Integration of Phenotypic, Accumulative, Physiological–Biochemical, and Transcriptomic Analyses Reveals New Insights in Lettuce Response to Iodine: Enhancement or Toxic Effects

Iodine interferes with plant gene expression and affects plant growth, but the molecular mechanisms behind plant responses to iodine are not yet fully understood. In this study, lettuce (Lactuca sativa L.) was exposed to varying levels (0, 1, 5, 10, 50, and 100 μM) of iodine (NaI and NaIO₃). The results indicate that NaI was more biologically effective than NaIO₃. Low concentrations of iodine increased plant biomass, photosynthetic pigment content, and protein content while maintaining reactive oxygen species homeostasis by regulating antioxidant enzyme activities and antioxidant content. However, high concentrations of iodine caused toxic phenotypic symptoms and increased oxidative stress. Transcriptomic analysis showed that high concentration (50 μM) of NaI led to the downregulation of most genes related to photosynthetic metabolism, disrupting the electron transfer process and Calvin cycle of photosynthesis. Furthermore, iodine exposure activated plant hormone signaling. In conclusion, this study revealed the morpho-physiological, biochemical, and transcriptional response mechanisms of lettuce to iodine exposure.