| 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. |
