Cambial-region-specific expression of the Agrobacterium iaa genes in transgenic aspen visualized by a linked uidA reporter gene

The level of indole-3-acetic acid (IAA) was locally modified in cambial tis sues of transgenic-aspen (Populus tremula L. x Populus tremuloides Michx.). We also demonstrate the use of a Linked reporter gene to visualize the exp ression of the iaa genes. The rate-limiting bacterial IAA-biosynthetic gene iaaM and the reporter gene for P-glucuronidase (GUS), uidA,:were each fuse d 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 activi ty allowed estimation of the level of de novo production of IAA in transgen ic 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 wi th wild-type plants. This increase did not result in any changes in the dev elopmental pattern of cambial derivatives or the cambial growth rate, which emphasizes the importance of the radial distribution pattern of IAA in con trolling the development of secondary xylem, and suggests that a moderate i ncrease in IAA concentration does not necessarily stimulate growth.