Hormonal interactions during the regulation of seed dormancy release and germination of Nicotiana (a) and Brassica (b) model species.
(a) Nicotiana sp. seed germination is two-step: testa rupture followed by endosperm rupture. Dormancy release and germination promotion
occur during seed after-ripening (dry storage at room temperature for several months) or via the light-gibberellin (GA) pathway during
imbibition. Abscisic acid (ABA) inhibits endosperm rupture but not testa rupture. GA, ethylene and brassinosteroids (BRs) promote endosperm
rupture and counteract the inhibitory effects of ABA. The ABA-inhibited class I β-1,3-glucanase genes (βGlu I) are transcriptionally induced
in the micropylar endosperm just before endosperm rupture (reviewed in Leubner-Metzger, 2003). This induction is highly localized in the
micropylar endosperm at the site of radicle emergence. Light, GA and ethylene promote and ABA, osmotica and darkness inhibit βGlu I
expression and endosperm rupture. This inhibition is partially reversed in transgenic tobacco (Nicotiana tabacum) seeds that over-express βGlu
I in the seed-covering layers under the control of an ABA-inducible transgene promoter. βGlu I is therefore causally involved in the promotion
of endosperm rupture. EREBP, ethylene-responsive element binding protein transcription factor. Part (a) is modified from Leubner-Metzger
(2006) and reprinted with permission from The Haworth Press, Inc.; article copies are available from The Haworth Document Delivery Service:
1-800-HAWORTH; Email: docdelivery@haworthpress.com. (b) Brassica napus seed germination is one-step. The mature seeds of these species
are without an endosperm and so testa rupture plus initial radicle elongation result in the completion of germination. ABA does not inhibit testa
rupture, but inhibits subsequent radicle growth (Schopfer & Plachy, 1984; for a review, see Kucera et al., 2005)
Hormonal interactions during the regulation of seed dormancy release and germination of Nicotiana (a) and Brassica (b) model species.(a) Nicotiana sp. seed germination is two-step: testa rupture followed by endosperm rupture. Dormancy release and germination promotionoccur during seed after-ripening (dry storage at room temperature for several months) or via the light-gibberellin (GA) pathway duringimbibition. Abscisic acid (ABA) inhibits endosperm rupture but not testa rupture. GA, ethylene and brassinosteroids (BRs) promote endospermrupture and counteract the inhibitory effects of ABA. The ABA-inhibited class I β-1,3-glucanase genes (βGlu I) are transcriptionally inducedin the micropylar endosperm just before endosperm rupture (reviewed in Leubner-Metzger, 2003). This induction is highly localized in themicropylar endosperm at the site of radicle emergence. Light, GA and ethylene promote and ABA, osmotica and darkness inhibit βGlu Iexpression and endosperm rupture. This inhibition is partially reversed in transgenic tobacco (Nicotiana tabacum) seeds that over-express βGluI in the seed-covering layers under the control of an ABA-inducible transgene promoter. βGlu I is therefore causally involved in the promotionof endosperm rupture. EREBP, ethylene-responsive element binding protein transcription factor. Part (a) is modified from Leubner-Metzger(2006) and reprinted with permission from The Haworth Press, Inc.; article copies are available from The Haworth Document Delivery Service:1-800-HAWORTH; Email: docdelivery@haworthpress.com. (b) Brassica napus seed germination is one-step. The mature seeds of these speciesare without an endosperm and so testa rupture plus initial radicle elongation result in the completion of germination. ABA does not inhibit testarupture, but inhibits subsequent radicle growth (Schopfer & Plachy, 1984; for a review, see Kucera et al., 2005)
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