APICAL AND LATERAL SHOOT APEX-SPECIFIC EXPRESSION IS CONFERRED BY A PROMOTER OF THE SEED STORAGE PROTEIN BETA-PHASEOLIN GENE

A previous analysis with deletion mutants of the native beta-phaseolin gene demonstrated that removal of a negative element 5' upstream of -107 permitted phaseolin expression in stem cortex and secondary root (Burow et al, 1992). Here we employed the beta-glucuronidase (GUS) reporter gene to visualize, by histochemical staining, the cell type-specificity of phaseolin expression in stem and root, and to understand further the spatial control of the beta-phaseolin gene. The 782 bp 5' upstream promoter and its deletion mutants were fused to the GUS gene, and these chimaeric genes were used to transform tobacco. Histochemical staining for GUS activity demonstrated that phaseolin promoters truncated downstream of -227 conferred cell-type specific expression in internal/external phloem and protoxylem of mature stem. Surprisingly, GUS staining was prominent in both apical and lateral shoot apices of plants that contain the full-length -782 promoter and mutant promoters deleted up to -64. GUS expression was extended to all cell types of shoot tips, including epidermis, cortex, vasculature, procambium and pith. Expression in vasculature of petioles was limited to plants with promoters truncated to -106 and -64. The current results are in agreement with our previous findings with the native phaseolin gene: that the major positive element (-295/-228) is sufficient for seed-specific late-temporal expression of the phaseolin gene. We conclude that the 5' upstream sequence of the beta-phaseolin gene directs spatially- and temporally-controlled gene expression in developing seeds during the reproductive phase, but also confers expression in shoot apices during the vegetative phase of plant development.