Chloroplasts prevent precocious flowering through a GOLDEN2-LIKE–B-BOX DOMAIN PROTEIN module

The timing of flowering is tightly controlled by signals that integrate environmental and endogenous cues.Sugars produced by carbon fixation in the chloroplast are a crucial endogenous cue for floral initiation.Chloroplasts also convey information directly to the nucleus through retrograde signaling to control plant growth and development.Here,we show that mutants defective in chlorophyll biosynthesis and chloroplast development flowered early,especially under long-day conditions,although low sugar accumulation was seen in some mutants.Plants treated with the bleaching herbicide norflurazon also flowered early,suggesting that chloroplasts have a role in floral repression.Among retrograde signaling mutants,the golden2-like 1(glk1)glk2 double mutants showed early flowering under long-day conditions.This early flowering was completely suppressed by constans(co)and flowering locus t(ft)mutations.Leaf vascular-specific knockdown of both GLK1 and GLK2 phenocopied the glk1 glk2 mutants.GLK1 and GLK2 repress flowering by directly activating the expression of B-BOX DOMAIN PROTEIN 14(BBX14),BBX15,and BBX16 via CCAATC cis-elements in the BBX genes.BBX14/15/16 physically interact with CO in the nucleus,and expression of BBXs hampered CO-mediated FT transcription.Simultaneous knockdown of BBX14/15/16 by artificial miRNA(35S::amiR-BBX14/15/16)caused early flowering with increased FT transcript levels,whereas BBX overexpression caused late flowering.Flowering of glk1/2 and 35S::amiR-BBX14/15/16 plants was insensitive to norflurazon treatment.Taking these observations together,we propose that the GLK1/2-BBX14/15/16 module provides a novel mechanism explaining how the chloroplast represses flowering to balance plant growth and reproductive development.

Arabidopsis ABF3 and ABF4 Transcription Factors Act with the NF-YC Complex to Regulate SOC1 Expression and Mediate Drought-Accelerated Flowering

The drought-escape response accelerates flowering in response to drought stress, allowing plants to adaptively shorten their life cycles. Abscisic acid (ABA) mediates plant responses to drought, but the role of ABA-responsive element (ABRE)-binding factors (ABFs) in the drought-escape response is poorly understood. Here, we show that Arabidopsis thaliana ABF3 and ABF4 regulate flowering in response to drought through transcriptional regulation of the floral integrator SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (SOC1). The abf3 abf4 mutant displayed ABA-insensitive late flowering under long-day conditions. Ectopic expression of ABF3 or ABF4 in the vasculature, but not in the shoot apex, induced early flowering, whereas expression of ABF3 fused with the SRDX transcript!onal repressor domain delayed flowering. We identified SOC1 as a direct downstream target of ABF3/4, and found that SOC1 mRNA levels were lower in abf3 abf4 than in wild-type plants. Moreover, induction of SOC1 by ABA was hampered in abf3 abf4 mutants. ABF3 and ABF4 were enriched at the -1028- to -657-bp region of the SOC1 promoter, which does not contain canonical ABF-ABRE-binding motifs but has the NF-Y binding element. We found that ABF3 and ABF4 interact with nuclear factor Y subunit C (NF-YC) 3/4/9 in vitro and in planta, and induction of SOC1 by ABA was hampered in nf-yc3 yc4 yc9 mutants. Interestingly, the abf3 abf4, nf-yc3 yc4 yc9, and sod mutants displayed a reduced drought-escape response. Taken together, these results suggest that ABF3 and ABF4 act with NF-YCs to promote flowering by inducing SOC1 transcription under drought conditions . This mechanism might contribute to adaptation by enabling plants to complete their life cycles under drought stress.

FLOWERING LOCUS M isoforms differentially affect the subcellular localization and stability of SHORT VEGETATIVE PHASE to regulate temperature-responsive flowering in Arabidopsis

Temperature is an important environmental cue that affects flowering time in plants.The MADS-box transcription factor FLOWERING LOCUS M(FLM)forms a heterodimeric complex with SHORT VEGETATIVE PHASE(SVP)and controls ambient temperature-responsive flowering in Arabidopsis.FLM-βand FLM-δ,two major splice variants produced from the FLM locus,exert opposite effects on flowering,but the molecular mechanism by which the interaction between FLM isoforms and SVP affects temperature-responsive flowering remains poorly understood.Here,we show that FLM-βand FLM-δplay important roles in modulating the temperature-dependent behavior,conformation,and stability of SVP.Nuclear localization of SVP decreases as temperature increases.FLM-βis required for SVP nuclear translocation at low temperature,whereas SVP interacts with FLM-δmainly in the cytoplasm at high temperature.SVP preferentially binds to FLM-βat low temperature in tobacco leaf cells.SVP shows high binding affinity to FLM-βat low temperature and to FLM-δat high temperature.SVP undergoes similar structural changes in the interactions with FLM-βand FLM-δ;however,FLM-δlikely causes more pronounced conformational changes in the SVP structure.FLM-δcauses rapid degradation of SVP at high temperature,compared with FLM-β,possibly via ubiquitination.Mutation of lysine 53 or lysine 165 in SVP causes increased abundance of SVP due to reduced ubiquitination of SVP and thus delays flowering at high temperature.Our findings suggest that temperature-dependent differential interactions between SVP and FLM isoforms modulate the temperature-responsive induction of flowering in Arabidopsis.