Upstream non-coding region which confers polar expression to Ri plasmid root inducing gene rolB

Abstract

Root differentiation could be elicited on carrot discs by transformation with the agropine Ri plasmid rolB gene cloned in the binary vector Bin19, provided two conditions were met. Firstly, an adequate auxin supply had to be provided. This was achieved by co-inoculation with a strain carrying only the auxin synthetic genes of the TR-DNA. Most of the resulting roots were then shown to harbour only rolB and no aux genes. Secondly, an extended non-coding region (∼1200 bp) at the 5′ end of rolB had to be included in the construction. A shorter (∼300 bp) 5′ region, including TATA and CCAAT boxes, was not sufficient to trigger root differentiation. Both the extended (B1185) and reduced (B310) 5′ regions of rolB were then cloned upstream of the β-glucuronidase (GUS) reporter gene and infections carried out both on the apical and on the basal side of carrot discs. Strong expression of GUS, visualized histochemically as an intense blue colouring of transformed cells was observed with B1185-GUS constructions on the apical side of the discs. Only occasionally could coloured cells be observed on the basal side of the discs with B1185-GUS and on both apical and basal sides with B310-GUS constructions. Strong GUS expression was, on the contrary, achieved on cells of both auxin-rich (apical) and auxin-depleted (basal) sides of the discs with the strong constitutive viral promoter, CaMV35S. These results indicate the presence of an upstream regulatory region which confers polar expression to the rolB gene and suggest a role for auxin in its activation.