Changes in levels of gibberellins, their putative conjugates, and ABA in zygotic embryos during late maturation and dry seed stages of Brassica napus cv Topaz
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Two late stages [days 35 and 40 after pollination (DAP)] in zygotic embryo (ZE) development of Brassica napus were utilized to quantify, by the stable isotope-labeled dilution method, levels of "free" and "aglycone" gibberellins (GAs), as well as abscisic acid (ABA), during the programmed dehydration of the seed. GAs from both the early 13 hydroxylation and early non-hydroxylation pathways were present in these ZEs of B napus. Between 35 and 40 DAP endogenous ABA dropped precipitously (almost 30-fold) and this drop in ABA was accompanied by a significant reduction in levels of GA 1 and even in levels of the inactive GA catabolites, GA8 and GA29. Levels of GA4 and putative GA85 also dropped appreciably, though not significantly. In contrast, the levels of GA20 and GA9 (the immediate precursors of GA1 and GA4, respectively) did not change in the ZEs during this transition. A fungal-derived cellulase was used to hydrolyze the highly water-soluble fraction, which will contain GA conjugates. Relatively high levels of several GAs (GA9, GA20) were thus quantified after hydrolysis as the aglycones, e.g., 56 and 25 ng/g DW of GA20 and 23 and 5 ng/g DW, of GA9, respectively at DAP 35 and DAP 40. Other GAs found after hydrolysis of the highly water-soluble fraction remained relatively constant between 35 and 40 DAP. An exception was the putative GA85 aglycone, which increased sixfold (free GA85 decreased by ca. half). The transition to the dry seed stage for ZEs of B. napus is thus accompanied not only by the expected reduction in ABA, but also by reduced levels of many "free" GAs, especially the bioactive, 3-hydroxylated GAs. In contrast, levels of 3-deoxy GAs remain relatively high, implying a partial block in the 3-hydroxylation "activation" step of GA biosynthesis. 2007 Springer Science+Business Media B.V.
