Cloning and characterization of IAA28, a gene involved in suppressing lateral root development and mediating auxin responses in Arabidopsis thaliana
Rogg, Luise Elizabeth
Doctor of Philosophy
Auxins are an important class of plant hormones implicated in most aspects of plant development, and therefore influence the overall size and shape of a plant. Whereas the signal transduction pathways that sense and respond to auxin remain mysterious, a number of genes do undergo dramatic transcriptional alterations in response to auxin. The Aux/IAA genes were originally isolated based on strong and rapid transcriptional up-regulation following auxin induction. Aux/IAA genes are primary response genes whose products are thought to regulate auxin-responsive transcription. A new member of the Aux/IAA gene family, IAA28, was cloned based on the abnormal auxin responses and unusual auxin-related adult phenotypes of a gain-of-function mutant, including decreased apical dominance and extremely reduced lateral root formation. In addition to auxin, the iaa28-1 mutant is also resistant to inhibition of root elongation by cytokinin and ethylene, but responds normally to other phytohormones. Northern analysis, promoter - reporter gene fusions and Western analysis demonstrate that IAA28 is strongly expressed in roots. IAA28 may be a primary response gene, as IAA28 mRNA levels increase in response to cycloheximide. However, IAA28 transcription decreases upon auxin treatment, a novel response that differs from other characterized members of the Aux/IAA gene family. Experiments with an auxin-inducible reporter construct suggest that IAA28 is a repressor of auxin-induced transcription. Levels of tagged IAA28 protein decrease dramatically after exogenous auxin treatment, thus IAA28 degradation may increase in response to auxin signaling. In addition, microarray analysis suggests that a number of genes have altered transcript levels in the gain-of-function iaa28-1 mutant background. From these findings, IAA28 has been proposed to encode a transcriptional repressor that functions to regulate expression of genes that promote lateral root initiation in response to auxin.
Genetics; Plant physiology