Structure-guided analysis of Arabidopsis JASMONATE-INDUCED OXYGENASE (JOX) 2 reveals key residues for recognition of jasmonic acid substrate by plant JOXs

The jasmonic acid (JA) signaling pathway is used by plants to control wound responses. The persistent accumulation of JA inhibits plant growth, and the hydroxylation of JA to 12-hydroxy-JA by JASMONATE-INDUCED OXYGENASEs (JOXs, also named jasmonic acid oxidases) is therefore vital for plant growth, while structural details of JA recognition by JOXs are unknown. Here, we present the 2.65 Å resolution X-ray crystal structure of Arabidopsis JOX2 in complex with its substrate JA and its co-substrates 2-oxoglutarate and Fe(II). JOX2 contains a distorted double-stranded β helix (DSBH) core flanked by α helices and loops. JA is bound in the narrow substrate pocket by hydrogen bonds with the arginine triad R225, R350, and R354 and by hydrophobic interactions mainly with the phenylalanine triad F157, F317, and F346. The most critical residues for JA binding are F157 and R225, both from the DSBH core, which interact with the cyclopentane ring of JA. The spatial distribution of critical residues for JA binding and the shape of the substrate-binding pocket together define the substrate selectivity of the JOXs. Sequence alignment shows that these critical residues are conserved among JOXs from higher plants. Collectively, our study provides insights into the mechanism by which higher plants hydroxylate the hormone JA.

Melatonin antagonizes ABA action to promote seed germination by regulating Ca2+ efflux and H2O2 accumulation

Seed germination is a vital stage in the plant life-cycle that greatly contributes to plant establishment. Melatonin has been shown to promote seed germination under various environmental stresses; however, the mechanism remains largely underexplored. Here, we reported that melatonin antagonized abscisic acid (ABA) to promote seed germination by regulating ABA and gibberellic acid (GA₃) balance. Transcriptomic analysis revealed that such a role of melatonin was associated with Ca²⁺ and redox signaling. Melatonin pretreatment induced Ca²⁺ efflux accompanied by an up-regulation of vacuolar H+/Ca²⁺ antiporter 3 (CAX3). AtCAX3 deletion in Arabidopsis exhibited reduced Ca²⁺ efflux. Inhibition of Ca²⁺ efflux in the seeds of melon and Arabidopsis mutant AtCAX3 compromised melatonin-induced germination under ABA stress. Melatonin increased H₂O₂ accumulation, and H₂O₂ pretreatment decreased ABA/GA₃ ratio and promoted seed germination under ABA stress. However, complete inhibition of H₂O₂ accumulation abolished melatonin-induced ABA and GA₃ balance and seed germination. Our study reveals a novel regulatory mechanism in which melatonin counteracts ABA to induce seed germination that essentially involves CAX3-mediated Ca²⁺ efflux and H₂O₂ accumulation, which, in turn, regulate ABA and GA₃ balance by promoting ABA catabolism and/or GA₃ biosynthesis.

Naphthylphthalamic acid associates with and inhibits PIN auxin transporters

N-1-naphthylphthalamic acid (NPA) is a key inhibitor of directional (polar) transport of the hormone auxin in plants. For decades, it has been a pivotal tool in elucidating the unique polar auxin transport-based processes underlying plant growth and development. Its exact mode of action has long been sought after and is still being debated, with prevailing mechanistic schemes describing only indirect connections between NPA and the main transporters responsible for directional transport, namely PIN auxin exporters. Here we present data supporting a model in which NPA associates with PINs in a more direct manner than hitherto postulated. We show that NPA inhibits PIN activity in a heterologous oocyte system and that expression of NPA-sensitive PINs in plant, yeast, and oocyte membranes leads to specific saturable NPA binding. We thus propose that PINs are a bona fide NPA target. This offers a straightforward molecular basis for NPA inhibition of PIN-dependent auxin transport and a logical parsimonious explanation for the known physiological effects of NPA on plant growth, as well as an alternative hypothesis to interpret past and future results. We also introduce PIN dimerization and describe an effect of NPA on this, suggesting that NPA binding could be exploited to gain insights into structural aspects of PINs related to their transport mechanism.

APAn, a Class of ABA Receptor Agonism/Antagonism Switching Probes

In plants, biosynthesized ABA undergoes two important physiological processes of signal transduction and metabolism simultaneously. In this study, we described a class of ABA receptor agonist/antagonist switching probes APAn, which can regulate the agonistic activity or antagonistic activity according to the length of a 6'-alkoxyl chain. From APA1 to APA6, with the extension of the alkoxyl chain, it showed a gradually increased receptor-binding potential and decreased HAB1 inhibition activity. Theoretical analysis based on molecular docking and molecular dynamics simulation revealed that some factors outside the ligand-binding pocket in receptors could also affect the binding of the ligand to the receptor, for example, the van der Waals interaction between the alkyl chain in APAn and the 3'-tunnel of ABA receptors made it bind more tightly than iso-PhABA. This enhanced binding made it an antagonist rather than a weakened agonist.

Anthocyanin accumulation is initiated by abscisic acid to enhance fruit color during fig (Ficus carica L.) ripening

Fig fruit is well-known for its attractive flavor, color, and nutritional and medicinal value. Anthocyanin contributes to the fruit's color and constitutes a high percentage of the total antioxidant content of the fig fruit. We quantified the major anthocyanins and characterized the expression levels of anthocyanin-biosynthesis and transcription factor genes in fruit treated on-tree with exogenous abscisic acid (ABA) or ethephon, or the ABA inhibitors nordihydroguaiaretic acid (NDGA) or fluridone. The major anthocyanins cyanidin 3-O-glucoside and cyanidin 3-O-rutinoside were found in significantly higher quantities in exogenous ABA- and ethephon-treated fruit, with early dark purple color compared to the controls. On the other hand, NDGA- and fluridone-treated fruit had significantly lower amounts of anthocyanins, with less purple color coverage than controls. Expression levels of the anthocyanin-biosynthesis genes FcPAL, FcCHS2, FcCHI, FcF3H, FcDFR, FcANS, FcUFGT and Fc3RT were upregulated by exogenous ABA and ethephon treatment, and downregulated by NDGA and fluridone treatment. The MYB-bHLH-WD40 complex-related genes of ripe fig fruit were identified. In particular, FcMYB113 was strongly upregulated by exogenous ABA and ethephon, and strongly downregulated by NDGA and fluridone. In addition, moderate upregulation of FcGL3 and FcWD40 was observed with exogenous ABA and ethephon treatment, and moderate downregulation in NDGA- and fluridone-treated fruit. These results indicate that ABA can initiate anthocyanin biosynthesis, which ultimately improves the color and nutritional value of fig fruit, enhancing their attractiveness to consumers.

Interplay between 1-aminocyclopropane-1-carboxylic acid, γ-aminobutyrate and D-glucose in the regulation of high nitrate-induced root growth inhibition in maize

Nitrogen is one of the main factors that affect plant growth and development. However, high nitrogen concentrations can inhibit both shoot and root growth, even though the processes involved in this inhibition are still unknown. The aim of this work was to identify the metabolic alterations that induce the inhibition of root growth caused by high nitrate supply, when the whole plant growth is also reduced. High nitrate altered nitrogen and carbon metabolism, reducing the content of sugars and inducing the accumulation of Ca²⁺ and amino acids, such as glutamate, alanine and γ-aminobutyrate (GABA), that could act to replenish the succinate pool in the tricarboxylic acid cycle and maintain its activity. Other metabolic alterations found were the accumulation of the polyamines spermidine and spermine, and the reduction of jasmonic acid (JA) and the ethylene precursor aminocyclopropane-1-carboxylic acid (ACC). These results indicate that the growth root inhibition by high NO₃^- is a complex metabolic response that involves GABA as a key link between C and N metabolism which, together with plant growth regulators such as auxins, cytokinins, abscisic acid, JA, and the ethylene precursor ACC, is able to regulate the metabolic response of root grown under high nitrate concentrations.

