Correlation between the expression of T‐DNA IAA biosynthetic genes from developmentally regulated promoters and the distribution of IAA in different organs of transgenic tobacco

Abstract

To alter the level and distribution of IAA in tobacco, the T‐DNA iaaM gene was fused to the TR‐DNA 1′ promoter, and this construct was used to transform transgenic tobacco SR1 plants containing the natural iaaH gene. Coexpression of the 1’‐iaaM and iaaH genes was associated with reduced plant height, leaf size, and internode diameter. Free and conjugated IAA levels were higher in 1′‐iaaMliaaH plants than in wild‐type, most notably in the basal leaves and internodes. Extending this work, hygromycin‐resistant plants containing the iaaM gene expressed from either the 1′, CaMV 35S or potato ST‐LS1 promoters were crossed with a kanamycin‐resistant 35S‐iaaH plant. Transcription of the iaaM and iaaH genes, and levels of IAA and ABA were monitored in hygromycin‐ and kanamycin‐resistant progeny. Growth was inhibited in all lines, particularly in 35S‐iaaM x 35S‐iaaH plants. The phenotypes of 1′‐iaaM x 35S‐iaaH and 35S‐iaaM x 35S‐iaaH plants were similar to those of 1′‐iaaMliaaH and 35S‐iaaMliaaH plants, respectively, indicating that iaaH expression does not limit IAA biosynthesis. Transcription of the iaaM gene varied between the lines, as well as within and between organs of the same line. An increased IAA level was associated with iaaM transcription in most organs. Overall, the ABA level was higher in wild‐type than in transgenic lines, but did not vary between the transgenic lines.