Putrescine, spermidine, and spermine play distinct roles in rice salt tolerance

Polyamines (PAs) play diverse roles in plant growth and development, as well as responses to environmental stimuli. In this study, the effects of PAs on rice salt tolerance were investigated. Salt stress resulted in the alteration of endogenous PAs levels in rice roots and leaves, where spermidine (Spd) and spermine (Spm) contents were increased, and putrescine (Put) content was decreased. RT-qPCR analysis revealed that PAs biosynthesis-related genes ADC1, ODC, and Arginase were significantly downregulated by salt treatment; however, SAMDC transcription was significantly upregulated. Exogenous Spm enhanced rice salt tolerance remarkably; however, exogenous Put and Spd undermined rice salt tolerance. Transgenic rice plants overexpressing SAMDC display a higher ratio of Spm/(Put+Spd) and enhanced salt tolerance. Salt stress also increased polyamine oxidase activities in rice, resulting in elevated reactive oxygen species (ROS) production. Our findings revealed that accumulation of Put and Spd substantially reduced salt tolerance in rice, likely by facilitating ROS production; whereas, conversion of Put and Spd to Spm contributes to rice salt tolerance.