Gibberellin Biosynthesis Inhibitor Research Articles

Shade-induced stem elongation in rice seedlings: Implication of tissue-specific phytohormone regulation.

A better understanding of shade avoidance syndrome (SAS) is an urgent need because of its effect on energy reallocation. Leverage-related mechanism in crops is of potential economic interest for agricultural applications. Here we report the SAS phenotype at tissue level rice seedlings. Tissue-specific RNA-sequencing indicates auxin plays different roles between coleoptile and the first leaf. Phenotypes of wild type treated by gibberellin and brassinosteroid biosynthesis inhibitors and of related mutants suggest these two hormones positively regulate SAS. Our work reveals the diversity of hormone responses in different organs and different species in shade conditions.

Xyloglucan Metabolism Differentially Impacts the Cell Wall Characteristics of the Endosperm and Embryo during Arabidopsis Seed Germination

Cell wall remodeling is an essential mechanism for the regulation of plant growth and architecture, and xyloglucans (XyGs), the major hemicellulose, are often considered as spacers of cellulose microfibrils during growth. In the seed, the activity of cell wall enzymes plays a critical role in germination by enabling embryo cell expansion leading to radicle protrusion, as well as endosperm weakening prior to its rupture. A screen for Arabidopsis (Arabidopsis thaliana) mutants affected in the hormonal control of germination identified a mutant, xyl1, able to germinate on paclobutrazol, an inhibitor of gibberellin biosynthesis. This mutant also exhibited reduced dormancy and increased resistance to high temperature. The XYL1 locus encodes an α-xylosidase required for XyG maturation through the trimming of Xyl. The xyl1 mutant phenotypes were associated with modifications to endosperm cell wall composition that likely impact on its resistance, as further demonstrated by the restoration of normal germination characteristics by endosperm-specific XYL1 expression. The absence of phenotypes in mutants defective for other glycosidases, which trim Gal or Fuc, suggests that XYL1 plays the major role in this process. Finally, the decreased XyG abundance in hypocotyl longitudinal cell walls of germinating embryos indicates a potential role in cell wall loosening and anisotropic growth together with pectin de-methylesterification.

Breadfruit (Artocarpus altilis) gibberellin 2-oxidase genes in stem elongation and abiotic stress response

Breadfruit (Artocarpus altilis) is a traditional staple tree crop in the Oceania. Susceptibility to windstorm damage is a primary constraint on breadfruit cultivation. Significant tree loss due to intense tropical windstorm in the past decades has driven a widespread interest in developing breadfruit with dwarf stature. Gibberellin (GA) is one of the most important determinants of plant height. GA 2-oxidase is a key enzyme regulating the flux of GA through deactivating biologically active GAs in plants. As a first step toward understanding the molecular mechanism of growth regulation in the species, we isolated a cohort of four full-length GA2-oxidase cDNAs, AaGA2ox1- AaGA2ox4 from breadfruit. Sequence analysis indicated the deduced proteins encoded by these AaGA2oxs clustered together under the C19 GA2ox group. Transcripts of AaGA2ox1, AaGA2ox2 and AaGA2ox3 were detected in all plant organs, but exhibited highest level in source leaves and stems. In contrast, transcript of AaGA2ox4 was predominantly expressed in roots and flowers, and displayed very low expression in leaves and stems. AaGA2ox1, AaGA2ox2 and AaGA2ox3, but not AaGA2ox4 were subjected to GA feedback regulation where application of exogenous GA3 or gibberellin biosynthesis inhibitor, paclobutrazol was shown to manipulate the first internode elongation of breadfruit. Treatments of drought or high salinity increased the expression of AaGA2ox1, AaGA2ox2 and AaGA2ox4. But AaGA2ox3 was down-regulated under salt stress. The function of AaGA2oxs is discussed with particular reference to their role in stem elongation and involvement in abiotic stress response in breadfruit.

