The high-affinity transporter BnPHT1;4 is involved in phosphorus acquisition and mobilization for facilitating seed germination and early seedling growth of Brassica napus

BACKGROUND

Seed germination and seedling establishment are two of the most critical phases in plant development. However, the molecular mechanisms underlying the effect of phosphorus on seed germination and post-germinated growth of oilseed rape are unclear so far. Here, we report the role of BnPHT1;4 in seed germination and early seedling development of Brassica napus.

RESULTS

Our results show that BnPHT1;4 is preferentially expressed in cotyledons of early developing seedlings. Overexpression of BnPHT1;4 in oilseed rape promoted seed germination and seedling growth. Expression levels of the genes related to ABA and GA biosynthesis and signaling were significantly altered in BnPHT1;4 transgenic seedlings. Consequently, active GA level was up-regulated, whereas ABA content was down-regulated in BnPHT1;4 transgenic seedlings. Furthermore, exogenous GA could promote seed germination of wild type, while exogenous ABA could partially recover the advanced-germination phenotype of BnPHT1;4 transgenic seeds. Total phosphorus content in cotyledons of the transgenic seedlings was decreased more rapidly than that in wild type when Pi was supplied or deficient, and Pi contents in shoots and roots of the BnPHT1;4 transgenic plants were higher than those in wild type under high and low Pi conditions.

CONCLUSIONS

Our data suggest that the high-affinity transporter BnPHT1;4 is involved in phosphorus acquisition and mobilization for facilitating seed germination and seedling growth of Brassica napus by modulating ABA and GA biosynthesis.

NIN Acts as a Network Hub Controlling a Growth Module Required for Rhizobial Infection

The symbiotic infection of root cells by nitrogen-fixing rhizobia during nodulation requires the transcription factor Nodule Inception (NIN). Our root hair transcriptomic study extends NIN's regulon to include Rhizobium Polar Growth and genes involved in cell wall modification, gibberellin biosynthesis, and a comprehensive group of nutrient (N, P, and S) uptake and assimilation genes, suggesting that NIN's recruitment to nodulation was based on its role as a growth module, a role shared with other NIN-Like Proteins. The expression of jasmonic acid genes in nin suggests the involvement of NIN in the resolution of growth versus defense outcomes. We find that the regulation of the growth module component Nodulation Pectate Lyase by NIN, and its function in rhizobial infection, are conserved in hologalegina legumes, highlighting its recruitment as a major event in the evolution of nodulation. We find that Nodulation Pectate Lyase is secreted to the infection chamber and the lumen of the infection thread. Gene network analysis using the transcription factor mutants for ERF Required for Nodulation1 and Nuclear Factor-Y Subunit A1 confirms hierarchical control of NIN over Nuclear Factor-Y Subunit A1 and shows that ERF Required for Nodulation1 acts independently to control infection. We conclude that while NIN shares functions with other NIN-Like Proteins, the conscription of key infection genes to NIN's control has made it a central regulatory hub for rhizobial infection.

Cytokinin-induced parthenocarpy of San Pedro type fig (Ficus carica L.) main crop: explained by phytohormone assay and transcriptomic network comparison

CPPU-induced San Pedro type fig main crop parthenocarpy exhibited constantly increasing IAA content and more significantly enriched KEGG pathways in the receptacle than in female flowers. N-(2-chloro-4-pyridyl)-N-phenylurea (CPPU) was applied to San Pedro fig (Ficus carica L.) main crop to induce parthenocarpy; the optimal effect was obtained with 25 mg L^-1 application to syconia when female flowers were at anthesis. To elucidate the key expression changes in parthenocarpy conversion, significant changes in phytohormone level and transcriptome of fig female flowers and receptacles were monitored. HPLC-MS revealed increased IAA content in female flowers and receptacle 2, 4 and 10 days after treatment (DAT), decreased zeatin level in the receptacle 2, 4 and 10 DAT, decreased GA₃ content 2 and 4 DAT, and increased GA₃ content 10 DAT. ABA level increased 2 and 4 DAT, and decreased 10 DAT. CPPU-treated syconia released more ethylene than the control except 2 DAT. RNA-Seq and bioinformatics analysis revealed notably more differentially expressed KEGG pathways in the receptacle than in female flowers. In the phytohormone gene network, GA-biosynthesis genes GA20ox and GA3ox were upregulated, along with GA signal-transduction genes GID1 and GID2, and IAA-signaling genes AUX/IAA and GH3. ABA-biosynthesis gene NCED and signaling genes PP2C and ABF were downregulated 10 DAT. One ACO gene showed consistent upregulation in both female flowers and receptacle after CPPU treatment, and more than a dozen of ERFs demonstrated opposing changes in expression. Our results revealed early-stage spatiotemporal phytohormone and transcriptomic responses in CPPU-induced San Pedro fig main crop parthenocarpy, which could be valuable for further understanding the nature of the parthenocarpy of different fig types.

SlHY5 Integrates Temperature, Light, and Hormone Signaling to Balance Plant Growth and Cold Tolerance

During the transition from warm to cool seasons, plants experience decreased temperatures, shortened days, and decreased red/far-red (R/FR) ratios of light. The mechanism by which plants integrate these environmental cues to maintain plant growth and adaptation remains poorly understood. Here, we report that low temperature induced the transcription of PHYTOCHROME A and accumulation of LONG HYPOCOTYL5 (SlHY5, a basic Leu zipper transcription factor) in tomato (Solanum lycopersicum) plants, especially under short day conditions with low R/FR light ratios. Reverse genetic approaches and physiological analyses revealed that silencing of SlHY5 increased cold susceptibility in tomato plants, whereas overexpression of SlHY5 enhanced cold tolerance. SlHY5 directly bound to and activated the transcription of genes encoding a gibberellin-inactivation enzyme, namely GIBBERELLIN2-OXIDASE4, and an abscisic acid biosynthetic enzyme, namely 9-CIS-EPOXYCAROTENOID DIOXYGENASE6 (SlNCED6). Thus, phytochrome A-dependent SlHY5 accumulation resulted in an increased abscisic acid/gibberellin ratio, which was accompanied by growth cessation and induction of cold response. Furthermore, silencing of SlNCED6 compromises short day- and low R/FR-induced tomato resistance to cold stress. These findings provide insight into the molecular genetic mechanisms by which plants integrate environmental stimuli with hormones to coordinate their growth with impending cold temperatures. Moreover, this work reveals a molecular mechanism that plants have evolved for growth and survival in response to seasonal changes.

Diterpene synthases facilitating production of the kaurane skeleton of eriocalyxin B in the medicinal plant Isodon eriocalyx

The Isodon plants (Lamiaceae) have been used in traditional Chinese medicine to alleviate sufferings from inflammations and cancers. This feature has been attributed to the presence of pharmacologically active ent-kaurane diterpenoids such as eriocalyxin B and oridonin. The Isodon eriocalyx (Dunn) Kudô species native to southwest China can accumulate a particularly high content of ent-kaurane diterpenoids (∼1.5% w/w of dried leaves). We previously identified diterpene synthases IeCPS1 and IeCPS2 as ent-copalyl diphosphate synthases (ent-CPS) potentially involved in Isodon ent-kaurane diterpenoids biosynthesis. In this study, analysis of RNA-seq transcriptome of the I. eriocalyx plant revealed three other diterpene synthase genes (IeCPS3, IeKS1, and IeKSL1). Their functional characterization through coupled in vitro enzyme assays has confirmed that IeCPS3 is an ent-CPS specifically producing ent-copalyl diphosphate (ent-CPP). IeKS1 accepted ent-CPP to produce exclusively ent-kaurene and may thus be defined as an ent-kaurene synthase (ent-KS). When IeKSL1 was combined with IeCPS2 or IeCPS3, no product was detected. Based on tissue-specific expression and metabolic localization studies, the IeCPS3 and IeKS1 transcripts were significantly accumulated in leaves where the ent-kaurane diterpenoid eriocalyxin B dominates, whereas weak expression of both were observed in germinating seeds in which gibberellin biosynthetic pathway is normally active. Our findings suggest that both IeCPS3 and IeKS1 possess dual roles in general (gibberellins) and specialized diterpenoid metabolism, such as that of the Isodon ent-kaurane diterpenoids.

