Apple (Malus x domestica Borkh.) Responds to a Simulated Severe Drought: Genes Common and Unique to Leaves and Bark

Dehydration is a feature of many abiotic stresses, but is more often an agricultural threat on its own.  Plants have evolved numerous mechanisms for coping with dehydration, including morphological, biochemical and molecular genetic responses. These mechanisms are complex and involve activating various combinations of  biochemical response pathways. Although a number of studies of drought response in herbaceous plants have been reported, there have been few studies on woody plants, particularly fruit tree crops. We have been studying dehydration responses in apple, using ‘Royal Gala’ as our model.  Young plants (~ 1yr old) were subjected to a two week period representing a severe drought (water was withheld to 40% of pot saturation). The control group was watered every other day to saturation. mRNA from leaves and bark sampled at the end of the drought period was converted to cDNA and used in a suppression subtractive hybridization (SSH) scheme to isolate genes both up- and down-regulated in response to the abiotic challenge to which the plants were exposed. As expected, many genes associated with photosynthesis in leaves were down-regulated; however, select photosynthetic genes were also up-regulated. In bark tissue consisting mainly of green cambium, several photosynthetic genes found in leaves were also up- or down-regulated in bark. There were a few genes unique to each tissue that responded to drought treatment. Comparison of these genes with those found in roots or identified by other methods reveals the incredible complexity of this process.