Repressor Of Growth Research Articles

DELLA proteins restrain germination and elongation growth in Arabidopsis thaliana COP9 signalosome mutants

The COP9 signalosome (CSN) is an evolutionarily conserved multiprotein complex with an essential role in the development of higher eukaryotes. CSN deconjugates the ubiquitin-related modifier NEDD8 from the cullin subunit of cullin-RING type E3 ubiquitin ligases (CRLs), and CSN-mediated cullin deneddylation is required for full CRL activity. Although several plant E3 CRL functions have been shown to be compromised in Arabidopsis csn mutants, none of these functions have so far been shown to limit growth in these mutants. Here, we examine the role of CSN in the context of the E3 ubiquitin ligase SCF SLEEPY1 (SLY1), which promotes gibberellic acid (GA)-dependent responses in Arabidopsis thaliana. We show that csn mutants are impaired in GA- and SCF SLY1-dependent germination and elongation growth, and we show that these defects correlate with an accumulation and reduced turnover of an SCF SLY1-degradation target, the DELLA protein REPRESSOR-OF- ga1-3 (RGA). Genetic interaction studies between csn mutants and loss-of-function alleles of RGA and its functional homologue GIBBERELLIC ACID INSENSITIVE ( GAI) further reveal that RGA and GAI repress defects of germination in strong csn mutants. In addition, we find that these two DELLA proteins are largely responsible for the elongation defects of a weak csn5 mutant allele. We thus conclude that an impairment of SCF SLY1 is at least in part causative for the germination and elongation defects of csn mutants, and suggest that DELLA proteins are major growth repressors in these mutants.

The bHLH Transcription Factor SPATULA Controls Final Leaf Size in Arabidopsis thaliana

Leaves possess intrinsic information about their final size, but the developmental mechanisms setting the limits of growth are not well characterized. By screening enhancer trap lines that show a specific expression pattern in leaf primordia, we isolated one line, 576. This line contains a T-DNA insertion upstream of the basic helix-loop-helix (bHLH) transcription factor SPATULA (SPT) gene, and shows expression in the basal region of young leaves, where cell proliferation is active. An spt loss-of-function mutation increased leaf size and total cell number within a leaf, while SPT overexpression decreased leaf size and total cell number within a leaf. Although spt mutations did not affect cell size, SPT overexpression decreased the cell size in fully expanded leaves. Genetic analysis suggested that SPT acts independently from another set of cell proliferation-dependent organ size regulators ANGUSTIFOLIA3 (AN3) and GROWTH REGULATING FACTOR5 (AtGRF5). Detailed analysis of spt leaf development showed that the spt mutation enlarged the size of the meristematic region in leaf primordia, while overexpression of AtGRF5 promoted cell proliferation without affecting the enlargement of the meristematic region. These results suggest that SPT functions as a repressor of leaf growth and that meristematic region size in young leaf primordia, in terms of proliferative cell number within leaf primordia, is another target of leaf size determination, which previously had not been identified.

Estrogen and IGF-I Independently Down-Regulate Critical Repressors of Breast Cancer Growth.

