Two binding sites for the plant transcription factor ASF-1 can respond to auxin treatments in transgenic tobacco.

The hormone, auxin, plays an important role in the differentiation and growth of plant cells. A number of auxin-responsive genes have been characterized but until now minimal auxin-responsive cis-elements within these promoter regions have not been identified. Here we show that two related DNA sequences of 21 base pairs can respond to auxin treatment in transgenic tobacco. In contrast, treatments with cytokinin or abscisic acid do not cause any apparent increase in promoter activity of these cis-acting elements. These sequences are present in the promoter regions of the nopaline synthase gene from the T-DNA of Agrobacterium tumefaciens and the 35 S promoter from cauliflower mosaic virus. Both sequences have been shown to be binding sites for the tobacco transcription factor ASF-1. Pretreatment of leaves with cycloheximide does not inhibit the response to auxin treatment, suggesting that hormone sensitivity of these promoter elements does not involve de novo synthesis of ASF-1. In addition, promoter elements from some auxin-responsive plant genes can bind ASF-1 in vitro. Based on these results, we propose that transcriptional activation by ASF-1 may be modulated by auxin through modification of pre-existing factors. Our results also suggest a role for ASF-1 in mediating some of the effects of auxin in vivo.