Growth suppression by exogenous abscisic acid and uniconazole for prolonged marketability of bell pepper transplants in commercial conditions

Vegetable transplants quickly outgrow the optimal size for shipping and transplanting, limiting sales, and marketing flexibility in commercial nurseries. Abscisic acid (ABA) and uniconazole can suppress shoot growth by inducing stress-adaptive responses and inhibiting gibberellin biosynthesis, respectively. We evaluated the effectiveness of the two growth regulators in prolonging marketability of ‘Excursion’ and ‘Revolution’ bell pepper (Capsicum annuum L.) transplants at commercial nursery greenhouses in Texas and Florida. Spray treatments in the Texas experiment were 0 and 3.8mM ABA at 7, 5, 3, or 1 day before the anticipated maturity date (DBM), and those in the Florida experiment were no spray control, 3.8mM ABA at 7, 5, 3, or 1 DBM, and 34μM uniconazole at 4 DBM. Both experiments showed similar results with minimal cultivar-specific effects. Different growth modifications were induced by ABA and uniconazole. First, suppression of stem elongation by ABA was reversible by 7 days after the anticipated maturity date (DAM), whereas that by uniconazole lasted for 20 days or until 16 DAM with up to 15% reductions in stem length. Second, only ABA inhibited shoot and root dry matter accumulation. The growth modulating effect of uniconazole appears to be limited to height control, which is beneficial for producing compact transplants, rather than as a growth holding strategy. By contrast, the overall growth suppression by ABA is desirable for prolonging transplant marketability. Importantly, the magnitude of this growth suppression was moderate (9%–12% shoot biomass reductions at 7–8 DAM) and mostly reversible by 14–16 DAM. Furthermore, root growth inhibition by ABA occurred with a time lag of over a week, allowing sufficient root development and increasing root-to-shoot ratio at 0 DBM. Although these growth holding effects of ABA were generally maximized when it was applied at 7 or 5 DBM, leaf chlorosis and cotyledon abscission were also induced by ABA in a similar age-dependent manner. These results suggest that ABA application 3 DBM is an effective growth holding strategy with minimal negative-side effects for bell pepper transplants.

Breadfruit (Artocarpus altilis) gibberellin 20-oxidase genes: sequence variants, stem elongation and abiotic stress response

Breadfruit (Artocarpus altilis) is a traditional staple tree crop throughout the tropics. Susceptibility to windstorm damage is the primary constraint on breadfruit cultivation. Significant tree loss due to intense tropical windstorm in the past decades has driven an increasing interest in developing dwarf varieties of breadfruit. As a first step toward understanding the molecular mechanism of growth regulation in the species, we investigated the role of gibberellin and the regulation of GA20-oxidase genes in breadfruit. We provided first evidence that the stem elongation in breadfruit could be manipulated by exogenous gibberellin-related growth regulators. We then cloned six GA20-oxidase cDNAs, AaGA20ox1–AaGA20ox6, in full length from breadfruit. Sequence analysis showed that the predicted proteins of the AaGA20ox1–AaGA20ox3 bear all the hallmarks of functional GA20-oxidase of other species, but predicted AaGA20ox4–AaGA20ox6 as expressed, unprocessed pseudogenes closely related to AaGA20ox2. AaGA20ox1, AaGA20ox3 and AaGA20ox4 were predominantly expressed in green vegetative organs, but displayed different expression pattern in roots and reproductive organs. AaGA20ox2, AaGA20ox5 and AaGA20ox6 were expressed mainly in leaves at low level. AaGA20ox1, AaGA20ox3–AaGA20ox6 were subjected to GA feedback regulation following treatment of exogenous gibberellin and/or gibberellin biosynthesis inhibitors. AaGA20ox1 and AaGA20ox3 were down-regulated under drought and salinity stress, but AaGA20ox2 was up-regulated under salt stress. Pseudogenes AaGA20ox4 and AaGA20ox5 were up-regulated under drought or/and salt stress condition. The function of AaGA20oxs is discussed with particular reference to their role in stem elongation and involvement in abiotic stress response in breadfruit.

Characterization of Arabidopsis mutants insensitive to high sugar concentrations

Sugar is appreciated to function as a signaling molecule by controlling gene expression, thereby regulating various developmental processes in plants. We screened sucrose-insensitive (sin) mutants in Arabidopsis that showed normal germination and seedling development under high concentration (438 mM) of sucrose. Compared to wild-type and other sugar mutants, sin mutants seedlings exhibited resistance to high glucose, high salt, and paclobutrazol, inhibitor of gibberellin biosynthesis. We discuss the unique characteristics of these sin mutants in this report.

