Repressing a Repressor: Gibberellin-Induced Rapid Reduction of the RGA Protein in Arabidopsis

Abstract

RGA (for repressor of ga1-3 ) and SPINDLY ( SPY ) are likely repressors of gibberellin (GA) signaling in Arabidopsis because the recessive rga and spy mutations partially suppressed the phenotype of the GA-deficient mutant ga1-3 . We found that neither rga nor spy altered the GA levels in the wild-type or the ga1-3 background. However, expression of the GA biosynthetic gene GA4 was reduced 26% by the rga mutation, suggesting that partial derepression of the GA response pathway by rga resulted in the feedback inhibition of GA4 expression. The green fluorescent protein (GFP)-RGA fusion protein was localized to nuclei in transgenic Arabidopsis. This result supports the predicted function of RGA as a transcriptional regulator based on sequence analysis. Confocal microscopy and immunoblot analyses demonstrated that the levels of both the GFP-RGA fusion protein and endogenous RGA were reduced rapidly by GA treatment. Therefore, the GA signal appears to derepress the GA signaling pathway by degrading the repressor protein RGA. The effect of rga on GA4 gene expression and the effect of GA on RGA protein level allow us to identify part of the mechanism by which GA homeostasis is achieved. INTRODUCTIONGibberellins (GAs) are members of a large family of diterpenoid compounds, some of which are plant growth regulators that control such diverse processes as seed germination, stem growth, and flower development. Although the GA biosynthetic pathway has been elucidated (reviewed in Lange, 1998;Hedden and Proebsting, 1999;Hedden and Phillips, 2000;Yamaguchi and Kamiya, 2000), much less is known about its signal transduction pathway in plants. Recent molecular and pharmacological studies in cereal aleurone showed that Ca 2 ϩ , calmodulin, cyclic GMP, heterotrimeric G proteins, GAMYB, and protein kinases may play a role in GA signaling (reviewed in Bethke and Jones, 1998;Lovegrove and Hooley, 2000). Isolation of GA response mutants and molecular cloning of corresponding genes in Arabidopsis also have identified several novel components of the GA signal transduction pathway (reviewed in Thornton et al., 1999;Sun, 2000). The putative repressors include SPINDLY ( SPY ;Jacobsen et al., 1996), RGA (for repressor of ga1-3 ; Silverstone et al., 1998), GAI (for GA insensitive; Peng et al., 1997), and SHORT INTERNODES ( SHI ;Fridborg et al., 1999), and the potential activators are SLEEPY ( SLY ;Steber et al., 1998) and PICKLE ( PKL ; Ogas et al., 1999).SPY was identified originally because spy mutations allowed the seed to germinate in the presence of the GA biosynthesis inhibitor paclobutrazol (PAC; Jacobsen and Olszewski, 1993). The defect in the SPY function also was able to partially suppress the phenotype of the GA biosynthetic mutant ga1-3 , which is a nongerminating, male-sterile, extreme dwarf (Silverstone et al., 1997). Sequence analysis of SPY and in vitro enzyme assays using the recombinant SPY protein suggest that SPY probably is a Ser/Thr O -linked N -acetylglucosamine transferase (OGT; Thornton et al., 1999).We identified RG...

Gibberellin-Induced Gene Expression Associated with Cytoplasmic Male Sterility in Sunflower

“…Alternatively auxins accumulation may enhance GA responses. This fi nding is in accordance with experiments which have shown that GAs de-repress gene expression by inducing proteolysis of GA signaling repressors -the DELLA proteins (47,48,53) and auxin modulate the rate of GA-mediated DELLA protein degradation through decreasing the protein's stability (8,34). Similar interactions between auxins and GA in male development and fertility were found with GA-reversible male-sterile tomato mutant, sl-2, revealing that reduction of indole-3-acetic acid by lowering growing temperatures restores anther fertility without GA applications (49).…”

“…Taken together, the work presented here appears to support the hypothesis based on the current homeostatic model (48), that an increase of the active GAs level derepresses the GA response (feed-forward mechanism). The increased activity of the GA response pathway resulting fi nally the expression of the gene responsible for male sterility as a GA-responsive genes causes a reduction in the levels of bioactive GAs by inhibiting GA biosynthesis (feedback mechanism) and activating GA catabolism.…”

Recessive-interfering mutations in the gibberellin signaling gene SLEEPY1 are rescued by overexpression of its homologue, SNEEZY

“…It seems that GA causes GA responses by triggering the disappearance of DELLA proteins (24)(25)(26). Whereas only a single DELLA gene is known in barley (SLN1, SLENDER1) (24) and in rice (SLR1, SLENDER RICE1) (27), there are five DELLA genes in Arabidopsis.…”

The plant stress hormone ethylene controls floral transition via DELLA-dependent regulation of floral meristem-identity genes

“…Downstream of GID1 is a family of nuclear growth repressor proteins, the DELLAs (23-25). The DELLAs are a subfamily of the GRAS family of putative transcriptional regulators (20, 26), a subfamily that in Arabidopsis comprises GAI, RGA, RGL1, RGL2, and RGL3 (23,[27][28][29]. DELLAs restrain plant growth, whereas GA promotes growth via relief of DELLA-mediated growth restraint (24,25,30,31).…”