Gibberellic Acid Signaling Pathway Research Articles

C-TERMINAL DOMAIN PHOSPHATASE-LIKE 3 contributes to GA-mediated growth and flowering by interaction with DELLA proteins.

Gibberellic acid (GA) plays a central role in many plant developmental processes and is crucial for crop improvement. DELLA proteins, the core suppressors in the GA signaling pathway, are degraded by GA via the 26S proteasomal pathway to release the GA response. However, little is known about the phosphorylation-mediated regulation of DELLA proteins. In this study, we combined GA response assays with protein-protein interaction analysis to infer the connection between Arabidopsis thaliana DELLAs and the C-TERMINAL DOMAIN PHOSPHATASE-LIKE 3 (CPL3), a phosphatase involved in the dephosphorylation of RNA polymerase II. We show that CPL3 directly interacts with DELLA proteins and promotes DELLA protein stability by inhibiting its degradation by the 26S proteasome. Consequently, CPL3 negatively modulates multiple GA-mediated processes of plant development, including hypocotyl elongation, flowering time, and anthocyanin accumulation. Taken together, our findings demonstrate that CPL3 serves as a novel regulator that could improve DELLA stability and thereby participate in GA signaling transduction.

Properties of INDETERMINATE DOMAIN Proteins from Physcomitrium patens: DNA-Binding, Interaction with GRAS Proteins, and Transcriptional Activity.

INDETERMINATE DOMAIN (IDD) proteins are plant-specific transcription factors that interact with GRAS proteins, such as DELLA and SHORT ROOT (SHR), to regulate target genes. The combination of IDD and DELLA proteins regulates genes involved in gibberellic acid (GA) synthesis and GA signaling, whereas the combination of IDD with the complex of SHR and SCARECROW, another GRAS protein, regulates genes involved in root tissue formation. Previous bioinformatic research identified seven IDDs, two DELLA, and two SHR genes in Physcomitrium patens, a model organism for non-vascular plants (bryophytes), which lack a GA signaling pathway and roots. In this study, DNA-binding properties and protein-protein interaction of IDDs from P. patens (PpIDD) were analyzed. Our results showed that the DNA-binding properties of PpIDDs were largely conserved between moss and seed plants. Four PpIDDs showed interaction with Arabidopsis DELLA (AtDELLA) proteins but not with PpDELLAs, and one PpIDD showed interaction with PpSHR but not with AtSHR. Moreover, AtIDD10 (JACKDAW) interacted with PpSHR but not with PpDELLAs. Our results indicate that DELLA proteins have modified their structure to interact with IDD proteins during evolution from moss lineage to seed plants, whereas the interaction of IDD and SHR was already present in moss lineage.

Genome-Wide Analysis of Genes Involved in the GA Signal Transduction Pathway in 'duli' Pear (Pyrus betulifolia Bunge).

Gibberellic acid (GA) is an important phytohormone that regulates every aspect of plant growth and development. While elements involved in GA signaling have been identified and, hence, their functions have been well studied in model plants, such as Arabidopsis and rice, very little is known in pear. We, therefore, analyzed the genes related to GA signaling from the recently sequenced genome of the wildtype ‘duli’ pear (Pyrus betulifolia Bunge), a widely used rootstock for grafting in pear cultivation in China due to its vigorous growth and resistance to abiotic and biotic stress. In total, 15 genes were identified, including five GA receptors PbGID1s (GA-INSENSTIVE DWARF 1), six GA negative regulators, PbDELLAs, and four GA positive regulators, PbSLYs. Exogenous application of GA could promote the expression of PbGID1s but inhibit that of PbDELLAs and PbSLYs in tissue culture ‘duli’ pear seedlings. The expression profiles of these genes in field-grown trees under normal growth conditions, as well as in tissue-cultured seedlings treated with auxin (IAA), GA, paclobutrazol (PAC), abscisic acid (ABA), and sodium chloride (NaCl), were also studied, providing further evidence of the involvement of these genes in GA signaling in ‘duli’ pear plants. The preliminary results obtained in this report lay a good foundation for future research into GA signaling pathways in pear. Importantly, the identification and preliminary functional verification of these genes could guide molecular breeding in order to obtain the highly desired dwarf pear rootstocks for high-density plantation to aid easy orchard management and high yielding of pear fruits.