Molecular basis of strigolactone perception in root-parasitic plants: aiming to control its germination with strigolactone agonists/antagonists

The genus Striga, also called "witchweed", is a member of the family Orobanchaceae, which is a major family of root-parasitic plants. Striga can lead to the formation of seed stocks in the soil and to explosive expansion with enormous seed production and stability once the crops they parasitize are cultivated. Understanding the molecular mechanism underlying the communication between Striga and their host plants through natural seed germination stimulants, "strigolactones (SLs)", is required to develop the technology for Striga control. This review outlines recent findings on the SL perception mechanism, which have been accumulated in Striga hermonthica by the similarity of the protein components that regulate SL signaling in nonparasitic model plants, including Arabidopsis and rice. HTL/KAI2 homologs were identified as SL receptors in the process of Striga seed germination. Recently, this molecular basis has further promoted the development of various types of SL agonists/antagonists as seed germination stimulants or inhibitors. Such chemical compounds are also useful to elucidate the dynamic behavior of SL receptors and the regulation of SL signaling.

Genome-wide transcriptional profiling for elucidating the effects of brassinosteroids on Glycine max during early vegetative development

Soybean is a widely grown grain legume and one of the most important economic crop species. Brassinosteroids play a crucial role in plant vegetative growth and reproductive development. However, it remains unclear how BRs regulate the developmental processes in soybean, and the molecular mechanism underlying soybean early development is largely unexplored. In this study, we first characterized how soybean early vegetative growth was specifically regulated by the BR biosynthesis inhibitor propiconazole; this characterization included shortened root and shoot lengths, reduced leaf area, and decreased chlorophyll content. In addition, the growth inhibition induced by Pcz could be rescued by exogenous brassinolide application. The RNA-seq technique was employed to investigate the BR regulatory networks during soybean early vegetative development. Identification and analysis of differentially expressed genes indicated that BRs orchestrate a wide range of cellular activities and biological processes in soybean under various BR concentrations. The regulatory networks between BRs and multiple hormones or stress-related pathways were investigated. The results provide a comprehensive view of the physiological functions of BRs and new insights into the molecular mechanisms at the transcriptional level of BR regulation of soybean early development.

Transcriptomic analysis reveals key factors in fruit ripening and rubbery texture caused by 1-MCP in papaya

BACKGROUND

Ethylene promotes fruit ripening whereas 1-methylcyclopropene (1-MCP), a non-toxic antagonist of ethylene, delays fruit ripening via the inhibition of ethylene receptor. However, unsuitable 1-MCP treatment can cause fruit ripening disorders.

RESULTS

In this study, we show that short-term 1-MCP treatment (400 nL•L- 1, 2 h) significantly delays papaya fruit ripening with normal ripening characteristics. However, long-term 1-MCP treatment (400 nL•L- 1, 16 h) causes a "rubbery" texture of fruit. The comparative transcriptome analysis showed that a total of 5529 genes were differently expressed during fruit ripening compared to freshly harvested fruits. Comprehensive functional enrichment analysis showed that the metabolic pathways of carbon metabolism, plant hormone signal transduction, biosynthesis of amino acids, and starch and sucrose metabolism are involved in fruit ripening. 1-MCP treatment significantly affected fruit transcript levels. A total of 3595 and 5998 differently expressed genes (DEGs) were identified between short-term 1-MCP, long-term 1-MCP treatment and the control, respectively. DEGs are mostly enriched in the similar pathway involved in fruit ripening. A large number of DEGs were also identified between long-term and short-term 1-MCP treatment, with most of the DEGs being enriched in carbon metabolism, starch and sucrose metabolism, plant hormone signal transduction, and biosynthesis of amino acids. The 1-MCP treatments accelerated the lignin accumulation and delayed cellulose degradation during fruit ripening. Considering the rubbery phenotype, we inferred that the cell wall metabolism and hormone signal pathways are closely related to papaya fruit ripening disorder. The RNA-Seq output was confirmed using RT-qPCR by 28 selected genes that were involved in cell wall metabolism and hormone signal pathways.

CONCLUSIONS

These results showed that long-term 1-MCP treatment severely inhibited ethylene signaling and the cell wall metabolism pathways, which may result in the failure of cell wall degradation and fruit softening. Our results reveal multiple ripening-associated events during papaya fruit ripening and provide a foundation for understanding the molecular mechanisms underlying 1-MCP treatment on fruit ripening and the regulatory networks.

Transcriptional landscape of soybean (Glycine max) embryonic axes during germination in the presence of paclobutrazol, a gibberellin biosynthesis inhibitor

Gibberellins (GA) are key positive regulators of seed germination. Although the GA effects on seed germination have been studied in a number of species, little is known about the transcriptional reprogramming modulated by GA during this phase in species other than Arabidopsis thaliana. Here we report the transcriptome analysis of soybean embryonic axes during germination in the presence of paclobutrazol (PBZ), a GA biosynthesis inhibitor. We found a number of differentially expressed cell wall metabolism genes, supporting their roles in cell expansion during germination. Several genes involved in the biosynthesis and signaling of other phytohormones were also modulated, indicating an intensive hormonal crosstalk at the embryonic axis. We have also found 26 photosynthesis genes that are up-regulated by PBZ at 24 hours after imbibition (HAI) and down-regulated at 36 HAI, which led us to suggest that this is part of a strategy to implement an autotrophic growth program in the absence of GA-driven mobilization of reserves. Finally, 30 transcription factors (mostly from the MYB, bHLH, and bZIP families) were down-regulated by PBZ and are likely downstream GA targets that will drive transcriptional changes during germination.

Synthesis and Biological Evaluation of Novel Triazole Derivatives as Strigolactone Biosynthesis Inhibitors

Strigolactones (SLs) are one of the plant hormones that control several important agronomic traits, such as shoot branching, leaf senescence, and stress tolerance. Manipulation of the SL biosynthesis can increase the crop yield. We previously reported that a triazole derivative, TIS108, inhibits SL biosynthesis. In this study, we synthesized a number of novel TIS108 derivatives. Structure-activity relationship studies revealed that 4-(2-phenoxyethoxy)-1-phenyl-2-(1 H-1,2,4-triazol-1-yl)butan-1-one (KK5) inhibits the level of 4-deoxyorobanchol in roots more strongly than TIS108. We further found that KK5-treated Arabidopsis showed increased branching phenotype with the upregulated gene expression of AtMAX3 and AtMAX4. These results indicate that KK5 is a specific SL biosynthesis inhibitor in rice and Arabidopsis.

Adventitious rooting of Chrysanthemum is stimulated by a low red:far-red ratio

Adventitious rooting, a critical process in the vegetative propagation of many ornamentals, can be affected by both light intensity and light quality. We investigated the use of spectral light quality to improve adventitious rooting of Chrysanthemum morifolium cuttings by applying different combinations of blue, red and far-red light. Additionally, unrooted cuttings were treated before planting with two auxin transport inhibitors (TIBA and NPA) to study the effect of light quality on auxin biosynthesis and/or transport. Results showed that lowering the R:FR ratio (decreasing the phytochrome photostationary state, PSS) improved rooting significantly and decreased the inhibiting effect of the auxin transport inhibitor NPA. An extra decrease of PSS by adding blue light to a red + far-red spectrum further enhanced rooting. In contrast, adding blue light to solely red light decreased rooting, an effect which was more pronounced in combination with the auxin transport inhibitors TIBA and NPA. Our results show that phytochrome plays a role in adventitious root formation through the action of auxin, but that also blue light receptors interact in this process.

