Differential regulation of an early and late responsive rice endosperm kinase mRNA in seedling leaves

The rice endosperm kinase (REK) gene belonging to the SNF (sucrosenonfermenting)1-like subfamily of the calcium-/calmodulin-dependent protein kinase group (whose physiological role(s) remain elusive) remains uncharacterized to date in photosynthetic tissues. Using an established in vitro rice (Oryza sativa L. cv. Nipponbare) seedling model system, we provide the first evidence for a rapid (within 30 min) enhancement of the REK mRNA in response to the critical signals mediating plant self-defense responses, namely jasmonic acid (JA) and salicylic acid (SA), and hydrogen peroxide. This enhanced expression was suppressed upon co-application of JA, SA or ethephon (ethylene generator), and may require a de novo synthesized protein factor(s) suggested by the use of cycloheximide (CHX). Besides signaling molecules, osmolytes, drought, heavy metals and protein phosphatase inhibitors, especially okadaic acid, also enhanced the REK mRNA expression. Interestingly, a long-term exposure to several of these stresses revealed a strong but differential regulation of the REK transcript with time under light and darkness. Furthermore, a fungal elicitor, chitosan, also enhanced the REK mRNA expression, which was completely abolished at either 12 or 24 h by CHX. However, high/low temperatures, and the environmental pollutants, ozone and sulfur dioxide, failed to enhance the REK mRNA expression, as determined using an in vivo system. These results strongly indicate a function for REK in defense/stress responses in rice photosynthetic tissues.