Transcriptional and posttranscriptional regulation of Arabidopsis TCH4 expression by diverse stimuli. Roles of cis regions and brassinosteroids.

The Arabidopsis TCH4 gene is up-regulated in expression by diverse environmental and hormonal stimuli. Because TCH4 encodes a xyloglucan endotransglucosylase/hydrolase, this change in expression may reflect a recruitment of cell wall-modifying activity in response to environmental stress and growth. How diverse stimuli lead to the common response of TCH4 expression regulation is not known. Here, we show that induction of expression by the diverse stimuli of touch, darkness, cold, heat, and brassinosteroids (BRs) is conferred to reporter genes by the same 102-bp 5'-untranscribed TCH4 region; this result is consistent with the idea that shared regulatory elements are employed by diverse stimuli. Distal regions influence magnitude and kinetics of expression and likely harbor regulatory elements that are redundant with those located more proximal to the transcriptional start site. Substitution of the proximal regulatory region sequences in the context of distal elements does not disrupt inducible expression. TCH4 expression induction is transcriptional, at least in part because 5'-untranscribed sequences are sufficient to confer this regulation. However, 5'-untranslated sequences are necessary and sufficient to confer the marked transience of TCH4 expression, most likely through an effect on mRNA stability. Perception of BR is not necessary for TCH4::GUS induction by environmental stimuli because regulation is intact in the BR-insensitive mutant, bri1-2. The full response to auxin, however, requires the functioning of BRI1. Developmental expression of TCH4 is unlikely to be meditated by BR because TCH4::GUS is expressed in BR perception and biosynthetic mutants bri1-2 and det2-1, respectively.