Transgenic rice expressing a methionine-rich protein

Abstract

Sulfur amino acid composition is an important determinant of seed protein quality. A chimeric gene encoding Sunflower Seed Albumin (SSA), one of the most sulfur-rich seed storage proteins so far identified, was introduced into rice ( Oryza sativa) in order to modify seed cysteine and methionine content. Analysis of a transgenic line expressing SSA at approximately 7% of total seed protein revealed that the mature grain showed little change in the total sulfur amino acid content compared to the parental genotype. This result indicated that the transgenic rice grain was unable to respond to the added demand for cysteine and methionine imposed by the production of SSA. The limited sulfur reserves appeared to be reallocated from endogenous proteins to the new sulfur sink in the transgenic grain. Analysis of the protein composition of the transgenic gram showed changes in the relative levels of the major seed storage proteins, as well as some non-storage proteins, compared to non-transgenic controls. Changes observed at the protein level were concomitant with differences in mRNA accumulation but not always with the level of transcription. It is hypothesised that the changes in the transgenic endosperm tissue were mediated by a signal transduction pathway that normally modulates seed storage protein composition in response to environmental fluctuations in sulfur availability, via both transcriptional and post-transcriptional control of gene expression. A developing rice panicle m1croarray was constructed to examme changes in gene expressiOn m the developing seeds of the SSA transgenic rice. The expressiOn of selected genes was also examined in the seeds of rice plants supplied with limited soil sulfur. Data from these experiments supported the hypothesis that the SSA-expressing seeds suffered a sulfur stress imposed by the increased demand for sulfur amino acids from the introduced transgene. The microarray experiments also identified several potential regulatory genes including genes for a transcription factor, an RNA-binding protein and signalling proteins. These genes may represent components of the signalling pathway that regulates changes in seed protein composition in response to sulfur availability.