Characterization and expression analysis of two mitogen-activated protein kinase kinase kinase genes from Antarctic moss Pohlia nutans

Mitogen-activated protein kinase (MAPK) signaling plays essential roles in plant growth, development, and responses to environmental stresses. However, only limited information is available on the MAPK signaling genes in the bryophytes. Two full-length mitogen-activated protein kinase kinase kinase genes (designated as PnMEKK1 and PnMEKK2) were identified from Antarctic moss Pohlia nutans. The full-length cDNAs of PnMEKK1 and PnMEKK2 were 3012 bp and 3096 bp, encoding the receptor-like kinases of 804 and 576 amino acids, respectively. Multiple sequence alignment showed that PnMEKK1 and PnMEKK2 possessed the conserved STKc_MAP3K-like domains, but they had relatively low identities with other protein kinases. Phylogenetic analysis showed that they clustered together with the protein kinases of ferns and mosses but not higher plants. In addition, the subcellular localization analysis by observing the transient expression of PnMEKK-green fluorescence protein in Arabidopsis mesophyll protoplasts revealed that PnMEKK1 and PnMEKK2 were cytoplasm-localized protein kinases. Meanwhile, the mRNA expression profiles of PnMEKK1 and PnMEKK2 were quantified by quantitative RT-PCR. Results showed that cold, salinity, drought, and UV-B radiation could motivate the up-regulation of PnMEKK1 and PnMEKK2 mRNA expression. In addition, the application of plant hormone abscisic acid (ABA) and methyl jasmonate (MeJA) also could up-regulate the mRNA expression level. Taken together, we purposed that these two isolated PnMEKKs might involve in Antarctic moss P. nutans adapting to the polar extreme environments.