The Gibberellic Acid Signaling Repressor RGL2 Inhibits Arabidopsis Seed Germination by Stimulating Abscisic Acid Synthesis and ABI5 Activity

Abstract

Seed germination is antagonistically controlled by the phytohormones gibberellic acid (GA) and abscisic acid (ABA). GA promotes seed germination by enhancing the proteasome-mediated destruction of RGL2 (for RGA-LIKE2), a key DELLA factor repressing germination. By contrast, ABA blocks germination by inducing ABI5 (for ABA-INSENSITIVE5), a basic domain/leucine zipper transcription factor repressing germination. Decreased GA synthesis leads to an increase in endogenous ABA levels through a stabilized RGL2, a process that may involve XERICO, a RING-H2 zinc finger factor promoting ABA synthesis. In turn, increased endogenous ABA synthesis is necessary to elevate not only ABI5 RNA and protein levels but also, critically, those of RGL2. Increased ABI5 protein is ultimately responsible for preventing seed germination when GA levels are reduced. However, overexpression of ABI5 was not sufficient to repress germination, as ABI5 activity requires phosphorylation. The endogenous ABI5 phosphorylation and inhibition of germination could be recapitulated by the addition of a SnRK2 protein kinase to the ABI5 overexpression line. In sleepy1 mutant seeds, RGL2 overaccumulates; germination of these seeds can occur under conditions that produce low ABI5 expression. These data support the notion that ABI5 acts as the final common repressor of germination in response to changes in ABA and GA levels.

The GATA-type transcription factors GNC and GNL/CGA1 repress gibberellin signaling downstream from DELLA proteins and PHYTOCHROME-INTERACTING FACTORS

“…At later stages, we found GNL expression to be largely restricted to the cotyledons, while GNC expression was particularly prominent in the embryonic root, a region that elongates during radicle emergence ( Fig. 2G; Belin et al 2006;Piskurewicz et al 2008). We thus concluded that GNC and GNL are expressed in the germinating embryo as well as in the endosperm, and that the two genes may act in the same but also in distinct tissues during specific stages of germination.…”

“…Immunoblots with anti-DELLA protein antibodies were preformed as described previously except that cycloheximide was omitted to test for DELLA protein accumulation rather than protein degradation (Willige et al 2007;Piskurewicz et al 2008). POR immunoblots were performed overnight at 4°C with an anti-POR antibody (Agrisera).…”

The Arabidopsis ZINC FINGER PROTEIN3 Interferes with Abscisic Acid and Light Signaling in Seed Germination and Plant Development

“…The ABA insensitivity and tolerance to paclobutrazol of ZFP3ox lines could be influenced by reduced transcription of the DELLA factor RGL2 in ZFP3ox seedlings, which is a negative regulator of GA responses (Lee et al, 2002;Tyler et al, 2004). RGL2 was shown to control germination by enhancing ABA-dependent ABI3 and ABI5 activities, which act as final repressors of germination (Piskurewicz et al, 2008(Piskurewicz et al, , 2009. Although the transcript levels of ABI5 and RGL2 were lower in ZFP3ox seedlings, they are probably not direct targets of ZFP3, as their expression was not altered by ZFP3 overexpression after short-term ABA induction (Fig.…”

Gibberellin-GID1-DELLA: A Pivotal Regulatory Module for Plant Growth and Development

“…PIL5 also reduces GA accumulation and increases ABA levels via SOM-NUS (a nuclear zinc-finger protein; Kim et al, 2008). Interestingly, ABA was shown to promote transcription of another AtDELLA gene (RGL2; Piskurewicz et al, 2008). In the light, phytochromes mediate light-induced germination by causing PIL5 degradation and GA accumulation, the combined effects of which allow down-regulation of DELLA activity (by reducing transcription and increasing protein turnover).…”