The makings of a gradient: spatiotemporal distribution of gibberellins in plant development

The gibberellin phytohormones regulate growth and development throughout the plant lifecycle. Upstream regulation and downstream responses to gibberellins vary across cells and tissues, developmental stages, environmental conditions, and plant species. The spatiotemporal distribution of gibberellins is the result of an ensemble of biosynthetic, catabolic and transport activities, each of which can be targeted to influence gibberellin levels in space and time. Understanding gibberellin distributions has recently benefited from discovery of transport proteins capable of importing gibberellins as well as novel methods for detecting gibberellins with high spatiotemporal resolution. For example, a genetically-encoded fluorescent biosensor for gibberellins was deployed in Arabidopsis and revealed gibberellin gradients in rapidly elongating tissues. Although cellular accumulations of gibberellins are hypothesized to regulate cell growth in developing embryos, germinating seeds, elongating stems and roots, and developing floral organs, understanding the quantitative relationship between cellular gibberellin levels and cellular growth awaits further investigation. It is also unclear how spatiotemporal gibberellin distributions result from myriad endogenous and environmental factors directing an ensemble of known gibberellin enzymatic and transport steps.

Aberrant Stamen Development is Associated with Parthenocarpic Fruit Set Through Up-Regulation of Gibberellin Biosynthesis in Tomato

Parthenocarpy, a process in which fruit set occurs without fertilization, leads to the production of seedless fruit. A number of floral homeotic mutants with abnormal stamen development exhibit parthenocarpic fruit set. Flower development is thought to repress ovary growth before anthesis. However, the mechanism of parthenocarpic fruit development caused by aberrant flower formation is poorly understood. To investigate the molecular mechanism of parthenocarpic fruit development in floral homeotic mutants, we performed functional analysis of Tomato APETALA3 (TAP3) by loss-of-function approaches. Organ-specific promoter was used to induce organ-specific loss of function in stamen and ovary/fruit. We observed increased cell expansion in tap3 mutants and TAP3-RNAi lines during parthenocarpic fruit growth. These were predominantly accompanied by the up-regulation of GA biosynthesis genes, including SlGA20ox1, SlGA20ox2, and SlGA20ox3, as well as reduced expression of the GA-inactivating gene SlGA2ox1 and the auxin signaling gene SlARF7 involved in a crosstalk between GA and auxin. These transcriptional profiles are in agreement with the GA levels in these lines. These results suggest that stamen development negatively regulates fruit set by repressing the GA biosynthesis.

Synthesis of Indoleacetic Acid, Gibberellic Acid and ACC-Deaminase by Mortierella Strains Promote Winter Wheat Seedlings Growth under Different Conditions

The endogenous pool of phytoregulators in plant tissues supplied with microbial secondary metabolites may be crucial for the development of winter wheat seedlings during cool springs. The phytohormones may be synthesized by psychrotrophic microorganisms in lower temperatures occurring in a temperate climate. Two fungal isolates from the Spitzbergen soils after the microscopic observations and “the internal transcribed spacer” (ITS) region molecular characterization were identified as Mortierellaantarctica (MA DEM7) and Mortierella verticillata (MV DEM32). In order to study the synthesis of indoleacetic acid (IAA) and gibberellic acid (GA), Mortierella strains were grown on media supplemented with precursor of phytohormones tryptophan at 9, 15 °C, and 20 °C for nine days. The highest amount of IAA synthesis was identified in MV DEM32 nine-day-culture at 15 °C with 1.5 mM of tryptophan. At the same temperature (15 °C), the significant promoting effect (about 40% root and shoot fresh weight) of this strain on seedlings was observed. However, only MA DEM-7 had the ACC (1-aminocyclopropane-1-carboxylate) deaminase activity with the highest efficiency at 9 °C and synthesized IAA without tryptophan. Moreover, at the same conditions, the strain was confirmed to possess the strong promoting effect (about 40% root and 24% shoot fresh weight) on seedlings. Both strains synthesized GA in all tested terms and temperatures. The studied Mortierella strains had some important traits that led them to be considered as microbial biofertilizers components, improving plant growth in difficult temperate climates.

A brassinosteroid responsive miRNA-target module regulates gibberellin biosynthesis and plant development

Plant growth and development are highly coordinated by hormones, including brassinosteroid (BR) and gibberellin (GA). Although much progress has been made in understanding the fundamental signaling transduction in BR and GA, their relationship remains elusive in rice. Here, we show that BR suppresses the level of OsmiR159d, which cleaves the target OsGAMYBL2 gene. The OsmiR159d-OsGAMYBL2 pair functions as an early BR-responsive module regulating the expression of BU1, a BR-regulated gene involved in BR signaling, and CPS1 and GA3ox2, two genes in GA biosynthesis, by binding to the promoters of these genes. Furthermore, OsGSK2, a key negative player in BR signaling, interacts with OsGAMYBL2 and prevents it from being degraded under 24-epibrassinolide treatment, whereas SLR1, a rice DELLA protein negatively regulating GA signaling, interacts with OsGAMYBL2 and prevents OsGAMYBL2 from binding to the target gene promoter. GA signaling induces degradation of OsGAMYBL2 and, consequently, enhances BR signaling. These results demonstrate that a BR-responsive module acts as a common component functioning in both BR and GA pathways, which connects BR signaling and GA biosynthesis, and thus coordinates the regulation of BR and GA in plant growth and development.

NbGIS regulates glandular trichome initiation through GA signaling in tobacco

KEY MESSAGE

A novel gene NbGIS positively regulates glandular trichome initiation through GA Signaling in tobacco. NbMYB123-like regulates glandular trichome initiation by acting downstream of NbGIS in tobacco. Glandular trichome is a specialized multicellular structure which has capability to synthesize and secrete secondary metabolites and protects plants from biotic and abiotic stresses. Our previous results revealed that a C2H2 zinc-finger transcription factor GIS and its sub-family genes act upstream of GL3/EGL3-GL1-TTG1 transcriptional activator complex to regulate trichome initiation in Arabidopsis. In this present study, we found that NbGIS could positively regulate glandular trichome development in Nicotiana benthamiana (tobacco). Our result demonstrated that 35S:NbGIS lines exhibited much higher densities of trichome on leaves, main stems, lateral branches and sepals than WT plants, while NbGIS:RNAi lines had the opposite phenotypes. Furthermore, our results also showed that NbGIS was required in response to GA signal to control glandular trichome initiation in Nicotiana benthamiana. In addition, our results also showed that NbGIS significantly influenced GA accumulation and expressions of marker genes of the GA biosynthesis, might result in the changes of growth and maturation in tobacco. Lastly, our results also showed that NbMYB123-like regulated glandular trichome initiation in tobacco by acting downstream of NbGIS. These findings provide new insights to discover the molecular mechanism by which C2H2 transcriptional factors regulates glandular trichome initiation through GA signaling pathway in tobacco.

Vegetative growth after flowering through gibberellin biosynthesis regulates pod setting rate in soybean (Glycine max (L.) Merr.)

Pod setting rate in soybean is an important trait that determines pod number, which is highly correlated with seed yield. Using two soybean cultivars with different pod setting rates, we examined the relationship between plant growth regulation by gibberellin (GA) and pod setting rate. Plant growth rate (PGR) after flowering was significantly higher in 'Fukuyutaka' (low pod setting rate) than in 'Kariyutaka' (high pod setting rate); this difference was caused by increasing of GA biosynthesis-related genes expression. Additionally, pod setting rate in 'Fukuyutaka' was lower than that in 'Kariyutaka'. Furthermore, when 'Kariyutaka' was treated with GA after flowering, the PGR increased and pod setting rate decreased. These results suggest that pod setting rate in soybean is regulated by vegetative growth after flowering through GA biosynthesis.

Silencing GRAS2 reduces fruit weight in tomato

GRAS family transcription factors are involved in multiple biological processes in plants. Here, we report that GRAS2 plays a vital role in regulating fruit weight in tomato (Solanum lycopersicum). We establish that the expression of GRAS2 was elevated in ovaries and maintained at a constant level in fertilized ovules. Reduction of GRAS2 expression in transgenic plants reduced fruit weight through modulating ovary growth and cell size. At the metabolic level, downregulation of GRAS2 decreased activities of the gibberellic acid biosynthesis and signal transduction pathways, leading to insufficient levels of active gibberellic acid during the initial ovary development of tomato. Moreover, genotypic diversity of GRAS2 was consistent with the molecular basis of fruit weight evolution, suggesting that GRAS2 contributes to the molecular basis of the evolution of fruit weight in tomato. Collectively, these findings enhance our understanding of GRAS2 functions, in fruit development of tomato, and demonstrate a strong association between the GRAS gene family and fruit development.

