The ASH1 HOMOLOG 2 (ASHH2) Histone H3 Methyltransferase Is Required for Ovule and Anther Development in Arabidopsis

Abstract

BackgroundSET-domain proteins are histone lysine (K) methyltransferases (HMTase) implicated in defining transcriptionally permissive or repressive chromatin. The Arabidopsis ASH1 HOMOLOG 2 (ASHH2) protein (also called SDG8, EFS and CCR1) has been suggested to methylate H3K4 and/or H3K36 and is similar to Drosophila ASH1, a positive maintainer of gene expression, and yeast Set2, a H3K36 HMTase. Mutation of the ASHH2 gene has pleiotropic developmental effects. Here we focus on the role of ASHH2 in plant reproduction.Methodology/Principal FindingsA slightly reduced transmission of the ashh2 allele in reciprocal crosses implied involvement in gametogenesis or gamete function. However, the main requirement of ASHH2 is sporophytic. On the female side, close to 80% of mature ovules lack embryo sac. On the male side, anthers frequently develop without pollen sacs or with specific defects in the tapetum layer, resulting in reduction in the number of functional pollen per anther by up to ∼90%. In consistence with the phenotypic findings, an ASHH2 promoter-reporter gene was expressed at the site of megaspore mother cell formation as well as tapetum layers and pollen. ashh2 mutations also result in homeotic changes in floral organ identity. Transcriptional profiling identified more than 300 up-regulated and 600 down-regulated genes in ashh2 mutant inflorescences, whereof the latter included genes involved in determination of floral organ identity, embryo sac and anther/pollen development. This was confirmed by real-time PCR. In the chromatin of such genes (AP1, AtDMC1 and MYB99) we observed a reduction of H3K36 trimethylation (me3), but not H3K4me3 or H3K36me2.Conclusions/SignificanceThe severe distortion of reproductive organ development in ashh2 mutants, argues that ASHH2 is required for the correct expression of genes essential to reproductive development. The reduction in the ashh2 mutant of H3K36me3 on down-regulated genes relevant to the observed defects, implicates ASHH2 in regulation of gene expression via H3K36 trimethylation in chromatin of Arabidopsis inflorescences.

Arabidopsis SET DOMAIN GROUP2Is Required for H3K4 Trimethylation and Is Crucial for Both Sporophyte and Gametophyte Development

“…sdg2 mutant flowers contain normal numbers of floral organs and a relatively normal morphology, except that the stamens remain short during later flower developmental stages (stage definition according to Smyth et al, 1990; Figure 1C). Homeotic conversion of floral organs, as had been previously reported for the atx1-1 (Alvarez-Venegas et al, 2003) and sdg8-1/ashh2-1 (Grini et al, 2009) mutants, was not observed in sdg2 mutants. In sdg2 mutants, the short stamens failed to reach the receptive stigmatic papillae of the pistil at anthesis for successful pollination.…”

“…By contrast, downregulation of AG, PI, AP2, and AP1 was shown in atx1-1, with flowers exhibiting homeotic conversions and variable aberrations (Alvarez-Venegas et al, 2003). sdg8-1/ashh2-1 has also been reported to display downregulation of PI, AP2, and AP1, with a low proportion of flowers exhibiting homeotic conversions (Grini et al, 2009). Furthermore, the sdg2 mutants differ in the flowering time phenotype from previously studied TrxG family gene mutants.…”

“…Five of the six characterized genes, SDG8/ASHH2/EFS/CCR1, SDG25/ ATXR7, SDG26/ASHH1, SDG27/ATX1, and SDG30/ATX2, are involved in flowering time regulation (Soppe et al, 1999;Kim et al, 2005;Zhao et al, 2005;Pien et al, 2008;Saleh et al, 2008;Xu et al, 2008;Berr et al, 2009;Tamada et al, 2009), whereas SDG4/ASHR3 is involved in late stages of pollen development (Cartagena et al, 2008;Thorstensen et al, 2008). In addition, ATX1 involved in H3K4 trimethylation (H3K4me3) is necessary for normal root, leaf, and floral organ growth (Alvarez-Venegas et al, 2003;Alvarez-Venegas and Avramova, 2005); and SDG8/ASHH2/EFS/CCR1, primarily involved in H3K36me2 and H3K36me3, is implicated in the regulation of organ size, shoot branching, fertility, and carotenoid composition (Soppe et al, 1999;Xu et al, 2008;Cazzonelli et al, 2009;Grini et al, 2009). In this study, we show that the previously uncharacterized TrxG family gene SDG2 (also named ATXR3) plays crucial roles in both sporophyte and gametophyte development.…”

Carotenoids in nature: insights from plants and beyond

“…The sdg8 mutant shows several pleiotropic phenotypes, such as early flowering, altered shoot and root architecture, altered pathogen resistance, male sterility, abnormal sporophyte and gametophyte development (Kim et al 2005;Cazzonelli et al 2009a;Grini et al 2009;Berr et al 2010). A few putative target genes responsible for these phenotypes have been described and target gene loci show a lower abundance of H3K4 or H3K36 methylation marks, as well as reduced mRNA levels in the sdg8 mutant.…”

Histone Lysine-to-Methionine Mutations Reduce Histone Methylation and Cause Developmental Pleiotropy

“…H3 Lys-36 trimethylation (H3K36me3) is a highly conserved histone mark and is primarily correlated with active gene expression in eukaryotes (Roudier et al, 2011;Wen et al, 2014). In Arabidopsis (Arabidopsis thaliana), H3K36me3 is enriched at the gene-body regions and plays an important role in regulating the expression of development-related genes involved in brassinosteroid signaling pathways, carbon and light responses, floral organ development, and flowering (Xu et al, 2008;Grini et al, 2009;He et al, 2011;Wang et al, 2014;Yang et al, 2014;Li et al, 2015). Deposition of H3K36 methylation is mediated by a collection of conserved SET domain-containing group (SDG) histone methyltransferases.…”