The distinct ripening processes in the reproductive and non-reproductive parts of the fig syconium are driven by ABA

The common fig bears a unique closed inflorescence structure, the syconium, composed of small individual drupelets that develop from the ovaries, which are enclosed in a succulent receptacle of vegetative origin. The fig ripening process is traditionally classified as climacteric; however, recent studies have suggested that distinct mechanisms exist in its reproductive and non-reproductive parts. We analysed ABA and ethylene production, and expression of ABA-metabolism, ethylene-biosynthesis, MADS-box, NAC, and ethylene response-factor genes in inflorescences and receptacles of on-tree fruit treated with ABA, ethephon, fluridone, and nordihydroguaiaretic acid (NDGA). Exogenous ABA and ethephon accelerated fruit ripening and softening, whereas fluridone and NDGA had the opposite effect, delaying endogenous ABA and ethylene production compared to controls. Expression of the ABA-biosynthesis genes FcNCED2 and FcABA2, ethylene-biosynthesis genes FcACS4, FcACOL, and FcACO2, FcMADS8, 14, 15, FcNAC1, 2, 5, and FcERF9006 was up-regulated by exogenous ABA and ethephon. NDGA down-regulated FcNCED2 and FcABA2, whereas fluridone down-regulated FcABA2; both down-regulated the ethylene-related genes. These results demonstrate the key role of ABA in regulation of ripening by promoting ethylene production, as in the climacteric model plant tomato, especially in the inflorescence. However, increasing accumulation of endogenous ABA until full ripeness and significantly low expression of ethylene-biosynthesis genes in the receptacle suggests non-climacteric, ABA-dependent ripening in the vegetative-originated succulent receptacle part of the fruit.

Molecular mechanism of seed dormancy release induced by fluridone compared with cod stratification in Notopterygium incisum

BACKGROUND

Notopterygium incisum is an important Chinese medicinal plant. Its mature seeds have underdeveloped embryos and are physiological dormant. We found the seeds with full developed embryos can germinate after treated by fluridone (FL), an inhibitor of abscisic acid (ABA). In order to understand the molecular mechanisms underlying seed dormancy release by FL, we compared the transcriptomic changes in dormancy release induced by two different methods, FL and cold stratification (CS) in N. incisum. We further analyzed the gene expression patterns involved in seed germination and dormancy using quantitative reverse-transcription PCR.

RESULTS

RNA-sequence analysis revealed more dramatic changes in the transcriptomes of FL than those in CS, particularly for genes involved in the biosynthesis and regulation of gibberellins (GAs) and ABA. The down-regulation of ABA biosynthesis genes and the dramatic up-regulation of NiCYP707As, an ABA catabolic gene, contributed to the reduced ABA levels in FL. The increased GA3 levels in CS-treated seeds were due to the up-regulation of NiGA3OX. Both NiABI5 (a positive ABA regulator) and NiGAI (a negative regulator of GA) were down-regulated in FL and CS. The upregulation of strigolactones (SLs; the metabolites with the same precursor as ABA) biosynthesis and regulatory genes in both FL- and CS-treated seeds indicates that SLs contribute positively to seed dormancy release in N. incisum.

CONCLUSIONS

Our results indicated that FL- and CS-seed dormancy release possibly depends on two totally different mechanisms: alleviation of the effects of ABA and potentiation of the effects of GA, respectively. However, NiABI5 and NiGAI probably function as common factors integrating the effects of ABA and GA on seed dormancy release.

Target-based selectivity of strigolactone agonists and antagonists in plants and their potential use in agriculture

Strigolactones (SLs) are small carotenoid-derived molecules that possess a wide spectrum of functions, including plant hormonal activities and chemical mediation of rhizosphere communication with both root parasitic plants and symbiotic arbuscular mycorrhizal fungi. Chemicals that regulate the functions of SLs may therefore have the potential to become widely used in agricultural applications. For example, various SL analogs and mimics have been developed to reduce the seed banks of root parasites in the field. Other analogs and mimics act selectively to suppress branching, with weak, or no stimulation, of germination in root parasites. In addition, some antagonists for SL receptors have been developed based on the mechanisms of SL perception. A better understanding of the modes of action of SL perception by various receptors will help to support the design of SL analogs, mimics, and antagonists with high activity and selectivity. Here, we review the compounds reported so far from the viewpoint of their selectivity to their targets, and the possibilities for their use in agriculture.

Polar auxin transport is implicated in vessel differentiation and spatial patterning during secondary growth in Populus

PREMISE OF THE STUDY

Dimensions and spatial distribution of vessels are critically important features of woody stems, allowing for adaptation to different environments through their effects on hydraulic efficiency and vulnerability to embolism. Although our understanding of vessel development is poor, basipetal transport of auxin through the cambial zone may play an important role.

METHODS

Stems of Populus tremula ×alba were treated with the auxin transport inhibitor N-1-naphthylphthalamic acid (NPA) in a longitudinal strip along the length of the lower stem. Vessel lumen diameter, circularity, and length; xylem growth; tension wood area; and hydraulic conductivity before and after a high pressure flush were determined on both NPA-treated and control plants.

KEY RESULTS

NPA-treated stems formed aberrant vessels that were short, small in diameter, highly clustered, and angular in cross section, whereas xylem formed on the untreated side of the stem contained typical vessels that were similar to those of controls. NPA-treated stems had reduced specific conductivity relative to controls, but this difference was eliminated by the high-pressure flush. The control treatment (lanolin + dimethyl sulfoxide) reduced xylem growth and increased tension wood formation, but never produced the aberrant vessel patterning seen in NPA-treated stems.

CONCLUSIONS

These results are consistent with a model of vessel development in which basipetal polar auxin transport through the xylem-side cambial derivatives is required for proper expansion and patterning of vessels and demonstrate that reduced auxin transport can produce stems with altered stem hydraulic properties.

Naphthylphthalamic acid and the mechanism of polar auxin transport

Our current understanding of how plants move auxin through their tissues is largely built on the use of polar auxin transporter inhibitors. Although the most important proteins that mediate auxin transport and its regulation have probably all been identified and the mapping of their interactions is well underway, mechanistically we are still surprisingly far away from understanding how auxin is transported. Such an understanding will only emerge after new data are placed in the context of the wealth of physiological data on which they are founded. This review will look back over the use of a key inhibitor called naphthylphthalamic acid (NPA) and outline its contribution to our understanding of the molecular mechanisms of polar auxin transport, before proceeding to speculate on how its use is likely still to be informative.

Apple fruit superficial scald resistance mediated by ethylene inhibition is associated with diverse metabolic processes

Fruits stored at low temperature can exhibit different types of chilling injury. In apple, one of the most serious physiological disorders is superficial scald, which is characterized by discoloration and brown necrotic patches on the fruit exocarp. Although this phenomenon is widely ascribed to the oxidation of α-farnesene, its physiology is not yet fully understood. To elucidate the mechanism of superficial scald development and possible means of prevention, we performed an integrated metabolite screen, including an analysis of volatiles, phenols and lipids, together with a large-scale transcriptome study. We also determined that prevention of superficial scald, through the use of an ethylene action inhibitor, is associated with the triggering of cold acclimation-related processes. Specifically, the inhibition of ethylene perception stimulated the production of antioxidant compounds to scavenge reactive oxygen species, the synthesis of fatty acids to stabilize plastid and vacuole membranes against cold temperature, and the accumulation of the sorbitol, which can act as a cryoprotectant. The pattern of sorbitol accumulation was consistent with the expression profile of a sorbitol 6-phosphate dehydrogenase, MdS6PDH, the overexpression of which in transgenic Arabidopsis thaliana plants confirmed its involvement in the cold acclimation and freezing tolerance.