Abstract Background: Estrogen receptor and insulin-like growth factor (IGF) signaling pathways are important for both normal mammary gland development and breast cancer pathogenesis. Despite this evidence, cross-talk between these two critical mitogenic and survival pathways remains poorly understood, particularly at the level of downstream gene transcriptional networks.Methods: We performed microarray analysis on ER-positive MCF-7 breast cancer cells following stimulation with either estradiol (10nM) or IGF-I (10nM) for 3hr or 24hr. We defined an E2-IGF gene expression signature and examined the effect of the gene signature on time to recurrence in ER+ tumors in the publicly available NKI dataset. Q-RT-PCR was used to validate the effects of IGF-I and E2 on mRNA of repressed genes. To examine gene regulation further, MCF-7 cells were incubated with various signaling inhibitors and gene expression measured.Results: We found 183 probesets to be co-regulated by E2 and IGF-I after 3hr of stimulation and 454 probesets to be co-regulated at the 24hr time point. Patients with tumors that had the same set of genes up- and down-regulated had a poor outcome (p=7.04E-07). E2-IGF co-regulated genes showed a significant enrichment for down-regulated genes (p<0.001). The top 10% of down-regulated genes contained four (BLNK, SOCS2, CCNG2, and ING4) that have previously been shown to have tumor suppressor function and/or show loss in human breast tumors. We confirmed that E2 and IGF-I indeed repressed levels of these candidate genes. Blockade of ER by ICI 182780 (ICI) completely reversed E2-mediated repression but had little or no effect on IGF-mediated repression of these genes. Similarly, blockade of IGF-IR was able to completely reverse IGF-I-mediated repression of all four genes, but had essentially no effect upon E2-mediated repression. Inhibition of the PI3K pathway with LY294002 (20μM) affected short-term (3hr) down-regulation of BLNK and SOCS2 by both E2 and IGF-I, and CCNG2 by IGF-I. By 24 hours, LY completely reversed both E2 and IGF-I mediated repression of CCNG2, and significantly reversed both E2 and IGF-I repression of BLNK and SOCS2. Blockade of MEK1/2 with U0126 (10μM) had no effect on short-term repression by E2 or IGF-I, though after 24hr it affected both E2 and IGF-I repression of BLNK and SOCS2.Conclusions: E2 and IGF-I co-regulate a set of genes that affect breast cancer outcome. E2 and IGF-I down-regulate several critical tumor suppressor genes. While the down-regulation is independent of each other at the level of ER and IGF-IR, for some genes, there is convergence on the PI3K pathway for repression. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 1126.

Myostatin represses physiological hypertrophy of the heart and excitation–contraction coupling

Although myostatin negatively regulates skeletal muscle growth, its function in heart is virtually unknown. Herein we demonstrate that it inhibits basal and IGF-stimulated proliferation and differentiation and also modulates cardiac excitation-contraction (EC) coupling. Loss of myostatin induced eccentric hypertrophy and enhanced cardiac responsiveness to beta-adrenergic stimulation in vivo. This was due to myostatin null ventricular myocytes having larger [Ca(2+)](i) transients and contractions and responding more strongly to beta-adrenergic stimulation than wild-type cells. Enhanced cardiac output and beta-adrenergic responsiveness of myostatin null mice was therefore due to increased SR Ca(2+) release during EC coupling and to physiological hypertrophy, but not to enhanced myofilament function as determined by simultaneous measurement of force and ATPase activity. Our studies support the novel concept that myostatin is a repressor of physiological cardiac muscle growth and function. Thus, the controlled inhibition of myostatin action could potentially help repair damaged cardiac muscle by inducing physiological hypertrophy.

Gibberellin Signaling in the Endodermis Controls Arabidopsis Root Meristem Size

Plant growth is driven by cell proliferation and elongation. The hormone gibberellin (GA) regulates Arabidopsis root growth by controlling cell elongation, but it is currently unknown whether GA also controls root cell proliferation. Here we show that GA biosynthetic mutants are unable to increase their cell production rate and meristem size after germination. GA signals the degradation of the DELLA growth repressor proteins GAI and RGA, promoting root cell production. Targeting the expression of gai (a non-GA-degradable mutant form of GAI) in the root meristem disrupts cell proliferation. Moreover, expressing gai in dividing endodermal cells was sufficient to block root meristem enlargement. We report a novel function for GA regulating cell proliferation where this signal acts by removing DELLA in a subset of, rather than all, meristem cells. We suggest that the GA-regulated rate of expansion of dividing endodermal cells dictates the equivalent rate in other root tissues. Cells must double in size prior to dividing but cannot do so independently, because they are physically restrained by adjacent tissues with which they share cell walls. Our study highlights the importance of probing regulatory mechanisms linking molecular- and cellular-scale processes with tissue and organ growth responses.

