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 h
ydrolysed conjugate fraction from mature wheat grains. Gibberellin A(2
0)-13-O-glucoside was also quantified directly as the permethyl deriva
tive 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 th
is period, whereas abscisic acid contents of embryo and endosperm decl
ined 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 7
0-78% decrease in transcripts for both alpha-amylase gene families in
aleurones of paclobutrazol-treated grains. It was concluded that the i
nitiation of alpha-amylase gene expression in wheat scutella was indep
endent of de novo GA biosynthesis, whereas that in the aleurone was la
rgely dependent on embryo-produced GAs.