Sly-miR398 Participates in Heat Stress Tolerance in Tomato by Modulating ROS Accumulation and HSP Response

Heat stress is one of the most important environmental problems in agriculture, which severely restricts the growth and yield of plants. In plants, microRNA398 (miR398) negatively regulates the activity of superoxide dismutase (SOD) by modulating the expression of its coding genes (CSDs) post-transcriptionally, thereby regulating reactive oxygen species (ROS) homeostasis and stress resistance. In this study, the role of miR398 in heat stress tolerance in tomato was investigated. Under heat stress, the expression of miR398 was upregulated in tomato, while the expression of its target genes (CSD1 and CSD2) and SOD activity was downregulated. Furthermore, by comparing the heat stress response in wild type (WT) and a transgenic line overexpressing MIR398 (miR398-OE), the results showed that overexpression of miR398 promoted tomato growth and the expression of genes encoding heat shock factor (HSF, transcription factor) and heat shock protein (HSP) under heat stress. Meanwhile, downregulated activity of antioxidant enzymes, including SOD, catalase (CAT), peroxidase (POD), and ascorbate peroxidase (APX), and enhanced ROS accumulation was observed in miR398-OE compared with that in WT under heat stress. Further study using dimethylthiourea (DMTU, a ROS scavenger) indicated that the enhanced plant growth and expression of HSFs/HSPs was based on the promoted accumulation of ROS in miR398-OE. Overall, the results of this study revealed that the upregulated expression of miR398 in response to heat stress would modulate the antioxidant system and enhance ROS accumulation, thereby enhancing the expression of HSFs and HSPs and heat stress tolerance in tomato.