Characterization of a midwinter-expressed dehydrin (DHN) gene from apple trees (Malus domestica)

To gain a better understanding of the proteins associated with cold acclimation and winter dormancy in apple trees (Malus domestica), we studied the expression of a dehydrin gene encoding one of the important proteins involved in these processes. A novel gene, designated mddhn, was obtained from M. domestica cv. "Golden Delicious". The full-length complementary DNA (cDNA) consists of 993 nucleotides with an open reading frame of 687 bp. The deduced amino acid sequence of 229 residues shows similarity to members of the dehydrin (DHN) family. The putative DHN presents a structure typical of DHN proteins that are composed of two consensus Y-segments, a stretch of serine residues, and four lysine-rich repeats (Y₂SK₄). This apple DHN shows the greatest similarity to members of both the YnKn and YnSKn classes of DHNs from peach and almond: 57% to peach (Y₂SK₃) and almond (Y₂SK₃ and Y₂K₄) and a lower percentage (35%) to peach (Y₂K₉). When we analyzed the available expressed sequence tag databases for additional apple DHNs, we identified a DHN of the class Y₂SK₄ with a deduced amino acid sequence with 79-98% identity among the cultivars "Royal Gala", "Goldrush", and the M9 rootstock that showed high identity to our DHN from "Golden Delicious" (92-98%). The transcripts of mddhn are highly expressed in bark and bud tissues when the plant is dormant and cold hardy in midwinter, but are not expressed in early spring when cold hardiness is lost and the buds are growing.