Plant Physiol, June 2000, Vol. 123, pp. 531-542

Cambial-Region-Specific Expression of the Agrobacterium iaa Genes in Transgenic Aspen Visualized by a Linked uidA Reporter Gene¹

Department of Forest Genetics and Plant Physiology, Swedish University of Agricultural Sciences, 90183 Umeå, Sweden (H.T., S.R., B.S.); Institut für Forstbotanik und Baumphysiologie, Albert-Ludwigs-Universität, Bertoldstrasse 17, 79085 Freiburg, Germany (L.P., S.F.); and Department of Molecular Biology, Göteborg University, Medicinaregatan 9C, 41390 Göteborg, Sweden (O.O.)

The level of indole-3-acetic acid (IAA) was locally modified in cambial tissues of transgenic aspen (Populus tremula L. × Populus tremuloides Michx.). We also demonstrate the use of a linked reporter gene to visualize the expression of the iaa genes. The rate-limiting bacterial IAA-biosynthetic gene iaaM and the reporter gene for -glucuronidase (GUS), uidA, were each fused to the cambial-region-specific Agrobacterium rhizogenes rolC promoter and linked on the same T-DNA. In situ hybridization of the iaaM gene confirmed that histochemical analysis of GUS activity could be used to predict iaaM gene expression. Moreover, quantitative fluorometric analysis of GUS activity allowed estimation of the level of de novo production of IAA in transgenic lines carrying a single-copy insert of the iaaM, uidA T-DNA. Microscale analysis of the IAA concentration across the cambial region tissues showed an increase in IAA concentration of about 35% to 40% in the two transgenic lines, but no changes in the radial distribution pattern of IAA compared with wild-type plants. This increase did not result in any changes in the developmental pattern of cambial derivatives or the cambial growth rate, which emphasizes the importance of the radial distribution pattern of IAA in controlling the development of secondary xylem, and suggests that a moderate increase in IAA concentration does not necessarily stimulate growth.