PIL5 represses floral transition in Arabidopsis under long day conditions

PHYTOCHROME INTERACING FACTOR 3 LIKE 5 (PIL5), also named PHYTOCHROME INTERACTING FACTOR 1 (PIF1) is an important b-HLH transcription factor in Arabidopsis thaliana. Here we show that mutant of pil5-1 displays early flowering phenotype. We demonstrate that the expressions of the major flowering promoter genes [FLOWERING LOCUS T (FT), SUPPRESOR OF OVEREXPRESSION OF CO 1 (SOC1), and LEAFY (LFY)] are upregulated in the mutant of pil5-1. There is a significant increase of the mRNA of PIL5 in the mutants of co2-1, ft-10, soc1-2, and lfy-4. These changes provide the molecular evidence that PIL5 interacts with the flowering regulators to control flowering time. Moreover, it is shown in our results that PIL5 mutation mediates the increased contents of gibberellic acid (GA). Which is further supported by the qRT-PCR analysis, an increased transcriptome level of the GA biosynthesis genes (GA3ox1, GA3ox2, GA20ox1, GA20ox2, and GA20ox3) has been observed in the pil5-1 mutants as compared to the wild type. Collectively, PIL5 is involved in floral transition interacting with flowering integrators and GA.

Isoprenoid-derived plant signaling molecules: biosynthesis and biological importance

The present review summarizes current knowledge of the biosynthesis and biological importance of isoprenoid-derived plant signaling compounds. Cellular organisms use chemical signals for intercellular communication to coordinate their growth, development, and responses to environmental cues. The skeletons of majority of plant signaling molecules, mediators of plant intercellular 'broadcasting', are built from C₅ units of isoprene and therefore belong to a huge and diverse group of natural substances called isoprenoids (terpenoids). They fill many important roles in nature. This review summarizes current knowledge of the biosynthesis and biological importance of a group of isoprenoid-derived plant signaling compounds.

Identification of the dwarf gene GmDW1 in soybean (Glycine max L.) by combining mapping-by-sequencing and linkage analysis

KEY MESSAGE

GmDW1 encodes an ent-kaurene synthase (KS) acting at the early step of the biosynthesis pathway for gibberellins (GAs) and regulates the development of plant height in soybean. Plant height is an important component of plant architecture, and significantly affects crop breeding practices and yield. Here, we report the characterization of an EMS-induced dwarf mutant (dw) of the soybean cultivar Zhongpin 661 (ZDD23893). The dw mutant displayed reduced plant height and shortened internodes, both of which were mainly attributed to the longitudinally decreased cell length. The bioactive GA1 (gibberellin A1) and GA4 (gibberellin A4) were not detectable in the stem of dw, and the dwarf phenotype could be rescued by treatment with exogenous GA3. Genetic analysis showed that the dwarf trait of dw was controlled by a recessive nuclear gene. By combining linkage analysis and mapping-by-sequencing, we mapped the GmDW1 gene to an approximately 460-kb region on chromosome (Chr.) 8, containing 36 annotated genes in the reference Willliams 82 genome. Of these genes, we identified two nonsynonymous single nucleotide polymorphisms (SNPs) that are present in the encoding regions of Gmdw1 and Glyma.08G165100 in dw, respectively. However, only the SNP mutation (T>A) at nucleotide 1224 in Gmdw1 cosegregated with the dwarf phenotype. GmDW1 encodes an ent-kaurene synthase, and was expressed in various tissues including root, stem, and leaf. Further phenotypic analysis of the allelic variations in soybean accessions strongly indicated that GmDW1 is responsible for the dwarf phenotype in dw. Our results provide important information for improving our understanding of the genetics of soybean plant height and crop breeding.

Isoprenoid-derived plant signaling molecules: biosynthesis and biological importance

The present review summarizes current knowledge of the biosynthesis and biological importance of isoprenoid-derived plant signaling compounds. Cellular organisms use chemical signals for intercellular communication to coordinate their growth, development, and responses to environmental cues. The skeletons of majority of plant signaling molecules, mediators of plant intercellular 'broadcasting', are built from C₅ units of isoprene and therefore belong to a huge and diverse group of natural substances called isoprenoids (terpenoids). They fill many important roles in nature. This review summarizes current knowledge of the biosynthesis and biological importance of a group of isoprenoid-derived plant signaling compounds.

Time-Course Transcriptomics Analysis Reveals Key Responses of Submerged Deepwater Rice to Flooding

Water submergence is an environmental factor that limits plant growth and survival. Deepwater rice (Oryza sativa) adapts to submergence by rapidly elongating its internodes and thereby maintaining its leaves above the water surface. We performed a comparative RNA sequencing transcriptome analysis of the shoot base region, including basal nodes, internodes, and shoot apices of seedlings at two developmental stages from two varieties with contrasting deepwater growth responses. A transcriptomic comparison between deepwater rice cv C9285 and nondeepwater rice cv Taichung 65 revealed both similar and differential expression patterns between the two genotypes during submergence. The expression of genes related to gibberellin biosynthesis, trehalose biosynthesis, anaerobic fermentation, cell wall modification, and transcription factors that include ethylene-responsive factors was significantly different between the varieties. Interestingly, in both varieties, the jasmonic acid content at the shoot base decreased during submergence, while exogenous jasmonic acid inhibited submergence-induced internode elongation in cv C9285, suggesting that jasmonic acid plays a role in the submergence response of rice. Furthermore, a targeted de novo transcript assembly revealed transcripts that were specific to cv C9285, including submergence-induced biotic stress-related genes. Our multifaceted transcriptome approach using the rice shoot base region illustrates a differential response to submergence between deepwater and nondeepwater rice. Jasmonic acid metabolism appears to participate in the submergence-mediated internode elongation response of deepwater rice.

Chromatin-remodeling factor OsINO80 is involved in regulation of gibberellin biosynthesis and is crucial for rice plant growth and development

The phytohormone gibberellin (GA) plays essential roles in plant growth and development. Here, we report that OsINO80, a conserved ATP-dependent chromatin-remodeling factor in rice (Oryza sativa), functions in both GA biosynthesis and diverse biological processes. OsINO80-knockdown mutants, derived from either T-DNA insertion or RNA interference, display typical GA-deficient phenotypes, including dwarfism, reduced cell length, late flowering, retarded seed germination and impaired reproductive development. Consistently, transcriptome analyses reveal that OsINO80 knockdown results in downregulation by more than two-fold of over 1,000 genes, including the GA biosynthesis genes CPS1 and GA3ox2, and the dwarf phenotype of OsINO80-knockdown mutants can be rescued by the application of exogenous GA3. Chromatin immunoprecipitation (ChIP) experiments show that OsINO80 directly binds to the chromatin of CPS1 and GA3ox2 loci. Biochemical assays establish that OsINO80 specially interacts with histone variant H2A.Z and the H2A.Z enrichments at CPS1 and GA3ox2 are decreased in OsINO80-knockdown mutants. Thus, our study identified a rice chromatin-remodeling factor, OsINO80, and demonstrated that OsINO80 is involved in regulation of the GA biosynthesis pathway and plays critical functions for many aspects of rice plant growth and development.

The Role of Arabidopsis Inositol Polyphosphate Kinase AtIPK2β in Glucose Suppression of Seed Germination and Seedling Development

Seed germination and subsequent seedling development are critical phases in plants. These processes are regulated by a complex molecular network in which sugar has been reported to play an essential role. However, factors affecting sugar responses remain to be fully elucidated. In this study, we demonstrate that AtIPK2β, known to participate in the synthesis of myo-inositol 1,2,3,4,5,6-hexakisphosphate (IP6, phytate), affects Arabidopsis responses to glucose during seed germination. The loss-of-function mutant atipk2β showed increased sensitivity to 6% glucose and paclobutrazol (PAC). Yeast two-hybrid assay showed that AtIPK2β interacts with sucrose non-fermenting-1-related protein kinase (SnRK1.1), and bimolecular fluorescence complementation (BiFC) and pull-down assay further confirmed this interaction. Moreover, AtIPK2β was phosphorylated by SnRK1.1 in vitro, and the effect of restoring AtIPK2β to yeast cells lacking IPK2 (Δipk2) was abolished by catalytically active SnRK1.1. Further analysis indicated that IP6 reduces the suppression of seed germination caused by glucose, accompanied by altered expression levels of glucose-/hormone-responsive genes. Collectively, these findings indicate that AtIPK2β and IP6 are involved in glucose suppression of seed germination and that AtIPK2β enzyme activity is likely to be regulated by SnRK1.1.

Comprehensive Transcriptome Analysis of Phytohormone Biosynthesis and Signaling Genes in the Flowers of Chinese Chinquapin (Castanea henryi)

Chinese chinquapin (Castanea henryi) nut provides a rich source of starch and nutrients as food and feed, but its yield is restricted by a low ratio of female to male flowers. Little is known about the developmental programs underlying sex differentiation of the flowers. To investigate the involvement of phytohormones during sex differentiation, we described the morphology of male and female floral organs and the cytology of flower sex differentiation, analyzed endogenous levels of indole-3-acetic acid (IAA), gibberellins (GAs), cytokinins (CKs), and abscisic acid (ABA) in the flowers, investigated the effects of exogenous hormones on flower development, and evaluated the expression profiles of genes related to biosyntheses and signaling pathways of these four hormones using RNA-Seq combined with qPCR. Morphological results showed that the flowers consisted of unisexual and bisexual catkins, and could be divided into four developmental stages. HPLC results showed that CK accumulated much more in the female flowers than that in the male flowers, GA and ABA showed the opposite results, while IAA did not show a tendency. The effects of exogenous hormones on sex differentiation were consistent with those of endogenous hormones. RNA-Seq combined with qPCR analyses suggest that several genes may play key roles in hormone biosynthesis and sex differentiation. This study presents the first comprehensive report of phytohormone biosynthesis and signaling during sex differentiation of C. henryi, which should provide a foundation for further mechanistic studies of sex differentiation in Castanea Miller species and other nonmodel plants.

