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

Abstract

The length of the Arabidopsis thaliana life cycle depends on the timing of the floral transition. Here, we define the relationship between the plant stress hormone ethylene and the timing of floral initiation. Ethylene signaling is activated by diverse environmental stresses, but it was not previously clear how ethylene regulates flowering. First, we show that ethylene delays flowering in Arabidopsis, and that this delay is partly rescued by loss-of-function mutations in genes encoding the DELLAs, a family of nuclear gibberellin (GA)-regulated growth-repressing proteins. This finding suggests that ethylene may act in part by modulating DELLA activity. We also show that activated ethylene signaling reduces bioactive GA levels, thus enhancing the accumulation of DELLAs. Next, we show that ethylene acts on DELLAs via the CTR1-dependent ethylene response pathway, most likely downstream of the transcriptional regulator EIN3. Ethylene-enhanced DELLA accumulation in turn delays flowering via repression of the floral meristem-identity genes LEAFY (LFY) and SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1). Our findings establish a link between the CTR1/EIN3-dependent ethylene and GA-DELLA signaling pathways that enables adaptively significant regulation of plant life cycle progression in response to environmental adversity.

Fig. 1.

Ethylene delays flowering by reducing bioactive GA levels. (A) Representative (25-day-old) WT Ler plants (two plants per box are shown) grown in LDs on growth medium containing 10 μM ACC (+ACC) and/or 10 μM GA (+GA) (and control). All plants shown have bolted, except for plants growing on ACC. (B) Representative (5-week-old) ctr1–1 and ga1–3 mutant plants grown in SDs and treated with GA (+GA) or control. (C) Mean vegetative rosette leaf number (± SD; n > 30) of WT Col, ctr1–1, WT Ler, and ga1–3 plants grown on soil in SDs and GA-treated (red) or control (blue). The asterisks represent plants that had not flowered by the end of the experiment (8 weeks). (D) Levels of GAs in WT Col and ctr1–1 mutant plants (expressed as picograms per gram of fresh weight; ±SD; n = 5). n.d. indicates not detected.

Fig. 2.

Activation of GA signaling accelerates the flowering of ctr1–1 plants. (A and B) Representative (5-week-old) WT Ler, ctr1–1 × Ler, ctr1–1 gai-t6, ctr1–1 rga-24, and ctr1–1 gai-t6 rga-24 mutant plants grown in SDs and treated with GA (B) or control (A). (C) Representative (5-week-old) ctr1–1 × Ler and ctr1–1 spy-5 mutant plants grown in SDs and treated with GA (+GA; Right) or control (Left). (D) Mean vegetative rosette leaf number (± SD; n > 30) of WT Ler, ctr1–1 × Ler, ctr1–1 gai-t6 rga-24, and ctr1–1 spy-5 plants grown on soil in SDs and treated with GA (red) or control (blue). The asterisk represents plants that had not flowered by the end of the experiment (8 weeks).

Fig. 3.

Ethylene delays flowering via DELLA-dependent repression of LFY and SOC1 transcript levels. (A) Levels of floral meristem identity LFY and SOC1, and GA biosynthesis AtGA3ox1 and AtGA20ox1 gene transcripts in SD, soil-grown, GA-treated WT Ler, ctr1–1 × Ler, ctr1–1 gai-t6, ctr1–1 rga-24, and ctr1–1 gai-t6 rga-24 mutant plants (and controls). ELF4a transcripts provide loading control. (B) Flowering time (time at which 50% of plants had bolted) expressed as time to bolt and number of rosette leaves (± SD; n > 15) of WT Ler and 35S:LFY overexpression plants grown in LDs on growth medium containing 10 μM ACC (light gray) or control (dark gray).

Fig. 4.

Ethylene regulation of floral transition is EIN3-dependent. (A) Representative (30-day-old) ebf1–1 ebf2–1 mutant plants grown in LDs and treated with GA (+GA; Right) or control (Left). (B) Flowering time (time at which 50% of plants had bolted) of selected lines as indicated (± SD; n > 30) grown in soil in LDs in the presence (red) or absence (blue) of GA treatment. The asterisk represents plants that had not bolted by the end of the experiment (50 days). (C) Immunodetection of EIN3 in 2-week-old selected lines as indicated. ebf1–1 ebf2–1 plants were treated with GA (+GA) or not. The asterisk marks EIN3 at the expected molecular size. β-tubulin (β-TUB; Middle) and Ponceau red (Bottom) staining of the membrane after transfer serve as a sample-loading controls. (D) Mean vegetative rosette leaf number (±SD; n > 15) of WT Col and 35S:ERF1 plants grown on soil in SDs (8-h photoperiod), GA-treated (red) or control (blue). (E) Levels of ERF1 and floral meristem identity LFY and SOC1 gene transcripts (determined by RT-PCR) in SDs, soil-grown, GA-treated WT Col, and 35S:ERF1 plants (and controls). ELF4a transcripts provide loading control.

Fig. 5.

Model for integration of the ethylene and GA–DELLA signaling pathways in the regulation of floral transition. Activation of ethylene signaling reduces bioactive GA levels, thus promoting the accumulation of DELLAs. DELLA accumulation in turn slows the plant life cycle and delays flowering. Ethylene production activates ethylene signaling by inhibiting CTR1 and increasing EIN3 levels via the SCF^EBF1/EBF2 ubiquitin pathway. Accumulation of DELLAs delays floral transition (via regulation of LFY and SOC1 transcript levels) and increases the abundance of GA-biosynthesis gene transcripts via a negative feedback loop.