THE EXPRESSION OF VVGA20OX1 AND VVGA3OX1 DURING FRUIT SET OF GRAPEVINE

Gibberellin plays an important role in fruit set in grapevine and is often used to improve berry fruit set percent. However, few studies have focused on the molecular mechanism(s) of how gibberellin improves fruit set. This research aims to explore the influence of gibberellin and paclobutrazol (PAC; an inhibitor of gibberellin biosynthesis) on grape berry fruit set. The expression of VvGA20ox1 and VvGA3ox1 were also analyzed. The result showed that application of gibberellin improved fruit set percent, PAC inhibited fruit set, and PAC inhibition was fully reversed with GA3 application. The fruit size of berries treated with gibberellin was smaller than controls. The expression of VvGA20ox1 and VvGA3ox1 showed different patterns; VvGA20ox1 seemed to be ubiquitously expressed, but VvGA3ox1 was expressed less in tendrils and stems. Before anthesis, VvGA20ox1 showed high expression, but it decreased after anthesis. VvGA20ox expression was lower in ovaries treated with gibberellin than PAC. This indicated that VvGA20ox1 may not or may negatively regulate gibberellin biosynthesis during grape berry fruit set. By contrast, the expression of VvGA3ox1 in inflorescence was low before anthesis, but expression increased rapidly after anthesis. Before anthesis, VvGA3ox1?s expression in inflorescences treated with gibberellin was higher than controls. After anthesis, there was no significant difference in VvGA3ox1 expression between control and gibberellin-treated berries. However, after using PAC, there was no difference with control before anthesis, but 8 days after anthesis, VvGA3ox1 expression in berries was much lower than control and gibberellin-treated berries, indicating that VvGA3ox1 may be involved in gibberellin biosynthesis during grape berry fruit set. There was no significant difference in tissue microstructure among different applications.

The ABA-Deficiency Suppressor Locus HAS2 Encodes the PPR Protein LOI1/MEF11 Involved in Mitochondrial RNA Editing

The hot ABA-deficiency suppressor2 (has2) mutation increases drought tolerance and the ABA sensitivity of stomata closure and seed germination. Here we report that the HAS2 locus encodes the MITOCHONDRIAL EDITING FACTOR11 (MEF11), also known as LOVASTATIN INSENSITIVE1. has2/mef11 mutants exhibited phenotypes very similar to the ABA-hypersensitive mutant, hai1-1 pp2ca-1 hab1-1 abi1-2, which is impaired in four genes encoding type 2C protein phosphatases (PP2C) that act as upstream negative regulators of the ABA signaling cascade. Like pp2c, mef11 plants were more resistant to progressive water stress and seed germination was more sensitive to paclobutrazol (a gibberellin biosynthesis inhibitor) as well as mannitol and NaCl, compared with the wild-type plants. Phenotypic alterations in mef11 were associated with the lack of editing of transcripts for the mitochondrial cytochrome c maturation FN2 (ccmFN2) gene, which encodes a cytochrome c-heme lyase subunit involved in cytochrome c biogenesis. Although the abundance of electron transfer chain complexes was not affected, their dysfunction could be deduced from increased respiration and altered production of hydrogen peroxide and nitric oxide in mef11 seeds. As minor defects in mitochondrial respiration affect ABA signaling, this suggests an essential role for ABA in mitochondrial retrograde regulation.

The mitochondrion-located protein OsB12D1 enhances flooding tolerance during seed germination and early seedling growth in rice.

B12D belongs to a function unknown subgroup of the Balem (Barley aleurone and embryo) proteins. In our previous work on rice seed germination, we identified a B12D-like protein encoded by LOC_Os7g41350 (named OsB12D1). OsB12D1 pertains to an ancient protein family with an amino acid sequence highly conserved from moss to angiosperms. Among the six OsB12Ds, OsB12D1 is one of the major transcripts and is primarily expressed in germinating seed and root. Bioinformatics analyses indicated that OsB12D1 is an anoxic or submergence resistance-related gene. RT-PCR results showed OsB12D1 is induced remarkably in the coleoptiles or roots by flooding during seed germination and early seedling growth. The OsB12D1-overexpressed rice seeds could protrude radicles in 8 cm deep water, further exhibiting significant flooding tolerance compared to the wild type. Moreover, this tolerance was not affected by the gibberellin biosynthesis inhibitor paclobutrazol. OsB12D1 was identified in the mitochondrion by subcellular localization analysis and possibly enhances electron transport through mediating Fe and oxygen availability under flooded conditions. This work indicated that OsB12D1 is a promising gene that can help to enhance rice seedling establishment in farming practices, especially for direct seeding.

