*2.7. Expression Profiles of MdSAP Genes*

Knowing the patterns of expression in various tissue types can help us understand gene functions. Firstly, we collected different tissues including young roots, stems, fully expanded leaves, flowers, and mature fruit (70 mm, red peel, 150 days after bloom), from apple plants that were five years old after bud grafting. The scion was *Malus domestica* "Golden Delicious", and the rootstock was *M. hupehensis*. Then, we isolated the full-length cDNA, 50 -UTR, and 30 -UTR sequences for 13 *MdSAP* genes and used specific primers for our qRT-PCR assays. These *MdSAP*s were constitutively expressed in the five tissues examined here, albeit at different levels of transcription (Figure 6A). For example, *MdSAP7*, -*8*, -*10*, -*15*, -*19*, and -*29* were most highly expressed in the leaves; *MdSAP12*, -*16*, -*21*, and -*28* were most highly expressed in the fruits; and *MdSAP14*, -*23*, and -*25* were most highly expressed in the roots. To induce a water deficit, irrigation was withheld up to 8 days, while the designated control plants continued to receive normally scheduled irrigation ("Golden Delicious" scions and *M. hupehensis* rootstocks). In response to drought stress, the expression of *MdSAP15*, -*25*, and -*28* was significantly induced from that detected in the non-stressed control plants (Figure 6B), transcripts of *MdSAP7* and -*21* mRNAs were significantly reduced, and expression of the other MdSAP genes remained constant.

**Figure 6.** Tissue-specific expression and drought response of 13 *MdSAP* genes. (**A**) Expression patterns of 13 *MdSAP* genes in apple tissues. (**B**) Expression patterns of 13 *MdSAP* genes in response to drought stress, i.e., normally scheduled irrigation withheld for 0, 4, or 8 days. Three independent replicates were used for calculations. Error bars indicate standard deviation.
