3), and combined salt–alkali stress (50 mM NaCl + 5 mM NaHCO3) in dishes, with ddH2O serving as the control (CK). After 7 days of germination, the seedlings were transferred to a hydroponic system containing Hoagland nutrient solution supplemented with the corresponding treatments. Following 32 days of stress exposure, leaf and root tissue samples were collected for morphological and physiological measurements, as well as mRNA and miRNA sequencing analyses. Physiological assays revealed significant growth inhibition and increased electrolyte leakage under stress conditions. Transcriptome profiling identified over 5000 common differentially expressed genes (DEGs) in both leaves and roots under stress conditions, mainly enriched in pathways related to “iron ion binding”, “flavonoid biosynthesis”, “MAPK signaling”, and “alpha-Linolenic acid metabolism”. MiRNA sequencing detected 453 miRNAs, including 188 novel candidates, with several differentially expressed miRNAs (DEMs) exhibiting tissue- and stress-specific patterns. Integrated analysis revealed 147, 81, and 140 negatively correlated miRNA–mRNA pairs across three treatment groups, highlighting key regulatory modules in hormone signaling and metabolic pathways. Notably, in the ethylene and abscisic acid signaling pathways, ERF (XLOC_006645) and PP2C (MsG0180000476.01) were found to be regulated by miR5255 and miR172c, respectively, suggesting a post-transcriptional layer of hormonal control. DEM target genes enrichment pathway analyses also identified stress-specific regulation of “Fatty acid degradation”, “Galactose metabolism”, and “Fructose and mannose metabolism”. qRT-PCR validation confirmed the expression trends of selected DEGs and DEMs. Collectively, these findings reveal the complexity of miRNA–mRNA regulatory networks in alfalfa’s response to salt–alkali stress and provide candidate regulators for breeding stress-resilient cultivars.