Gibberellins and α-amylase gene expression in germinating wheat grains

Abstract

Gibberellins (GAs), GA₈, GA₁₇, GA₁₉, GA₂₀, GA₂₉, and GA₇₉ were identified by full-scan gas chromatography-mass spectrometry in a purified acidic fraction and GA₈, GA₂₀, GA₇₉, and GA₉₀ in a hydrolysed conjugate fraction from mature wheat grains. Gibberellin A₂₀-13-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°C. Bioactive GA₁ and GA₃ 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 GA₁ plus GA₃ content of scutella by 70–80% over 4 days without affecting significantly the steady-state pool of α-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 α-amylase gene families in aleurones of paclobutrazol-treated grains. It was concluded that the initiation of α-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.