Overexpression of type II rice metacaspase, OsMC4, increases endoplasmic reticulum stress tolerance in transgenic rice calli

The endoplasmic reticulum (ER) is an organelle responsible for regulating protein synthesis in plants. High salinity can lead to the accumulation of misfolded proteins, resulting in an ER stress response mechanism known as the unfolded protein response (UPR). Failure of the UPR to reverse the effect of protein misfolding will activate programmed cell death (PCD). Metacaspase genes are known regulate PCD in plants. This study aims to provide a comprehensive analysis of the expression patterns of type II rice metacaspase (OsMC) genes in response to ER and salinity stress in rice leaf. Among five type II metacaspases in the rice genome, OsMC4, OsMC5, and OsMC8 expressions were found to be upregulated during treatment with tunicamycin (ER stress) and sodium chloride, NaCl (salinity stress). A construct of taqRFP::OsMC4, controlled by the CaMV35S promoter, was generated and transformed into rice calli. The transgenic rice calli overexpressing taqRFP::OsMC4 demonstrated significant changes in the expression of the ER stress-marker genes, protein disulfide isomerase (OsPDI), and binding immunoglobulin protein (OsBiP). The results from this analysis provide preliminary evidence that at least one of the type II metacaspases, OsMC4, is be able to reduce ER and salinity stresses in rice. Further functional analysis of OsMC genes in ER and salinity stress tolerance could be carried out in transgenic rice overexpressing OsMCs in the future to improve stress tolerance. Simple summary: Oryza sativa, commonly known as rice, is one of the most consumed crops in the world. In response to multiple biotic and abiotic factors, a series of endoplasmic reticulum (ER) stress response regulators are activated. There is evidence that high salinity triggers ER stress in plants. This study aims to determine the level of gene expression among type II metacaspases in rice in response to ER and salinity stress and to assess how they may be linked to PCD in rice calli. Three metacaspase genes, OsMC4, OsMC5, and OsMC8, have been observed to have significant expression post-treatment with tunicamycin in rice leaf. Overexpression of taqRFP:: OsMC4 in rice calli significantly reduces the expression level of the stress markers, OsBiP and OsPDI, indicating that the stress level is relatively lower in the transgenic calli compared to the wild-type calli. Therefore, overexpression of taqRFP::OsMC4 in rice may increase rice tolerance towards ER and salinity stress. These expression analyses of the OsMCs family provide valuable information for further functional studies on the biological roles of OsMCs in ER and salinity stress responses.