ional genes at different levels. Upon the completion of whole genome and Tipifarnib supplier transcriptome sequencing of potato DM1-3 516R44, a number of genes related to drought-stress response and water use efficiency have been identified by quantitative trait locus mapping, and transcriptome analyses of stress response in potato leaves has been performed with plants grown under a single condition. However, molecular mechanism of potato plant, especially the tuber or stolon responsing to water-stimulus are not very well understood with the results available now. Tubers are the main “sink” organ and it also the most important agronomic trait of potato plants. In addition, the tuber bulking stage is a critical period of time for potato tuberization, during which plants are extremely sensitive to water deficit. Therefore, we explored the molecular mechanisms underpinning drought stress responses in the stolon tissue of a potato variety. Plant cultivated under severe drought stress condition during tuber bulking stage were used for next-generation transcriptome sequencing to fully resolve gene expression profiles in response to water deficit and re-watering. Materials and Methods Plant materials and drought treatments The potato strain Ningshu 4 was from our own lab. All plant materials were collected from our Guyuan Experimental Field, which does not involve any endangered or protected species. Before tuber bulking 2 / 20 Transcriptome Profiling of Potato stage, 40 healthy and uniform ball-plants were 
transplanted into buckets and cultivated indoors at room temperature under 12-hr light/dark cycles at 4,000 lux. Experimental soil was collected from a test site in Guyuan City, Ningxia Hui Autonomous Region, with the maximum field water-holding capacity of 21%. At flowering stage, the above plants were split in to three groups with ten plants in each group. The first group of plants was exposed to severe drought stress treatment for three days, during which relative soil moisture was controlled at 3540% of the maximum field waterholding capacity. The second group of plants was exposed to severe drought stress treatment for 3 days, followed by re-watering to the control level and grown for 3 days. And the last group of plants was maintained under normal watering condition throughout the time period as control. Then, stolon tips from three randomly selected plants of each group were pooled together, immediately grounded into fine powder in liquid nitrogen for sequencing analysis. Total RNA extraction Total RNA was extracted from plant materials using a TransZol Plant kit according to the manufacturer’s instruction. Digestion of gDNA in each sample was conducted using DNase I. The integrity and purity of RNA was confirmed using 1.2% non-denaturing agarose gel electrophoresis and the PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19697363 Agilent 2100 RNA 6000 Kit. Strand-specific cDNA library construction and sequencing From each sample, 3 g of total RNA was taken to construct strand-specific cDNA libraries using the NEBNext Ultra RNA Library Prep Kit. The quality of cDNA libraries was tested, and quantified cDNA was subjected to purification, elution, end repair, poly addition and adaptor ligation. Then, agarose gel electrophoresis was used for size selection of gene fragments, followed by PCR amplification. The established cDNA libraries were sequenced using the Illumina Hiseq 2000 PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19698726 platform to generate 100-bp paired-end reads. Sequence analysis Image data obtained from cDNA sequencing was converted to the correspon