Linking hydrogen-enhanced rice aluminum tolerance with the reestablishment of GA/ABA balance and miRNA-modulated gene expression: A case study on germination

Although previous results showed that exogenous hydrogen (H₂) alleviated aluminum (Al) toxicity, the detailed mechanism remains unclear. Here, we reported that the exposure of germinating rice seeds to Al triggered H₂ production, followed by a decrease of GA/ABA ratio and seed germination inhibition. Compared to inert gas (argon), H₂ pretreatment not only strengthened H₂ production and alleviated Al-induced germination inhibition, but also partially reestablished the balance between GA and ABA. By contrast, a GA biosynthesis inhibitor paclobutrazol (PAC) could block the H₂-alleviated germination inhibition. The expression of GA biosynthesis genes (GA20ox1 and GA20ox2) and ABA catabolism genes (ABA8ox1 and ABA8ox2), was also induced by H₂. Above results indicated that GA/ABA might be partially involved in H₂ responses. Subsequent results revealed that compared with Al alone, transcripts of miR398a and miR159a were decreased by H₂, and expression levels of their target genes OsSOD2 and OsGAMYB were up-regulated. Whereas, miR528 and miR160a transcripts were increased differentially, and contrasting tendencies were observed in the changes of their target genes (OsAO and OsARF10). The transcripts of Al-tolerant gene OsSTAR1/OsSTAR2 and OsFRDL4 were up-regulated. Above results were consistent with the anti-oxidant defense, decreased Al accumulation, and enhanced citrate efflux. Together, our results provided insight into the mechanism underlying H₂-triggered Al tolerance in plants.

Exogenous auxin represses soybean seed germination through decreasing the gibberellin/abscisic acid (GA/ABA) ratio

Auxin is an important phytohormone which mediates diverse development processes in plants. Published research has demonstrated that auxin induces seed dormancy. However, the precise mechanisms underlying the effect of auxin on seed germination need further investigation, especially the relationship between auxins and both abscisic acid (ABA) and gibberellins (GAs), the latter two phytohormones being the key regulators of seed germination. Here we report that exogenous auxin treatment represses soybean seed germination by enhancing ABA biosynthesis, while impairing GA biogenesis, and finally decreasing GA1/ABA and GA4/ABA ratios. Microscope observation showed that auxin treatment delayed rupture of the soybean seed coat and radicle protrusion. qPCR assay revealed that transcription of the genes involved in ABA biosynthetic pathway was up-regulated by application of auxin, while expression of genes involved in GA biosynthetic pathway was down-regulated. Accordingly, further phytohormone quantification shows that auxin significantly increased ABA content, whereas the active GA1 and GA4 levels were decreased, resulting insignificant decreases in the ratiosGA1/ABA and GA4/ABA.Consistent with this, ABA biosynthesis inhibitor fluridone reversed the delayed-germination phenotype associated with auxin treatment, while paclobutrazol, a GA biosynthesis inhibitor, inhibited soybean seed germination. Altogether, exogenous auxin represses soybean seed germination by mediating ABA and GA biosynthesis.

Gene expression and metabolite profiling of gibberellin biosynthesis during induction of somatic embryogenesis in Medicago truncatula Gaertn

Gibberellins (GAs) are involved in the regulation of numerous developmental processes in plants including zygotic embryogenesis, but their biosynthesis and role during somatic embryogenesis (SE) is mostly unknown. In this study we show that during three week- long induction phase, when cells of leaf explants from non-embryogenic genotype (M9) and embryogenic variant (M9-10a) were forming the callus, all the bioactive gibberellins from non-13-hydroxylation (GA4, GA7) and 13-hydroxylation (GA1, GA5, GA3, GA6) pathways were present, but the contents of only a few of them differed between the tested lines. The GA53 and GA19 substrates synthesized by the 13-hydroxylation pathway accumulated specifically in the M9-10a line after the first week of induction; subsequently, among the bioactive gibberellins detected, only the content of GA3 increased and appeared to be connected with acquisition of embryogenic competence. We fully annotated 20 Medicago truncatula orthologous genes coding the enzymes which catalyze all the known reactions of gibberellin biosynthesis. Our results indicate that, within all the genes tested, expression of only three: MtCPS, MtGA3ox1 and MtGA3ox2, was specific to embryogenic explants and reflected the changes observed in GA53, GA19 and GA3 contents. Moreover, by analyzing expression of MtBBM, SE marker gene, we confirmed the inhibitory effect of manipulation in GAs metabolism, applying exogenous GA3, which not only impaired the production of somatic embryos, but also significantly decreased expression of this gene.

An operon for production of bioactive gibberellin A4 phytohormone with wide distribution in the bacterial rice leaf streak pathogen Xanthomonas oryzae pv. oryzicola

Phytopathogens have developed elaborate mechanisms to attenuate the defense response of their host plants, including convergent evolution of complex pathways for production of the GA phytohormones, which were actually first isolated from the rice fungal pathogen Gibberella fujikuroi. The rice bacterial pathogen Xanthomonas oryzae pv. oryzicola (Xoc) has been demonstrated to contain a biosynthetic operon with cyclases capable of producing the universal GA precursor ent-kaurene. Genetic (knock-out) studies indicate that the derived diterpenoid serves as a virulence factor for this rice leaf streak pathogen, serving to reduce the jasmonic acid-mediated defense response. Here the functions of the remaining genes in the Xoc operon are elucidated and the distribution of the operon in X. oryzae is investigated in over 100 isolates. The Xoc operon leads to production of the bioactive GA4 , an additional step beyond production of the penultimate precursor GA9 mediated by the homologous operons recently characterized from rhizobia. Moreover, this GA biosynthetic operon was found to be widespread in Xoc (> 90%), but absent in the other major X. oryzae pathovar. These results indicate selective pressure for production of GA4 in the distinct lifestyle of Xoc, and the importance of GA to both fungal and bacterial pathogens of rice.

Impact of blue, red, and far-red light treatments on gene expression and steviol glycoside accumulation in Stevia rebaudiana

Stevia rebaudiana (Bertoni) Bertoni is a plant that biosynthesizes a group of natural sweeteners that are up to approximately 400 times sweeter than sucrose. The sweetening components of S. rebaudiana are steviol glycosides (SGs) that partially share their biosynthesis pathway with gibberellins (GAs). However, the molecular mechanisms through which SGs levels can be improved have not been studied. Therefore, transcription levels of several SG biosynthesis-related genes were analyzed under several light treatments involved in GA biosynthesis. We detected higher transcription of UGT85C2, which is one of the UDP-glycosyltransferases (UGTs) involved in catalyzing the sugar-transfer reaction, under red/far-red (R/FR) 1.22 light-emitting diodes (LEDs) and blue LEDs treatment. In this study, it was demonstrated that transcription levels of SG-related genes and the SGs content are affected by light treatments known to affect the GA contents. It is expected that this approach could serve as a practical way to increase SG contents using specific light treatments.

Investigating the Phylogenetic Range of Gibberellin Biosynthesis in Bacteria

Certain plant-associated microbes can produce gibberellin (GA) phytohormones, as first described for the rice fungal pathogen Gibberella fujikuroi and, more recently, for bacteria, including several rhizobia and the rice bacterial pathogen Xanthomonas oryzae pv. oryzicola. The relevant enzymes are encoded by a biosynthetic operon that exhibits both a greater phylogenetic range and scattered distribution among plant-associated bacteria. Here, the phylogenetic distribution of this operon was investigated. To demonstrate conserved functionality, the enzymes encoded by the disparate operon from X. translucens pv. translucens, along with those from the most divergent example, found in Erwinia tracheiphila, were biochemically characterized. In both of these phytopathogens, the operon leads to production of the bioactive GA4. Based on these results, it seems that this operon is widely dedicated to GA biosynthesis. However, there is intriguing variation in the exact product. In particular, although all plant pathogens seem to produce bioactive GA4, rhizobia generally only produce the penultimate hormonal precursor GA9. This is suggested to reflect their distinct interactions with plants, because production of GA4 counteracts the jasmonic-acid-mediated defense response, reflecting the importance of wounds as the entry point for these phytopathogens, whereas such suppression presumably is detrimental in the rhizobial symbiotic relationship.

