|University of Freiburg, Faculty of Biology, Institute for Biology II, Botany / Plant Physiology, Schänzlestr. 1, D-79104 Freiburg, Germany, Web: 'The Seed Biology Place' http://www.seedbiology.de
*Department of Biological Sciences, Simon Fraser University, 9 8888, University Drive, Burnaby BC, V5A 1S6, Canada
Received 14 March 2011; Revised 22 May 2011; Accepted 13 June 2011
Advance Acess publication 21 June 2011
Germination of endospermic seeds is partly regulated by the micropylar endosperm, which acts as constraint to radicle protrusion. Gibberellin (GA) signalling pathways control coat-dormancy release, endosperm weakening, and organ expansion during seed germination. Three GIBBERELLIN INSENSITIVE DWARF1 (GID1) GA receptors are known in Arabidopsis thaliana: GID1a, GID1b and GID1c. Molecular phylogenetic analysis of angiosperm GID1s reveals that they cluster into two eudicot (GID1ac, GID1b) groups and one monocot group. Eudicots have at least one gene from each of the two groups, indicating that the different GID1 receptors fulfill distinct roles during plant development. We used a comparative Brassicaceae approach, in which gid1 mutant and whole-seed transcript analyses in Arabidopsis were combined with seed tissue-specific analyses of its close relative Lepidium sativum (garden cress), for which we cloned three GID1 orthologs. GA signalling via the GID1ac receptors is required for Arabidopsis seed germination, GID1b can not compensate the impaired germination of the gid1agid1c mutant. Transcript expression patterns differed temporarily, spatially and hormonally, with GID1b being distinct from GID1ac in both species. Endosperm weakening is mediated, at least in part, through GA-induced genes encoding cell-wall modifying proteins. A suppression subtraction hybridization (SSH) cDNA library enriched for sequences that are highly expressed during early germination in the micropylar endosperm contained expansins and xyloglucan endo-transglycosylases/hydrolases (XTHs). Their transcript expression patterns in both species strongly suggest that they are regulated by distinct GID1-mediated GA signalling pathways. The GID1ac and GID1b pathways seem to fulfill distinct regulatory roles during Brassicaceae seed germination and seem to control their downstream targets distinctly.
Key words: Arabidopsis thaliana, endosperm weakening, expansin, GIBBERELLIN INSENSITIVE DWARF1 (GID1), Lepidium sativum (garden cress), seed germination, xyloglucan endo-transglycosylase/hydrolase (XTH).
Financial support: Our work is funded by grants from the Deutsche Forschungsgemeinschaft (Grant DFG LE720/6 and MU3114/1-1) and the Deutscher Akademischer Austauschdienst (Grant DAAD D/0628197) to G.L.-M. and K.M., and the Wissenschaftliche Gesellschaft Freiburg to A.L. and A.V., which is gratefully acknowledged.