Sdr4 dominates pre-harvest sprouting and facilitates adaptation to local climatic condition in Asian cultivated rice.

Pre-harvest sprouting (PHS), which reduces grain yield and quality, is controlled by seed dormancy genes. Because few dormancy-related genes have been cloned, the genetic basis of seed dormancy in rice (Oryza sativa L.) remains unclear. Here, we performed a genome-wide association study and linkage mapping to dissect the genetic basis of seed dormancy in rice. Our findings suggest that Seed Dormancy4 (Sdr4), a central modulator of seed dormancy, integrates the abscisic acid and gibberellic acid signaling pathways at the transcriptional level. Haplotype analysis revealed that three Sdr4 alleles in rice cultivars already existed in ancestral Oryza rufipogon accessions. Furthermore, like the semi-dwarf 1 (SD1) and Rc loci, Sdr4 underwent selection during the domestication and improvement of Asian cultivated rice. The distribution frequency of the Sdr4-n allele in different locations in Asia is negatively associated with local annual temperature and precipitation. Finally, we developed functional molecular markers for Sdr4, SD1, and Rc for use in molecular breeding. Our results provide clues about the molecular basis of Sdr4-regulated seed dormancy. Moreover, these findings provide guidance for utilizing the favorable alleles of Sdr4 and Rc to synergistically boost PHS resistance, yield, and quality in modern rice varieties.

The high concentrations of abscisic, jasmonic, and salicylic acids produced under long days do not accelerate flowering in Chenopodium ficifolium 459

The survival and adaptation of angiosperms depends on the proper timing of flowering. The weedy species Chenopodium ficifolium serves as a useful diploid model for comparing the transition to flowering with the important tetraploid crop Chenopodium quinoa due to the close phylogenetic relationship. The detailed transcriptomic and hormonomic study of the floral induction was performed in the short-day accession C. ficifolium 459. The plants grew more rapidly under long days but flowered later than under short days. The high levels of abscisic, jasmonic, and salicylic acids at long days were accompanied by the elevated expression of the genes responding to oxidative stress. The increased concentrations of stress-related phytohormones neither inhibited the plant growth nor accelerated flowering in C. ficifolium 459 at long photoperiods. Enhanced content of cytokinins and the stimulation of cytokinin and gibberellic acid signaling pathways under short days may indicate the possible participation of these phytohormones in floral initiation. The accumulation of auxin metabolites suggests the presence of a dynamic regulatory network in C. ficifolium 459.

Interplays between auxin and GA signaling coordinate early fruit development.

Phytohormones and their interactions are critical for fruit development and are key topics in horticulture research. Auxin, together with gibberellic acid (GA), promotes cell division and expansion, thereby regulating fruit development and enlargement after fertilization. Auxin- and GA-related mutants show parthenocarpy (fruit formation without fertilization of the ovule) in many plant species, indicating that these hormones and possibly their interactions play a key role in the regulation of fruit initiation and development. Recent studies have shown clear molecular and genetic evidence that ARF/IAA and DELLA proteins interact with one another and regulate both auxin and GA signaling pathways in response to auxin and GA during fruit growth in horticultural plants such as tomato (the most studied fleshy fruit) and strawberry (the model for Rosaceae). These recent findings provide new insights into the mechanisms by which the plant hormones auxin and GA regulate fruit development.

