| Summary: | Previous research has shown that miR398 contributed to plant thermotolerance by silencing its target gene <i>COPPER/ZINC SUPEROXIDE DISMUTASE1</i> (<i>CSD1</i>) in <i>Arabidopsis thaliana</i>. However, the phylogenesis of miR398 and <i>CSD1</i> in Brassica crop and their role in regulating leaf cell death under heat stress remains unexplored. Here, we characterized the homologous genes of miR398a and <i>CSD1</i> in <i>Brassica rapa</i> ssp. <i>pekinensis</i> (Chinese cabbage) and found miR398a abundance was accumulated under heat stress (38 °C and 46 °C for 1 h) in Chinese cabbage, while the expression level of its targets <i>BraCSD1-1</i> and <i>BraCSD2-1</i> were downregulated. To further explore their role in heat response, we constructed the transgenic plants overexpressing artificial miR398a (aBra-miR398a), Bra-miR398a target mimic (Bra-MIM398a), and <i>BraCSD1-1</i> in Chinese cabbage for genetic study. Under high temperatures, <i>p35S::aBra-miR398a</i> lines reduced the areas of leaf cell death and delayed the leaf cell death. By contrast, <i>p35S::Bra-MIM398a</i> and <i>p35S::BraCSD1-1</i> plants enlarged the areas of leaf cell death and displayed the earliness of leaf cell death. Finally, we found that the expression level of stress-responsive genes <i>BraLEA76</i>, <i>BraCaM1</i>, <i>BraPLC</i>, <i>BraDREB2A</i>, and <i>BraP5CS</i> increased in transgenic plants overexpressing aBra-miR398a, which may contribute to their resistance to heat-induced leaf cell death. Taken together, these results revealed the function of Bra-miR398a in attenuating leaf cell death to ensure plant thermotolerance, indicating that the miR398-CSD1 module could be potential candidates for heat-resistant crop breeding.
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