Arabidopsis RGL1 Encodes a Negative Regulator of Gibberellin Responses

Abstract

In Arabidopsis, the DELLA subfamily of GRAS regulatory genes consists of GAI , RGA , RGA-LIKE1 ( RGL1 ), RGL2 , and RGL3 . GAI and RGA are known to be negative regulators of gibberellin (GA) responses. We found that RGL1 is a similar repressor of GA responses, as revealed by RGL1 gain-of-function and loss-of-function phenotypes. Repression of GA responses in Arabidopsis was conferred by a dominant 35S-rgl1 transgene carrying a DELLA domain deletion analogous to the GA-insensitive gai-1 mutation. As in GA-deficient Arabidopsis, the transgenic plants were dark green dwarfs with underdeveloped trichomes and flowers. Expression levels of GA4 , a feedback-regulated GA biosynthetic gene, were increased correspondingly. Conversely, a loss-of-function rgl1 line had reduced GA4 expression and exhibited GA-independent activation of seed germination, leaf expansion, flowering, stem elongation, and floral development, as detected by resistance to the GA biosynthesis inhibitor paclobutrazol. RGL1 plays a greater role in seed germination than do GAI and RGA . The expression profile of RGL1 differed from those of the four other DELLA homologs. RGL1 message levels were predominant in flowers, with transcripts detected in developing ovules and anthers. As with RGA, green fluorescent protein (GFP)-tagged RGL1 protein was localized to the nucleus, but unlike GFP-RGA, there was no degradation after GA treatment. These findings indicate that RGL1 is a partially redundant, but distinct, negative regulator of GA responses and suggest that all DELLA subfamily members might possess separate as well as overlapping roles in GA signaling. INTRODUCTIONGibberellins (GAs) are a large family of tetracyclic diterpenoid compounds, many of which play important roles in plant growth and development (Hooley, 1994;Swain and Olszewski, 1996). Bioactive GAs are known to promote diverse processes, including seed germination, leaf expansion, shoot/stem elongation, floral initiation, floral organ development, and fruit development (Hooley, 1994;Swain and Olszewski, 1996). Mutants in GA biosynthesis have helped to determine the GA biosynthesis pathway (Hedden and Phillips, 2000;Yamaguchi and Kamiya, 2000), but less is known at present about the GA signal transduction pathway. A receptor for GA has been localized to the plasma membrane, but the identity of the receptor remains unknown (Gilroy and Jones, 1994). Biochemical and pharmacological data have indicated that Ca 2 ϩ , calmodulin, cyclic GMP, heterotrimeric G proteins, GAMYB, and protein kinases all are involved in GA signaling (Bethke and Jones, 1998;Lovegrove and Hooley, 2000;Sun, 2000).Genes involved in GA signal transduction have been uncovered through GA response mutants. These mutants are either GA-insensitive dwarfs or constitutive GA response mutants (Sun, 2000). The GA-insensitive dwarfs resemble GA-deficient mutants, but they are not rescued by GA treatment. They typically exhibit compact, dark green leaves, inhibition of seed germination, delay of flowering, and abnormal flower development...

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.…”

Common mechanisms regulating expression of rice aleurone genes that contribute to the primary response for gibberellin

“…For example, GAI, RGA and RGL1 are negative regulators of stem elongation [11,19,21,22]. RGL2 mainly controls seed germination [20,23].…”