An auxin-repressed gene (RpARP) from black locust (Robinia pseudoacacia) is posttranscriptionally regulated and negatively associated with shoot elongation.

The plant hormone auxin regulates various growth and developmental processes by controlling the expression of auxin-response genes. While many genes up-regulated by auxin have been characterized, less is known about the genes that are down-regulated by auxin. We isolated and characterized an auxin-repressed gene (RpARP) from the tree legume, Robinia pseudoacacia L. A sequence similarity search in public databases showed that the RpARP gene has homologs in various higher plants including monocots and dicots. The deduced amino acid sequences are highly conserved among these homologs (up to 85% identity). Northern blot analysis showed that auxin repressed RpARP gene expression and that repression was dependent on the presence of metabolizable sugar and on protein synthesis. In addition, cold treatment abolished the auxin-mediated repression of RpARP gene expression. Results from transgenic plant analyses suggest that RpARP gene expression is posttranscriptionally regulated by auxin and by the untranslated regions. Sequence analysis of the promoter region (-70 and -500 bp upstream of the putative transcription initiation site) of the RpARP gene identified four sucrose-repressible response elements (TATCCAT-motifs; Huang et al. 1990), suggesting that the cis-elements responsible for regulation by sucrose are located in the promoter region. In fact, the expression of the transgenic RpARP gene was unaffected by sucrose when driven by a CaMV 35S promoter. We present evidence that RpARP gene expression is negatively associated with hypocotyl elongation.