Transcriptomic analysis reveals the GRAS family genes respond to gibberellin in Salvia miltiorrhiza hairy roots

BackgroundSalvia miltiorrhiza is one of the most important traditional Chinese medicinal plants with high medicinal value. Gibberellins are growth-promoting phytohormones that regulate numerous growth and developmental processes in plants. However, their role on the secondary metabolism regulation has not been investigated.ResultsIn this study, we found that gibberellic acid (GA) can promote hairy roots growth and increase the contents of tanshinones and phenolic acids. Transcriptomic sequencing revealed that many genes involved in the secondary metabolism pathway were the GA-responsive. After further analysis of GA signaling pathway genes, which their expression profiles have significantly changed, it was found that the GRAS transcription factor family had a significant response to GA. We identified 35 SmGRAS genes in S. miltiorrhiza, which can be divided into 10 subfamilies. Thereafter, members of the same subfamily showed similar conserved motifs and gene structures, suggesting possible conserved functions.ConclusionsMost SmGRAS genes were significantly responsive to GA, indicating that they may play an important role in the GA signaling pathway, also participating in the GA regulation of root growth and secondary metabolism in S. miltiorrhiza.

The APC/CTE E3 Ubiquitin Ligase Complex Mediates the Antagonistic Regulation of Root Growth and Tillering by ABA and GA.

The antagonistic regulation of seed germination by the phytohormones abscisic acid (ABA) and gibberellic acid (GA) has been well-established. However, how these phytohormones antagonistically regulate root growth and branching (tillering in rice, Oryza sativa) remains obscure. Rice TILLER ENHANCER (TE) encodes an activator of the APC/CTE E3 ubiquitin ligase complex that represses tillering but promotes seed germination. In this study, we identified a dual role of GA and APC/CTE in regulating root growth. High GA levels can activate APC/CTE to promote the degradation of rice SHORT-ROOT1 (OsSHR1, a key factor promoting root growth) in the root meristem (RM) or MONOCULM1 (MOC1, a key factor promoting tillering) in the axillary meristem (AM), leading to restricted root growth and tillering, while low GA levels can activate the role of APC/CTE in stimulating RM cell division to promote root growth. In addition, moderate enhancement of ABA signaling helps maintain the RM and AM size, sustaining root growth and tillering by antagonizing the GA-promoted degradation of OsSHR1 and MOC1 through the SnRK2-APC/CTE regulatory module. We conclude that APC/CTE plays a key role in regulating plant architecture by mediating the crosstalk between ABA and GA signaling pathways.

Endophytic Pseudomonas fluorescens induced sesquiterpenoid accumulation mediated by gibberellic acid and jasmonic acid in Atractylodes macrocephala Koidz plantlets

Atractylodes macrocephala is a traditional Chinese medicinal plant, its main bioactive components were sesquiterpenoids. An endophytic bacterium Pseudomonas fluorescens ALEB7B isolated from the stem of Atractylodes lancea was found to enhance sesquiterpenoid content by 20–74% in the root of A. macrocephala. However, the mechanism of endophytes inducing secondary metabolites biosynthesis in A. macrocephala is unknown. To elucidate the signaling mechanism and molecular basis of ALEB7B induced-sesquiterpenoid accumulation in A. macrocephala, the signaling involved in this process and the activity of key enzymes 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR) and 1-deoxy-d-xylulose 5-phosphate reductoisomerase (DXR) were detected. The results showed that gibberellic acid (GA) and jasmonic acid (JA) signals cooperatively contributed to the bacterium inducing sesquiterpenoid accumulation. The bacterium, JA and GA signals can increase the activities of both HMGR and DXR. By comparing the results of endophytic bacterium interactions with A. lancea, we found that ALEB7B can enhance the sesquiterpenoids content in A. macrocephala and A. lancea; however, the responsive signaling pathways induced by ALEB7B in A. macrocephala and A. lancea were different. In conclusion, endophytes may enhance sesquiterpenoid production in A. macrocephala plants by modulating the GA and JA signaling pathways. This study found that Pseudomonas fluorescens ALEB7B can efficiently promote sesquiterpenoid accumulation in Atractylodes macrocephala root by inducing GA and JA bursting and increasing activities of HMGR and DXR.

