An Auxin-Inducible Element in Soybean SAUR Promoters

The soybean SAUR (Small Auxin-Up RNA) genes are transcriptionally induced by exogenous auxins within a few minutes after hormone application. This response is specifically induced by auxins primarily in epidermal and cortical cells within elongation zones of hypocotyls and epicotyls. We have previously shown that an 832-bp soybean SAUR promoter/beta-glucuronidase (GUS) reporter gene fusion is responsive to auxin in transgenic tobacco plants (Y. Li, G. Hagen, T.J. Guilfoyle [1991] Plant Cell 3: 1167-1175). Similar results were obtained with an 868-bp SAUR 15A promoter-GUS reporter gene in transgenic tobacco (Y. Li, unpublished results). We have now analyzed a soybean SAUR 15A promoter in transgenic tobacco plants using 5' unidirectional deletions, internal deletions and mutations, and gain-of-function assays with a minimal cauliflower mosaic virus 35S promoter. Our results indicate that the distal upstream element/NdeI restriction endonuclease site element (NDE) (B.A. McClure, G. Hagen, C.S. Brown, M.A. Gee, T.J. Guilfoyle [1989] Plant Cell 1: 229-239) in the SAUR 15A promoter is necessary and sufficient for auxin induction. Our results also show that the 30-bp NDE portion of this element is responsible for most, if not all, of the auxin inducibility of the SAUR 15A promoter. The NDE contains two adjacent sequences, TGTCTC and GGTCCCAT, which have been previously identified as putative auxin-responsive elements. We propose that these elements might function independently or together, possibly with an additional element(s), to confer auxin inducibility to the SAUR promoters.