In addition, previous research has shown that plants overexpressing constitutively activated MKK2 show freezing tolerance without cold acclimation, whereas mkk2 null plants are hypersensitive to freezing temperatures. These results indicated that the MEKK1-MKK2-MPK4 cascade function in the acquisition of freezing tolerance through membrane rigidification in Arabidopsis. Furthermore, plants overexpressing constitutively activated MKK2 show up-regulation of CBF/DREB1 genes, which are transcription factors that play an important role in acquiring freezing tolerance. Moreover, we indicated that the cold-induced gene, COR15a, is up-regulated by membrane rigidification. The COR15a is one of the downstream targets of CBF/DREB1. These observations suggest that the CBF pathway is located downstream of this MAPK cascade in the cold acclimation response viamembrane rigidification.