Activation of Mitochondrial Protein Phosphatase SLP2 by MIA40 Regulates Seed Germination

Reversible protein phosphorylation catalyzed by protein kinases and phosphatases represents the most prolific and well-characterized posttranslational modification known. Here, we demonstrate that Arabidopsis (Arabidopsis thaliana) Shewanella-like protein phosphatase 2 (AtSLP2) is a bona fide Ser/Thr protein phosphatase that is targeted to the mitochondrial intermembrane space (IMS) where it interacts with the mitochondrial oxidoreductase import and assembly protein 40 (AtMIA40), forming a protein complex. Interaction with AtMIA40 is necessary for the phosphatase activity of AtSLP2 and is dependent on the formation of disulfide bridges on AtSLP2. Furthermore, by utilizing atslp2 null mutant, AtSLP2 complemented and AtSLP2 overexpressing plants, we identify a function for the AtSLP2-AtMIA40 complex in negatively regulating gibberellic acid-related processes during seed germination. Results presented here characterize a mitochondrial IMS-localized protein phosphatase identified in photosynthetic eukaryotes as well as a protein phosphatase target of the highly conserved eukaryotic MIA40 IMS oxidoreductase.

Gene expression and metabolite profiling of gibberellin biosynthesis during induction of somatic embryogenesis in Medicago truncatula Gaertn

Gibberellins (GAs) are involved in the regulation of numerous developmental processes in plants including zygotic embryogenesis, but their biosynthesis and role during somatic embryogenesis (SE) is mostly unknown. In this study we show that during three week- long induction phase, when cells of leaf explants from non-embryogenic genotype (M9) and embryogenic variant (M9-10a) were forming the callus, all the bioactive gibberellins from non-13-hydroxylation (GA4, GA7) and 13-hydroxylation (GA1, GA5, GA3, GA6) pathways were present, but the contents of only a few of them differed between the tested lines. The GA53 and GA19 substrates synthesized by the 13-hydroxylation pathway accumulated specifically in the M9-10a line after the first week of induction; subsequently, among the bioactive gibberellins detected, only the content of GA3 increased and appeared to be connected with acquisition of embryogenic competence. We fully annotated 20 Medicago truncatula orthologous genes coding the enzymes which catalyze all the known reactions of gibberellin biosynthesis. Our results indicate that, within all the genes tested, expression of only three: MtCPS, MtGA3ox1 and MtGA3ox2, was specific to embryogenic explants and reflected the changes observed in GA53, GA19 and GA3 contents. Moreover, by analyzing expression of MtBBM, SE marker gene, we confirmed the inhibitory effect of manipulation in GAs metabolism, applying exogenous GA3, which not only impaired the production of somatic embryos, but also significantly decreased expression of this gene.

Gibberellins producing Bacillus methylotrophicus KE2 supports plant growth and enhances nutritional metabolites and food values of lettuce

The nutritional quality of green leafy vegetables can be enhanced by application of plant beneficial micro-organisms. The present study was aimed to increase the food values of lettuce leaves by bacterial treatment. We isolated bacterial strain KE2 from Kimchi food and identified as Bacillus methylotrophicus by phylogenetic analysis. The beneficial effect of B. methylotrophicus KE2 on plants was confirmed by increasing the percentage of seed germination of Lactuca sativa L., Cucumis melo L., Glycine max L. and Brassica juncea L. It might be the secretion of array of gibberellins (GA1, GA3, GA7, GA8, GA9, GA12, GA19, GA20, GA24, GA34 and GA53) and indole-acetic acid from B. methylotrophicus KE2. The mechanism of plant growth promotion via their secreted metabolites was confirmed by a significant increase of GA deficient mutant rice plant growth. Moreover, the bacterial association was favor to enhance shoot length, shoot fresh weight and leaf width of lettuce. The higher concentration of protein, amino acids (Asp, Thr, Ser, Glu, Gly, Ala, Leu, Tyr and His), gama-aminobutric acid and fructose was found in bacterial culture (KE2) applied plants. The macro and micro minerals such as K, Mg, Na, P, Fe, Zn and N were also detected as significantly higher quantities in bacteria treated plants than untreated control plants. In addition, the carotenoids and chlorophyll a were also increased in lettuce at bacterial inoculation. The results of this study suggest that B. methylotrophicus KE2 application to soil helps to increase the plant growth and food values of lettuce.

Seed-borne endophytic Bacillus amyloliquefaciens RWL-1 produces gibberellins and regulates endogenous phytohormones of Oryza sativa

Some microorganisms are adapted to an endophytic mode, living symbiotically with plants through vertical transmission in seeds. The role of plant growth-promoting endophytes has been well studied, but those of seed-associated endophytic bacteria are less understood. The current study aimed to isolate and identify bacterial endophytes associated with rice (Oryza sativa L. 'Jin so mi') seeds, their potential to produce gibberellins (GAs), and role in improving host-plant physiology. The isolated bacterial endophyte RWL-1 was identified as Bacillus amyloliquefaciens by using 16S rRNA sequencing and phylogenetic analysis. The pure culture of B. amyloliquefaciens RWL-1, supplied with deuterated internal standards, was subjected to gas chromatography and mass spectrometric selected ion monitoring (GC-MS/SIM) for quantification of GAs. Results showed the presence of GAs in various quantities (ng/mL) viz., GA20 (17.88 ± 4.04), GA36 (5.75 ± 2.36), GA24 (5.64 ± 2.46), GA4 (1.02 ± 0.16), GA53 (0.772 ± 0.20), GA9 (0.12 ± 0.09), GA19 (0.093 ± 0.13), GA5 (0.08 ± 0.04), GA12 (0.014 ± 0.34), and GA8 (0.013 ± 0.01). Since endogenous seed GAs are essential for prolonged seed growth and subsequent plant development, we used exogenous GA3 as a positive control and water as a negative control for comparative analysis of the application of B. amyloliquefaciens RWL-1 to rice plants. The growth parameters of rice plants treated with endophytic bacterial cell application was significantly increased compared to the plants treated with exogenous GA3 and water. This was also revealed by the significant up-regulation of endogenous GA1 (17.54 ± 2.40 ng), GA4 (310 ± 5.41 ng), GA7 (192.60 ± 3.32 ng), and GA9 (19.04 ± 2.49 ng) as compared to results of the positive and negative control treatments. Rice plants inoculated with B. amyloliquefaciens RWL-1 exhibited significantly higher endogenous salicylic acid (1615.06 ± 10.81 μg), whereas endogenous abscisic acid (23.31 ± 2.76 ng) and jasmonic acid (25.51 ± 4.20 ng) were observed to be significantly lower in these inoculated plants than in those treated with exogenous GA3 and water. Results of the present study suggest that B. amyloliquefaciens RWL-1 has the ability to produce GAs and that its inoculation in seedlings can be beneficial to rice plants. Broader field trials should be conducted to determine its use as an alternative biofertilizer.

The ERF11 Transcription Factor Promotes Internode Elongation by Activating Gibberellin Biosynthesis and Signaling

The phytohormone gibberellin (GA) plays a key role in promoting stem elongation in plants. Previous studies show that GA activates its signaling pathway by inducing rapid degradation of DELLA proteins, GA signaling repressors. Using an activation-tagging screen in a reduced-GA mutant ga1-6 background, we identified AtERF11 to be a novel positive regulator of both GA biosynthesis and GA signaling for internode elongation. Overexpression of AtERF11 partially rescued the dwarf phenotype of ga1-6 AtERF11 is a member of the ERF (ETHYLENE RESPONSE FACTOR) subfamily VIII-B-1a of ERF/AP2 transcription factors in Arabidopsis (Arabidopsis thaliana). Overexpression of AtERF11 resulted in elevated bioactive GA levels by up-regulating expression of GA3ox1 and GA20ox genes. Hypocotyl elongation assays further showed that overexpression of AtERF11 conferred elevated GA response, whereas loss-of-function erf11 and erf11 erf4 mutants displayed reduced GA response. In addition, yeast two-hybrid, coimmunoprecipitation, and transient expression assays showed that AtERF11 enhances GA signaling by antagonizing the function of DELLA proteins via direct protein-protein interaction. Interestingly, AtERF11 overexpression also caused a reduction in the levels of another phytohormone ethylene in the growing stem, consistent with recent finding showing that AtERF11 represses transcription of ethylene biosynthesis ACS genes. The effect of AtERF11 on promoting GA biosynthesis gene expression is likely via its repressive function on ethylene biosynthesis. These results suggest that AtERF11 plays a dual role in promoting internode elongation by inhibiting ethylene biosynthesis and activating GA biosynthesis and signaling pathways.