Transcriptomic analysis of light-dependent anthocyanin accumulation in bicolored cherry fruits

Sweet cherry (Prunus avium L.) fruits are classified into dark-red and bicolored cultivars based on their anthocyanin contents; however, the mechanisms regulating the accumulation of these pigments are unclear. Here, we reveal that anthocyanin accumulation is highly dependent on light in bicolored ‘Rainier’ cherries, while it is only slightly light dependent in the dark-red ‘Hongdeng’ fruits. To reveal the transcriptional mechanisms regulating light-dependent anthocyanin accumulation in bicolored ‘Rainier’ cherries, we sequenced the transcriptomes of fruits grown in light or in darkness. Genes encoding the anthocyanin biosynthesis enzymes chalcone synthase, chalcone isomerase, flavanone 3-hydroxylase, and flavonoid 3′-hydroxylase were significantly upregulated by light in the bicolored fruits. Most of the differentially expressed regulatory genes were known to be involved in the light or hormone signal transduction pathways, such as those encoding protein phosphatase 2Cs, PHYTOCHROME INTERACTING FACTOR 3, phytochromes, and ELONGATED HYPOCOTYL 5. The expression levels of 32 highly expressed transcription factors were found to be significantly altered by light in the bicolored fruits, including members of the basic leucine zipper, R2R3-MYB, and WRKY transcription factor families. A co-expression network analysis further revealed that many of the light-regulated genes were co-expressed with genes involved in the abscisic acid and gibberellic acid signaling pathways, suggesting that these phytohormones play important roles in light-dependent anthocyanin biosynthesis. Together, our data reveal multiple roles for light in regulating anthocyanin biosynthesis in differently colored cherries.

Gibberellin DELLA signaling targets the retromer complex to redirect protein trafficking to the plasma membrane

The plant hormone gibberellic acid (GA) is a crucial regulator of growth and development. The main paradigm of GA signaling puts forward transcriptional regulation via the degradation of DELLA transcriptional repressors. GA has also been shown to regulate tropic responses by modulation of the plasma membrane incidence of PIN auxin transporters by an unclear mechanism. Here we uncovered the cellular and molecular mechanisms by which GA redirects protein trafficking and thus regulates cell surface functionality. Photoconvertible reporters revealed that GA balances the protein traffic between the vacuole degradation route and recycling back to the cell surface. Low GA levels promote vacuolar delivery and degradation of multiple cargos, including PIN proteins, whereas high GA levels promote their recycling to the plasma membrane. This GA effect requires components of the retromer complex, such as Sorting Nexin 1 (SNX1) and its interacting, microtubule (MT)-associated protein, the Cytoplasmic Linker-Associated Protein (CLASP1). Accordingly, GA regulates the subcellular distribution of SNX1 and CLASP1, and the intact MT cytoskeleton is essential for the GA effect on trafficking. This GA cellular action occurs through DELLA proteins that regulate the MT and retromer presumably via their interaction partners Prefoldins (PFDs). Our study identified a branching of the GA signaling pathway at the level of DELLA proteins, which, in parallel to regulating transcription, also target by a nontranscriptional mechanism the retromer complex acting at the intersection of the degradation and recycling trafficking routes. By this mechanism, GA can redirect receptors and transporters to the cell surface, thus coregulating multiple processes, including PIN-dependent auxin fluxes during tropic responses.

DELLA proteins interact with FLC to repress flowering transition.

