Two novel mitogen-activated protein signaling components, OsMEK1 and OsMAP1, are involved in a moderate low-temperature signaling pathway in rice

Rice (Oryza sativa) anther development is easily damaged by moderately low temperatures above 12degreesC. Subtractive screening of cDNA that accumulated in 12degreesC-treated anthers identified a cDNA clone, OsMEK1, encoding a protein with features characteristic of a mitogen-activated protein (MAP) kinase kinase. The putative OsMEK1 protein shows 92% identity to the maize (Zea mays) MEK homolog, ZmMEK1. OsMEK1 transcript levels were induced in rice anthers by 12degreesC treatment for 48 h. Similar OsMEK1 induction was observed in shoots and roots of seedlings that were treated at 12degreesC for up to 24 h. It is interesting that no induction of OsMEK1 transcripts was observed in 4degreesC-treated seedlings. In contrast, rice lip19, encoding a bZIP protein possibly involved in low temperature signal transduction, was not induced by 12degreesC treatment but was induced by 4degreesC treatment. Among the three MAP kinase homologs cloned, only OsMAP1 displayed similar 12degreesC-specific induction pattern as OsMEK1. A yeast two-hybrid system revealed that OsMEK1 interacts with OsMAP1, but not with OsMAP2 and OsMAP3, suggesting that OsMEK1 and OsMAP1 probably function in the same signaling pathway. An in-gel assay of protein kinase activity revealed that a protein kinase (approximately 43 kD), which preferentially uses myelin basic protein as a substrate, was activated by 12degreesC treatment but not by 4degreesC treatment. Taken together, these results lead us to conclude that at least two signaling pathways for low temperature stress exist in rice, and that a MAP kinase pathway with OsMEK1 and OsMAP1 components is possibly involved in the signaling for the higher range low-temperature stress.