Genome-wide identification and expression profiling reveal tissue-specific expression and differentially-regulated genes involved in gibberellin metabolism between Williams banana and its dwarf mutant

BACKGROUND

Dwarfism is one of the most valuable traits in banana breeding because semi-dwarf cultivars show good resistance to damage by wind and rain. Moreover, these cultivars present advantages of convenient cultivation, management, and so on. We obtained a dwarf mutant '8818-1' through EMS (ethyl methane sulphonate) mutagenesis of Williams banana 8818 (Musa spp. AAA group). Our research have shown that gibberellins (GAs) content in 8818-1 false stems was significantly lower than that in its parent 8818 and the dwarf type of 8818-1 could be restored by application of exogenous GA3. Although GA exerts important impacts on the 8818-1 dwarf type, our understanding of the regulation of GA metabolism during banana dwarf mutant development remains limited.

RESULTS

Genome-wide screening revealed 36 candidate GA metabolism genes were systematically identified for the first time; these genes included 3 MaCPS, 2 MaKS, 1 MaKO, 2 MaKAO, 10 MaGA20ox, 4 MaGA3ox, and 14 MaGA2ox genes. Phylogenetic tree and conserved protein domain analyses showed sequence conservation and divergence. GA metabolism genes exhibited tissue-specific expression patterns. Early GA biosynthesis genes were constitutively expressed but presented differential regulation in different tissues in Williams banana. GA oxidase family genes were mainly transcribed in young fruits, thus suggesting that young fruits were the most active tissue involved in GA metabolism, followed by leaves, bracts, and finally approximately mature fruits. Expression patterns between 8818 and 8818-1 revealed that MaGA20ox4, MaGA20ox5, and MaGA20ox7 of the MaGA20ox gene family and MaGA2ox7, MaGA2ox12, and MaGA2ox14 of the MaGA2ox gene family exhibited significant differential expression and high-expression levels in false stems. These genes are likely to be responsible for the regulation of GAs content in 8818-1 false stems.

CONCLUSION

Overall, phylogenetic evolution, tissue specificity and differential expression analyses of GA metabolism genes can provide a better understanding of GA-regulated development in banana. The present results revealed that MaGA20ox4, MaGA20ox5, MaGA20ox7, MaGA2ox7, MaGA2ox12, and MaGA2ox14 were the main genes regulating GA content difference between 8818 and 8818-1. All of these genes may perform important functions in the developmental processes of banana, but each gene may perform different functions in different tissues or during different developmental stages.

Gibberellic Acid: A Key Phytohormone for Spikelet Fertility in Rice Grain Production

The phytohormone gibberellic acid (GA) has essential signaling functions in multiple processes during plant development. In the "Green Revolution", breeders developed high-yield rice cultivars that exhibited both semi-dwarfism and altered GA responses, thus improving grain production. Most studies of GA have concentrated on germination and cell elongation, but GA also has a pivotal role in floral organ development, particularly in stamen/anther formation. In rice, GA signaling plays an important role in spikelet fertility; however, the molecular genetic and biochemical mechanisms of GA in male fertility remain largely unknown. Here, we review recent progress in understanding the network of GA signaling and its connection with spikelet fertility, which is tightly associated with grain productivity in cereal crops.

Role of reactive oxygen species produced by NADPH oxidase in gibberellin biosynthesis during barley seed germination

NADPH oxidase catalyzes the production of the superoxide anion (O2(-)), a reactive oxygen species (ROS), and regulates the germination of barley (Hordeum vulgare L.). Diphenyleneiodonium (DPI) chloride, an NADPH oxidase inhibitor, delayed barley germination, and exogenous H2O2 (an ROS) partially rescued it. Six enzymes, ent-copalyl diphosphate synthase (CPS), ent-kaurene synthase (KS), ent-kaurene oxidase (KO), ent-kaurenoic acid oxidase (KAO), GA20-oxidase (GA20ox) and GA3-oxidase (GA3ox), catalyze the transformation of trans-geranylgeranyl diphosphate to active gibberellin, which promotes germination. Exogenous H2O2 promoted the expressions of HvKAO1 and HvGA3ox1 in barley embryos. These results suggest that ROS produced by NADPH oxidase are involved in gibberellin biosynthesis through the regulation of HvKAO1 and HvGA3ox1.

Modularity of Conifer Diterpene Resin Acid Biosynthesis: P450 Enzymes of Different CYP720B Clades Use Alternative Substrates and Converge on the Same Products

Cytochrome P450 enzymes of the CYP720B subfamily play a central role in the biosynthesis of diterpene resin acids (DRAs), which are a major component of the conifer oleoresin defense system. CYP720Bs exist in families of up to a dozen different members in conifer genomes and fall into four different clades (I-IV). Only two CYP720B members, loblolly pine (Pinus taeda) PtCYP720B1 and Sitka spruce (Picea sitchensis) PsCYP720B4, have been characterized previously. Both are multisubstrate and multifunctional clade III enzymes, which catalyze consecutive three-step oxidations in the conversion of diterpene olefins to DRAs. These reactions resemble the sequential diterpene oxidations affording ent-kaurenoic acid from ent-kaurene in gibberellin biosynthesis. Here, we functionally characterized the CYP720B clade I enzymes CYP720B2 and CYP720B12 in three different conifer species, Sitka spruce, lodgepole pine (Pinus contorta), and jack pine (Pinus banksiana), and compared their activities with those of the clade III enzymes CYP720B1 and CYP720B4 of the same species. Unlike the clade III enzymes, clade I enzymes were ultimately found not to be active with diterpene olefins but converted the recently discovered, unstable diterpene synthase product 13-hydroxy-8(14)-abietene. Through alternative routes, CYP720B enzymes of both clades produce some of the same profiles of conifer oleoresin DRAs (abietic acid, neoabietic acid, levopimaric acid, and palustric acid), while clade III enzymes also function in the formation of pimaric acid, isopimaric acid, and sandaracopimaric acid. These results highlight the modularity of the specialized (i.e. secondary) diterpene metabolism, which produces conifer defense metabolites through variable combinations of different diterpene synthase and CYP720B enzymes.

Insight on Genes Affecting Tuber Development in Potato upon Potato spindle tuber viroid (PSTVd) Infection

Potato (Solanum tuberosum L) is a natural host of Potato spindle tuber viroid (PSTVd) which can cause characteristic symptoms on developing plants including stunting phenotype and distortion of leaves and tubers. PSTVd is the type species of the family Pospiviroidae, and can replicate in the nucleus and move systemically throughout the plant. It is not well understood how the viroid can affect host genes for successful invasion and which genes show altered expression levels upon infection. Our primary focus in this study is the identification of genes which can affect tuber formation since viroid infection can strongly influence tuber development and especially tuber shape. In this study, we used a large-scale method to identify differentially expressed genes in potato. We have identified defence, stress and sugar metabolism related genes having altered expression levels upon infection. Additionally, hormone pathway related genes showed significant up- or down-regulation. DWARF1/DIMINUTO, Gibberellin 7-oxidase and BEL5 transcripts were identified and validated showing differential expression in viroid infected tissues. Our study suggests that gibberellin and brassinosteroid pathways have a possible role in tuber development upon PSTVd infection.

OsWOX3A is involved in negative feedback regulation of the gibberellic acid biosynthetic pathway in rice (Oryza sativa)

The plant-specific WUSCHEL-related homeobox (WOX) nuclear proteins have important roles in the transcriptional regulation of many developmental processes. Among the rice (Oryza sativa) WOX proteins, a loss of OsWOX3A function in narrow leaf2 (nal2) nal3 double mutants (termed nal2/3) causes pleiotropic effects, such as narrow and curly leaves, opened spikelets, narrow grains, more tillers, and fewer lateral roots, but almost normal plant height. To examine OsWOX3A function in more detail, transgenic rice overexpressing OsWOX3A (OsWOX3A-OX) were generated; unexpectedly, all of them consistently exhibited severe dwarfism with very short and wide leaves, a phenotype that resembles that of gibberellic acid (GA)-deficient or GA-insensitive mutants. Exogenous GA3 treatment fully rescued the developmental defects of OsWOX3A-OX plants, suggesting that constitutive overexpression of OsWOX3A downregulates GA biosynthesis. Quantitative analysis of GA intermediates revealed significantly reduced levels of GA20 and bioactive GA1 in OsWOX3A-OX, possibly due to downregulation of the expression of KAO, which encodes ent-kaurenoic acid oxidase, a GA biosynthetic enzyme. Yeast one-hybrid and electrophoretic mobility shift assays revealed that OsWOX3A directly interacts with the KAO promoter. OsWOX3A expression is drastically and temporarily upregulated by GA3 and downregulated by paclobutrazol, a blocker of GA biosynthesis. These data indicate that OsWOX3A is a GA-responsive gene and functions in the negative feedback regulation of the GA biosynthetic pathway for GA homeostasis to maintain the threshold levels of endogenous GA intermediates throughout development.

