Abstract
ATBS1-INTERACTING FACTOR 2 (AIF2) is a non-DNA-binding basic helix–loop–helix (bHLH) transcription factor. We demonstrated that AIF2 retards dark-triggered and brassinosteroid (BR)-induced leaf senescence in Arabidopsis thaliana. Dark-triggered BR synthesis and the subsequent activation of BRASSINAZOLE RESISTANT 1 (BZR1), a BR signaling positive regulator, result in BZR1 binding to the AIF2 promoter in a dark-dependent manner, reducing AIF2 transcript levels and accelerating senescence. BR-induced down-regulation of AIF2 protein stability partly contributes to the progression of dark-induced leaf senescence. Furthermore, AIF2 interacts with INDUCER OF CBF EXPRESSION 1 (ICE1) via their C-termini. Formation of the AIF2–ICE1 complex and subsequent up-regulation of C-REPEAT BINDING FACTORs (CBFs) negatively regulates dark-triggered, BR-induced leaf senescence. This involves antagonistic down-regulation of PHYTOCHROME INTERACTING FACTOR 4 (PIF4), modulated through AIF2-dependent inhibition of ICE1’s binding to the promoter. PIF4-dependent activities respond to dark-induced early senescence and may promote BR synthesis and BZR1 activation to suppress AIF2 and accelerate dark-induced senescence. Taken together, these findings suggest a coordination of AIF2 and ICE1 functions in maintaining stay-green traits.
Highlights
Aging or leaf senescence is the final stage of leaf development
CHLOROPHYLL A/B BINDING PROTEIN 1 (CAB1) and RIBULOSE BISPHOSPHATE CARBOXYLASE SMALL CHAIN 1A (RBCS1A), two genes involved in active photosynthesis, were relatively highly expressed in AIF2-overexpressing p35S::AIF2-EGFP transgenic plants (AIF2ox) compared with those in Col-0 and aif2-1 plants (Fig. 1G).The similar retardation of age-dependent senescence was observed in AIF2overexpressing leaves
INDUCER OF C-REPEAT BINDING FACTORs (CBFs) EXPRESSION 1 (ICE1) is a MYC-like basic helix–loop–helix (bHLH) transcription factor that binds to MYC-recognition motifs (Chinnusamy et al, 2003; Kim et al, 2015).We showed that ICE1 directly binds to CBF2 and PHYTOCHROME INTERACTING FACTOR 4 (PIF4) in an ATBS1-INTERACTING FACTOR 2 (AIF2)-dependent manner and negatively controls the progression of dark-induced leaf senescence
Summary
Aging or leaf senescence is the final stage of leaf development. Nutrients and energy from leaves are redirected to new and developing tissues or storage organs. Many developmental and environmental signals such as age, drought, lack of nutrients, oxidative and temperature stresses, and pathogen attack influence the onset of leaf senescence in plants (Woo et al, 2013). Hormones are endogenous factors regulating the onset and progression of leaf senescence, 1476 | Kim et al. PHYTOCHROME-INTERACTING FACTORS (PIFs) orchestrate dark-induced senescence (DIS) by targeting chloroplast maintenance, chlorophyll metabolism, hormone signaling, and expression of senescence master regulators, suggesting potential molecular links to the energy deprivation signaling pathway (Liebsch and Keech, 2016). PIF3, PIF4, and PIF5 promote age-triggered and dark-induced leaf senescence (Song et al, 2014). PIF4 negatively regulates chloroplast activity and decreases dark-induced ethylene biosynthesis and ethylene-induced leaf senescence
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