GIBBERELLINS AND ALPHA-AMYLASE GENE-EXPRESSION IN GERMINATING WHEAT GRAINS

Gibberellins (GAs), GA(8), GA(17), GA(19), GA(20), GA(29), and GA(79) were identified by full-scan gas chromatography-mass spectrometry in a purified acidic fraction and GA(8), GA(20), GA(79), and GA(90) in a hydrolysed conjugate fraction from mature wheat grains. Gibberellin A(20)-13-O-glucoside was also quantified directly as the permethyl derivative in dry seed. The scutellum was identified as the major site of de novo GA biosynthesis by measuring ent-kaurene accumulation in vivo in grains treated with 2S,3S-paclobutrazol. Several GAs of the early 13-hydroxylation GA pathway began to accumulate in the axis and scutellum between 24 and 48 hours in untreated grains germinated at 15 degrees C. Bioactive GA(1) and GA(3) also increased in the endosperm during this period, whereas abscisic acid contents of embryo and endosperm declined rapidly over 48 hours following imbibition. Treating grains with 2S,3S-paclobutrazol reduced GA(1) plus GA(3) content of scutella by 70-80% over 4 days without affecting significantly the steady-state pool of alpha-amylase mRNA transcripts. In contrast, a 50-80% reduction in the content of bioactive GAs in the endosperm was associated with a 70-78% decrease in transcripts for both alpha-amylase gene families in aleurones of paclobutrazol-treated grains. It was concluded that the initiation of alpha-amylase gene expression in wheat scutella was independent of de novo GA biosynthesis, whereas that in the aleurone was largely dependent on embryo-produced GAs.