Shielding Flowers Developing under Stress: Translating Theory to Field Application.

Developing reproductive organs within a flower are sensitive to environmental stress. A higher incidence of environmental stress during this stage of a crop plants’ developmental cycle will lead to major breaches in food security. Clearly, we need to understand this sensitivity and try and overcome it, by agricultural practices and/or the breeding of more tolerant cultivars. Although passion fruit vines initiate flowers all year round, flower primordia abort during warm summers. This restricts the season of fruit production in regions with warm summers. Previously, using controlled chambers, stages in flower development that are sensitive to heat were identified. Based on genetic analysis and physiological experiments in controlled environments, gibberellin activity appeared to be a possible point of horticultural intervention. Here, we aimed to shield flowers of a commercial cultivar from end of summer conditions, thus allowing fruit production in new seasons. We conducted experiments over three years in different settings, and our findings consistently show that a single application of an inhibitor of gibberellin biosynthesis to vines in mid-August can cause precocious flowering of ~2–4 weeks, leading to earlier fruit production of ~1 month. In this case, knowledge obtained on phenology, environmental constraints and genetic variation, allowed us to reach a practical solution.

MORPHOLOGICAL AND PHYSIOLOGICAL RESPONSES OF YOUNG APPLE TREES TO FOLIAR APPLIED UNICONAZOLE

Uniconazole (UCZ) is a gibberellin biosynthesis inhibitor reported to be more effective than paclobutrazol at similar concentrations. It was labeled in Australia to improve avocado yield, received a supplemental label in the USA for vegetable trans-plants, and was reported to improve Arabidopsis drought tolerance. In this experiment one-year-old 'Golden Delicious'/MM. 106 (Malus ×domestica) trees were pruned to 15 cm above the graft union, one shoot selected and trained as a leader. Trees were sprayed with 0, 100, 200, 300, 400, or 800 mg/L UCZ on 15 June (≈ 12 new leaves formed). The topmost fully expanded leaf was tagged at treatment and referred to as mature leaf. Net photosynthesis (Pn), stomatal conductance (gs), and transpiration (E) of mature leaves and recently fully expanded leaves were measured at 0, 21, 52, and 102 days after treatment (DAT). The Pn of young leaves on UCZ treated trees were higher at 21 and 102 (P≤0.05) DAT than control leaves. However, the Pn of old leaves were numerically lower than controls at 21 DAT but higher at 52 and 102 DAT. New and old leaves of 300 mg/L treated trees had higher numerically maximum daily Pn (in the diurnal cycle) than control leaves and remained higher throughout the afternoon. In a separate experiment, trees were saturated for three days, and then water withheld from 10 July until 3 Aug. New leaves on trees treated with 300 mg/L had higher Pn than controls at four, eight, and 14 days after imposed water stress.

Functional characterization of xanthoxin dehydrogenase in rice

Abscisic acid (ABA) is a phytohormone that plays a key role in biotic and abiotic stress responses. ABA metabolic genes are promising targets for molecular breeding work to improve stress tolerance in crops. The accumulation of ABA does not always improve stress tolerance since stress-induced accumulation of ABA in pollen inhibits the normal course of gametogenesis, affecting grain yields in cereals. This effect highlights the importance of manipulating the ABA levels according to the type of tissues. The aim of this study was to assign an ABA biosynthetic enzyme, xanthoxin dehydrogenase (XanDH), as a functional marker to modulate ABA levels in rice. XanDH is a member of the short-chain dehydrogenase/reductase family that catalyzes the conversion of xanthoxin to abscisyl aldehyde (ABAld). Previously, this enzyme had only been identified in Arabidopsis, as AtABA2. In this study, a XanDH named OsABA2 was identified in rice. Phylogenetic analysis indicated that a single gene encodes for OsABA2 in the rice genome. Its amino acid sequence contains two motifs that are essential for cofactor binding and catalytic activity. Expression analysis of OsABA2 mRNA showed that the transcript level did not change in response to treatment with ABA or dehydration. Recombinant OsABA2 protein expressed in Escherichia coli converted xanthoxin to ABAld in an NAD-dependent manner. Moreover, expression of OsABA2 in an Arabidopsis aba2 mutant rescued the aba2 mutant phenotypes, characterized by reduced growth, increased water loss, and germination in the presence of paclobutrazol, a gibberellin biosynthesis inhibitor or high concentration of glucose. These results indicate that OsABA2 is a rice XanDH that functions in ABA biosynthesis.