Nitric oxide is essential for the development of aerenchyma in wheat roots under hypoxic stress

In response to flooding/waterlogging, plants develop various anatomical changes including the formation of lysigenous aerenchyma for the delivery of oxygen to roots. Under hypoxia, plants produce high levels of nitric oxide (NO) but the role of this molecule in plant-adaptive response to hypoxia is not known. Here, we investigated whether ethylene-induced aerenchyma requires hypoxia-induced NO. Under hypoxic conditions, wheat roots produced NO apparently via nitrate reductase and scavenging of NO led to a marked reduction in aerenchyma formation. Interestingly, we found that hypoxically induced NO is important for induction of the ethylene biosynthetic genes encoding ACC synthase and ACC oxidase. Hypoxia-induced NO accelerated production of reactive oxygen species, lipid peroxidation, and protein tyrosine nitration. Other events related to cell death such as increased conductivity, increased cellulase activity, DNA fragmentation, and cytoplasmic streaming occurred under hypoxia, and opposing effects were observed by scavenging NO. The NO scavenger cPTIO (2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide potassium salt) and ethylene biosynthetic inhibitor CoCl₂ both led to reduced induction of genes involved in signal transduction such as phospholipase C, G protein alpha subunit, calcium-dependent protein kinase family genes CDPK, CDPK2, CDPK 4, Ca-CAMK, inositol 1,4,5-trisphosphate 5-phosphatase 1, and protein kinase suggesting that hypoxically induced NO is essential for the development of aerenchyma.

Salt and methyl jasmonate aggravate growth inhibition and senescence in Arabidopsis seedlings via the JA signaling pathway

Numerous studies have demonstrated the function of salinity or jasmonic acid (JA) in plant growth and senescence. This study evaluated how the combination of salinity and methyl jasmonate (MeJA) (SaM) worked as a novel stress and then regulated plant growth in Arabidopsis. Firstly, we found that compared with MeJA or NaCl treatment alone, SaM would significantly intensified plant growth inhibition and senescence in wild-type (WT) seedlings, and these phenotypes could be partially compromised after SaM stress in JA-insensitive mutants. Meanwhile, genes involved in JA signaling and Senescence Associated Gene 13 (SAG13) were dramatically increased by SaM stress than that by MeJA or NaCl alone in WT. Moreover, a group of secondary metabolite - indolic glucosinolates (IGs) showed obvious over-accumulation after SaM treatment than that after each single one in WT, and the seedlings treated with IGs' metabolites performed similar inhibited growth and chlorotic leaves phenotypes compared with those caused by SaM stress. All these indicated the toxicity of IGs and their metabolites would prevent the growth progress of plants. Therefore, we concluded that SaM worked as a novel stress and intensified plant growth inhibition and senescence, which was dependent on JA-dependent and -independent signaling pathways.

Abscisic acid regulates seed germination of Vellozia species in response to temperature

The relationship between the phytohormones, gibberellin (GA) and abscisic acid (ABA) and light and temperature on seed germination is still not well understood. We aimed to investigate the role of the ABA and GA on seed germination of Vellozia caruncularis, V. intermedia and V. alutacea in response to light/dark conditions on different temperature. Seeds were incubated in GA (GA₃ or GA₄ ) or ABA and their respective biosynthesis inhibitors (paclobutrazol - PAC, and fluridone - FLU) solutions at two contrasting temperatures (25 and 40 °C). Furthermore, endogenous concentrations of active GAs and those of ABA were measured in seeds of V. intermedia and V. alutacea during imbibition/germination. Exogenous ABA inhibited the germination of Vellozia species under all conditions tested. GA, FLU and FLU + GA₃ stimulated germination in the dark at 25 °C (GA₄ being more effective than GA₃ ). PAC reduced seed germination in V. caruncularis and V. alutacea, but did not affect germination of V. intermedia at 40 °C either under light or dark conditions. During imbibition in the dark, levels of active GAs decreased in the seeds of V. intermedia, but were not altered in those of V. alutacea. Incubation at 40 °C decreased ABA levels during imbibition in both V. caruncularis and V. alutacea. We conclude that the seeds of Vellozia species studied here require light or high temperature to germinate and ABA has a major role in the regulation of Vellozia seed germination in response to light and temperature.

Inhibition of auxin transport and auxin signaling and treatment with far red light induces root coiling in the phospholipase-A mutant ppla-I-1. Significance for surface penetration?

When grown on a non-penetretable at a surface angle of 45°, Arabidopsis roots form wave-like structures and, in wild type rarely, but in certain mutants the tip root even may form circles. These circles are called coils. The formation of coils depends on the complex interaction of circumnutation, gravitropism and negative thigmotropism where - at least - gravitropism is intimately linked to auxin transport and signaling. The knockout mutant of patatin-related phospholipase-AI-1 (pplaI-1) is an auxin-signaling mutant which forms moderately increased numbers of coils on tilted agar plates. We tested the effects of the auxin efflux transport inhibitor NPA (1-naphthylphtalamic acid) and of the influx transport inhibitor 1-NOA (1-naphthoxyacetic acid) which both further increased root coil formation. The pPLAI-1 inhibitors HELSS (haloenol lactone suicide substrate=E-6-(bromomethylene)tetrahydro-3-(1-naphthalenyl)-2H-pyran-2-one) and ETYA (eicosatetraynoic acid) which are auxin signaling inhibitors also increased coil formation. In addition, far red light treatment increased coil formation. The results point out that a disturbance of auxin transport and signaling is one potential cause for root coils. As we show that the mutant pplaI-1 penetrates horizontal agar plates better than wild type plants root movements may help penetrating the soil.

Non-specificity of ethylene inhibitors: 'double-edged' tools to find out new targets involved in the root morphogenetic programme

In the last decade, genetic and pharmacological approaches have been used to explore ethylene biosynthesis and perception in order to study the role of ethylene and ethylene/auxin interaction in root architecture development. However, recent findings with pharmacological approaches highlight the non-specificity of commonly used inhibitors. This suggests that caution is required for interpreting these studies and that the use of pharmacological agents is a 'double-edged' tool. On one hand, non-specific effects make interpretation difficult unless other experiments, such as with different mutants or with multiple diversely acting chemicals, are conducted. On the other hand, the non-specificity of inhibitors opens up the possibility of uncovering some ligands or modulators of new receptors such as plant glutamate-like receptors and importance of some metabolic hubs in carbon and nitrogen metabolism such as the pyridoxal phosphate biosynthesis involved in the regulation of the root morphogenetic programme. Identification of such targets is a critical issue to improve the efficiency of absorption of macronutrients in relation to root the morphogenetic programme.

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.

CsTFL1, a constitutive local repressor of flowering, modulates floral initiation by antagonising florigen complex activity in chrysanthemum

Chrysanthemums require repeated cycles of short-day (SD) photoperiod for successful anthesis, but their vegetative state is strictly maintained under long-day (LD) or night-break (NB) conditions. We have previously demonstrated that photoperiodic flowering of a wild diploid chrysanthemum (Chrysanthemum seticuspe f. boreale) is controlled by a pair of systemic floral regulators, florigen (CsFTL3) and anti-florigen (CsAFT), produced in the leaves. Here, we report the functional characterisation of a local floral regulator, CsTFL1, a chrysanthemum orthologue of TERMINAL FLOWER 1 gene in Arabidopsis. Constitutive expression of CsTFL1 in C. seticuspe (CsTFL1-ox) resulted in extremely late flowering under SD and prevented up-regulation of floral meristem identity genes in shoot tips and leaves. Bimolecular fluorescence complementation assay showed that both CsTFL1 and CsFTL3 interacted with CsFDL1, a bZIP transcription factor FD homologue, in the nucleus. The transient gene expression assay indicated that CsTFL1 suppresses flowering by directly antagonising the flower inductive activity of the CsFTL3-CsFDL1 complex. Our results suggest that strict maintenance of vegetative state under non-inductive photoperiod is achieved by the coordinated action of both the systemic floral inhibitor and local floral inhibitor CsTFL1, which is constitutively expressed in shoot tips.

