Cold Acclimation-Induced WAP27 Localized in Endoplasmic Reticulum in Cortical Parenchyma Cells of Mulberry Tree Was Homologous to Group 3 Late-Embryogenesis Abundant Proteins

We have shown that two 27-kD proteins, designated as WAP27A and WAP27B, were abundantly accumulated in endoplasmic reticulum-enriched fractions isolated from cortical parenchyma cells of mulberry tree (Morus bombycis Koidz.) during winter (N. Ukaji, C. Kuwabara, D. Takezawa, K. Arakawa, S. Yoshida, S. Fujikawa [1999] Plant Physiol 120: 480–489). In the present study, cDNA clones encoding WAP27A and WAP27B were isolated and characterized. The deduced amino acid sequences of WAP27A and WAP27B cDNAs had 12 repeats of an 11-mer amino acid motif that was the common feature of group 3 late-embryogenesis-abundant proteins. Under field conditions, transcripts of WAP27 genes were initially detected in mid-October, reached maximum level from mid-November to mid-December, and then gradually decreased. The transcript levels of WAP27 genes in cortical parenchyma cells harvested in October was drastically induced by cold treatment within a few days, whereas those in cortical parenchyma cells harvested in August were low even by cold treatment for 3 weeks. Immunocytochemical analysis by electron microscopy confirmed that WAP27 was localized specifically in vesicular-form ER and also localized in dehydration-induced multiplex lamellae-form ER. The role of WAP27 in the ER is discussed in relation to acquisition of freezing tolerance of cortical parenchyma cells in mulberry tree during winter.