Karrikins delay soybean seed germination by mediating abscisic acid and gibberellin biogenesis under shaded conditions

Karrikins (KAR) are a class of signal compounds, discovered in wildfire smoke, which affect seed germination. Currently, numerous studies have focused on the model plant Arabidopsis in the KAR research field, rather than on crops. Thus the regulatory mechanisms underlying KAR regulation of crop seed germination are largely unknown. Here, we report that KAR delayed soybean seed germination through enhancing abscisic acid (ABA) biosynthesis, while impairing gibberellin (GA) biogenesis. Interestingly, KAR only retarded soybean seed germination under shaded conditions, rather than under dark and white light conditions, which differs from in Arabidopsis. Phytohormone quantification showed that KAR enhanced ABA biogenesis while impairing GA biosynthesis during the seed imbibition process, and subsequently, the ratio of active GA4 to ABA was significantly reduced. Further qRT-PCR analysis showed that the transcription pattern of genes involved in ABA and GA metabolic pathways are consistent with the hormonal measurements. Finally, fluridone, an ABA biogenesis inhibitor, remarkably rescued the delayed-germination phenotype of KAR-treatment; and paclobutrazol, a GA biosynthesis inhibitor, inhibited soybean seed germination. Taken together, these evidences suggest that KAR inhibit soybean seed germination by mediating the ratio between GA and ABA biogenesis.

Wounding induces changes in cytokinin and auxin content in potato tuber, but does not induce formation of gibberellins

Cytokinin, auxin and gibberellin contents in resting and wound-responding potato tubers have not been fully determined and coordinated with wound-healing processes. Using a well-defined wound-healing model system, hormone content and expression of genes associated with hormone turnover were determined in tubers following wounding. Changes in hormone content were coordinated with: (I) formation and completion of the wound closing layer (0-5/6 days), and (II) initiation of phellogen and wound periderm formation (∼ 7 days). Quantifiable amounts of biologically active cytokinins (Z, DZ and IP) were not detected in resting or wound-responding tubers. However, the precursor IPA and catabolic product c-ZOG were found in small amounts in resting and wound-responding tubers. Wound-induced activation of cytokinin biosynthesis was suggested by an increase in t-ZR and c-ZR content at 0.5 days and large increases in IPA and c-ZR content by 3 days and throughout 7 days after wounding suggesting roles in II, but little or no role in I. Expression of key genes involved in cytokinin metabolism followed similar profiles with transcripts decreasing through 3 days and then increasing at 5-7 days after wounding. Both free IAA and IAA-Asp were present in resting tubers. While IAA-Asp was no longer present by 3 days after wounding, IAA content nearly doubled by 5 days and was more than 4-fold greater at 7 days compared to that in resting tuber (0 day) suggesting roles in II, but little or no role in I. Gibberellins were not present in quantifiable amounts in resting or wound-responding tubers. These results suggest that bio-active cytokinins are wound-induced, but their residency is temporal and highly regulated. The transient presence of active cytokinins and corresponding increases in IAA content strongly suggest their involvement in the regulation of wound periderm development. The absence of gibberellins indicates that they are not a regulatory component of wound-healing processes.

Genetic Variation for Thermotolerance in Lettuce Seed Germination Is Associated with Temperature-Sensitive Regulation of ETHYLENE RESPONSE FACTOR1 (ERF1)

Seeds of most lettuce (Lactuca sativa) cultivars are susceptible to thermoinhibition, or failure to germinate at temperatures above approximately 28°C, creating problems for crop establishment in the field. Identifying genes controlling thermoinhibition would enable the development of cultivars lacking this trait and, therefore, being less sensitive to high temperatures during planting. Seeds of a primitive accession (PI251246) of lettuce exhibited high-temperature germination capacity up to 33°C. Screening a recombinant inbred line population developed from PI215246 and cv Salinas identified a major quantitative trait locus (Htg9.1) from PI251246 associated with the high-temperature germination phenotype. Further genetic analyses discovered a tight linkage of the Htg9.1 phenotype with a specific DNA marker (NM4182) located on a single genomic sequence scaffold. Expression analyses of the 44 genes encoded in this genomic region revealed that only a homolog of Arabidopsis (Arabidopsis thaliana) ETHYLENE RESPONSE FACTOR1 (termed LsERF1) was differentially expressed between PI251246 and cv Salinas seeds imbibed at high temperature (30°C). LsERF1 belongs to a large family of transcription factors associated with the ethylene-signaling pathway. Physiological assays of ethylene synthesis, response, and action in parental and near-isogenic Htg9.1 genotypes strongly implicate LsERF1 as the gene responsible for the Htg9.1 phenotype, consistent with the established role for ethylene in germination thermotolerance of Compositae seeds. Expression analyses of genes associated with the abscisic acid and gibberellin biosynthetic pathways and results of biosynthetic inhibitor and hormone response experiments also support the hypothesis that differential regulation of LsERF1 expression in PI251246 seeds elevates their upper temperature limit for germination through interactions among pathways regulated by these hormones. Our results support a model in which LsERF1 acts through the promotion of gibberellin biosynthesis to counter the inhibitory effects of abscisic acid and, therefore, promote germination at high temperatures.

A Role for Reactive Oxygen Species Produced by NADPH Oxidases in the Embryo and Aleurone Cells in Barley Seed Germination

Reactive oxygen species (ROS) promote the germination of several seeds, and antioxidants suppress it. However, questions remain regarding the role and production mechanism of ROS in seed germination. Here, we focused on NADPH oxidases, which produce ROS. After imbibition, NADPH oxidase mRNAs were expressed in the embryo and in aleurone cells of barley seed; these expression sites were consistent with the sites of ROS production in the seed after imbibition. To clarify the role of NADPH oxidases in barley seed germination, we examined gibberellic acid (GA) / abscisic acid (ABA) metabolism and signaling in barley seeds treated with diphenylene iodonium chloride (DPI), an NADPH oxidase inhibitor. DPI significantly suppressed germination, and suppressed GA biosynthesis and ABA catabolism in embryos. GA, but not ABA, induced NADPH oxidase activity in aleurone cells. Additionally, DPI suppressed the early induction of α-amylase by GA in aleurone cells. These results suggest that ROS produced by NADPH oxidases promote GA biosynthesis in embryos, that GA induces and activates NADPH oxidases in aleurone cells, and that ROS produced by NADPH oxidases induce α-amylase in aleurone cells. We conclude that the ROS generated by NADPH oxidases regulate barley seed germination through GA / ABA metabolism and signaling in embryo and aleurone cells.

Jasmonic Acid, Abscisic Acid, and Salicylic Acid Are Involved in the Phytoalexin Responses of Rice to Fusarium fujikuroi, a High Gibberellin Producer Pathogen

Fusarium fujikuroi, the causal agent of bakanae disease, is the main seedborne pathogen on rice. To understand the basis of rice resistance, a quantitative method to simultaneously detect phytohormones and phytoalexins was developed by using HPLC-MS/MS. With this method dynamic profiles and possible interactions of defense-related phytohormones and phytoalexins were investigated on two rice cultivars, inoculated or not with F. fujikuroi. In the resistant cultivar Selenio, the presence of pathogen induced high production of phytoalexins, mainly sakuranetin, and symptoms of bakanae were not observed. On the contrary, in the susceptible genotype Dorella, the pathogen induced the production of gibberellin and abscisic acid and inhibited jasmonic acid production, phytoalexins were very low, and bakanae symptoms were observed. The results suggested that a wide range of secondary metabolites are involved in plant defense against pathogens and phytoalexin synthesis could be an important factor for rice resistance against bakanae disease.

Loss of Arabidopsis thaliana Seed Dormancy is Associated with Increased Accumulation of the GID1 GA Hormone Receptors

Dormancy prevents seeds from germinating under favorable conditions until they have experienced dormancy-breaking conditions, such as after-ripening through a period of dry storage or cold imbibition. Abscisic acid (ABA) hormone signaling establishes and maintains seed dormancy, whereas gibberellin (GA) signaling stimulates germination. ABA levels decrease and GA levels increase with after-ripening and cold stratification. However, increasing GA sensitivity may also be critical to dormancy loss since increasing seed GA levels are detectable only with long periods of after-ripening and imbibition. After-ripening and cold stratification act additively to enhance GA hormone sensitivity in ga1-3 seeds that cannot synthesize GA. Since the overexpression of the GA receptor GID1 (GIBBERELLIN-INSENSITIVE DWARF1) enhanced this dormancy loss, and because gid1a gid1b gid1c triple mutants show decreased germination, the effects of dormancy-breaking treatments on GID1 mRNA and protein accumulation were examined. Partial after-ripening resulted in increased GID1b, but not GID1a or GID1c mRNA levels. Cold imbibition stimulated the accumulation of all three GID1 transcripts, but resulted in no increase in GA sensitivity during ga1-3 seed germination unless seeds were also partially after-ripened. This is probably because after-ripening was needed to enhance GID1 protein accumulation, independently of transcript abundance. The rise in GID1b transcript with after-ripening was not associated with decreased ABA levels, suggesting there is ABA-independent GID1b regulation by after-ripening and the 26S proteasome. GA and the DELLA RGL2 repressor of GA responses differentially regulated the three GID1 transcripts. Moreover, DELLA RGL2 appeared to switch between positive and negative regulation of GID1 expression in response to dormancy-breaking treatments.