A novel lily anther-specific gene encodes adhesin-like proteins associated with exine formation during anther development

The anther-specific gene LLA1271 isolated from lily (Lilium longiflorum Thunb.) anthers is novel and exists in two forms. The protein encoded by LLA1271 may represent an adhesin-like protein first found in higher plants. The protein contains a typical N-terminal signal peptide followed by a highly conserved repeat domain. The LLA1271 gene is temporally expressed at the phase of microspore development. RNA blot and RNA in situ hybridization analyses demonstrated that the gene was expressed both in the tapetum and in the microspore. The gene is endo- and exogenously induced by gibberellin. Studies with the gibberellin biosynthesis inhibitor uniconazole and an inhibitor of ethylene activity, 2,5-norbornadien (NBD), revealed that LLA1271 is negatively regulated by ethylene, and a cross-talk of regulation between gibberellin and ethylene occurs in young anthers. The treatment with NBD caused the tapetum to become densely cytoplasmic and highly polarized, whereas uniconazole arrested tapetal development in a state close to that of a tapetum without treatment. The LLA1271 protein is heat stable and heterogeneous. An immunoblot of separated protein fractions of the anther revealed that the LLA1271 protein was detected in protein fraction of the microspore released from the cell wall by treatment with either 0.5% or 2% Triton X-100. Ectopic expression of LLA1271 resulted in impaired stamen and low pollen germination. Scanning electron microscopy of TAP::LLA1271 pollen showed distorted exine formation and patterning. The LLA1271 protein once synthesized in both the tapetum and microspore is secreted and deposited on the surface of microspores, moderately affecting exine formation and patterning.

SIS8, a putative mitogen‐activated protein kinase kinase kinase, regulates sugar‐resistant seedling development in Arabidopsis

Sugar signaling pathways have been evolutionarily conserved among eukaryotes and are postulated to help regulate plant growth, development and responses to environmental cues. Forward genetic screens have identified sugar signaling or response mutants. Here we report the identification and characterization of Arabidopsis thaliana sugar insensitive8 (sis8) mutants, which display a sugar-resistant seedling development phenotype. Unlike many other sugar insensitive mutants, sis8 mutants exhibit wild-type responses to the inhibitory effects of abscisic acid and paclobutrazol (an inhibitor of gibberellin biosynthesis) on seed germination. Positional cloning of the SIS8 gene revealed that it encodes a putative mitogen-activated protein kinase kinase kinase (MAPKKK; At1g73660). SIS8mRNA is expressed ubiquitously among Arabidopsis organs. A UDP-glucosyltransferase, UGT72E1 (At3g50740), was identified as an interacting partner of SIS8 based on a yeast two-hybrid screen and in planta bimolecular fluorescence complementation. Both SIS8-yellow fluorescent protein (YFP) and UGT72E1-YFP fusion proteins localize to the nucleus when transiently expressed in tobacco leaf cells. T-DNA insertions in At3g50740 cause a sugar-insensitive phenotype. These results indicate that SIS8, a putative MAPKKK, is a regulator of sugar response in Arabidopsis and interacts with a UDP-glucosyltransferase in the nucleus.

