Abstract
Dehydrins are a family of plant proteins that accumulate in response to dehydration stresses, such as low temperature, drought, high salinity, or during seed maturation. We have previously constructed cDNA libraries from Rhododendron catawbiense leaves of naturally non-acclimated (NA; leaf LT50, temperature that results in 50% injury of maximum, approximately −7°C) and cold-acclimated (CA; leaf LT50 approximately −50°C) plants and analyzed expressed sequence tags (ESTs). Five ESTs were identified as dehydrin genes. Their full-length cDNA sequences were obtained and designated as RcDhn 1-5. To explore their functionality vis-à-vis winter hardiness, their seasonal expression kinetics was studied at two levels. Firstly, in leaves of R. catawbiense collected from the NA, CA, and de-acclimated (DA) plants corresponding to summer, winter and spring, respectively. Secondly, in leaves collected monthly from August through February, which progressively increased freezing tolerance from summer through mid-winter. The expression pattern data indicated that RcDhn 1-5 had 6- to 15-fold up-regulation during the cold acclimation process, followed by substantial down-regulation during deacclimation (even back to NA levels for some). Interestingly, our data shows RcDhn 5 contains a histidine-rich motif near N-terminus, a characteristic of metal-binding dehydrins. Equally important, RcDhn 2 contains a consensus 18 amino acid sequence (i.e., ETKDRGLFDFLGKKEEEE) near the N-terminus, with two additional copies upstream, and it is the most acidic (pI of 4.8) among the five RcDhns found. The core of this consensus 18 amino acid sequence is a 11-residue amino acid sequence (DRGLFDFLGKK), recently designated in the literature as the F-segment (based on the pair of hydrophobic F residues it contains). Furthermore, the 208 orthologs of F-segment-containing RcDhn 2 were identified across a broad range of species in GenBank database. This study expands our knowledge about the types of F-segment from the literature-reported single F-segment dehydrins (FSKn) to two or three F-segment dehydrins: Camelina sativa dehydrin ERD14 as F2S2Kn type; and RcDhn 2 as F3SKn type identified here. Our results also indicate some consensus amino acid sequences flanking the core F-segment in dehydrins. Implications for these cold-responsive RcDhn genes in future genetic engineering efforts to improve plant cold hardiness are discussed.
Highlights
Survival and growth of woody plants in cold climate is important for traditional sectors of horticulture and forestry
BLASTX search of PIR-NREF protein database revealed that several expressed sequence tag (EST) from the cold acclimated (CA) library were identified as dehydrin transcripts encoding five distinct dehydrins (Table 2)
Based on the presence of certain consensus regions of amino acids in their sequence, dehydrins are conventionally described by the “YSK” shorthand, according to which plant dehydrins can be categorized into five distinct structural types: (1) YnSKn, (2) SKn, (3) YnKn, (4) Kn, and (5) KnS (Close, 1996, 1997)
Summary
Survival and growth of woody plants in cold climate is important for traditional sectors of horticulture and forestry. Rhododendron, like many other woody perennials, can adapt to harsh winter through a process called cold acclimation (CA), by which they develop tolerance to low temperature and freezing seasonally, with hardiness increasing through the autumn, peaking in midwinter, declining during the spring, and reaching the lowest in summer (Arora and Taulavuori, 2016). It has been shown that dehydrins are located in the nucleus or cytoplasm of the cell (Close, 1997), in the vicinity of the plasma membrane (Danyluk et al, 1998), cytoplasmic endomembrane (Egerton-Warburton et al, 1997), and plasmodesmata (Karlson et al, 2003) Their high concentrations in cells (Baker et al, 1988), add to the appeal as engineering targets for enhancing plant stress defense capacity
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