Changes in the proteome and water state in bark and xylem of Hydrangea paniculata during loss of freezing tolerance

Appropriate timing and rate of cold deacclimation is a critical component of winter-survival of temperate perennials. However, little is known about the physiological and biochemical changes involved in the deacclimation process. In this study differential expression of proteins in bark and xylem and changes in the quantity and biophysical nature of water in stems of Hydrangea paniculata during deacclimation were investigated. Increasing temperature caused a sigmoid deacclimation pattern in H. paniculata, which was characterized by a distinct decrease in the abundance of several stress- or defence-related proteins, most of which are known to be associated with increased freezing tolerance. In addition to quantitative reduction of stress- or defence-related proteins, proteins changing in abundance during deacclimation mainly fell into the functional categories of carbohydrate and energy metabolism, translation regulation and protein metabolism. Several tissue-specific responses in the accumulation patterns of proteins involved in stress, defence, carbohydrate metabolism or regulation/signalling and structural components were observed between bark and xylem. This indicates different stress responses in the two tissue types or is related to their different structure and functions. Deacclimation of bark was associated with rehydration and a decrease in the accumulation of dehydrins. The latter was correlated with changes in the least mobile stem water fraction, indicating that dehydrins may be involved in binding water in freezing tolerant cells. In xylem, no clear seasonal trend in the content and biophysical nature of water was observed and no dehydrins changing in abundance were identified. The results offer insight into tissue-specific responses during deacclimation of a moderately freezing tolerant woody perennial.