Plant growth enhancement and associated physiological responses are coregulated by ethylene and gibberellin in response to harpin protein Hpa1

The harpin protein Hpa1 produced by the bacterial blight pathogen of rice induces several growth-promoting responses in plants, activating the ethylene signaling pathway, increasing photosynthesis rates and EXPANSIN (EXP) gene expression levels, and thereby enhancing the vegetative growth. This study was attempted to analyze any mechanistic connections among the above and the role of gibberellin in these responses. Hpa1-induced growth enhancement was evaluated in Arabidopsis, tomato, and rice. And growth-promoting responses were determined mainly as an increase of chlorophyll a/b ratio, which indicates a potential elevation of photosynthesis rates, and enhancements of photosynthesis and EXP expression in the three plant species. In Arabidopsis, Hpa1-induced growth-promoting responses were partially compromised by a defect in ethylene perception or gibberellin biosynthesis. In tomato and rice, compromises of Hpa1-induced growth-promoting responses were caused by a pharmacological treatment with an ethylene perception inhibitor or a gibberellin biosynthesis inhibitor. In the three plant species, moreover, Hpa1-induced growth-promoting responses were significantly impaired, but not totally eliminated, by abolishing ethylene perception or gibberellin synthesis. However, simultaneous nullifications in both ethylene perception and gibberellin biosynthesis almost canceled the full effects of Hpa1 on plant growth, photosynthesis, and EXP2 expression. Theses results suggest that ethylene and gibberellin coregulate Hpa1-induced plant growth enhancement and associated physiological and molecular responses.Electronic supplementary materialThe online version of this article (doi:10.1007/s00425-013-2013-y) contains supplementary material, which is available to authorized users.

Histological study on tuberisation of potato (Solanum tuberosum L.) promoted by inhibitors of gibberellin biosynthesis

The application of antigibberellins to potato plant cultures results in an accelerated tuber appearance, caused by stimulation of pith-,and cortex-cell growth, particularly in the radial direction. Their growth considerably precedes the initiation of parenchymatous cell divisions, which are the first steps of naturally occurrring tuberisation, regulated by a different mechanism than the quantitative proportion of endogenous gibberellins and cytokinins. Part of the cortex cells change the orientation of their divisions to a periclinal one. The xylem parenchyma cells also undergo hypertrophy.

The effect of the plant growth retardants AMO-1618 and CCC on the synthesis of ribonucleic acids and proteins in triticale embryos during the initial phase of germination

Triticale var. Grado caryopses were subjected to imbibition and germination in the presence of the growth retardants, AMO-1618 (2-isopropyl-4-dimethylamino-5-methylphenyl-1-piperidinecarboxylate methyl chloride) and CCC (2-chloroethyl)-trimethylammonium chloride) at the following concentrations, 3 x 10<sup>-4</sup> M and 10<sup>-3</sup> M. These compounds exerted a very strong inhibitory effect on the initiation of germination processes, growth of embryos and the germination capacity of the caryopses. At the concentration of 10<sup>-3</sup> M, AMO-1618 showed an especially strong effect, lowering the germination capacity of the caryopses to about 50%. It was also shown that both retardants are decidedly more effective on the germination of whole, intact caryopses than on that of isolated embryos. During the very earliest hours of germination, these retardants already inhibited RNA synthesis. The participation of the polyribosome fraction in the total ribosome fraction of embryos in the control sample after 24 hrs of germination of caryopses equalled about 70%, while in the samples treated with CCC (10<sup>-3</sup> M)- about 57%, in the samples treated with AMO-1618 (10<sup>-3</sup> M) about 35%,. The inhibition of incorporation of <sup>14</sup>C-amino acids into ribosomal proteins in the polyribosome fraction was in the case of CCC about 13%, while in the samples treated with AMO-1618, about 55%. In the monosome fraction (80S), the inhibition by CCC was about 23%, whereas in the samples treated with AMO-1618 it reached around 73%. From this data it is evident that the studied retardants have a significant influence on the synthesis of ribonucleic acids as well as on ribosome proteins. These results also suggest the existance of another mechanism, aside from that of inhibition of gibberellin biosynthesis, inhibiting the growth and development of cells. The high percentage of ribosome subunits in the samples treated with CCC, in comparison with controls and samples treated with AMO-1618, points to different mechanisms by which these two compounds affect protein biosynthesis.

Gibberellin mediates the development of gelatinous fibres in the tension wood of inclined Acacia mangium seedlings.

