Stylianos Poulios

GENERAL CONTROL NON-DEREPRESSIBLE 5 (GCN5) is a histone acetyltransferase that plays a central role in regulating plant development and responses to environmental stimuli. In root development, GCN5 promotes the maintenance of the stem cell niche and proliferation of transit-amplifying cells. SPATULA (SPT) is a basic helix-loop-helix (bHLH) transcription factor with established roles in gynoecium and carpel development, as well as broader functions in plant growth. In roots, SPT negatively regulates quiescent center size, root meristem size, and overall root growth, without substantially affecting root patterning or cell-type differentiation. To investigate a potential genetic interaction between SPT and GCN5 in root meristem development, we generated and characterized spt;gcn5 double mutants. Notably, loss of SPT partially suppressed some of the developmental defects associated with gcn5, including abnormalities in root meristem size, meristem organization, and root growth. These data suggest that a complicated genetic interaction between SPT and GCN5 regulates important aspects of root meristem growth and organization.

GENERAL CONTROL NON-DEREPRESSIBLE 5 (GCN5) is a histone acetyltransferase that plays a central role in regulating plant development and responses to environmental stimuli. In root development, GCN5 promotes the maintenance of the stem cell niche and proliferation of transit-amplifying cells. SPATULA (SPT) is a basic helix-loop-helix (bHLH) transcription factor with established roles in gynoecium and carpel development, as well as broader functions in plant growth. In roots, SPT negatively regulates quiescent center size, root meristem size, and overall root growth, without substantially affecting root patterning or cell-type differentiation. To investigate a potential genetic interaction between SPT and GCN5 in root meristem development, we generated and characterized spt;gcn5 double mutants. Notably, loss of SPT substantially suppressed some developmental defects associated with gcn5 mutations, including abnormalities in root meristem size, meristem organization, and root growth. These data suggest that a complex genetic interaction between SPT and GCN5 regulates important aspects of root meristem growth and organization.

Sterol biosynthesis underlies significant physiological functions in plants, including the production of membrane structural sterols and hormones such as brassinosteroids and cytokinins. Inhibition of sterol biosynthesis has been shown to disrupt multiple aspects of Arabidopsis thaliana development. Here, the effects of lovastatin, an inhibitor of HMG-CoA reductase, on root development were investigated, focusing on auxin–cytokinin distribution and transport. Lovastatin inhibited primary root growth, especially cell elongation, in a dose-dependent manner. Additionally, lateral root density was considerably increased and lateral root primordia (LRP) emerged ectopically. In accordance to the above defects, auxin/cytokinin imbalance was recorded by the ectopic presence of the synthetic auxin marker DR5 and a significant decrease of cytokinins, as revealed by depletion of the TCS (two-component signaling) marker. Because auxin distribution appeared disturbed, auxin transport impairment was further examined. Plasma membrane localization of PIN auxin efflux carriers declined significantly, showing additional diffuse cytoplasmic localization in LRP cells. However, the cell-specific localization patterns of several PINs and their abundance at the transcript and protein level appeared unaffected or slightly increased. Fluorescence recovery after photobleaching (FRAP) analysis regarding membrane kinetics of PIN2 revealed altered PIN2 membrane dynamics and transmission electron microscopy (TEM) observations showed structural defects at the plasma membrane-cell wall interface. Together, these results support that sterol biosynthesis is essential for maintaining plasma membrane organization, which, in turn, is key factor for the distribution of hormones that control root development. Highlights: Lovastatin treatment inhibits root growth and causes deregulated formation of lateral roots. Consistently, lovastatin causes altered patterns of auxin distribution relevant to PIN protein mis-localization and decreases cytokinin levels. These changes could be attributed to reduced structural sterols as exemplified from alteration in PIN2 membrane dynamics.

