A miR172e/TOE3 Module from the Halophyte Halostachys caspica Regulates Plant Multiple Abiotic Stress Tolerance via Cellular Homeostasis

Salt, drought and freezing stress were major abiotic factors limiting plant growth, development and yield. Halostachys caspica (Amaranthaceae), a halophyte native to saline-arid desert regions, tolerated multiple abiotic stresses, but its molecular mechanisms of stress tolerance remain unclear. By integrating the small RNA library and transcriptome data of H. caspica under high salinity, HcmiR172e was identified as a differentially expressed miRNA and selected for the study of multiple abiotic stress responses. Using its mature sequence (20 nt) to align with upregulated genes from the transcriptome, HcTOE3 (AP2 subfamily transcription factor belonging to the AP2/ERF family) was preliminarily predicted as its target gene through bioinformatic analysis. Our previous work demonstrated that HcTOE3 was strongly upregulated by multiple abiotic stresses, including salinity, drought, heat and low temperature. Furthermore, overexpression of HcTOE3 conferred freezing tolerance to Arabidopsis throughout the entire growth period. In this study, miRNA expression analyses showed that HcmiR172e was significantly downregulated in the assimilating branches of H. caspica under low temperature, heat, salt, drought, oxidative stress and abscisic acid (ABA) application. Tobacco transient expression assays and 5′RLM-RACE confirmed that HcmiR172e directly cleaved HcTOE3 transcripts in the region close to the 5′end of the ORF. HcmiR172e-overexpressing Arabidopsis displayed increased sensitivity to salt, drought, freezing stresses and ABA treatment, along with enhanced growth inhibition, elevated reactive oxygen species (ROS) accumulation, decreased osmolyte content and downregulation of stress-responsive genes. In contrast, HcTOE3-overexpressing Arabidopsis exhibited the opposite phenotypes, physiological responses and corresponding gene expression patterns under multiple stress treatments. These findings collectively elucidated the antagonistic regulatory roles of HcmiR172e and HcTOE3 in plant abiotic stress responses, providing novel molecular targets for engineering stress-tolerant crops for saline, arid, freezing environments.