Abstract
Previous work suggests that gibberellins (GAs) play an important role in early seed development. To more fully understand the roles of GAs throughout seed development, tissue-specific transcription profiles of GA metabolism genes and quantitative profiles of key GAs were determined in pea (Pisum sativum) seeds during the seed-filling development period (8-20 d after anthesis [DAA]). These profiles were correlated with seed photoassimilate acquisition and storage as well as morphological development. Seed coat growth (8-12 DAA) and the subsequent dramatic expansion of branched parenchyma cells were correlated with both transcript abundance of GA biosynthesis genes and the concentration of the growth effector GA, GA(1). These results suggest GA(1) involvement in determining the rate of seed coat growth and sink strength. The endosperm's PsGA20ox transcript abundance and the concentration of GA(20) increased markedly as the endosperm reached its maximum volume (12 DAA), thus providing ample GA(20) substrate for the GA 3-oxidases present in both the embryo and seed coat. Furthermore, PsGA3ox transcript profiles and trends in GA(1) levels in embryos at 10 to 16 DAA and also in embryo axes at 18 DAA suggest localized GA(1)-induced growth in these tissues. A shift from synthesis of GA(1) to that of GA(8) occurred after 18 DAA in the embryo axis, suggesting that deactivation of GA(1) to GA(8) is a likely mechanism to limit embryo axis growth and allow embryo maturation to proceed. We hypothesize that GA biosynthesis and catabolism are tightly regulated to bring about the unique developmental events that occur during seed growth, development, and maturation.
Highlights
Previous work suggests that gibberellins (GAs) play an important role in early seed development
There are tissue-specific developmental patterns in the seed coat, embryo, embryo axis, and endosperm tissues that we conclude are likely in part regulated by tissue-specific developmental regulation of GA1 biosynthesis and catabolism (Fig. 10; Table I)
We cannot exclude the possibility that other GA20ox, GA3ox, and GA2ox genes occur within the pea genome that may be expressed in seed tissues, the expression patterns of the known pea GA20ox, GA3ox, and GA2ox genes assessed in this study correlate well, in general, with the tissue-specific levels of 3-deoxy GA precursors, bioactive GA1, and the 2b-hydroxylated deactivated GA metabolites as development proceeds from a seed that is mainly seed coat tissue with liquid endosperm to one that is mainly embryo tissue with no liquid endosperm
Summary
Previous work suggests that gibberellins (GAs) play an important role in early seed development. Fertilization of lh-2 ovaries with wild-type pollen partially reversed these seed mutant phenotypes Taken together, this suggests that GAs play an important role in early pea seed development (Swain et al, 1993, 1995). Consistent with the hypothesis that GAs play an important role in early pea seed development, transcript levels of PsGA3ox (which are responsible for the production of biologically active 3b-hydroxylated GAs) peak in the seeds immediately following fertilization (0 DAA; Ozga et al, 2003) and during early seed development (approximately 4–7 DAA; Ozga et al, 2003, 2009). The seed coat, made up of several discernible layers, provides a protective environment as well as
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