Flowering is a highly orchestrated and extremely critical process in a plant's life cycle. Previous study has demonstrated that SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) and FLOWERING LOCUS T (FT) integrate the gibberellic acid (GA) signaling pathway and vernalization pathway in regulating flowering time, but detailed molecular mechanisms remain largely unclear. In GA signaling pathway, DELLA proteins are a group of master transcriptional regulators, while in vernalization pathway FLOWERING LOCUS C (FLC) is a core transcriptional repressor that down-regulates the expression of SOC1 and FT. Here, we report that DELLA proteins interact with FLC in vitro and in vivo, and the LHRI domains of DELLAs and the C-terminus of MADS domain of FLC are required for these interactions. Phenotypic and gene expression analysis showed that mutation of FLC reduces while over-expression of FLC enhances the GA response in the flowering process. Further, DELLA-FLC interactions promote the repression ability of FLC on its target genes. In summary, these findings report that the interaction between MADS box transcription factor FLC and GRAS domain regulator DELLAs may integrate various signaling inputs in flowering time control, and shed new light on the regulatory mechanism both for FLC and DELLAs in regulating gene expression.

GID1 modulates stomatal response and submergence tolerance involving abscisic acid and gibberellic acid signaling in rice.

Plant responses to abiotic stresses are coordinated by arrays of growth and developmental programs. Gibberellic acid (GA) and abscisic acid (ABA) play critical roles in the developmental programs and environmental responses, respectively, through complex signaling and metabolism networks. However, crosstalk between the two phytohormones in stress responses remains largely unknown. In this study, we report that GIBBERELLIN-INSENSITIVE DWARF 1 (GID1), a soluble receptor for GA, regulates stomatal development and patterning in rice (Oryza sativa L.). The gid1 mutant showed impaired biosynthesis of endogenous ABA under drought stress conditions, but it exhibited enhanced sensitivity to exogenous ABA. Scanning electron microscope and infrared thermal image analysis indicated an increase in the stomatal conductance in the gid1 mutant under drought conditions. Interestingly, the gid1 mutant had increased levels of chlorophyll and carbohydrates under submergence conditions, and showed enhanced reactive oxygen species (ROS)-scavenging ability and submergence tolerance compared with the wild-type. Further analyses suggested that the function of GID1 in submergence responses is partially dependent on ABA, and GA signaling by GID1 is involved in submergence tolerance by modulating carbohydrate consumption. Taken together, these findings suggest GID1 plays distinct roles in stomatal response and submergence tolerance through both the ABA and GA signaling pathways in rice.

Arabidopsis bZIP16 Transcription Factor Integrates Light and Hormone Signaling Pathways to Regulate Early Seedling Development

Transcriptomic adjustment plays an important role in Arabidopsis thaliana seed germination and deetiolation in response to environmental light signals. The G-box cis-element is commonly present in promoters of genes that respond positively or negatively to the light signal. In pursuing additional transcriptional regulators that modulate light-mediated transcriptome changes, we identified bZIP16, a basic region/Leu zipper motif transcription factor, by G-box DNA affinity chromatography. We confirmed that bZIP16 has G-box-specific binding activity. Analysis of bzip16 mutants revealed that bZIP16 is a negative regulator in light-mediated inhibition of cell elongation but a positive regulator in light-regulated seed germination. Transcriptome analysis supported that bZIP16 is primarily a transcriptional repressor regulating light-, gibberellic acid (GA)-, and abscisic acid (ABA)-responsive genes. Chromatin immunoprecipitation analysis revealed that bZIP16 could directly target ABA-responsive genes and RGA-like2, a DELLA gene in the GA signaling pathway. bZIP16 could also indirectly repress the expression of phytochrome interacting factoR3-like5, which encodes a basic helix-loop-helix protein coordinating hormone responses during seed germination. By repressing the expression of these genes, bZIP16 functions to promote seed germination and hypocotyl elongation during the early stages of Arabidopsis seedling development.

