Physiological and Transcriptional Analyses Provide Insight into Maintaining Ion Homeostasis of Sweet Sorghum under Salt Stress

Sweet sorghum is an important bioenergy grass and valuable forage with a strong adaptability to saline environments. However, little is known about the mechanisms of sweet sorghum coping with ion toxicity under salt stresses. Here, we first evaluated the salt tolerance of a sweet sorghum cultivar "Lvjuren" and determined its ion accumulation traits under NaCl treatments; then, we explored key genes involved in Na⁺, Cl^-, K⁺ and NO₃^- transport using transcriptome profiling and the qRT-PCR method. The results showed that growth and photosynthesis of sweet sorghum were unaffected by 50 and 100 mM NaCl treatments, indicative of a strong salt tolerance of this species. Under NaCl treatments, sweet sorghum could efficiently exclude Na⁺ from shoots and accumulate Cl^- in leaf sheaths to avoid their overaccumulation in leaf blades; meanwhile, it possessed a prominent ability to sustain NO₃^- homeostasis in leaf blades. Transcriptome profiling identified several differentially expressed genes associated with Na⁺, Cl^-, K⁺ and NO₃^- transport in roots, leaf sheaths and leaf blades after 200 mM NaCl treatment for 6 and 48 h. Moreover, transcriptome data and qRT-PCR results indicated that HKT1;5, CLCc and NPF7.3-1 should be key genes involved in Na⁺ retention in roots, Cl^- accumulation in leaf sheaths and maintenance of NO₃^- homeostasis in leaf blades, respectively. Many TFs were also identified after NaCl treatment, which should play important regulatory roles in salt tolerance of sweet sorghum. In addition, GO analysis identified candidate genes involved in maintaining membrane stability and photosynthetic capacity under salt stresses. This work lays a preliminary foundation for clarifying the molecular basis underlying the adaptation of sweet sorghum to adverse environments.