Revisiting the COP9 signalosome as a transcriptional regulator

The COP9 signalosome (CSN) is a highly conserved protein complex that was originally described as a repressor of light-dependent growth and transcription in Arabidopsis. The most studied CSN function is the regulation of protein degradation, which occurs primarily through the removal of the ubiquitin-like modifier Nedd8 from cullin-based E3 ubiquitin ligases. This activity can regulate transcription-factor stability and, therefore, transcriptional activity. Recent data suggest that the CSN also regulates transcription on the chromatin by mechanisms that are not yet clearly understood. Furthermore, the CSN subunits CSN5 and CSN2 seem to act as transcriptional coactivators and corepressors, respectively. Here, I re-evaluate the mechanisms by which the CSN acts as a transcriptional regulator, and suggest that they could extend beyond the regulation of protein stability.

Inhibition of DLEU1 Following siRNA Transfection in Burkett's Lymphoma (BL): Implication for Tumor Repressor Role of DLEU1 in C-Myc-Activated BL Lymphomagenesis

Background: The progress of childhood BL and DLBCL has improved dramatically in the past three decades; however, patients with a 13q-deletion have a significantly poorer outcome (Cairo/Patte et al Blood, 2007 and Patte/Cairo et al Blood, 2007; Poirel/Cairo et al Leukemia 2008). DLEU1, a potential tumor suppressor gene, is located within the 13q-deletion. DLEU1 was reported to be a key gene in the Burkitt classifier genes (Dave/Staudt et al NEJM, 2006) and c-myc binds to the promoter region of DLEU1. DLEU1-network proteins include, among others, E3 ubiquitin-protein ligase (UBR1), Tubulin beta-2C (TUBB2C) and RASSF1A. We previously demonstrated that UBR1, TUBB2C, and RASSF1A, were differentially expressed in BL vs DLBCL patients and cell lines by global gene profiles and real time RT-PCR studies (Day/Cairo et al AACR 2008; Day/Cairo et al ICML 2008). We further demonstrated decreased expression of UBR1 (33.2±4.5% reduction compared to control (p<0.02)) and TUBB2C (30.0±3.5% reduction compared to control (p<0.001)) by DLEU1 gene siRNA knock down, while expression of RASSF1A was not changed (Day/Cairo, et al SIOP 2008). Taken together, these data suggest the hypothesis that DLEU1 interacting with UBR1 may interfere with microtubule function, and therefore act as a tumor repressor in c-myc-activated BL lymphomagenesis, by arrest of the cell cycle at G2/M and subsequent inducion of apoptosis.Objective: In this study, we investigated the role of DLEU1 in regulation of apoptosis in BL by inhibition of DLEU1 gene expression by a DLEU1 siRNA and evaluated it effects on the apoptotic rate in a BL cell line.Methods: The Ramos BL cell line was transiently transfected with a 25-nucleotide modified DLEU1 siRNA (5′-AUACUUGGCAUGAAUGAACUUAUGU-3′ and 3′-UAUGAACCGUACUUACUUGAAUACA-5′). Stealth RNAi whose GC content is similar to that of this DLEU1 siRNA was used as negative control. The transient transfection of DLEU1 siRNA (10 – 20 nM) was achieved using Lipofectamine RNAiMAX. The transfection efficiency of siRNA was evaluated using Alexa Fluor Red Fluorescent Oligo. DLEU1 contents were measured by qRT-PCR with ddCt relative quantitative determination. GAPDH was used as endogenous control. Statistical analysis was conducted by one-way analysis of variance (ANOVA) followed by Tukey-Kramer multiple comparisons test. To determine the early and late stages of apoptosis, we transfected Ramos BL cells with DLEU1 siRNA, and then incubated cells with Annexin V-FITC and Propidium Iodide for 15 minutes, respectively (BD Pharmingen), followed by FACS using BD LSRII with FACSDiva.Results: The DLEU1 siRNA decreased the expression of DLEU1 RNA (52±13%; p<0.0006). The transfection efficiency of siRNA was 85 – 90%. Comparing to untreated cells, DLEU1 siRNA treatment significantly reduced early apoptosis (16.90±0.37%; p<0.001) and late stage apoptosis (14.70±0.27%; p<0.0001).Conclusion: These results suggest that when DLEU1 gene expression is decreased in BL cells, there is a significant reduction in both early and late apoptosis. The results strongly support a relationship between DLEU1 gene and regulation of BL apoptotic mechanisms. In concert with previous investigations, this data suggests that DLEU1 may function as a tumor growth repressor via UBR1 and TUBB2C-regulated mechanism in the cellular apoptotic process. Since c-myc binds promoter region of DLEU1 and these two genes are a part of the c-myc signaling network, this further underscores the importance of DLEU1 and its network proteins may play in c-myc-activated BL lymphomagenesis.

