Distinct Dynamics of HISTONE3 Variants between the Two Fertilization Products in Plants

Abstract

Sexual reproduction involves epigenetic reprogramming comprising DNA methylation and histone modifications. In addition, dynamics of HISTONE3 (H3) variant H3.3 upon fertilization are conserved in animals, suggesting an essential role. In contrast to H3, H3.3 marks actively transcribed regions of the genome and can be deposited in a replication-independent manner. Although H3 variants are conserved in plants, their dynamics during fertilization have remained unexplored. We overcame technical limitations to live imaging of the fertilization process in Arabidopsis thaliana and studied dynamics of the male-gamete-specific H3.3 and the centromeric Histone Three Related 12 (HTR12). The double-fertilization process in plants produces the zygote and the embryo-nourishing endosperm. We show that the zygote is characterized by replication-independent removal of paternal H3.3 and homogeneous incorporation of parental chromatin complements. In the endosperm, the paternal H3.3 is passively diluted by replication while the paternal chromatin remains segregated apart from the maternal chromatin (gonomery). Hence epigenetic regulations distinguish the two products of fertilization in plants. H3.3-replication-independent dynamics and gonomery also mark the first zygotic divisions in animal species. We thus propose the convergent selection of parental epigenetic imbalance involving H3 variants in sexually reproducing organisms.

BiP-mediated polar nuclei fusion is essential for the regulation of endosperm nuclei proliferation in Arabidopsis thaliana

“…After fertilization of a b2 ovule (i.e., one with fused polar nuclei) with the wild-type pollen, both the zygote and the endosperm contained single nuclei, which were each labeled with both GFP and monomeric red fluorescent protein (mRFP) (Fig. 2 F-H) (18). Similar results were obtained when b1 b2 ovules producing histone H2B-GFP were fertilized with wild-type pollen producing HTR10-mRFP1.…”

“…The nfd1 mutant is defective in these nuclear fusion processes; sperm nuclei were found to persist within the egg and the central cell when nfd1/+ pistils were pollinated with wild-type pollen (6). To analyze sperm nuclear fusion in the bip mutants, we generated transgenic b1/+ b2/b2 plants producing histone H2B-GFP (17) in their female gametophytes and crossed with wild-type plants producing HTR10-monomeric red fluorescent protein 1 (mRFP1) (18) in their pollen. After fertilization of a b2 ovule (i.e., one with fused polar nuclei) with the wild-type pollen, both the zygote and the endosperm contained single nuclei, which were each labeled with both GFP and monomeric red fluorescent protein (mRFP) (Fig.…”

Embryo and Endosperm Inherit Distinct Chromatin and Transcriptional States from the Female Gametes in Arabidopsis

“…It has recently been shown that virtually the entire endosperm genome is demethylated at 8 DAP, in sharp contrast with the embryo genome (Gehring et al, 2009;Hsieh et al, 2009). Moreover, the paternal chromatin undergoes distinct remodeling in the zygote and the endosperm following fertilization (Ingouff et al, 2007), and imprinting (i.e., parent-of-origin-dependent, monoallelic expression) is more prevalent in the endosperm than in the embryo. With the exception of a limited number of imprinted loci, which are differentially methylated between the egg and central cells (Gutierrez-Marcos et al, 2006), or specifically demethylated in the central cell by DME (Choi et al, 2002;Gehring et al, 2006;Jullien et al, 2008), little is known about the epigenetic state of the two female gametes.…”

“…By contrast, the zygote was enriched in H3K9me2 signals with strong and well-defined foci (Figures 2A to 2C). The polarization observed in the primary endosperm nuclei is reminiscent of the nuclear distribution of a paternally transmitted centromeric H3 fused to green fluorescent protein (HTR12-GFP; Ingouff et al, 2007), suggesting that the H3K9me2 distribution pattern might reflect polarization of paternally inherited chromatin in endosperm nuclei. This polarization was only transient, and after the third cleavage, distinct foci were visible and colocalized with 4',6-diamidino-2-phenylindole (DAPI)-stained chromocenters in all endosperm nuclei, similar to the pattern seen in somatic nuclei, but distinct from the zygotic pattern at that stage (Figures 2C and 2D; n = 50 ovules).…”

Cis-Regulatory Elements Determine Germline Specificity and Expression Level of an Isopentenyltransferase Gene in Sperm Cells of Arabidopsis

“…DUO POLLEN 1 (DUO1) encodes a MYB transcription factor that is expressed specifically in generative cell and sperm cells and serves a key regulatory function for generative cell division and sperm cell differentiation in Arabidopsis (Rotman et al, 2005;Brownfield et al, 2009a;Borg et al, 2011), whereas DUO3 functions to activate expression of target germline genes of DUO1 and is required for cell cycle progression, sperm cell specification, fertilization, and embryogenesis (Brownfield et al, 2009b). Male gamete-expressed histone variants have also been identified from lily (Lilium longiflorum) and Arabidopsis (Ingouff et al, 2007;Okada et al, 2005aOkada et al, , 2005b. In Arabidopsis, AtMGH3/HTR10 encodes a variant histone H3 detected in the generative cell of late bicellular pollen and sperm cells of anthesis pollen.…”