Epigenetic Regulation of Phase Transitions in Arabidopsis thaliana

Transition from Seeds to Seedlings: Hormonal and Epigenetic Aspects

Transition from seed to seedling is one of the critical developmental steps, dramatically affecting plant growth and viability. Before plants enter the vegetative phase of their ontogenesis, massive rearrangements of signaling pathways and switching of gene expression programs are required. This results in suppression of the genes controlling seed maturation and activation of those involved in regulation of vegetative growth. At the level of hormonal regulation, these events are controlled by the balance of abscisic acid and gibberellins, although ethylene, auxins, brassinosteroids, cytokinins, and jasmonates are also involved. The key players include the members of the LAFL network-the transcription factors LEAFY COTYLEDON1 and 2 (LEC 1 and 2), ABSCISIC ACID INSENSITIVE3 (ABI3), and FUSCA3 (FUS3), as well as DELAY OF GERMINATION1 (DOG1). They are the negative regulators of seed germination and need to be suppressed before seedling development can be initiated. This repressive signal is mediated by chromatin remodeling complexes-POLYCOMB REPRESSIVE COMPLEX 1 and 2 (PRC1 and PRC2), as well as PICKLE (PKL) and PICKLE-RELATED2 (PKR2) proteins. Finally, epigenetic methylation of cytosine residues in DNA, histone post-translational modifications, and post-transcriptional downregulation of seed maturation genes with miRNA are discussed. Here, we summarize recent updates in the study of hormonal and epigenetic switches involved in regulation of the transition from seed germination to the post-germination stage.

Epigenomic Regulatory Mechanism in Vegetative Phase Transition of Malus hupehensis

In woody plants, phase transitions substantially affect growth and development. Although there has been considerable interest in the regulatory mechanisms underlying phase changes, the associated epigenetic modifications remain relatively uncharacterized. We examined the DNA methylation changes and the transcriptional responses in adult and juvenile Malus hupehensis leaves. The DNA methylations were 66.61% and 68.3% in the CG context, 49.12% and 52.44% in the CHG context, and 7.02% and 8.22% in the CHH context for the adult and juvenile leaves, respectively. The number of differentially methylated regions in all contexts distributed in the genic regions varied. Additionally, inhibited DNA methylation in adult leaves activated the transcription of indole-3-acetic acid related genes in the signaling, response, and transport pathways. Moreover, the opposite methylation and expression patterns were observed for the SPL and AP2 family genes between the adult and juvenile leaves. Both gene families contribute to the M. hupehensis vegetative phase transition. Furthermore, the hyper-/hypomethylation of the gene body or promoter of transcription factor genes may lead to up-/downregulated gene expression. The methylation levels of the WRKY (22), NAC (21), ERF (8), WOX (2), KNAT (6), EIN3 (2), SCL (7), ZAT (7), and HSF (4) genes were higher in the adult leaves than in the juvenile leaves, whereas the opposite pattern was observed for the TCP (2), MADS-box (11), and DOF (3) genes. An analysis of the correlation between methylation and transcription indicated the methylation of the gene body in all contexts and the methylation of the promoter in the CG and CHG contexts are negatively correlated with gene expression. However, the methylation of the promoter in the CHH context is positively correlated with gene expression. These findings reflect the diversity in the epigenetic regulation of gene expression and may be useful for elucidating the epigenetic regulatory mechanism underlying the M. hupehensis vegetative phase transition.

Who Rules the Cell? An Epi-Tale of Histone, DNA, RNA, and the Metabolic Deep State

Epigenetics refers to the mode of inheritance independent of mutational changes in the DNA. Early evidence has revealed methylation, acetylation, and phosphorylation of histones, as well as methylation of DNA as part of the underlying mechanisms. The recent awareness that many human diseases have in fact an epigenetic basis, due to unbalanced diets, has led to a resurgence of interest in how epigenetics might be connected with, or even controlled by, metabolism. The Next-Generation genomic technologies have now unleashed torrents of results exposing a wondrous array of metabolites that are covalently attached to selective sites on histones, DNA and RNA. Metabolites are often cofactors or targets of chromatin-modifying enzymes. Many metabolites themselves can be acetylated or methylated. This indicates that the acetylome and methylome can actually be deep and pervasive networks to ensure the nuclear activities are coordinated with the metabolic status of the cell. The discovery of novel histone marks also raises the question on the types of pathways by which their corresponding metabolites are replenished, how they are corralled to the specific histone residues and how they are recognized. Further, atypical cytosines and uracil have also been found in eukaryotic genomes. Although these new and extensive connections between metabolism and epigenetics have been established mostly in animal models, parallels must exist in plants, inasmuch as many of the basic components of chromatin and its modifying enzymes are conserved. Plants are chemical factories constantly responding to stress. Plants, therefore, should lend themselves readily for identifying new endogenous metabolites that are also modulators of nuclear activities in adapting to stress.

Molecular and epigenetic regulations and functions of the LAFL transcriptional regulators that control seed development

The LAFL (i.e. LEC1, ABI3, FUS3, and LEC2) master transcriptional regulators interact to form different complexes that induce embryo development and maturation, and inhibit seed germination and vegetative growth in Arabidopsis. Orthologous genes involved in similar regulatory processes have been described in various angiosperms including important crop species. Consistent with a prominent role of the LAFL regulators in triggering and maintaining embryonic cell fate, their expression appears finely tuned in different tissues during seed development and tightly repressed in vegetative tissues by a surprisingly high number of genetic and epigenetic factors. Partial functional redundancies and intricate feedback regulations of the LAFL have hampered the elucidation of the underpinning molecular mechanisms. Nevertheless, genetic, genomic, cellular, molecular, and biochemical analyses implemented during the last years have greatly improved our knowledge of the LALF network. Here we summarize and discuss recent progress, together with current issues required to gain a comprehensive insight into the network, including the emerging function of LEC1 and possibly LEC2 as pioneer transcription factors.

BABA-Primed Histone Modifications in Potato for Intergenerational Resistance to Phytophthora infestans

In this paper we analyzed β-aminobutyric acid (BABA)-primed epigenetic adjustment of potato cv. "Sarpo Mira" to Phytophthora infestans. The first stress-free generation of the potato genotype obtained from BABA-primed parent plants via tubers and seeds showed pronounced resistance to the pathogen, which was tuned with the transcriptional memory of SA-responsive genes. During the early priming phase before the triggering stress, we found robust bistable deposition of histone marks (H3K4me2 and H3K27me3) on the NPR1 (Non-expressor of PR genes) and the SNI1 gene (Suppressor of NPR1, Inducible), in which transcription antagonized silencing. Switchable chromatin states of these adverse systemic acquired resistance (SAR) regulators probably reprogrammed responsiveness of the PR1 and PR2 genes and contributed to stress imprinting. The elevated levels of heritable H3K4me2 tag in the absence of transcription on SA-dependent genes in BABA-primed (F0) and its vegetative and generative progeny (F1) before pathogen challenge provided evidence for the epigenetic mark for intergenerational memory in potato. Moreover, our study revealed that histone acetylation was not critical for maintaining BABA-primed defense information until the plants were triggered with the virulent pathogen when rapid and boosted PRs gene expression probably required histone acetyltransferase (HAT) activity both in F0 and F1 progeny.