SMXLs regulate seed germination under salinity and drought stress in soybean

Soybeans are one of the most important crops worldwide, but yield and quality can be severely affected by abiotic stresses. Genes in the Suppressor of MAX2 1-Like (SMXL) family are known downstream elements of the karrikin (KAR) and strigolactone (SL) signaling pathways, which play important roles in plant growth and development regulation under abiotic stresses. However, this family has not been well studied in soybeans. Here, seventeen soybean SMXL orthologues were identified, which phylogenetic analysis revealed can be divided into four groups. SMXLs further evinced a highly conserved evolution, with their origin in the liverworts. In a stress treatment experiment, germination of soybean seeds was delayed for two days relative to controls by treatment with 200 mM NaCl, and completely inhibited by 20% polyethylene glycol-induced drought treatment. Alongside the germination delay of salt treatment, GmSMXL2.1, GmSMXL6.1, GmSMXL7.1, and GmSMXL8.1 were significantly upregulated at 26 h, and remained high at 42 h. In contrast, drought stress downregulated all eight examined GmSMXL genes. Similar results were found for upstream regulators of GmSMXLs, including GmKAI2, GmD14, and GmMAX2 in the KAR and SL signaling pathways; these were significantly upregulated in germinating seeds after salt treatment, but significantly downregulated or not affected under drought treatment. In summary, GmSMXLs regulate drought and salt stress responses during the seed germination stage in soybean, potentially via MAX2-mediated KAR- and SL-dependent and independent pathways.