Abstract
We are developing transgenic rice plants accumulating pharmaceutical products in endosperm. In some cases, however, the seeds displayed an opaque phenotype with floury and shrunken features. In these seeds, the endoplasmic reticulum (ER)-resident chaperons such as BiP were upregulated. Therefore, we thought that the ER stress was caused by the accumulation of foreign gene products in these seeds. Recently, several bZIP transcription factors with transmembrane domain (TMD) have been shown to be involved in the ER stress response in Arabidopsis. We identified three bZIP transcription factors possessing a putative TMD among 89 predicted bZIP in the rice genome. OsbZIP39 has a bZIP DNA binding domain followed by a putative TMD. A truncated form without C-terminal region containing putative TMD (OsbZIP39ΔC) fused with GFP localized to the nucleus in rice protoplasts. Overexpression of OsbZIP39ΔC activated a BiP promoter in a transient assay. In addition, BiP protein was highly accumulated in the OsbZIP39ΔC expressing transgenic lines without any ER stress inducer. These results indicated that OsbZIP39 regulates the ER stress response in rice.