PROCERAencodes a DELLA protein that mediates control of dissected leaf form in tomato

Leaves of seed plants can be described as simple, where the leaf blade is entire, or dissected, where the blade is divided into distinct leaflets. Mechanisms that define leaflet number and position are poorly understood and their elucidation presents an attractive opportunity to understand mechanisms controlling organ shape in plants. In tomato (Solanum lycopersicum), a plant with dissected leaves, KNOTTED1-like homeodomain proteins (KNOX) are positive regulators of leaflet formation. Conversely, the hormone gibberellin (GA) can antagonise the effects of KNOX overexpression and reduce leaflet number, suggesting that GA may be a negative regulator of leaflet formation. However, when and how GA acts on leaf development is unknown. The reduced leaflet number phenotype of the tomato mutant procera (pro) mimics that of plants to which GA has been applied during leaf development, suggesting that PRO may define a GA signalling component required to promote leaflet formation. Here we show that PRO encodes a DELLA-type growth repressor that probably mediates GA-reversible growth restraint. We demonstrate that PRO is required to promote leaflet initiation during early stages of growth of leaf primordia and conversely that reduced GA biosynthesis increases the capability of the tomato leaf to produce leaflets in response to elevated KNOX activity. We propose that, in tomato, DELLA activity regulates leaflet number by defining the correct timing for leaflet initiation.

The GTPase domain of Galphao contributes to the functional interaction of Galphao with the promyelocytic leukemia zinc finger protein

Go, one of the most abundant heterotrimeric G proteins in the brain, is classified as a member of the Gi/Go family based on its homology to Gi proteins. Recently, we identified promyelocytic leukemia zinc finger protein (PLZF) as a candidate downstream effector for the alpha subunit of Go (Gαo). Activated Gαo interacts with PLZF and augments its function as a repressor of transcription and cell growth. G protein-coupled receptor-mediated Gαo activation also enhanced PLZF function. In this study, we determined that the GTPase domain of Gαo contributes to Gαo:PLZF interaction. We also showed that the Gαo GTPase domain is important in modulating the function of PLZF. This data indicates that the GTPase domain of Gαo may be necessary for the functional interaction of Gαo with PLZF.

Two wobble-splicing events affect ING4 protein subnuclear localization and degradation

ING4 (inhibitor of growth 4) is a candidate tumor suppressor gene that is implicated as a repressor of cell growth, angiogenesis, cell spreading and cell migration and can suppress loss of contact inhibition in vitro. Another group and we identified four wobble-splicing isoforms of ING4 generated by alternative splicing at two tandem splice sites, GC(N) 7GT and NAGNAG, which caused canonical (GT-AG) and non-canonical (GC-AG) splice site wobbling selection. Expression of the four ING4 wobble-splicing isoforms did not vary significantly in any of the cell lines examined. Here we show that ING4_v1 is translocated to the nucleolus, indicating that ING4 contains an intrinsic nucleolar localization signal. We further demonstrate that the subcellular localization of ING4 is modulated by two wobble-splicing events at the exon 4–5 boundary, causing displacement from the nucleolus to the nucleus. We also observed that ING4 is degraded through the ubiquitin–proteasome pathway and that it is subjected to N-terminal ubiquitination. We demonstrate that nucleolar accumulation of ING4 prolongs its half-life, but lack of nucleolar targeting potentially increases ING4 degradation. Taken together, our data suggest that the two wobble-splicing events at the exon 4–5 boundary influence subnuclear localization and degradation of ING4.

