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Water-use efficiency(WUE) is crucial to plant survival,yield and vegetation dynamics.Many researches trended to reduce water use by more crop per drop.Althoughmuch progresses have been achieved in determining the physiological mechanisms of WUE,there is few report on the genetic controls of WUE.The first gene ERECTA,which t promotes WUE,was shown to decrease stomatal density and be involved in the positive regulation of photosynthetic capacity. The EPIDERMAL PATTERNING FACTOR 1(EPF1) gene identified in Arabidopsis is expressed specifically in stomatal cells and precursors.EPF1 controls stomatal patterning through the regulation of asymmetric cell division,its activity depends on the TOO MANY MOUTHS receptor-like protein and ERECTA family receptor kinases,suggesting that EPF1 provides a positional cue interpreted by these receptors.A mutation of EPF1 results in increased stomatal density and violation of the one-cell-spacing rule(clustering of stomata).However,there is no report on the relationship between AtEPF1 and WUE. The cottonwood poplar,Populus deltoides×Populus nigra,is one of the most important species in large-scale forestation in China.However,its high water consumption is a key factor limiting its growth in arid and semiarid regions.Identifcation of genes increasing WUE in this species is important for understanding stress responses and improvement of this species. In this study,microarray analysis between poplar genotypes NE-19(P.nigra×(P.deltoides×P.nigra)) with the higher WUE and DN-2(P.deltoides×P.nigra) with a lower WUE,was performed.PdEPF1 from NE-19,one of the WUE-related genes,was identified,strongly expressed in immature leaves and roots,but not in senescent leaves,functional leaves and the stem.This gene is strongly induced by drought,abscisic acid and high salt levels,and enhanced by gibberellic acid.To unvarel the mechanism responsible for the inducible expression of PdEPF1 by different stresses,a 1090-bp PdEPF1 promoter fragment was isolated by the adaptor PCR with reference to the transcriptional start site determined from the PdEPF1 cDNA sequence.The PdEPF1 promoter contains CAAT and TATA motifs located at nucleotides-204 and-35 relative to the transcriptional start site,respectively.Each of these motifs has characteristics of eukaryotic gene promoters.Interestingly,,the 1090-bp promoter region contained several motifs based on the computer prediction,such as a W-box(a GA-responsive element),an MYC binding site,an early response to dehydration binding site,and a controlling guard cell-specific gene expression binding site.This indicate,that the stress-inducible expression of PdEPF1 is regulated at the level of transcription.