Targeting Hormone-Related Pathways to Improve Grain Yield in Rice: A Chemical Approach

Sink/source relationships, regulating the mobilization of stored carbohydrates from the vegetative tissues to the grains, are of key importance for grain filling and grain yield. We used different inhibitors of plant hormone action to assess their effects on grain yield and on the expression of hormone-associated genes. Among the tested chemicals, 2-indol-3-yl-4-oxo-4-phenylbutanoic acid (PEO-IAA; antagonist of auxin receptor), nordihydroguaiaretic acid (NDGA; abscisic acid (ABA) biosynthesis inhibitor), and 2-aminoisobutyric acid (AIB; ethylene biosynthesis inhibitor) improved grain yield in a concentration dependent manner. These effects were also dependent on the plant developmental stage. NDGA and AIB treatments induced an increase in photosynthesis in flag leaves concomitant to the increments of starch content in flag leaves and grains. NDGA inhibited the expression of ABA-responsive gene, but did not significantly decrease ABA content. Instead, NDGA significantly decreased jasmonic acid and jasmonic acid-isoleucine. Our results support the notion that the specific inhibition of jasmonic acid and ethylene biosynthesis resulted in grain yield increase in rice.

Involvement of abscisic acid and salicylic acid in signal cascade regulating bacterial endophyte-induced volatile oil biosynthesis in plantlets of Atractylodes lancea

The enormous biological diversity of endophytes, coupled with their potential to enhance the production of bioactive metabolites in plants, has driven research efforts focusing on endophytes. However, limited information is available on the impacts of bacterial endophytes on plant secondary metabolism and signaling pathways involved. This work showed that an endophytic Acinetobacter sp. ALEB16, capable of activating accumulation of plant volatile oils, also induced abscisic acid (ABA) and salicylic acid (SA) production in Atractylodes lancea. Pre-treatment of plantlets with biosynthetic inhibitors of ABA or SA blocked the bacterium-induced volatile production. ABA inhibitors suppressed not only the bacterium-induced volatile accumulation but also the induced ABA and SA generation; nevertheless, SA inhibitors did not significantly inhibit the induced ABA biosynthesis, implying that SA acted downstream of ABA production. These results were confirmed by observations that exogenous ABA and SA reversed the inhibition of bacterium-induced volatile accumulation by inhibitors. Transcriptional activities of genes in sesquiterpenoid biosynthesis also increased significantly with bacterium, ABA and SA treatments. Mevalonate pathway proved to be the main source of isopentenyldiphosphate for bacterium-induced sesquiterpenoids, as assessed in experiments using specific terpene biosynthesis inhibitors. These results suggest that Acinetobacter sp. acts as an endophytic elicitor to stimulate volatile biosynthesis of A. lancea via an ABA/SA-dependent pathway, thereby yielding additional insight into the interconnection between ABA and SA in biosynthesis-related signaling pathways.

BRASSINOSTEROID INSENSITIVE2 interacts with ABSCISIC ACID INSENSITIVE5 to mediate the antagonism of brassinosteroids to abscisic acid during seed germination in Arabidopsis

Seed germination and postgerminative growth are regulated by a delicate hormonal balance. Abscisic acid (ABA) represses Arabidopsis thaliana seed germination and postgerminative growth, while brassinosteroids (BRs) antagonize ABA-mediated inhibition and promote these processes. However, the molecular mechanism underlying BR-repressed ABA signaling remains largely unknown. Here, we show that the Glycogen Synthase Kinase 3-like kinase BRASSINOSTEROID INSENSITIVE2 (BIN2), a critical repressor of BR signaling, positively regulates ABA responses during seed germination and postgerminative growth. Mechanistic investigation revealed that BIN2 physically interacts with ABSCISIC ACID INSENSITIVE5 (ABI5), a bZIP transcription factor. Further genetic analysis demonstrated that the ABA-hypersensitive phenotype of BIN2-overexpressing plants requires ABI5. BIN2 was found to phosphorylate and stabilize ABI5 in the presence of ABA, while application of epibrassinolide (the active form of BRs) inhibited the regulation of ABI5 by BIN2. Consistently, the ABA-induced accumulation of ABI5 was affected in BIN2-related mutants. Moreover, mutations of the BIN2 phosphorylation sites on ABI5 made the mutant protein respond to ABA improperly. Additionally, the expression of several ABI5 regulons was positively modulated by BIN2. These results provide evidence that BIN2 phosphorylates and stabilizes ABI5 to mediate ABA response during seed germination, while BRs repress the BIN2-ABI5 cascade to antagonize ABA-mediated inhibition.

Biosynthetic pathway of the phytohormone auxin in insects and screening of its inhibitors

Insect galls are abnormal plant tissues induced by galling insects. The galls are used for food and habitation, and the phytohormone auxin, produced by the insects, may be involved in their formation. We found that the silkworm, a non-galling insect, also produces an active form of auxin, indole-3-acetic acid (IAA), by de novo synthesis from tryptophan (Trp). A detailed metabolic analysis of IAA using IAA synthetic enzymes from silkworms indicated an IAA biosynthetic pathway composed of a three-step conversion: Trp → indole-3-acetaldoxime → indole-3-acetaldehyde (IAAld) → IAA, of which the first step is limiting IAA production. This pathway was shown to also operate in gall-inducing sawfly. Screening of a chemical library identified two compounds that showed strong inhibitory activities on the conversion step IAAld → IAA. The inhibitors can be efficiently used to demonstrate the importance of insect-synthesized auxin in gall formation in the future.

Plant cells use auxin efflux to explore geometry

Cell movement is the central mechanism for animal morphogenesis. Plant cell development rather relies on flexible alignment of cell axis adjusting cellular differentiation to directional cues. As central input, vectorial fields of mechanical stress and gradients of the phytohormone auxin have been discussed. In tissue contexts, mechanical and chemical signals will always act in concert; experimentally it is difficult to dissect their individual roles. We have designed a novel approach, based on cells, where directionality has been eliminated by removal of the cell wall. We impose a new axis using a microfluidic set-up to generate auxin gradients. Rectangular microvessels are integrated orthogonally with the gradient. Cells in these microvessels align their new axis with microvessel geometry before touching the wall. Auxin efflux is necessary for this touch-independent geometry exploration and we suggest a model, where auxin gradients can be used to align cell axis in tissues with minimized mechanical tensions.

Syntheses and Biological Activities of Pyranyl-substituted Cinnamates

Twenty-two kinds of pyranyl-substituted cinnamates were synthesized by the reaction of 4-hydroxy-6-(2-phenylethyl)-2H-pyran-2-one or 4-hydroxy-6-methyl-2H-pyran-2-one (HMP) with a variety of substituted cinnamic acids, and their antifungal and plant growth inhibitory activities were investigated. Among the compounds prepared, 6-methyl-2-oxo-2H-pyran-4-yl 3-(4-isopropylphenyl)propenoate (H5) showed the strongest antifungal activity against Rhizoctonia solani and Sclerotium dellfinii, and 6-methyl-2-oxo-2H-pyran-4-yl 3-(2-methylphenyl)propenoate (H2) had the highest plant growth inhibitory activity toward Brassica rapa.

Physiology and toxicology of hormone-disrupting chemicals in higher plants

Higher plants are exposed to natural environmental organic chemicals, associated with plant-environment interactions, and xenobiotic environmental organic chemicals, associated with anthropogenic activities. The effects of these chemicals result not only from interaction with metabolic targets, but also from interaction with the complex regulatory networks of hormone signaling. Purpose-designed plant hormone analogues thus show extensive signaling effects on gene regulation and are as such important for understanding plant hormone mechanisms and for manipulating plant growth and development. Some natural environmental chemicals also act on plants through interference with the perception and transduction of endogenous hormone signals. In a number of cases, bioactive xenobiotics, including herbicides that have been designed to affect specific metabolic targets, show extensive gene regulation effects, which are more in accordance with signaling effects than with consequences of metabolic effects. Some of these effects could be due to structural analogies with plant hormones or to interference with hormone metabolism, thus resulting in situations of hormone disruption similar to animal cell endocrine disruption by xenobiotics. These hormone-disrupting effects can be superimposed on parallel metabolic effects, thus indicating that toxicological characterisation of xenobiotics must take into consideration the whole range of signaling and metabolic effects. Hormone-disruptive signaling effects probably predominate when xenobiotic concentrations are low, as occurs in situations of residual low-level pollutions. These hormone-disruptive effects in plants may thus be of importance for understanding cryptic effects of low-dosage xenobiotics, as well as the interactive effects of mixtures of xenobiotic pollutants.

