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...

Successful floral meristem (FM) determinacy is critical for subsequent reproductive development and the plant life cycle. Although the phytohormones cytokinin and auxin interact to coregulate many aspects of plant development, whether and how cytokinin and auxin function in FM determinacy remain unclear. Here, we show that in , cytokinin homeostasis is critical for FM determinacy. In this developmental context, auxin promotes the expression of () to repress cytokinin activity. ARF3 directly represses the expression of () family genes and indirectly represses () family genes, both of which encode enzymes required for cytokinin biosynthesis. ARF3 also directly inhibits the expression of , a cytokinin receptor gene, resulting in reduced cytokinin activity. Consequently, ARF3 controls cell division by regulating cell cycle gene expression through cytokinin. In flowers, we show that AGAMOUS (AG) dynamically regulates the expression of and s, resulting in coordinated regulation of FM maintenance and termination through cell division. Moreover, genome-wide transcriptional profiling revealed both repressive and active roles for ARF3 in early flower development. Our findings establish a molecular link between AG and auxin/cytokinin and shed light on the mechanisms of stem cell maintenance and termination in the FM.

Termination of the stem cells in the floral meristem (also known as floral determinacy) is critical for the reproductive success of plants, and the molecular activities regulating floral determinacy are precisely orchestrated during the course of floral development. In Arabidopsis thaliana, regulators of floral determinacy include several transcription factor genes, such as APETALA2 (AP2), AGAMOUS (AG), SUPERMAN (SUP), and CRABSCLAW (CRC), as well as a microRNA (miRNA), miR172, which targets AP2. How the transcription factor and miRNA genes are coordinately regulated to achieve floral determinacy is unknown. A mutation in POWERDRESS (PWR), a previously uncharacterized gene encoding a SANT-domain-containing protein, was isolated in this study as an enhancer of the weakly indeterminate ag-10 allele. PWR was found to promote the transcription of CRC, MIR172a, b, and c and/or enhance Pol II occupancy at their promoters, without affecting MIR172d or e. A mutation in mature miR172d was additionally found to enhance the determinacy defects of ag-10 in an AP2-dependent manner, providing direct evidence that miR172d is functional in repressing AP2 and thereby contributes to floral determinacy. Thus, while PWR promotes floral determinacy by enhancing the expression of three of the five MIR172 members as well as CRC, MIR172d, whose expression is PWR-independent, also functions in floral stem cell termination. Taken together, these findings demonstrate how transcriptional diversification and functional redundancy of a miRNA family along with PWR-mediated co-regulation of miRNA and transcription factor genes contribute to the robustness of the floral determinacy network.