A 27 kDa Rhododendron protein is associated with constitutive freezing tolerance and is related to the ABA / water deficit stress-inducible family of proteins

Abstract

SummaryUp-regulation of specific genes (or accumulation of certain proteins) has been widely reported to be associated with increased freezing tolerance during cold acclimation in plants.In the present study, we focused on specific alterations in protein metabolism associated with constitutive freezing tolerance (i.e., the level of freezing tolerance in non-acclimated and de-acclimated states) in leaf tissues of Rhododendron, a broad–leaf evergreen species. Current-year leaves from three Rhododendron cvs. (‘Chionoides’, ‘Grumpy Yellow’ and ‘Vulcan’s Flame’) were collected for leaf freezing tolerance estimations, and protein extractions in non-acclimated, cold acclimated, and de-acclimated states. Two groups of containerised plants were used: 1) those exposed to natural field conditions during the course of the study; and 2) those maintained in environmentally-controlled growth chambers. Data indicated a distinct seasonal (group 1) or cyclical (group 2) pattern in leaf freezing tolerance in all cultivars in response to non-, cold-, and de-acclimating conditions. However, the maximum freezing tolerance of cold-acclimated plants was substantially lower in growth chamber-acclimated plants than in field plants.The constitutive (non-acclimated or de-acclimated) leaf freezing tolerance of these cultivars ranged from –2.2°C to –7.4°C with a mean of –5.4°C. SDS-PAGE revealed the presence of a 27 kDa protein (RhCFT27) in non-acclimated and de-acclimated tissues, which was barely detectable in cold-acclimated leaves from both groups of plants. Micro-sequencing of four peptides produced by partial cleavage of RhCFT27 revealed considerable homology with two proteins (from Mesembryanthemum crystallinum and Pinus taeda) that belong to the class of ABA stress-, and ripening or water deficit stress-inducible proteins. We hypothesise that non- and de-acclimated leaf freezing tolerance in Rhododendron, which is higher than typically observed for constitutive leaf freezing tolerance in herbaceous species, might be regulated by other ‘constitutive’, ABA-responsive gene expression which differs from that which typically occurs during cold-acclimation.