In plants, genomic DNA methylation which contributes to development and stress responses can be actively removed by DEMETER-like DNA demethylases (DMLs). Indeed, in Arabidopsis DMLs are important for maternal imprinting and endosperm demethylation, but only a few studies demonstrate the developmental roles of active DNA demethylation conclusively in this plant. Here, we show a direct cause and effect relationship between active DNA demethylation mainly mediated by the tomato DML, SlDML2, and fruit ripeningan important developmental process unique to plants. RNAi SlDML2 knockdown results in ripening inhibition via hypermethylation and repression of the expression of genes encoding ripening transcription factors and rate-limiting enzymes of key biochemical processes such as carotenoid synthesis. Our data demonstrate that active DNA demethylation is central to the control of ripening in tomato.active DNA demethylation | DNA glycosylase lyase | epigenetic | tomato | fruit ripening G enomic DNA methylation is a major epigenetic mark that is instrumental to many aspects of chromatin function, including gene expression, transposon silencing, or DNA recombination (1-4). In plants, DNA methylation can occur at cytosine both in symmetrical (CG or CHG) and nonsymmetrical (CHH) contexts and is controlled by three classes of DNA methyltransferases, namely, the DNA Methyltransferase 1, Chromomethylases, and the Domain Rearranged Methyltransferases (5-7). Indeed, in all organisms, cytosine methylation can be passively lost after DNA replication in the absence of methyltransferase activity (1). However, plants can also actively demethylate DNA via the action of DNA GlycosylaseLyases, the so-called DEMETER-Like DNA demethylases (DMLs), that remove methylated cytosine, which is then replaced by a nonmethylated cytosine (8

As one of the most promising nonfullerene acceptors for organic photovoltaics, perylene diimide (PDI)-based multibranched molecules with twisted or three-dimensional (3D) geometric structures have been developed, which effectively increase the power conversion efficiency (PCE) of organic solar cells. Understanding the structure–property relationships in multichromophoric molecular architectures at molecular and ultrafast time levels is a crucial step in establishing new design principles in organic electronic materials. For this, photodriven excited-state symmetry-breaking charge separation (SB-CS) of PDI-based multichromophoric acceptors has been proposed to improve the PCE by reducing the self-aggregation of the planar PDI monomer. Herein, we investigated the intramolecular excited-state SB-CS and charge recombination (CR) dynamics of two symmetric phenyl-methane-based PDI derivatives, a twist dimer PM-PDI2 (phenyl-methane-based PDI dimer) and a 3D configuration tetramer PM-PDI4 (phenyl-methane-based PDI tetramer), in different solvents using ultrafast femtosecond transient absorption (fs-TA) spectroscopy and quantum chemical calculations. The quantum chemical calculations and steady-state spectra show that the two PDI derivatives undergo conformational changes upon excitation, leading to their emission states that have the characteristics of partial charge-transfer (CT) exciton in all solvents. Based on the evolution of the fs-TA data, it is observed that the evolution from the CT state to SB-CS state is disfavored in a weak polar solvent, whereas clear SB-CS spectroscopic signatures of cationic and anionic PDI are observed in polar solvents. Faster CS and slower CR processes of PM-PDI4 are observed in comparison to those of PM-PDI2. The crowded space in the 3D structure shortens the distance between the branches, leading to a stronger electronic coupling at the lowest excited state and a larger negative Gibbs free energy change of PM-PDI4 relative to that of PM-PDI2, which benefits the charge separation among PDI units in PM-PDI4. Besides, the 3D structure of PM-PDI4 also restricts rotation to a surface crossing region between the excited state and ground state, thus inhibiting nonradiative CR process and increasing the CS state lifetime. Our results suggest that the kinetics of CS and CR processes are strongly related to the molecular geometric structure, and the excited-state symmetry breaking in the 3D structure acceptor has superior photogenerated charge and photovoltaic properties from the perspective of ultrafast dynamics.

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Naturally-occurring epimutants are rare and have mainly been described in plants. However how these mutants maintain their epigenetic marks and how they are inherited remain unknown. Here we report that CHROMOMETHYLASE3 (SlCMT3) and other methyltransferases are required for maintenance of a spontaneous epimutation and its cognate Colourless non-ripening (Cnr) phenotype in tomato. We screened a series of DNA methylation-related genes that could rescue the hypermethylated Cnr mutant. Silencing of the developmentally-regulated SlCMT3 gene results in increased expression of LeSPL-CNR, the gene encodes the SBP-box transcription factor residing at the Cnr locus and triggers Cnr fruits to ripen normally. Expression of other key ripening-genes was also up-regulated. Targeted and whole-genome bisulfite sequencing showed that the induced ripening of Cnr fruits is associated with reduction of methylation at CHG sites in a 286-bp region of the LeSPL-CNR promoter, and a decrease of DNA methylation in differentially-methylated regions associated with the LeMADS-RIN binding sites. Our results indicate that there is likely a concerted effect of different methyltransferases at the Cnr locus and the plant-specific SlCMT3 is essential for sustaining Cnr epi-allele. Maintenance of DNA methylation dynamics is critical for the somatic stability of Cnr epimutation and for the inheritance of tomato non-ripening phenotype.

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