Roots are usually underground plant organs, responsible for anchoring to the soil, absorbing water and nutrients, and interacting with the rhizosphere. During root development, roots respond to a variety of environmental signals, contributing to plant survival. Histone post-translational modifications play essential roles in gene expression regulation, contributing to plant responses to environmental cues. Histone acetylation is one of the most studied post-translational modifications, regulating numerous genes involved in various biological processes, including development and stress responses. Although the effect of histone acetylation on plant responses to biotic and abiotic stimuli has been extensively reviewed, no recent reviews exist focusing on root development regulation by histone acetylation. Therefore, this review brings together all the knowledge about the impact of histone acetylation on root development in several plant species, mainly focusing on Arabidopsis thaliana. Here, we summarize the role of histone acetylation and deacetylation in numerous aspects of root development, such as stem cell niche maintenance, cell division, expansion and differentiation, and developmental zone determination. We also emphasize the gaps in current knowledge and propose new perspectives for research toward deeply understanding the role of histone acetylation in root development.

Histone acetyltransferases (HATs) modify the amino-terminal tails of the core histone proteins via acetylation, regulating chromatin structure and transcription. GENERAL CONTROL NON-DEREPRESSIBLE 5 (GCN5) is a HAT that specifically acetylates H3K14 residues. GCN5 has been associated with cell division and differentiation, meristem function, root, stem, foliar, and floral development, and plant environmental response. The flowers of gcn5 plants display a reduced stamen length and exhibit male sterility relative to the wild-type plants. We show that these effects may arise from gibberellin (GA)-signaling defects. The signaling pathway of bioactive GAs depends on the proteolysis of their repressors, DELLA proteins. The repressor GA (RGA) DELLA protein represses plant growth, inflorescence, and flower and seed development. Our molecular data indicate that GCN5 is required for the activation and H3K14 acetylation of genes involved in the late stages of GA biosynthesis and catabolism. We studied the genetic interaction of the RGA and GCN5; the RGA can partially suppress GCN5 action during the whole plant life cycle. The reduced elongation of the stamen filament of gcn5–6 mutants is reversed in the rga–t2;gcn5–6 double mutants. RGAs suppress the GCN5 effect on the gene expression and histone acetylation of GA catabolism and GA signaling. Interestingly, the RGA and RGL2 do not suppress ADA2b function, suggesting that ADA2b acts downstream of GA signaling and is distinct from GCN5 activity. In conclusion, we propose that the action of GCN5 on stamen elongation is partially mediated by RGA and GA signaling.

Lavender essential oil (EO) is widely used for medicinal purposes. The significant monoterpenes’ abundance of linalool and linalool acetate accounts for more than 50% of lavender EO compounds. Monoterpenes synthesis differs throughout plant development as a result of the differential gene expression patterns in distinct cell types. Previously, we have reported that the chemical composition of Lavandula angustifolia cv. etherio EO was affected by diurnal harvest time. The aim of this was to evaluate if the gene expression of lavender monoterpenes synthases is altered during the day length and correlated with the accumulation of the major components of lavender EO. The relative expression of linalool synthase (LaLINS), limonene synthase (LaLIMS) and terpene synthase-like (LaTPS-l) was recorded in flowers at the 3rd to 5th stage every 3 h during two consecutive days using quantitative real-time PCR. The composition of the lavender EO was also monitored during the day length using GC-MS analysis. Our results indicate that the expression of genes involved in the synthesis of lavender EO, including linalool and limonene synthases, is accompanied by oscillations, picking at mid-day and leading to linalool acetate accumulation in the afternoon. In conclusion, the monoterpenes synthase expression in lavender flowers is rhythmically affected during the day, leading to a higher accumulation of EO compounds in the afternoon. These results will be helpful to monitor the biosynthesis of lavender EO to ensure a high-quality product. Furthermore, the outcome of this study will be useful for breeding programs in the lavender field to modulate the biosynthesis of linalool and linalool acetate during the flowering harvest period.