GsGASA1 mediated root growth inhibition in response to chronic cold stress is marked by the accumulation of DELLAs

Low, but not freezing temperatures are a major factor limiting plant vegetative growth and development. It is not clear what signaling mechanism is used to limit plant growth in response to chronic low temperatures, thus it is important to continue to isolate and characterize genes whose activity/expression correlate well with cold responses. In this study, a novel GASA gene was isolated from Glycine soja and named GsGASA1. Quantitative real-time PCR analysis indicated that GsGASA1 expression responded to exogenous gibberellic acid (GA) and abscisic acid (ABA) treatments. Compared with wild-type plants under long-term cold treatment, the constitutive expression of GsGASA1 in transgenic Arabidopsis plants enhanced the inhibition of root elongation, while also increasing the transcript levels of RGL2 and RGL3, two of five DELLA genes in Arabidopsis. DELLA is a class of transcriptional regulators in GA signaling pathway restraining plant growth. Our results imply that GsGASA1 participates in chronic cold-induced root growth inhibition with the accumulation of DELLA genes. Lastly, a subcellular localization study using a yellow fluorescent protein (YFP) fusion protein indicated that GsGASA1 was localized to the plasma membrane, cytoplasm and nucleus.

Integrative Study on Proteomics, Molecular Physiology, and Genetics Reveals an Accumulation of Cyclophilin-Like Protein, TaCYP20-2, Leading to an Increase of Rht Protein and Dwarf in a Novel GA-Insensitive Mutant (gaid) in Wheat

Dwarfism with a "Green Revolution" phenotype is a desirable agronomic trait for crop cultivators as associated with increased yield, improved lodging resistance and higher fertility. Few dwarf mutants of hexaploid wheat (Triticum aestivum), except for Rht-B1 and Rht-D1, have been identified. Here, we report on a novel dwarf natural wheat mutant, which is identified as a gibberellic acid (GA)-insensitive dwarf (gaid) mutant for its semidominant blocking GA signaling pathway. Physiological and morphological investigations showed that the shoot elongation of gaid mutant plants is insensitive to exogenous GA(3) treatment. Expression of TaGA20ox1 in the gaid mutant was enhanced after GA(3) treatment. The short stem of gaid resulted from reduced cell elongation. The transcript expression of Rht, encoding a DELLA protein negatively regulating GA signaling in wheat, displayed similar patterns between gaid and wild type. Contrarily, the degradation of Rht induced by GA(3) treatment was suppressed in the mutant. 2-DE screening assay showed that the expression patterns of the mutant, as well as their responses to GA(3), were changed as compared with the wild type. In the mutant, one of enriched proteins was identified as TaCYP20-2 by Q-TOF MS approach and immunoblotting. TaCYP20-2 was localized in the chloroplast and cell plasma membrane. The transcript of TaCYP20-2 was higher in gaid than that in wild type. Molecular genetic data showed that overexpression of TaCYP20-2 in wheat resulted in a dwarfism similar to that of gaid. It suggests that TaCYP20-2 is a new member that regulates wheat stem development mediated by DELLA protein degradation of GA signaling pathway.

ABA and GA signaling pathways interact and regulate seed germination and seedling development under salt stress

Soil salinity is one of the most significant abiotic stresses limiting plant growth. The ability of plant cells to adapt and survive under salt stress conditions involves triggering a network of signaling events including hormones such as abscisic acid (ABA) known to regulate many important aspects of growth and development. ABA is also known to play a critical role in stress responses such as the regulation of seed germination under salt and osmotic stress. Components of the gibberellic acid (GA) signaling pathway have also been shown to regulate germination; however, the involvement of GA signaling in salt and osmotic stress is largely unexamined. Here, we examined the responses of mutants in the GA signaling pathway (rgl2 and spy) and in the ABA signaling pathway (abi3 and abi5) to salt (NaCl) and osmotic (mannitol) stress during seed germination and early seedling development. Several mutants show resistance to increased levels of both salt and osmotic stress at germination and later stages of seedling development suggesting a role for ABA and GA signaling in these processes. qRT-PCR was employed to determine the effect of salt stress on seed germination via transcriptional control of the components in GA or ABA signaling pathways. We found that RGL2, ABI3, and ABI5 transcripts are greatly induced by NaCl in wildtype plants, but show little if any induction by NaCl in mutant backgrounds suggesting that this regulation of induction during salt stress may occur through ABA–GA crosstalk. Overall, our results indicate that each of the ABA and GA signaling pathways is individually involved in regulation of various seedling developmental stages under stress conditions. In addition, these two hormone pathways appear to be interacting in the regulation of germination and early seedling growth under salt and osmotic stress conditions.