Loss of function mutations in the rice chromomethylase OsCMT3a cause a burst of transposition

Methylation patterns of plants are unique as, in addition to the methylation at CG dinucleotides that occurs in mammals, methylation also occurs at non-CG sites. Genes are methylated at CG sites, but transposable elements (TEs) are methylated at both CG and non-CG sites. The role of non-CG methylation in transcriptional silencing of TEs is being extensively studied at this time, but only very rare transpositions have been reported when non-CG methylation machineries have been compromised. To understand the role of non-CG methylation in TE suppression and in plant development, we characterized rice mutants with changes in the chromomethylase gene, OsCMT3a. oscmt3a mutants exhibited a dramatic decrease in CHG methylation, changes in the expression of some genes and TEs, and pleiotropic developmental abnormalities. Genome resequencing identified eight TE families mobilized in oscmt3a during normal propagation. These TEs included tissue culture-activated copia retrotransposons Tos17 and Tos19 (Lullaby), a pericentromeric clustered high-copy-number non-autonomous gypsy retrotransposon Dasheng, two copia retrotransposons Osr4 and Osr13, a hAT-tip100 transposon DaiZ, a MITE transposon mPing, and a LINE element LINE1-6_OS. We confirmed the transposition of these TEs by polymerase chain reaction (PCR) and/or Southern blot analysis, and showed that transposition was dependent on the oscmt3a mutation. These results demonstrated that OsCMT3a-mediated non-CG DNA methylation plays a critical role in development and in the suppression of a wide spectrum of TEs. These in planta mobile TEs are important for studying the interaction between TEs and the host genome, and for rice functional genomics.

Maize (Zea mays) seeds can detect above-ground weeds; thiamethoxam alters the view

BACKGROUND

Far red light is known to penetrate soil and delay seed germination. Thiamethoxam as a seed treatment has been observed to enhance seed germination. No previous work has explored the effect of thiamethoxam on the physiological response of buried maize seed when germinating in the presence of above-ground weeds. We hypothesised that the changes in red:far red reflected from above-ground weeds would be detected by maize seed phytochrome and delay seed germination by decreasing the level of GA and increasing ABA. We further hypothesised that thiamethoxam would overcome this delay in germination.

RESULTS

Thiamethoxam enhanced seed germination in the presence of above-ground weeds by increasing GA signalling and downregulating DELLA protein and ABA signalling genes. An increase in amylase activity and a degradation of starch were also observed.

CONCLUSIONS

Far red reflected from the above-ground weeds was capable of penetrating below the soil surface and was detected by maize seed phytochrome. Thiamethoxam altered the effect of far red on seed germination by stimulating GA and inhibiting ABA synthesis. This is the first study to suggest that the mode of action of thiamethoxam involves both GA synthesis and ABA inhibition.

Brassinosteroids Are Master Regulators of Gibberellin Biosynthesis in Arabidopsis

Plant growth and development are highly regulated processes that are coordinated by hormones including the brassinosteroids (BRs), a group of steroids with structural similarity to steroid hormones of mammals. Although it is well understood how BRs are produced and how their signals are transduced, BR targets, which directly confer the hormone's growth-promoting effects, have remained largely elusive. Here, we show that BRs regulate the biosynthesis of gibberellins (GAs), another class of growth-promoting hormones, in Arabidopsis thaliana. We reveal that Arabidopsis mutants deficient in BR signaling are severely impaired in the production of bioactive GA, which is correlated with defective GA biosynthetic gene expression. Expression of the key GA biosynthesis gene GA20ox1 in the BR signaling mutant bri1-301 rescues many of its developmental defects. We provide evidence that supports a model in which the BR-regulated transcription factor BES1 binds to a regulatory element in promoters of GA biosynthesis genes in a BR-induced manner to control their expression. In summary, our study underscores a role of BRs as master regulators of GA biosynthesis and shows that this function is of major relevance for the growth and development of vascular plants.

[Mechanism of dwarfing effect of tomato (Solanum lycopersicon) seedlings induced by cold-shock treatment under high temperature stress]

To explore the dwarfing mechanism of tomato seedlings induced by cold-shock treatment followed by high temperature, tomato seedlings were subjected to cold-shock treatment once a day at 8:00 with temperature of 5, 10 and 15 °C for 10, 20 and 30 min, respectively, and ethylene production rate was measured. Plant height, ethylene production and gibberellin (GA3) content of the seedlings treated with T10 °C D10 min (cold-shock with 10 °C for 10 min), coupled with utilization of growth regulators, were also evaluated. The results showed that the release of ethylene was increased with the decrease of cold-shock temperature and extension of treatment time. The cold-shock treatment of 5 °C and 30 min had the highest ethylene production rate of 60.3 nL h-1 . g-1, which was 6.5 times of the control. None of ethephon (ETH), silver thiosulphate (STS), GA, or paclobutrazol (PP333) could completely block high ethylene production induced by cold-shock treatment. Tomato seedlings with cold-shock treatment (T10 °C D10 min ) resulted in reduction in GA3 content by 38.1% compared with the value of control (130.6 µg . g-1). Neither ethephon nor STS had significant effect on the dwarfing induced by cold-shock. However, GA3 weakened the dwarfing effect induced by cold-shock treatment (T10 °C D10 min), while PP333 greatly enhanced it. The dwarfing effect by cold-shock treatment of T10 °C D10 min was equivalent to that of application of 4.0 mg . L-1 PP333 based on the seedling height as an evaluation indicator. It was concluded that cold-shock treatment stimulated shoot ethylene production and blocked GA3 synthesis. GA3 played a vital role in dwarfing effect on tomato seedling induced by cold-shock treatment. Cold-shock with 10 °C and duration of 10 min could promote the growth of tomato seedlings with shorter stem and higher dry mass accumulation.

Heterologous expression and transcript analysis of gibberellin biosynthetic genes of grasses reveals novel functionality in the GA3ox family

BACKGROUND

The gibberellin (GA) pathway plays a central role in the regulation of plant development, with the 2-oxoglutarate-dependent dioxygenases (2-ODDs: GA20ox, GA3ox, GA2ox) that catalyse the later steps in the biosynthetic pathway of particularly importance in regulating bioactive GA levels. Although GA has important impacts on crop yield and quality, our understanding of the regulation of GA biosynthesis during wheat and barley development remains limited. In this study we identified or assembled genes encoding the GA 2-ODDs of wheat, barley and Brachypodium distachyon and characterised the wheat genes by heterologous expression and transcript analysis.

RESULTS

The wheat, barley and Brachypodium genomes each contain orthologous copies of the GA20ox, GA3ox and GA2ox genes identified in rice, with the exception of OsGA3ox1 and OsGA2ox5 which are absent in these species. Some additional paralogs of 2-ODD genes were identified: notably, a novel gene in the wheat B genome related to GA3ox2 was shown to encode a GA 1-oxidase, named as TaGA1ox-B1. This enzyme is likely to be responsible for the abundant 1β-hydroxylated GAs present in developing wheat grains. We also identified a related gene in barley, located in a syntenic position to TaGA1ox-B1, that encodes a GA 3,18-dihydroxylase which similarly accounts for the accumulation of unusual GAs in barley grains. Transcript analysis showed that some paralogs of the different classes of 2-ODD were expressed mainly in a single tissue or at specific developmental stages. In particular, TaGA20ox3, TaGA1ox1, TaGA3ox3 and TaGA2ox7 were predominantly expressed in developing grain. More detailed analysis of grain-specific gene expression showed that while the transcripts of biosynthetic genes were most abundant in the endosperm, genes encoding inactivation and signalling components were more highly expressed in the seed coat and pericarp.

CONCLUSIONS

The comprehensive expression and functional characterisation of the multigene families encoding the 2-ODD enzymes of the GA pathway in wheat and barley will provide the basis for a better understanding of GA-regulated development in these species. This analysis revealed the existence of a novel, endosperm-specific GA 1-oxidase in wheat and a related GA 3,18-dihydroxylase enzyme in barley that may play important roles during grain expansion and development.