Delay of iris flower senescence by cytokinins and jasmonates

It is not known whether tepal senescence in Iris flowers is regulated by hormones. We applied hormones and hormone inhibitors to cut flowers and isolated tepals of Iris × hollandica cv. Blue Magic. Treatments with ethylene or ethylene antagonists indicated lack of ethylene involvement. Auxins or auxin inhibitors also did not change the time to senescence. Abscisic acid (ABA) hastened senescence, but an inhibitor of ABA synthesis (norflurazon) had no effect. Gibberellic acid (GA3 ) slightly delayed senescence in some experiments, but in other experiments it was without effect, and gibberellin inhibitors [ancymidol or 4-hydroxy-5-isopropyl-2-methylphenyltrimethyl ammonium chloride-1-piperidine carboxylate (AMO-1618)] were ineffective as well. Salicylic acid (SA) also had no effect. Ethylene, auxins, GA3 and SA affected flower opening, therefore did reach the flower cells. Jasmonates delayed senescence by about 2.0 days. Similarly, cytokinins delayed senescence by about 1.5-2.0 days. Antagonists of the phosphatidylinositol signal transduction pathway (lithium), calcium channels (niguldipine and verapamil), calmodulin action [fluphenazine, trifluoroperazine, phenoxybenzamide and N-(6-aminohexyl)-5-chloro-1-naphtalenesulfonamide hydrochloride (W-7)] or protein kinase activity [1-(5-isoquinolinesulfonyl)-2-methylpiperazine hydrochloride (H-7), N-[2-(methylamino)ethyl]-5-isoquinolinesulfonamide hydrochloride (H-8) and N-(2-aminoethyl)-5-isoquinolinesulfonamide dihydrochloride (H-9)] had no effect on senescence, indicating no role of a few common signal transduction pathways relating to hormone effects on senescence. The results indicate that tepal senescence in Iris cv. Blue Magic is not regulated by endogenous ethylene, auxin, gibberellins or SA. A role of ABA can at present not be excluded. The data suggest the hypothesis that cytokinins and jasmonates are among the natural regulators.

Gibberellin is required for the formation of tension wood and stem gravitropism in Acacia mangium seedlings

BACKGROUND AND AIMS

Angiosperm trees generally form tension wood on the upper sides of leaning stems. The formation of tension wood is an important response to gravitational stimulus. Gibberellin appears to be involved in the differentiation of secondary xylem, but it remains unclear whether gibberellin plays a key role in the formation of tension wood and plant gravitropism. Therefore, a study was designed to investigate the effects of gibberellin and of inhibitors of the synthesis of gibberellin, namely paclobutrazole and uniconazole-P, on the formation of tension wood and negative stem gravitropism in Acacia mangium seedlings.

METHODS

Gibberellic acid (GA(3)), paclobutrazole and uniconazole-P were applied to seedlings via the soil in which they were growing. Distilled water was applied similarly as a control. Three days after such treatment, seedlings were tilted at an angle of 45° from the vertical, and samples of stems were collected for analysis 2 weeks, 2 months and 6 months after tilting. The effects of treatments on the stem recovery degree (Rº) were analysed as an index of the negative gravitropism of seedlings, together the width of the region of tension wood in the upper part of inclined stems.

KEY RESULTS

It was found that GA(3) stimulated the negative gravitropism of tilted seedling stems of A. mangium, while paclobutrazole and uniconazole-P inhibited recovery to vertical growth. Moreover, GA(3) stimulated the formation of tension wood in tilted A. mangium seedlings, while paclobutrazole and uniconazole-P strongly suppressed the formation of tension wood, as assessed 2 weeks after tilting.

CONCLUSIONS

The results suggest that gibberellin plays an important role at the initial stages of formation of tension wood and in stem gravitropism in A. mangium seedlings in response to a gravitational stimulus.

ethylene receptor 1 (etr1) Is Sufficient and Has the Predominant Role in Mediating Inhibition of Ethylene Responses by Silver in Arabidopsis thaliana

Ethylene influences many processes in Arabidopsis thaliana through the action of five receptor isoforms. All five isoforms use copper as a cofactor for binding ethylene. Previous research showed that silver can substitute for copper as a cofactor for ethylene binding activity in the ETR1 ethylene receptor yet also inhibit ethylene responses in plants. End-point and rapid kinetic analyses of dark-grown seedling growth revealed that the effects of silver are mostly dependent upon ETR1, and ETR1 alone is sufficient for the effects of silver. Ethylene responses in etr1-6 etr2-3 ein4-4 triple mutants were not blocked by silver. Transformation of these triple mutants with cDNA for each receptor isoform under the promoter control of ETR1 revealed that the cETR1 transgene completely rescued responses to silver while the cETR2 transgene failed to rescue these responses. The other three isoforms partially rescued responses to silver. Ethylene binding assays on the binding domains of the five receptor isoforms expressed in yeast showed that silver supports ethylene binding to ETR1 and ERS1 but not the other isoforms. Thus, silver may have an effect on ethylene signaling outside of the ethylene binding pocket of the receptors. Ethylene binding to ETR1 with silver was ∼30% of binding with copper. However, alterations in the K(d) for ethylene binding to ETR1 and the half-time of ethylene dissociation from ETR1 do not underlie this lower binding. Thus, it is likely that the lower ethylene binding activity of ETR1 with silver is due to fewer ethylene binding sites generated with silver versus copper.

The effects of auxin and strigolactones on tuber initiation and stolon architecture in potato

Various transcriptional networks and plant hormones have been implicated in controlling different aspects of potato tuber formation. Due to its broad impact on many plant developmental processes, a role for auxin in tuber initiation has been suggested but never fully resolved. Here, auxin concentrations were measured throughout the plant prior to and during the process of tuber formation. Auxin levels increase dramatically in the stolon prior to tuberization and remain relatively high during subsequent tuber growth, suggesting a promoting role for auxin in tuber formation. Furthermore, in vitro tuberization experiments showed higher levels of tuber formation from axillary buds of explants where the auxin source (stolon tip) had been removed. This phenotype could be rescued by application of auxin on the ablated stolon tips. In addition, a synthetic strigolactone analogue applied on the basal part of the stolon resulted in fewer tubers. The experiments indicate that a system for the production and directional transport of auxin exists in stolons and acts synergistically with strigolactones to control the outgrowth of the axillary stolon buds, similar to the control of above-ground shoot branching.

Synthesis of 2RS,4RS-1-[2-phenyl-4-[2-(2-trifluromethoxy-phenoxy)-ethyl]-1,3-dioxolan-2-yl-methyl]-1H-1,2,4-triazole derivatives as potent inhibitors of brassinosteroid biosynthesis

Brassinosteroids are important phytohormones that affect many aspects of plant growth and development. In order to manipulate brassinosteroid levels in plant tissues by using specific biosynthesis inhibitors, we have carried out a systemic search for specific inhibitors of brassinosteroid biosynthesis. Synthesis of triazole derivatives based on the ketoconazole scaffold revealed a series of novel brassinosteroid biosynthesis inhibitors (the YCZ series). To explore the structure-activity relationships of this synthetic series, we now report the synthesis of new triazole derivatives with different aromatic structures at position 2 of 1,3-dioxolane skeleton. We found that the variation of aromatic substituent significantly affect the inhibitory potency. Structure-activity relationships studies indicated that 4-chlorophenyl analogue is the most potent inhibitor of BR biosynthesis with an IC₅₀ value approximately 0.12 ± 0.04 µM, while a bulky biphenyl group exhibited a great negative effect on promoting the inhibitory potency with an IC₅₀ larger than 10 µM.