Gibberellin stimulates negative gravitropism and the formation of tension wood in tilted Acacia mangium seedlings, while inhibitors of gibberellin synthesis strongly inhibit the return to vertical growth and suppress the formation of tension wood. To characterize the role of gibberellin in tension wood formation and gravitropism, this study investigated the role of gibberellin in the development of gelatinous fibres and in the changes in anatomical characteristics of woody elements in Acacia mangium seedlings exposed to a gravitational stimulus. Gibberellin, paclobutrazol and uniconazole-P were applied to the soil in which seedlings were growing, using distilled water as the control. Three days after the start of treatment, seedlings were inclined at 45 ° to the vertical and samples were harvested 2 months later. The effects of the treatments on wood fibres, vessel elements and ray parenchyma cells were analysed in tension wood in the upper part of inclined stems and in the opposite wood on the lower side of inclined stems. Application of paclobutrazol or uniconazole-P inhibited the increase in the thickness of gelatinous layers and prevented the elongation of gelatinous fibres in the tension wood of inclined stems. By contrast, gibberellin stimulated the elongation of these fibres. Application of gibberellin and inhibitors of gibberellin biosynthesis had only minor effects on the anatomical characteristics of vessel and ray parenchyma cells. The results suggest that gibberellin is important for the development of gelatinous fibres in the tension wood of A. mangium seedlings and therefore in gravitropism.

A novel Arabidopsis MYB-like transcription factor, MYBH, regulates hypocotyl elongation by enhancing auxin accumulation

Critical responses to developmental or environmental stimuli are mediated by different transcription factors, including members of the ERF, bZIP, MYB, MYC, and WRKY families. Of these, MYB genes play roles in many developmental processes. The overexpression of one MYB gene, MYBH, significantly increased hypocotyl elongation in Arabidopsis thaliana plants grown in the light, and the expression of this gene increased markedly in the dark. The MYBH protein contains a conserved motif, R/KLFGV, which was implicated in transcriptional repression. Interestingly, the gibberellin biosynthesis inhibitor paclobutrazol blocked the increase in hypocotyl elongation in seedlings that overexpressed MYBH. Moreover, the function of MYBH was dependent on phytochrome-interacting factor (PIF) proteins. Taken together, these results suggest that hypocotyl elongation is regulated by a delicate and efficient mechanism in which MYBH expression is triggered by challenging environmental conditions such as darkness, leading to an increase in PIF accumulation and subsequent enhanced auxin biosynthesis. These results indicate that MYBH is one of the molecular components that regulate hypocotyl elongation in response to darkness.

Transcriptomic Analysis of the Role of Carboxylic Acids in Metabolite Signaling in Arabidopsis Leaves

The transcriptional response to metabolites is an important mechanism by which plants integrate information about cellular energy and nutrient status. Although some carboxylic acids have been implicated in the regulation of gene expression for select transcripts, it is unclear whether all carboxylic acids have the same effect, how many transcripts are affected, and how carboxylic acid signaling is integrated with other metabolite signals. In this study, we demonstrate that perturbations in cellular concentrations of citrate, and to a lesser extent malate, have a major impact on nucleus-encoded transcript abundance. Functional categories of transcripts that were targeted by both organic acids included photosynthesis, cell wall, biotic stress, and protein synthesis. Specific functional categories that were only regulated by citrate included tricarboxylic acid cycle, nitrogen metabolism, sulfur metabolism, and DNA synthesis. Further quantitative real-time polymerase chain reaction analysis of specific citrate-responsive transcripts demonstrated that the transcript response to citrate is time and concentration dependent and distinct from other organic acids and sugars. Feeding of isocitrate as well as the nonmetabolizable citrate analog tricarballylate revealed that the abundance of selected marker transcripts is responsive to citrate and not downstream metabolites. Interestingly, the transcriptome response to citrate feeding was most similar to those observed after biotic stress treatments and the gibberellin biosynthesis inhibitor paclobutrazol. Feeding of citrate to mutants with defects in plant hormone signaling pathways did not completely abolish the transcript response but hinted at a link with jasmonic acid and gibberellin signaling pathways. Our results suggest that changes in carboxylic acid abundances can be perceived and signaled in Arabidopsis (Arabidopsis thaliana) by as yet unknown signaling pathways.