Robinia pseudoacacia inner-bark lectin promoter expresses GUS also predominantly in phloem of transgenic tobacco

Robinia pseudoacacia L., a leguminous deciduous tree, accumulates lectins in the inner bark (phloem) and seeds. The lectin gene Rplec2 is expressed predominantly in the inner bark. To examine the expression pattern directed by the 5′-upstream region (from -793 bp to +26 bp relative to the transcription start site) of Rplec2 in heterologous plants, a transcriptional fusion of this region with the β-glucuronidase (GUS) reporter gene was introduced into tobacco plants. The resulting transformants showed higher GUS activity in the stems than in the leaves and roots. Histochemical assay of GUS activity demonstrated that the expression of the GUS gene was confined to internal and external phloem, parenchyma cells adjacent to the phloem, and xylem parenchyma cells in the stems. In petioles, the GUS gene was expressed in phloem and nearby parenchyma cells, but not in leaf mesophyll cells. GUS activity was also detected in the phloem of the root. In addition, sequence analysis of the 5′-upstream region of Rplec2 revealed three conserved motifs shared with several previously reported phloem-specific promoters. These results indicate that the 5′-upstream region of Rplec2 functions as a promoter that directs preferential expression of the GUS gene to the phloem of transgenic tobacco plants.