SlERF.F12 modulates the transition to ripening in tomato fruit by recruiting the co-repressor TOPLESS and histone deacetylases to repress key ripening genes.

Ethylene response factors (ERFs) are downstream components of ethylene-signaling pathways known to play critical roles in ethylene-controlled climacteric fruit ripening, yet little is known about the molecular mechanism underlying their mode of action. Here, we demonstrate that SlERF.F12, a member of the ERF.F subfamily containing Ethylene-responsive element-binding factor-associated Amphiphilic Repression (EAR) motifs, negatively regulates the onset of tomato (Solanum lycopersicum) fruit ripening by recruiting the co-repressor TOPLESS 2 (TPL2) and the histone deacetylases (HDAs) HDA1/HDA3 to repress the transcription of ripening-related genes. The SlERF.F12-mediated transcriptional repression of key ripening-related genes 1-AMINO-CYCLOPROPANE-1-CARBOXYLATE SYNTHASE 2 (ACS2), ACS4, POLYGALACTURONASE 2a, and PECTATE LYASE is dependent on the presence of its C-terminal EAR motif. We show that SlERF.F12 interacts with the co-repressor TPL2 via the C-terminal EAR motif and recruits HDAs SlHDA1 and SlHDA3 to form a tripartite complex in vivo that actively represses transcription of ripening genes by decreasing the level of the permissive histone acetylation marks H3K9Ac and H3K27Ac at their promoter regions. These findings provide new insights into the ripening regulatory network and uncover a direct link between repressor ERFs and histone modifiers in modulating the transition to ripening of climacteric fruit.

Ethylene-MPK8-ERF.C1-PR module confers resistance against Botrytis cinerea in tomato fruit without compromising ripening.

The plant hormone ethylene plays a critical role in fruit defense against Botrytis cinerea attack, but the underlying mechanisms remain poorly understood. Here, we showed that ethylene response factor SlERF.C1 acts as a key regulator to trigger the ethylene-mediated defense against B. cinerea in tomato fruits without compromising ripening. Knockout of SlERF.C1 increased fruit susceptibility to B. cinerea with no effect on ripening process, while overexpression enhanced resistance. RNA-Seq, transactivation assays, EMSA and ChIP-qPCR results indicated that SlERF.C1 activated the transcription of PR genes by binding to their promoters. Moreover, SlERF.C1 interacted with the mitogen-activated protein kinase SlMPK8 which allowed SlMPK8 to phosphorylate SlERF.C1 at the Ser174 residue and increases its transcriptional activity. Knocking out of SlMPK8 increased fruit susceptibility to B. cinerea, whereas overexpression enhanced resistance without affecting ripening. Furthermore, genetic crosses between SlMPK8-KO and SlERF.C1-OE lines reduced the resistance to B. cinerea attack in SlERF.C1-OE fruits. In addition, B. cinerea infection induced ethylene production which in turn triggered SlMPK8 transcription and enhanced the phosphorylation of SlERF.C1. Overall, our findings reveal the regulatory mechanism of the 'Ethylene-MPK8-ERF.C1-PR' module in resistance against B. cinerea and provide new insight into the manipulation of gray mold disease in fruits.

Regulation of Heat Shock Factor Pathways by γ-aminobutyric Acid (GABA) Associated with Thermotolerance of Creeping Bentgrass.

Activation and enhancement of heat shock factor (HSF) pathways are important adaptive responses to heat stress in plants. The γ-aminobutyric acid (GABA) plays an important role in regulating heat tolerance, but it is unclear whether GABA-induced thermotolerance is associated with activation of HSF pathways in plants. In this study, the changes of endogenous GABA level affecting physiological responses and genes involved in HSF pathways were investigated in creeping bentgrass during heat stress. The increase in endogenous GABA content induced by exogenous application of GABA effectively alleviated heat damage, as reflected by higher leaf relative water content, cell membrane stability, photosynthesis, and lower oxidative damage. Contrarily, the inhibition of GABA accumulation by the application of GABA biosynthesis inhibitor further aggravated heat damage. Transcriptional analyses showed that exogenous GABA could significantly upregulate transcript levels of genes encoding heat shock factor HSFs (HSFA-6a, HSFA-2c, and HSFB-2b), heat shock proteins (HSP17.8, HSP26.7, HSP70, and HSP90.1-b1), and ascorbate peroxidase 3 (APX3), whereas the inhibition of GABA biosynthesis depressed these genes expression under heat stress. Our results indicate GABA regulates thermotolerance associated with activation and enhancement of HSF pathways in creeping bentgrass.