Forkhead-box A1 (FOXA1) expression in breast cancer and its prognostic significance

The forkhead-box A1 (FOXA1) controls downstream transcription of oestrogen receptor (ER)-regulated genes. In this study, the biological and prognostic value of FOXA1 expression was assessed immunohistochemically in a large and well-characterised series of invasive breast carcinoma with a long term follow-up using tissue microarray. FOXA1 expression was associated with steroid hormone receptors (ERα, PgR and AR), other variables of good prognosis such as smaller tumour size, lower histological grade, luminal cytokeratins (CK18 and CK7/8), BRCA1 and E-cadherin. Its expression showed an inverse relation with basal CKs (CK14 and CK5/6) and P-cadherin. We found an association between high FOXA1 expression and a better survival in the whole series however; multivariate analysis showed that FOXA1 was not an independent prognostic marker. In conclusion, our results show that FOXA1 protein is associated with markers of good prognosis supporting its role as a growth repressor in breast cancer. In this series, FOXA1 was found not to be of an independent prognostic significance in breast cancer and as such its immunohistochemical assessment alone does not appear to have relevance in routine practice to stratify ER-positive (luminal-like) tumours into clinically significant subgroups.

Global identification of DELLA target genes during Arabidopsis flower development.

Gibberellin (GA) plays important roles in regulating many aspects of plant development. GA derepresses its signaling pathway by promoting the degradation of DELLA proteins, a family of nuclear growth repressors. Although the floral organ identity is established in flowers of the GA-deficient mutant ga1-3, the growth of all floral organs is severely retarded. In particular, abortive anther development in ga1-3 results in male sterility. Genetic analysis has revealed that various combinations of null mutants of DELLA proteins could gradually rescue floral organ defects in ga1-3 and that RGA is the most important DELLA protein involved in floral organ development. To elucidate the early molecular events controlled by RGA during flower development, we performed whole-genome microarray analysis to identify genes in response to the steroid-inducible activation of RGA in ga1-3 rgl2 rga 35S:RGA-GR. Although DELLA proteins were suggested as transcriptional repressors, similar numbers of genes were down-regulated or up-regulated by RGA during floral organ development. More than one-third of RGA down-regulated genes were specifically or predominantly expressed in stamens. A significant number of RGA-regulated genes are involved in phytohormone signaling or stress response. Further expression analysis through activation of RGA by steroid induction combined with cycloheximide identified eight genes as immediate targets of RGA. In situ hybridization and transgenic studies further showed that the expression pattern and function of several selected genes were consistent with the predictions from microarray analysis. These results suggest that DELLA regulation of floral organ development is modulated by multiple phytohormones and stress signaling pathways.

DELLAs Control Plant Immune Responses by Modulating the Balance of Jasmonic Acid and Salicylic Acid Signaling

In Arabidopsis, the flagellin-derived peptide flg22 elevates antibacterial resistance [1] and inhibits growth [2] upon perception via the leucine-rich repeat receptor-like kinase Flagellin-Sensitive 2 (FLS2) [3]. DELLA proteins are plant growth repressors whose degradation is promoted by the phytohormone gibberellin [4]. Here, we show that DELLA stabilization contributes to flg22-induced growth inhibition. In addition, we show that DELLAs promote susceptibility to virulent biotrophs and resistance to necrotrophs, partly by altering the relative strength of salicylic acid and jasmonic acid (JA) signaling. A quadruple-DELLA mutant (which lacks four out of the five Arabidopsis DELLA proteins [5]) was partially insensitive to gene induction by Methyl-Jasmonate (MeJA), whereas the constitutively active dominant DELLA mutant gai[6] was sensitized for JA-responsive gene induction, implicating DELLAs in JA-signaling and/or perception. Accordingly, the elevated resistance of gai to the necrotrophic fungus Alternaria brassicicola and susceptibility to the hemibiotroph Pseudomonas syringae pv. tomato strain DC3000 (Pto DC3000) was attenuated in the JA-insensitive coi1-16 mutant [7]. These findings suggest an explanation for why the necrotrophic fungus Gibberella fujikuroi, causal agent of the foolish-seedling disease of rice, makes gibberellin.