General Control Non-Derepressible 5 (GCN5) is a histone acetyltransferase that targets multiple genes and is essential for the acetylation of Lysine residues in the N-terminal tail of histone H3 in Arabidopsis. GCN5 interacts with the transcriptional coactivator Alteration/Deficiency in Activation 2b (ADA2b), which enhances its activity functioning in multiprotein complexes, such as the Spt-Ada-Gcn5-Acetyltransferase complex (SAGA). Mutations in GCN5 and ADA2b result in pleiotropic phenotypes, including alterations in the growth of roots. Auxin is known to regulate root development by modulating gene expression patterns. Auxin moves polarly during plant growth via the Pin-formed (PIN) auxin efflux transport proteins. The effect of GCN5 and ADA2b on auxin distribution at different stages of early root growth (4 to 7 days post-germination) was studied using the reporter lines DR5rev::GFP and PIN1::PIN1-GFP. In wild-type plants, auxin efflux transporter PIN1 expression increases from the fourth to the seventh day of root growth. The PIN1 expression was reduced in the roots of gcn5-1 and ada2b-1 compared to the wild type. The expression of PIN1 in ada2b-1 mutants is confined only to the meristematic zone, specifically in the stele cells, whereas it is almost abolished in the elongation zone. Gene expression analysis showed that genes associated with auxin transport, PIN1, PIN3 and PIN4, are downregulated in gcn5-1 and ada2b-1 mutants relative to the wild type. As a result, auxin accumulation was also reduced in gcn5-1 and ada2b-1 compared to wild-type roots. Furthermore, acetylation of Lysine 14 of histone H3 (H3K14) was also affected in the promoter and coding region of PIN1, PIN3 and PIN4 genes during root growth of Arabidopsis in gcn5 mutants. In conclusion, GCN5 acts as a positive regulator of auxin distribution in early root growth by modulating histone H3 acetylation and the expression of auxin efflux transport genes.

In Arabidopsis thaliana, the histone acetyltransferase GCN5 and the associated coactivator ADA2b regulate root growth and affect gene expression. The cytokinin signaling reporter TCS::GFP was introduced into gcn5-1, ada2b-1, and ada2a-2, as well as the ada2a-2ada2b-1 mutants. The early root growth (4 to 7 days post-germination) was analyzed using cellular and molecular approaches. TCS signal accumulated from the fourth to seventh days of root growth in the wild-type columella cells. In contrast, ada2b-1 and gcn5-1 and ada2a-2ada2b-1 double mutants displayed reduced TCS expression relative to wild type. Gene expression analysis showed that genes associated with cytokinin homeostasis were downregulated in the roots of gcn5-1 and ada2b-1 mutants compared to wild-type plants. H3K14 acetylation was affected in the promoters of cytokinin synthesis and catabolism genes during root growth of Arabidopsis. Therefore, GCN5 and ADA2b are positive regulators of cytokinin signaling during root growth by modulating histone acetylation and the expression of genes involved in cytokinin synthesis and catabolism. Auxin application in the roots of wild-type seedlings increased TCS::GFP expression. In contrast, ada2b and ada2ada2b mutant plants do not show the auxin-induced TCS signal, suggesting that GCN5 and ADA2b are required for the auxin-induced cytokinin signaling in early root growth.

Histone acetylation is directly related to gene expression. In yeast, the acetyltransferase general control nonderepressible-5 (GCN5) targets histone H3 and associates with transcriptional co-activators alteration/deficiency in activation-2 (ADA2) and alteration/deficiency in activation-3 (ADA3) in complexes like SAGA. Arabidopsis thaliana has two genes encoding proteins, designated ADA3a and ADA3b, that correspond to yeast ADA3. We investigated the role of ADA3a and ADA3b in regulating gene expression during flowering time. Specifically, we found that knock out mutants ada3a-2 and the double mutant ada3a-2 ada3b-2 lead to early flowering compared to the wild type plants under long day (LD) conditions and after moving plants from short days to LD. Consistent with ADA3a being a repressor of floral initiation, FLOWERING LOCUS T (FT) expression was increased in ada3a mutants. In contrast, other genes involved in multiple pathways leading to floral transition, including FT repressors, players in GA signaling, and members of the SPL transcriptional factors, displayed reduced expression. Chromatin immunoprecipitation analysis revealed that ADA3a affects the histone H3K14 acetylation levels in SPL3, SPL5, RGA, GAI, and SMZ loci. In conclusion, ADA3a is involved in floral induction through a GCN5-containing complex that acetylates histone H3 in the chromatin of flowering related genes.