RcDhn5, a cold acclimation-responsive dehydrin from Rhododendron catawbiense rescues enzyme activity from dehydration effects in vitro and enhances freezing tolerance in RcDhn5-overexpressing Arabidopsis plants

Dehydrins (DHNs) are typically induced in response to abiotic stresses that impose cellular dehydration. As extracellular freezing results in cellular dehydration, accumulation of DHNs and development of desiccation tolerance are believed to be key components of the cold acclimation (CA) process. The present study shows that RcDhn5, one of the DHNs from Rhododendron catawbiense leaf tissues, encodes an acidic, SK2 type DHN and is upregulated during seasonal CA and downregulated during spring deacclimation (DA). Data from in vitro partial water loss assays indicate that purified RcDhn5 protects enzyme activity against a dehydration treatment and that this protection is comparable with acidic SKn DHNs from other species. To investigate the contribution of RcDhn5 to freezing tolerance (FT), Arabidopsis plants overexpressing RcDhn5 under the control of 35S promoter were generated. Transgenic plants exhibited improved 'constitutive' FT compared with the control plants. Furthermore, a small but significant improvement in FT of RcDhn5-overexpressing plants was observed after 12 h of CA; however, this gained acclimation capacity was not sustained after a 6-day CA. Transcript profiles of cold-regulated native Arabidopsis DHNs (COR47, ERD10 and ERD14) during a CA time-course suggests that the apparent lack of improvement in cold-acclimated FT of RcDhn5-overexpressing plants over that of wild-type controls after a 6-day CA might have been because of the dilution of the effect of RcDhn5 overproduction by a strong CA-induced expression of native Arabidopsis DHNs. This study provides evidence that RcDhn5 contributes to freezing stress tolerance and that this could be, in part, because of its dehydration stress-protective ability.