Chemical biology of abscisic acid

Chemical biology is a discipline that utilizes chemicals to elucidate biological mechanisms and physiological functions. Various abscisic acid (ABA) derivatives have revealed the structural requirement for the perception by ABA receptors while biotin or caged derivatives of ABA have disclosed the localization of several ABA-binding proteins. Recently, selective ABA agonist has been used to identify ABA receptors. Furthermore, ABA biosynthesis and catabolic inhibitors have contributed to the identification of new ABA functions in plant growth and development. The physiological function of ABA in non-plant organisms has gradually been revealed. In this review, we discuss the development of small bioactive chemicals and their significance in ABA research.

Ntann12 annexin expression is induced by auxin in tobacco roots

Ntann12, encoding a polypeptide homologous to annexins, was found previously to be induced upon infection of tobacco with the bacterium Rhodococcus fascians. In this study, Ntann12 is shown to bind negatively charged phospholipids in a Ca(2+)-dependent manner. In plants growing in light conditions, Ntann12 is principally expressed in roots and the corresponding protein was mainly immunolocalized in the nucleus. Ntann12 expression was inhibited following plant transfer to darkness and in plants lacking the aerial part. However, an auxin (indole-3-acetic acid) treatment restored the expression of Ntann12 in the root system in dark conditions. Conversely, polar auxin transport inhibitors such as 1-naphthylphthalamic acid (NPA) or 2,3,5-triiodobenzoic acid (TIBA) inhibited Ntann12 expression in light condition. These results indicate that the expression of Ntann12 in the root is linked to the perception of a signal in the aerial part of the plant that is transmitted to the root via polar auxin transport.

OsDOG, a gibberellin-induced A20/AN1 zinc-finger protein, negatively regulates gibberellin-mediated cell elongation in rice

The A20/AN1 zinc-finger proteins (ZFPs) play pivotal roles in animal immune responses and plant stress responses. From previous gibberellin (GA) microarray data and A20/AN1 ZFP family member association, we chose Oryza sativa dwarf rice with overexpression of gibberellin-induced gene (OsDOG) to examine its function in the GA pathway. OsDOG was induced by gibberellic acid (GA(3)) and repressed by the GA-synthesis inhibitor paclobutrazol. Different transgenic lines with constitutive expression of OsDOG showed dwarf phenotypes due to deficiency of cell elongation. Additional GA(1) and real-time PCR quantitative assay analyses confirmed that the decrease of GA(1) in the overexpression lines resulted from reduced expression of GA3ox2 and enhanced expression of GA2ox1 and GA2ox3. Adding exogenous GA rescued the constitutive expression phenotypes of the transgenic lines. OsDOG has a novel function in regulating GA homeostasis and in negative maintenance of plant cell elongation in rice.

Epigenetic and physiological effects of gibberellin inhibitors and chemical pruners on the floral transition of azalea

The ability to control the timing of flowering is a key strategy in planning the production of ornamental species such as azaleas; however, it requires a thorough understanding of floral transition. DNA methylation is involved in controlling the functional state of chromatin and gene expression during floral induction pathways in response to environmental and developmental signals. Plant hormone signalling is also known to regulate suites of morphogenic processes in plants and its role in flowering-time control is starting to emerge as a key controlling step. This work investigates if the gibberellin (GA) inhibitors and chemical pinching applied in improvement of azalea flowering alter the dynamics of DNA methylation or the levels of polyamines (PAs), GAs and cytokinins (CKs) during floral transition, and whether these changes could be related to the effects observed on flowering ability. DNA methylation during floral transition and endogenous content of PAs, GAs and CKs were analysed after the application of GA synthesis inhibitors (daminozide, paclobutrazol and chlormequat chloride) and a chemical pruner (fatty acids). The application of GA biosynthesis inhibitors caused alterations in levels of PAs, GAs and CKs and in global DNA methylation levels during floral transition; also, these changes in plant growth regulators and DNA methylation were correlated with flower development. DNA methylation, PA, GA and CK levels can be used as predictive markers of plant floral capacity in azalea.

The phenylquinazoline compound S-4893 is a non-competitive cytokinin antagonist that targets Arabidopsis cytokinin receptor CRE1 and promotes root growth in Arabidopsis and rice

We identified two phenylquinazoline compounds in a large-scale screening for cytokinin antagonists in yeast expressing the Arabidopsis cytokinin receptor cytokinin response 1/histidine kinase 4 (CRE1). After chemical modifications, we obtained compound S-4893, which non-competitively inhibited binding of the natural ligand 2-isopentenyladenine to CRE1. S-4893 antagonized cytokinin-induced activation of the Arabidopsis response regulator 5 promoter in Arabidopsis. Importantly, S-4893 had no detectable intrinsic cytokinin agonist activity in Arabidopsis or in the transformed yeast system. Cytokinin bioassay further demonstrated that S-4893 antagonized cytokinin-induced stimulation of callus formation and inhibition of root elongation. S-4893 also promoted seminal, crown and lateral root growth in rice, suggesting that S-4893 could potentially promote root growth in a variety of agronomically important plants. We believe S-4893 will be a useful tool in functional studies of cytokinin action in a wide range of plants and a lead compound for the development of useful root growth promoters in agriculture.

Arabidopsis CYP72C1 is an atypical cytochrome P450 that inactivates brassinosteroids

Cytochrome P450 monooxygenases (P450s) are a diverse family of proteins that have specialized roles in secondary metabolism and in normal cell development. Two P450s in particular, CYP734A1 and CYP72C1, have been identified as brassinosteroid-inactivating enzymes important for steroid-mediated signal transduction in Arabidopsis thaliana. Genetic analyses have demonstrated that these P450s modulate growth throughout plant development. While members of the CYP734A subfamily inactivate brassinosteroids through C-26 hydroxylation, the biochemical activity of CYP72C1 is unknown. Because CYP734A1 and CYP72C1 in Arabidopsis diverge more than brassinosteroid inactivating P450s in other plants, this study examines the structure and biochemistry of each enzyme. Three-dimensional models were generated to examine the substrate binding site structures and determine how they might affect the function of each P450. These models have indicated that the active site of CYP72C1 does not contain several conserved amino acids typically needed for substrate hydroxylation. Heterologous expression of these P450s followed by substrate binding analyses have indicated that CYP734A1 binds active brassinosteroids, brassinolide and castasterone, as well as their upstream precursors whereas CYP72C1 binds precursors more effectively. Seedling growth assays have demonstrated that the genetic state of CYP734A1, but not CYP72C1, affected responsiveness to high levels of exogenous brassinolide supporting our observations that CYP72C1 acts on brassinolide precursors. Although there may be some overlap in their physiological function, the distinct biochemical functions of these proteins in Arabidopsis has significant potential to fine-tune the levels of different brassinosteroid hormones throughout plant growth and development.

A new lead chemical for strigolactone biosynthesis inhibitors

Several triazole-containing chemicals have previously been shown to act as efficient inhibitors of cytochrome P450 monooxygenases. To discover a strigolactone biosynthesis inhibitor, we screened a chemical library of triazole derivatives to find chemicals that induce tiller bud outgrowth of rice seedlings. We discovered a triazole-type chemical, TIS13 [2,2-dimethyl-7-phenoxy-4-(1H-1,2,4-triazol-1-yl)heptan-3-ol], which induced outgrowth of second tiller buds of wild-type seedlings, as observed for non-treated strigolactone-deficient d10 mutant seedlings. TIS13 treatment reduced strigolactone levels in both roots and root exudates in a concentration-dependent manner. Co-application of GR24, a synthetic strigolactone, with TIS13 canceled the TIS13-induced tiller bud outgrowth. Taken together, these results indicate that TIS13 inhibits strigolactone biosynthesis in rice seedlings. We propose that TIS13 is a new lead compound for the development of specific strigolactone biosynthesis inhibitors.