The Pea DELLA proteins LA and CRY are important regulators of gibberellin synthesis and root growth.

The theory that bioactive gibberellins (GAs) act as inhibitors of inhibitors of plant growth was based originally on the slender pea (Pisum sativum) mutant (genotype la cry-s), but the molecular nature of this mutant has remained obscure. Here we show that the genes LA and CRY encode DELLA proteins, previously characterized in other species (Arabidopsis [Arabidopsis thaliana] and several grasses) as repressors of growth, which are destabilized by GAs. Mutations la and cry-s encode nonfunctional proteins, accounting for the fact that la cry-s plants are extremely elongated, or slender. We use the la and cry-s mutations to show that in roots, DELLA proteins effectively promote the expression of GA synthesis genes, as well as inhibit elongation. We show also that one of the DELLA-regulated genes is a second member of the pea GA 3-oxidase family, and that this gene appears to play a major role in pea roots.

FOXO-regulated transcription restricts overgrowth of Tsc mutant organs

FOXO is thought to function as a repressor of growth that is, in turn, inhibited by insulin signaling. However, inactivating mutations in Drosophila melanogaster FOXO result in viable flies of normal size, which raises a question over the involvement of FOXO in growth regulation. Previously, a growth-suppressive role for FOXO under conditions of increased target of rapamycin (TOR) pathway activity was described. Here, we further characterize this phenomenon. We show that tuberous sclerosis complex 1 mutations cause increased FOXO levels, resulting in elevated expression of FOXO-regulated genes, some of which are known to antagonize growth-promoting pathways. Analogous transcriptional changes are observed in mammalian cells, which implies that FOXO attenuates TOR-driven growth in diverse species.

Ethylene-induced Arabidopsis hypocotyl elongation is dependent on but not mediated by gibberellins

Ethylene, or its precursor 1-aminocyclopropane-1-carboxylic acid (ACC), can stimulate hypocotyl elongation in the light. It is questioned whether gibberellins (GAs) play a role in this response. Tests with light of different wavelengths demonstrated that the ethylene response depends on blue light and functional cryptochrome signalling. Levels of bio-active GA(4) were reduced in seedlings showing an ethylene response. Furthermore, ACC treatment of seedlings caused accumulation of the DELLA protein RGA, a repressor of growth. Concurrently, transcript levels of several GA biosynthesis genes were up-regulated and GA inactivation genes down-regulated by ACC. Hypocotyl elongation in response to ACC was strongly reduced in seedlings with a diminished GA signal, while being vigorously stimulated in a quadruple DELLA knock-out mutant with constitutive GA signalling. These data show that ethylene-driven hypocotyl elongation is mainly blue light-dependent and that this ethylene response, although GA dependent, hence needing a basal GA level, is not mediated by GA, but rather acts via a separate pathway.

Global Analysis of DELLA Direct Targets in Early Gibberellin Signaling inArabidopsis

Bioactive gibberellins (GAs) are phytohormones that regulate growth and development throughout the life cycle of plants. DELLA proteins are conserved growth repressors that modulate all aspects of GA responses. These GA-signaling repressors are nuclear localized and likely function as transcriptional regulators. Recent studies demonstrated that GA, upon binding to its receptor, derepresses its signaling pathway by binding directly to DELLA proteins and targeting them for rapid degradation via the ubiquitin-proteasome pathway. Therefore, elucidating the signaling events immediately downstream of DELLA is key to our understanding of how GA controls plant development. Two sets of microarray studies followed by quantitative RT-PCR analysis allowed us to identify 14 early GA-responsive genes that are also early DELLA-responsive in Arabidopsis thaliana seedlings. Chromatin immunoprecipitation provided evidence for in vivo association of DELLA with promoters of eight of these putative DELLA target genes. Expression of all 14 genes was downregulated by GA and upregulated by DELLA. Our study reveals that DELLA proteins play two important roles in GA signaling: (1) they help establish GA homeostasis by direct feedback regulation on the expression of GA biosynthetic and GA receptor genes, and (2) they promote the expression of downstream negative components that are putative transcription factors/regulators or ubiquitin E2/E3 enzymes. In addition, one of the putative DELLA targets, XERICO, promotes accumulation of abscisic acid (ABA) that antagonizes GA effects. Therefore, DELLA may restrict GA-promoted processes by modulating both GA and ABA pathways.

