lic pathways. Heat shock responsive genes was one of the functional gene that facilitate protein refolding and stabilize polypeptides and membranes under stress condition by preventing protein from aggregation, protecting non-native enzymes from degradation and assisting protein refolding. Expression induction of these genes under drought stress has been reported in barley and rice. Our data showed different expression of heat shock protein-encoding genes, indicating that they were also directly involved in regulating drought-stress responses in potato. Aquaporins, which facilitate transmembrane transport of small molecules such as water, have been showed to be involved in PF-562271 site response to drought and salt stress, and to ABA treatment. In our data, aquaporin genes were significantly expressed in Ningshu 4 stolons under drought stress condition. Similar expression pattern was also observed in Arabidopsis, chickpea , foxtail millet and maize. Additionally, ScPIP2a, an aquaporin, was reported to act in photoperiod perception, generating flower-inducing signals in the leaves. The existing researches indicated these proteins could play an important role in water stimulus response and the 
15 / 20 Transcriptome Profiling of Potato maintenance of intracellular osmotic potential stability and organelle integrity. Therefore, we hypothesize that aquaporin may play a similar regulatory role in potato tuber bulking and growth under drought stress. Potato tuber formation is a complex biological process governed by both environmental factors and genes. Cell division and expansion, together with the associated starch and protein accumulation, are the major events of tuber bulking in potato. Photoperiod perception in leaves initiates tuberization in the subapical region of the underground stolons. Dioxygenases, which catalyze the oxygenation of polyunsaturated fatty acids such as linoleic and linolenic acids, contribute to 9-hydroperoxy linolenic acid -HPOT) and 13-hydroperoxy linolenic acid -HPOT) accumulation. Jasmonic acid is synthesized from 13-HPOT and then metabolized to tuberonic acid and tuberonic acid glucoside, both of which induce tuber formation. In this study, expression of PHYB, GA and LOXencoding gene was significantly different in drought and re-watering plants, suggesting that these genes could function in regulating starch accumulation and tuber formation of potato plants. As an important plant hormone, ABA plays important roles in plant response to drought stress by inducing the expression of TF-, heat shock protein-, transporter-, and osmotic regulator-encoding genes downstream of stress signaling pathways in both ABA-dependent and ABA-independent manners. We observed that expression of ABA anabolism- or signal transduction-related genes such as abscisic acid PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19697363 8′-hydroxylase 1-like, Beta-carotene hydroxylase, AREB, and PP2C5 was all significantly altered during drought stress and re-watering treatment, indicating that ABA and anabolism related genes were also PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19697345 important regulators for drought-stress and re-watering stimulus-response in potato. This hypothesis is also supported by previous findings in corn and cotton . In addition, expression of some gene involved in auxin, cytokinin and gibberellin anabolism was significantly affected, which, together with ABA, jointly regulates the response of potato stolons to drought stress and re-watering. Similar results were also found in Chinese cabbage and barley . Collectively, our findings demonst