Proteomics of Rac GTPase signaling reveals its predominant role in elicitor-induced defense response of cultured rice cells.

We have previously shown that a human small GTPase Rac homolog, OsRac1, from rice (Oryza sativa) induces cascades of defense responses in rice plants and cultured cells. Sphingolipid elicitors (SEs) have been similarly shown to activate defense responses in rice. Therefore, to systematically analyze proteins whose expression levels are altered by OsRac1 and/or SE treatment, we performed a differential display analysis of proteins by the use of two-dimensional gel electrophoresis and mass spectrometry. A total of 271 proteins whose expression levels were altered by constitutively active (CA)-OsRac1 or SE were identified. Interestingly, of 100 proteins that were up-regulated by a SE, 87 were also induced by CA-OsRac1, suggesting that OsRac1 plays a pivotal role in defense responses induced by SE in cultured rice cells. In addition, CA-OsRac1 induces the expression of 119 proteins. Many proteins, such as pathogenesis-related proteins, SGT1, and prohibitin, which are known to be involved in the defense response, were found among these proteins. Proteins involved in redox regulation, chaperones such as heat shock proteins, BiP, and chaperonin 60, proteases and protease inhibitors, cytoskeletal proteins, subunits of proteasomes, and enzymes involved in the phenylpropanoid and ethylene biosynthesis pathways were found to be induced by CA-OsRac1 or SE. Results of our proteomic analysis revealed that OsRac1 is able to induce many proteins in various signaling and metabolic pathways and plays a predominant role in the defense response in cultured rice cells.