Loss-of-function of DELLA protein SLN1 activates GA signaling in barley aleurone

Gibberellic acid (GA) is an important signaling molecule that participates in many aspects of plant growth and development. While the importance of this hormone is clear, the transcriptional regulatory networks involved are still being characterized. The cereal aleurone, particularly the barley aleurone, has been used as a classic model to study GA and GA signaling for many years, and these studies have significantly contributed to our understanding of GA in plant biology. The objective of this study was to characterize the transcripts regulated through the DELLA protein SLN1, a negative regulator of the GA signaling pathway. To detect the transcripts, Affymetrix Barley 1 GeneChips were hybridized with RNA extracted from barley aleurone treated with GA or aleurone of the DELLA mutant sln1c without GA treatment. The transcripts detected, in term of both expressed genes and their function, were highly similar between the GA-treatment and the sln1c mutant. These results from a genome-wide transcript analysis provide evidence that SLN1 in the GA signal transduction pathway controls almost all GA-induced genes in the barley aleurone.

Biochemical Insights on Degradation ofArabidopsisDELLA Proteins Gained From a Cell-Free Assay System

The phytohormone gibberellic acid (GA) regulates diverse aspects of plant growth and development. GA responses are triggered by the degradation of DELLA proteins, which function as repressors in GA signaling pathways. Recent studies in Arabidopsis thaliana and rice (Oryza sativa) have implied that the degradation of DELLA proteins occurred via the ubiquitin-proteasome system. Here, we developed an Arabidopsis cell-free system to recapitulate DELLA protein degradation in vitro. Using this cell-free system, we documented that Lys-29 of ubiquitin is the major site for ubiquitin chain formation to mediate DELLA protein degradation. We also confirmed the specific roles of GA receptors and multisubunit E3 ligase components in regulating DELLA protein degradation. In addition, blocking DELLA degradation with a PP1/PP2A phosphatase inhibitor in our cell-free assay suggested that degradation of DELLA proteins required protein Ser/Thr dephosphorylation activity. Furthermore, our data revealed that the LZ domain of Arabidopsis DELLA proteins is essential for both their stability and activity. Thus, our in vitro degradation system provides biochemical insights into the regulation of DELLA protein degradation. This in vitro assay system could be widely adapted for dissecting cellular signaling pathways in which regulated proteolysis is a key recurrent theme.

Gibberellic acid-mediated salt signaling in seed germination

Seed germination initiates the postembryonic development of plants, which determines successful seedling establishment and plant propagation. It is therefore tightly regulated by diverse environmental conditions, including high salinity and drought, as well as by intrinsic developmental programs, among which gibberellic acid (GA) is best understood. Regulatory roles of GA in seed germination have been extensively studied. It is also known that high salinity inhibits germination by repressing genes encoding GA biosynthetic enzymes. However, it is still unclear how salt signals are coordinately incorporated into the GA signaling pathway at the molecular level. We recently demonstrated that a membrane-bound NAC transcription factor, NTL8, mediates salt signaling, primarily through a RGL2-independent GA pathway, in regulating seed germination. High salinity promotes NTL8 transcription and proteolytic activation of NTL8. Notably, the NTL8-mediated salt signaling is independent of abscisic acid (ABA). These observations indicate that membrane-mediated transcription control is an important component of salt signaling during seed germination.Addendum to: Kim SG, Lee AK, Yoon HK, Park CM. A membrane-bound NAC transcription factor NTL8 regulates gibberellic acid-mediated salt signaling in Arabidopsis seed germination. Plant J 2008; doi: 10.1111/j.1365-313X.2008.03493.x