Gibberellin regulates Arabidopsis seed germination via RGL2, a GAI/RGA-like gene whose expression is up-regulated following imbibition

Abstract

The germination of Arabidopsis seeds is promoted by gibberellin (GA). Arabidopsis GAI, and RGA are genes encoding key GA signal-transduction components (GAI and RGA) that mediate GA regulation of stem elongation. The Arabidopsis genome contains two further genes, RGL1 and RGL2, that encode proteins (RGL1 and RGL2) that are closely related to GAI and RGA. Here, we show that RGL2 regulates seed germination in response to GA, and that RGL1, GAI, and RGA do not. In addition, we show that RGL2 transcript levels rise rapidly following seed imbibition, and then decline rapidly as germination proceeds. In situ GUS staining revealed that RGL2 expression in imbibed seeds is restricted to elongating regions of pre-emergent and recently emerged radicles. These observations indicate that RGL2 is a negative regulator of GA responses that acts specifically to control seed germination rather than stem elongation. Furthermore, as RGL2 expression is imbibition inducible, RGL2 may function as an integrator of environmental and endogenous cues to control seed germination.

Large-scale analysis of the GRAS gene family in Arabidopsis thaliana

“…SHR, SCL29, and SCL32 belong to the branch III, and only SHR has been analyzed in detail (Helariutta et al 2000;Nakajima et al 2001). The branch IV comprises so-called ''DELLA'' proteins: GAI, RGA, RGL1, RGL2, and RGL3, which are negative regulators of GA signaling (Peng et al 1997;Silverstone et al 1998;Dill and Sun 2001;Lee et al 2002;Wen and Chang 2002;Tyler et al 2004). LAS, SCR, and SCL23 appear to belong to the branch V, but only the robustness of node for SCR and SCL23 is strongly supported in our bootstrap analysis ( Supplementary Fig.…”

“…1a). Of 33 GRAS genes, only a third was genetically analyzed: GAI, RGA, RGL1, RGL2, RGL3, SCR, SHR, PAT1, SCL13, and SCL18/ LAS (Di Laurenzio et al 1996;Peng et al 1997;Silverstone et al 1998;Bolle et al 2000;Helariutta et al 2000;Dill and Sun 2001;Lee et al 2002;Wen and Chang 2002;Greb et al 2003;Tyler et al 2004;Torres-Galea et al 2006). Typically, GRAS proteins share signature motifs of VHIID flanked by two leucine heptad repeats (LHRI and LHRII), PFYRE and SAW (designated after the profound amino acid residues in the conserved regions) with rather variable N-terminal regions (Pysh et al 1999;Bolle 2004).…”

“…Of 28 bait clones, 20 (71.4%) clones were able to activate the marker genes without prey clones (autoactivation), suggesting that these 20 GRAS members could act as transcription activators in yeast (Supplementary Table S4). Intriguingly, 8 of 20 autoactivating bait clones were previously reported as transcription regulators (Peng et al 1997;Silverstone et al 1998;Helariutta et al 2000; Lee et al 2002;Wen and Chang 2002;Greb et al 2003;Tyler et al 2004;Torres-Galea et al 2006). Excluding these 20 autoactivating bait clones, we performed pair-wise Y2H assay.…”

“…According to the current genome annotation (version 7.0), a total of 33 GRAS genes are predicted in Arabidopsis thaliana (TAIR, http://www.arabidopsis.org). Of them, only 10 GRAS genes were analyzed in detail thus far: GAI, RGA, RGL1, RGL2, RGL3, SCR, SHR, PAT1, SCL13, and SCL18/LAS (Di Laurenzio et al 1996;Peng et al 1997;Silverstone et al 1998;Bolle et al 2000;Helariutta et al 2000;Dill and Sun 2001;Lee et al 2002;Wen and Chang 2002;Greb et al 2003;Tyler et al 2004;Torres-Galea et al 2006). These genes play a crucial role in diverse plant growth and development, ranging from GA signaling, root radial pattering, light signal transduction, and axillary shoot meristem formation.…”

Signalling for developmental plasticity

“…The RGL2 gene (Accession number NM111216) negatively regulates seed germination responses to gibberellin [48]. The expression of RGL2 is up-regulated by seed imbibition, which is a pre-requisite for germination.…”