THE AGROBACTERIUM-RHIZOGENES ROLB AND ROLC PROMOTERS ARE EXPRESSED INPERICYCLE CELLS COMPETENT TO SERVE AS ROOT INITIALS IN TRANSGENIC HYBRID ASPEN

Expression of the Agrobacterium rhizogenes rolB and rolC promoters was studied in transgenic hybrid aspen (Populus tremula L. x P. tremuloid es Michx.) lines containing a chimeric fusion of either the rolB or th e rolC promoter and the reporter gene uid4. The resultant GUS activity was monitored by histochemical analysis in aerial tissues as well as in developing roots. Both the rolC and rolB promoters were shown to be expressed in the phloem and in the root tips, which is similar to the expression pattern previously described fur annual plants. However, a strong expression of the rolB promoter in the rays of the phloem and the cambial zone of the stem, and of the rolC promoter in groups of pe ricycle cells prior to and during lateral root initiation was unique f or hybrid aspen. In both stem and root tissues, the expression of the rolB and rolC promoters was localised primarily in a subset of cells c ompetent to form adventitious or lateral roots, suggesting that these cells might serve as the target for A. rhizogenes infection, The biolo gical significance of the cell-specific rol gene expression in establi shing the hairy root disease is discussed.