Physiological Mechanisms and Core Genes in Response to Saline-Alkali Stress in Foxtail Millet ( L.).

Soil salinization and alkalization are becoming increasingly severe in recent decades, which poses serious threats to crop production and food security in the world. Foxtail millet ( L.) is an important cereal crop in China, and it is important to elucidate its saline-alkali tolerance mechanisms for the breeding of new saline-alkali tolerant varieties. In this study, we used 75% seawater to treat two foxtail millet varieties with different saline-alkali tolerances (JK3, saline-alkali tolerant; B175, saline-alkali sensitive) during the seedling stage, and conducted morphological, cellular ultrastructure, physiological, and transcriptomic analyses on the two varieties. The morphological analysis of the saline-alkali response indicated that JK3 exhibited stronger saline-alkali tolerance than B175. The results of the cellular ultrastructure showed that under saline-alkali stress, JK3 had a more intact leaf cell structure than B175, indicating that saline-alkali stress causes less damage to its cells. The physiological analysis of saline-alkali response indicated that JK3 had consistently higher activities of catalase and polyphenol oxidase, as well as higher contents of soluble sugars and soluble proteins at 48-120 h than B175. Transcriptomic analysis revealed that JK3 enhanced its saline-alkali tolerance by positively regulating pathways such as tryptophan/fatty acid metabolism, the MAPK signaling pathway, and peroxisome pathways. Further, WGCNA combining morphological and physiological indicators identified four key modules and five functional pathways (MAPK signaling, glycerolipid metabolism, phosphate and phosphonate metabolism, galactose metabolism, and endoplasmic reticulum protein processing) in response to saline-alkali stress, and identified a total of 24 core genes. Functional annotation indicated that these genes may be involved in the response to saline-alkali stress. These findings lay a foundation for in-depth studies of the molecular mechanisms for saline-alkali tolerance in foxtail millet.