Short-term transcriptomic responses of Populus euphratica roots and leaves to drought stress

Populus euphratica Oliv., the Euphrates poplar, is the tallest tree species in the arid desert areas of Northwest China. Investigation of its drought-resistance genes is valuable to increase understanding of drought resistance mechanisms. RNA-seq of leaves and roots under drought simulation by 25% polyethylene glycol-6000 (PEG 6000) were performed at 0, 4, and 12 h. Leaves and roots responded differently to drought via differentially upregulated and downregulated genes; more genes were downregulated than upregulated in both leaves and roots. Additionally, these differentially expressed genes were enriched in different GO terms and KEGG pathways. For example, GO term ‘response to organic substance’ and KEGG pathway ‘nitrogen metabolism’ were enriched for drought-stressed leaves, while GO term ‘cell wall organization or biogenesis’ and KEGG pathway ‘zeatin biosynthesis’ were enriched for drought-stressed roots. The enrichment of the GO term ‘enzyme linked receptor protein signalling pathway’ in both leaf and root drought responses suggests that these tissues may also have similar mechanisms. However, roots under drought stress for four hs responded by activating programed cell death. The KEGG pathway ‘plant hormone signal transduction’ was detected for 4- and 12-h drought-stressed leaves and 12-h drought-stressed roots, suggesting that plant hormone signal transduction plays an important role in both roots and leaves. GO enrichment of upregulated and downregulated genes for leaves and roots reflect differentially regulatory mechanisms of response to drought stress via different biological processes such as the regulation of photosynthesis and auxin signalling pathway in leaves, and the regulation of defence response and water homeostasis in roots. Fifteen candidate genes, including transcription factors, protein kinase, transporter, late embryogenesis abundant protein and mannitol dehydrogenase, were further selected to determine their response to drought using qRT-PCR. The results show that the expression patterns of 13 of the 15 genes correspond to the RNA-seq results. This study provides new insights into the drought response mechanism of P. euphratica and suggests new candidate gene resources.