Negative feedback regulation of microbe-associated molecular pattern-induced cytosolic Ca2+ transients by protein phosphorylation.

Microbe/pathogen-associated molecular patterns (MAMPs/PAMPs) often induce rises in cytosolic free Ca(2+) concentration (Ca(2+)) and protein phosphorylation. Though they are postulated to play pivotal roles in plant innate immunity, their molecular links and the regulatory mechanisms remain largely unknown. To investigate the regulatory mechanisms for MAMP-induced Ca(2+) mobilization, we have established a transgenic rice (Oryza sativa) cell line stably expressing apoaequorin, and characterized the interrelationship among MAMP-induced changes in Ca(2+), production of reactive oxygen species (ROS) and protein phosphorylation. Oligosaccharide and sphingolipid MAMPs induced Ca(2+) transients mainly due to plasma membrane Ca(2+) influx, which were dramatically suppressed by a protein phosphatase inhibitor, calyculin A (CA). Hydrogen peroxide and hypo-osmotic shock triggered similar Ca(2+) elevations, which were not affected by CA. MAMP-induced protein phosphorylation, which is promoted by CA, has been shown to be required for ROS production and MAPK activation, while it negatively regulates MAMPs-induced Ca(2+) mobilization and may play a crucial role in temporal regulation of Ca(2+) signature.