GA action: turning on de-DELLA repressing signaling

Phytohormone gibberellins (GA) are a large family of tetracyclic diterpenoids and play the important roles in modulation of plant growth and development throughout the plant life cycle. GA depresses its signaling by the GA-promoted destabilization of the DELLA protein growth repressors via 26S proteasome pathway. Recent evidences indicate that the DELLA proteins interact with multiple environmental and other hormonal response pathways and confer plant growth restraint. Furthermore, the discovery of rice GIBBERELLIN INSENSITIVE DWARF1 (GID1) and three Arabidopsis AtGID1 homologs as soluble GA receptors opens new prospects for understanding of de-DELLA repressing system in GA signaling.

Silencing of DELLA induces facultative parthenocarpy in tomato fruits

DELLA proteins are plant nuclear factors that restrain growth and proliferation in response to hormonal signals. The effects of the manipulation of the DELLA pathway in the making of a berry-like fruit were investigated. The expression of the Arabidopsis thaliana gain-of-function DELLA allele Atgai (del) in tomato (Solanum lycopersicum L.) produced partially sterile dwarf plants and compacted influorescences, as expected for a constitutively activated growth repressor. In contrast, antisense silencing of the single endogenous tomato DELLA gene homologue (SlDELLA) produced slender-like plants with elongated flower trusses. Interestingly, the depletion of SlDELLA in tomato was sufficient to overcome the growth arrest normally imposed on the ovary at anthesis, resulting in parthenocarpic fruits in the absence of pollination. Antisense SlDELLA-engineered fruits were smaller in size and elongated in shape compared with wild type. Cell number estimations showed that fruit set, resulting from reduced SlDELLA expression, arose from activated cell elongation at the longitudinal and lateral axes of the fruit pericarp, bypassing phase-II (post-pollination) cell divisions. Parthenocarpy caused by SlDELLA depletion is facultative, as hand pollination restored wild-type fruit phenotype. This indicates that fertilization-associated SlDELLA-independent signals are operational in ovary-fruit transitions. SlDELLA was also found to restrain growth in other reproductive structures, affecting style elongation, stylar hair primordial growth and stigma development.

Genetic and Molecular Regulation by DELLA Proteins of Trichome Development in Arabidopsis

Gibberellins (GA) are known to influence phase change in Arabidopsis (Arabidopsis thaliana) as well as the development of trichomes, which are faithful epidermal markers of shoot maturation. They modulate these developmental programs in part by antagonizing DELLA repressors of growth, GIBBERELLIC ACID INSENSITIVE (GAI) and REPRESSOR OF ga1-3 (RGA). In this study, we have probed the relative roles played by RGA, GAI, and two homologs, RGA-LIKE1 (RGL1) and RGL2, in these processes and investigated molecular mechanisms through which they influence epidermal differentiation. We found that the DELLAs act collectively to regulate trichome initiation on all aerial organs and that the onset of their activity is accompanied by the repression of most genes known to regulate trichome production. These effects are consistent with the results of genetic analysis, which conclusively place theses genes downstream of the DELLAs. We find that repression of trichome regulatory genes is rapid, but involves an indirect, rather than a direct, molecular mechanism, which requires de novo protein synthesis. DELLA activity also influences postinitiation events and we show that GAI is a major repressor of trichome branching, a role in which it is antagonized by RGL1 and RGL2. Finally, we report that, in contrast to most other effects, the repression by GA applications of flower trichome initiation is not dependent on RGA, GAI, RGL1, or RGL2. In summary, our data show that DELLA proteins are central to trichome development in Arabidopsis and that their effect can be largely explained by their transcriptional influence on trichome initiation activators.