Characterization of a shoot-specific, GA3- and ABA-regulated gene from tomato.

A tomato (Lycopersicon esculentum) gene (GAST1) that encodes an RNA whose abundance increases > 20-fold in shoots of the GA-deficient gib1 mutant following spraying with GA3 has been characterized. An increase in GAST1 RNA levels is detectable 2 h after treatment and levels continue to increase for at least an additional 10 h. Between 12 and 24 h following treatment, the amount of GAST1 RNA begins to decline and at 48 h the level is nearly equivalent to that of water-treated control plants. Nuclear runoff analysis indicates that 8 h after treatment with GA3, transcription of the GAST1 gene has increased only threefold, suggesting that GA acts both transcriptionally and post-transcriptionally. ABA partially inhibits the GA-mediated increase in GAST1 RNA abundance while ethephon, kinetin, and 2,4-D have little effect. GAST1 RNA is detectable in untreated leaves, stems, petioles and flowers, but not in roots. The GAST1 gene encodes a 0.7 kb transcript. The sequence of the GAST1 cDNA and genomic clones indicates that the gene is interrupted by three introns and potentially encodes a 112 amino acid protein of unknown function.