SUMMARYThe endoplasmic reticulum (ER) stress sensor IRE1 transduces signals by inducing the unconventional splicing of mRNAs encoding key transcription factors: HAC1 in yeast and XBP1 in animals. However, no HAC1 or XBP1 homologues have been found in plants, and until recently the substrate for plant IRE1 has remained unknown. This study demonstrates that the Oryza sativa (rice) OsbZIP50 transcription factor, an orthologue of Arabidopsis AtbZIP60, is regulated by IRE1-mediated splicing of its RNA. Despite the presence of a transcriptional activation domain, OsbZIP50 protein is not translocated into the nucleus efficiently in the absence of OsbZIP50 mRNA splicing. Unconventional splicing of OsbZIP50 mRNA causes a frame shift, which results in the appearance of a nuclear localization signal in the newly translated OsbZIP50. OsbZIP50 mRNA is spliced in a similar manner to HAC1 and XBP1 mRNAs; however, this splicing has very different effects on the translation products, a finding that shows the diversity of IRE1-related transcription factors in eukaryotes. In addition, the expression of OsbZIP50 is affected by ER stress sensor proteins OsIRE1, OsbZIP39 and OsbZIP60. ER stress-related genes differ with respect to their dependency on OsbZIP50 for their expression. The findings of this study improve our understanding of the molecular mechanisms underlying the plant ER stress response.

SUMMARYBinding protein (BiP) is the key chaperone involved in folding of secretory proteins such as seed storage proteins in the ER lumen. To obtain functional information about BiP1, a gene that is predominantly expressed during rice seed maturation, we generated several transgenic rice plants in which various levels of BiP1 protein accumulated in an endosperm-specific manner. Severe suppression (BiP1 KD) or significant over-expression (BiP1 OEmax) of BiP1 not only altered seed phenotype and the intracellular structure of endosperm cells, but also reduced seed storage protein content, starch accumulation and grain weight. Microarray and RT-PCR analyses indicated that expression of many chaperone and co-chaperone genes was induced in transgenic plants, with more prominent expression in the BiP1 KD line than in the BiP1 OEmax line. Transcriptional induction of most chaperones was observed in calli treated with dithiothreitol or tunicamycin, treatments that trigger ER stress, indicating that induction of the chaperone genes in transgenic rice was caused by an ER stress response. In transient assays using rice protoplasts, the ortholog (Os06g0622700) of the AtbZIP60 transcription factor was shown to be involved in activation of some chaperone genes. Slight increases in the BiP1 level compared with wild-type, accompanied by increased levels of calnexin and protein disulfide isomerase-like proteins, resulted in significant enhancement of seed storage protein content, without any change in intracellular structure or seed phenotype. Judicious modification of BiP1 levels in transgenic rice can provide suitable conditions for the production of secretory proteins by alleviating ER stress.

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SummaryThe common neurodegenerative disorder known as Alzheimer's disease is characterized by cerebral neuritic plaques of amyloid b (Ab) peptide. Plaque formation is related to the highly aggregative property of this peptide, because it polymerizes to form insoluble plaques or fibrils causing neurotoxicity. Here, we expressed Ab peptide as a new causing agent to endoplasmic reticulum (ER) stress to study ER stress occurred in plant. When the dimer of Ab 1-42 peptide was expressed in maturing seed under the control of the 2.3-kb glutelin GluB-1 promoter containing its signal peptide, a maximum of about 8 lg peptide per grain accumulated and was deposited at the periphery of distorted ER-derived PB-I protein bodies. Synthesis of Ab peptide in the ER lumen severely inhibited the synthesis and deposition of seed storage proteins, resulting in the generation of many small and abnormally appearing PB bodies. This ultrastructural change was accounted for by ER stress leading to the accumulation of aggregated Ab peptide in the ER lumen and a coordinated increase in ER-resident molecular chaperones such as BiPs and PDIs in Ab-expressing plants. Microarray analysis also confirmed that expression of several BiPs, PDIs and OsbZIP60 containing putative transmembrane domains was affected by the ER stress response. Ab-expressing transgenic rice kernels exhibited an opaque and shrunken phenotype. When grain phenotype and expression levels were compared among transgenic rice grains expressing several different recombinant peptides, such detrimental effects on grain phenotype were correlated with the expressed peptide causing ER stress rather than expression levels.