Histone acetylation is a major epigenetic control mechanism that is tightly linked to the promotion of gene expression. Histone acetylation levels are balanced through the opposing activities of histone acetyltransferases (HATs) and histone deacetylases (HDACs). Arabidopsis HDAC genes (AtHDACs) compose a large gene family, and distinct phenotypes among AtHDAC mutants reflect the functional specificity of individual AtHDACs. However, the mechanisms underlying this functional diversity are largely unknown. Here, we show that POWERDRESS (PWR), a SANT (SWI3/DAD2/N-CoR/TFIII-B) domain protein, interacts with HDA9 and promotes histone H3 deacetylation, possibly by facilitating HDA9 function at target regions. The developmental phenotypes of pwr and hda9 mutants were highly similar. Three lysine residues (K9, K14, and K27) of H3 retained hyperacetylation status in both pwr and hda9 mutants. Genome-wide H3K9 and H3K14 acetylation profiling revealed elevated acetylation at largely overlapping sets of target genes in the two mutants. Highly similar gene-expression profiles in the two mutants correlated with the histone H3 acetylation status in the pwr and hda9 mutants. In addition, PWR and HDA9 modulated flowering time by repressing AGAMOUS-LIKE 19 expression through histone H3 deacetylation in the same genetic pathway. Finally, PWR was shown to physically interact with HDA9, and its SANT2 domain, which is homologous to that of subunits in animal HDAC complexes, showed specific binding affinity to acetylated histone H3. We therefore propose that PWR acts as a subunit in a complex with HDA9 to result in lysine deacetylation of histone H3 at specific genomic targets.SANT domain | POWERDRESS | HDA9 | histone deacetylation | AGL19 P osttranslational modifications of histones-including acetylation, methylation, phosphorylation, and ubiquitinationplay important roles in plant development, genome integrity, and stress responses. Histone acetylation/deacetylation, a reversible process, promotes/represses gene expression (1) and occurs at lysine residues within histone N-terminal tails. The histone acetylation status is regulated by counteracting enzymes: histone acetyltransferases (HATs) and histone deacetylases (HDACs). The 18 HDACs identified in Arabidopsis (2) can be categorized into three groups based on phylogenetic analysis: reduced potassium dependency-3/histone deacetylase-1 (RPD3/HDA1), histone deacetylase-2 (HD2), and silent information regulator-2 (SIR2)-like (3). Twelve HDACs belong to the RPD3/HDA1 group (3) and are involved in various biological processes, such as organ development, reproductive processes, hormone signaling, and DNA methylation (4-9). They can be further classified into three classes based on sequence homology (3). The HD2 group is plant-specific and includes four HDACs that act in plant development and stress responses (10-13). The two HDACs encoded by the SIR2 family genes in Arabidopsis, SRT1 and SRT2, regulate mitochondrial energy metabolism and cellular dedifferentiation, respectively (14,15).I...
SUMMARYIn flowering plants, male gametes arise via meiosis of diploid pollen mother cells followed by two rounds of mitotic division. Haploid microspores undergo polar nuclear migration and asymmetric division at pollen mitosis I to segregate the male germline, followed by division of the germ cell to generate a pair of sperm cells. We previously reported two gemini pollen (gem) mutants that produced twin-celled pollen arising from polarity and cytokinesis defects at pollen mitosis I in Arabidopsis. Here, we report an independent mutant, gem3, with a similar division phenotype and severe genetic transmission defects through pollen. Cytological analyses revealed that gem3 disrupts cell division during male meiosis, at pollen mitosis I and during female gametophyte development. We show that gem3 is a hypomorphic allele (aug6-1) of AUGMIN subunit 6, encoding a conserved component in the augmin complex, which mediates microtubule (MT)-dependent MT nucleation in acentrosomal cells. We show that MT arrays are disturbed in gem3/aug6-1 during male meiosis and pollen mitosis I using fluorescent MT-markers. Our results demonstrate a broad role for the augmin complex in MT organization during sexual reproduction, and highlight gem3/aug6-1 mutants as a valuable tool for the investigation of augmin-dependent MT nucleation and dynamics in plant cells.
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