Gibberellin and abscisic acid regulate GAST1 expression at the level of transcription

Both gibberellic acid (GA(3)) and abscisic acid (ABA) regulate the expression of the GAST1 gene of tomato. Treatment with GA(3) increases the abundance of GAST1 RNA while treatment with ABA blocks this effect. In this study, the effects of GA(3) and ABA on the rate of transcription of the GAST1 gene and the stability of GAST1 RNA were examined. Nuclear run-on analyses detected an increase in transcription of the GAST1 gene 1 h after GA(3) treatment with transcription increasing to a maximum at 9 h after treatment. The half-life of GAST1 RNA in GA(3)-treated leaves was similar to that in control leaves. In addition, the extent of overexpression of GAST1 RNA in transgenic tomato plants containing the CaMV 35S promoter driving the expression of the GAST1 transcribed region was largely unaffected by GA(3). These results suggest that GA(3) stimulates the expression of the GAST1 gene by acting only at the level of transcription. ABA treatment dramatically reduced the abundance of GAST1 RNA in gib1 shoots through an effect at the level of transcription and did not appear to affect the stability of this RNA. Midcourse ABA addition to the GA(3)-incubated shoots reversed the GA(3)-mediated increase in the transcription of GAST1 gene within 15 min. Transgenic plants that either overexpressed or underexpressed GAST1 RNA exhibited no phenotypic differences from wild type.