The tryptophan conjugates of jasmonic and indole-3-acetic acids are endogenous auxin inhibitors

Most conjugates of plant hormones are inactive, and some function to reduce the active hormone pool. This study characterized the activity of the tryptophan (Trp) conjugate of jasmonic acid (JA-Trp) in Arabidopsis (Arabidopsis thaliana). Unexpectedly, JA-Trp caused agravitropic root growth in seedlings, unlike JA or nine other JA-amino acid conjugates. The response was dose dependent from 1 to 100 microm, was independent of the COI1 jasmonate signaling locus, and unlike the jasmonate signal JA-isoleucine, JA-Trp minimally inhibited root growth. The Trp conjugate with indole-3-acetic acid (IAA-Trp) produced a similar response, while Trp alone and conjugates with benzoic and cinnamic acids did not. JA-Trp and IAA-Trp at 25 microm nearly eliminated seedling root inhibition caused by 2 microm IAA. The TIR1 auxin receptor is required for activity because roots of tir1-1 grew only approximately 60% of wild-type length on IAA plus JA-Trp, even though tir1-1 is auxin resistant. However, neither JA-Trp nor IAA-Trp interfered with IAA-dependent interaction between TIR1 and Aux/IAA7 in cell-free assays. Trp conjugates inhibited IAA-stimulated lateral root production and DR5-beta-glucuronidase gene expression. JA-deficient mutants were hypersensitive to IAA and a Trp-overaccumulating mutant was less sensitive, suggesting endogenous conjugates affect auxin sensitivity. Conjugates were present at 5.8 pmol g(-1) fresh weight or less in roots, seedlings, leaves, and flowers, and the values increased approximately 10-fold in roots incubated in 25 microm Trp and IAA or JA at 2 microm. These results show that JA-Trp and IAA-Trp constitute a previously unrecognized mechanism to regulate auxin action.

Low levels of gibberellic acid control the biosynthesis of ajmalicine in Catharanthus roseus cell suspension cultures

In periwinkle cell suspensions, amounts of gibberellic acid ranging from 10 ( - 10) M to 10 ( - 7) M significantly antagonized, in a dose-dependant manner, the stimulation of ajmalicine biosynthesis by cytokinins (CKs). This inhibitory effect was strictly correlated with the abolition of the expression of two genes encoding enzymes of the monoterpenoid indole alkaloid (MIA) biosynthetic pathway and was normally upregulated after CK treatments. Moreover, low concentrations of the gibberellin biosynthesis inhibitor paclobutrazol could reverse the inhibitory effects of low auxin levels on ajmalicine accumulation in the cells. On the other hand, gibberellic acid could not affect the expression of two type-A response regulators considered to be CK primary response genes in periwinkle cells. The antagonistic effects of gibberellins and cytokinins on MIA biosynthesis and their possible impact on elements of the signal transduction are discussed.

The AtGenExpress hormone and chemical treatment data set: experimental design, data evaluation, model data analysis and data access

We analyzed global gene expression in Arabidopsis in response to various hormones and in related experiments as part of the AtGenExpress project. The experimental agents included seven basic phytohormones (auxin, cytokinin, gibberellin, brassinosteroid, abscisic acid, jasmonate and ethylene) and their inhibitors. In addition, gene expression was investigated in hormone-related mutants and during seed germination and sulfate starvation. Hormone-inducible genes were identified from the hormone response data. The effects of each hormone and the relevance of the gene lists were verified by comparing expression profiles for the hormone treatments and related experiments using Pearson's correlation coefficient. This approach was also used to analyze the relationships among expression profiles for hormone responses and those included in the AtGenExpress stress-response data set. The expected correlations were observed, indicating that this approach is useful to monitor the hormonal status in the stress-related samples. Global interactions among hormones-inducible genes were analyzed in a pairwise fashion, and several known and novel hormone interactions were detected. Genome-wide transcriptional gene-to-gene correlations, analyzed by hierarchical cluster analysis (HCA), indicated that our data set is useful for identification of clusters of co-expressed genes, and to predict the functions of unknown genes, even if a gene's function is not directly related to the experiments included in AtGenExpress. Our data are available online from AtGenExpressJapan; the results of genome-wide HCA are available from PRIMe. The data set presented here will be a versatile resource for future hormone studies, and constitutes a reference for genome-wide gene expression in Arabidopsis.

The AtGenExpress hormone and chemical treatment data set: experimental design, data evaluation, model data analysis and data access

We analyzed global gene expression in Arabidopsis in response to various hormones and in related experiments as part of the AtGenExpress project. The experimental agents included seven basic phytohormones (auxin, cytokinin, gibberellin, brassinosteroid, abscisic acid, jasmonate and ethylene) and their inhibitors. In addition, gene expression was investigated in hormone-related mutants and during seed germination and sulfate starvation. Hormone-inducible genes were identified from the hormone response data. The effects of each hormone and the relevance of the gene lists were verified by comparing expression profiles for the hormone treatments and related experiments using Pearson's correlation coefficient. This approach was also used to analyze the relationships among expression profiles for hormone responses and those included in the AtGenExpress stress-response data set. The expected correlations were observed, indicating that this approach is useful to monitor the hormonal status in the stress-related samples. Global interactions among hormones-inducible genes were analyzed in a pairwise fashion, and several known and novel hormone interactions were detected. Genome-wide transcriptional gene-to-gene correlations, analyzed by hierarchical cluster analysis (HCA), indicated that our data set is useful for identification of clusters of co-expressed genes, and to predict the functions of unknown genes, even if a gene's function is not directly related to the experiments included in AtGenExpress. Our data are available online from AtGenExpressJapan; the results of genome-wide HCA are available from PRIMe. The data set presented here will be a versatile resource for future hormone studies, and constitutes a reference for genome-wide gene expression in Arabidopsis.

Regulatory involvement of abscisic acid in potato tuber wound-healing

Rapid wound-healing is crucial in protecting potato tubers from infection and dehydration. Wound-induced suberization and the accumulation of hydrophobic barriers to reduce water vapour conductance/loss are principal protective wound-healing processes. However, little is known about the cognate mechanisms that effect or regulate these processes. The objective of this research was to determine the involvement of abscisic acid (ABA) in the regulation of wound-induced suberization and tuber water vapour loss (dehydration). Analysis by liquid chromatography-mass spectrometry showed that ABA concentrations varied little throughout the tuber, but were slightly higher near the periderm and lowest in the pith. ABA concentrations increase then decrease during tuber storage. Tuber wounding induced changes in ABA content. ABA content in wound-healing tuber discs decreased after wounding, reached a minimum by 24 h, and then increased from the 3rd to the 7th day after wounding. Wound-induced ABA accumulations were reduced by fluridone (FLD); an inhibitor of de novo ABA biosynthesis. Wound-induced phenylalanine ammonia lyase activity was slightly reduced and the accumulation of suberin poly(phenolics) and poly(aliphatics) noticeably reduced in FLD-treated tissues. Addition of ABA to the FLD treatment restored phenylalanine ammonia lyase activity and suberization, unequivocally indicating that endogenous ABA is involved in the regulation of these wound-healing processes. Similar experiments showed that endogenous ABA is involved in the regulation of water vapour loss, a process linked to wax accumulation in wound-healing tubers. Rapid reduction of water vapour loss across the wound surface is essential in preventing desiccation and death of cells at the wound site; live cells are required for suberization. These results unequivocally show that endogenous ABA is involved in the regulation of wound-induced suberization and the processes that protect surface cells from water vapour loss and death by dehydration.