Identification and functional analysis of a promoter sequence for phloem tissue specific gene expression from Populus trichocarpa

Unlike xylem, which is primarily composed of dead cells in vascular bundles, phloem has living cells. It transports organic nutrients and long-distance communication signals to all parts of plants. In this report, we describe a promoter from Populus trichocarpa that drives strong gene expression in a phloem tissue-specific manner. First, we identified five candidate genes with strong expression in the developing phloem (DP) tissue from whole-transcriptome gene chip analyses of different tissue types of poplar. The putative promoter sequences of them were isolated and tested for their promoter activity in transgenic Arabidopsis plants. Among them, a promoter of the Potri.001G340200.1 gene (called the PtrDP3 promoter) was identified that has the strongest activity in phloem tissue. PtrDP3 promoter activity was found exclusively in phloem cells of the stem and root tissues of transgenic Arabidopsis plants, which was reproduced in the transgenic poplar plants. The phloemspecific activity of the PtrDP3 promoter was detected as early as in three-day-old seedlings and was not affected by abiotic stresses or exogenously applied plant hormones in transgenic Arabidopsis plants. Promoter deletion analysis identified a 100-bp regulatory region of the PtrDP3 promoter, which is necessary to drive phloem specific expression. This study provides evidence of a strong phloem-specific promoter that is suitable for phloem-specific biotechnological modifications in plants.