Characterization of a Novel Stress-Response Member of the MAPK Family in Malus hupehensis Rehd

Malus hupehensis Rehd. var. pinyiensis Jiang (Pingyi Tiancha, PYTC) is a botanical variety of Malus (tea crabapple) originating from China. This species is characterized as apomictic, and it is highly capable of resisting water-logging, shade, cold, and various diseases. Mitogen-activated protein kinase (MAPK) cascades have been implicated in the regulation of stress and developmental signals in plants. In this study, an MAPK gene, MhMAPK, has been isolated from a PYTC complementary DNA (cDNA) library using rapid amplification of cDNA ends. The gene encodes a 373-amino-acid protein with high-sequence similarity to other previously reported plants MAPKs. MhMAPK contains all 11 MAPK conserved sub-domains and the phosphorylation motif TEY, and when fused to the green fluorescent protein, it is found to be localized in the nucleus of epidermal cells of onion. Transcripts of MhMAPK accumulate when PYTC is treated with 20% polyethylene glycol and 200 mM NaCl. These results indicated that MhMAPK may be functional within the nucleus by phosphorylating transcriptional factors. This, in turn, allows plants to rapidly respond to the environmental signals.