Developmental regulation of indole-3-acetic acid turnover in Scots pine seedlings

Developmental Morphology, Physiology, and Molecular Basis of the Pentagram Fruit of Averrhoa carambola

Averrhoa carambola, a key tropical and subtropical economic tree in the Oxalidaceae family, is distinguished by its unique pentagram-shaped fruit. This study investigates the developmental processes shaping the polarity of A. carambola fruit and their underlying hormonal and genetic mechanisms. By analyzing the Y1, Y2, and Y3 developmental stages-defined by the fruit diameters of 3-4 mm, 4-6 mm, and 6-12 mm, respectively-we observed that both cell number and cell size contribute to fruit development. Our findings suggest that the characteristic pentagram shape is established before flowering and is maintained throughout development. A hormonal analysis revealed that indole-3-acetic acid (IAA) and abscisic acid (ABA) show differential distribution between the convex and concave regions of the fruit across the developmental stages, with IAA playing a crucial role in polar auxin transport and shaping fruit morphology. A transcriptomic analysis identified several key genes, including AcaGH3.8, AcaIAA20, AcaYAB2, AcaXTH6, AcaYAB3, and AcaEXP13, which potentially regulate fruit polarity and growth. This study advances our comprehension of the molecular mechanisms governing fruit shape, offering insights for improving fruit quality through targeted breeding strategies.

N-glucosyltransferase GbNGT1 from ginkgo complements the auxin metabolic pathway

As auxins are among the most important phytohormones, the regulation of auxin homeostasis is complex. Generally, auxin conjugates, especially IAA glucosides, are predominant at high auxin levels. Previous research on terminal glucosylation focused mainly on the O-position, while IAA-N-glucoside and IAA-Asp-N-glucoside have been neglected since their discovery in 2001. In our study, IAA-Asp-N-glucoside was found to be specifically abundant (as high as 4.13 mg/g) in the seeds of 58 ginkgo cultivars. Furthermore, a novel N-glucosyltransferase, termed GbNGT1, was identified via differential transcriptome analysis and in vitro enzymatic testing. It was found that GbNGT1 could catalyze IAA-Asp and IAA to form their corresponding N-glucosides. The enzyme was demonstrated to possess a specific catalytic capacity toward the N-position of the IAA-amino acid or IAA from 52 substrates. Docking and site-directed mutagenesis of this enzyme confirmed that the E15G mutant could almost completely abolish its N-glucosylation ability toward IAA-Asp and IAA in vitro and in vivo. The IAA modification of GbNGT1 and GbGH3.5 was verified by transient expression assay in Nicotiana benthamiana. The effect of GbNGT1 on IAA distribution promotes root growth in Arabidopsis thaliana.

Broadening the roles of UDP-glycosyltransferases in auxin homeostasis and plant development

The levels of the important plant growth regulator indole-3-acetic acid (IAA) are tightly controlled within plant tissues to spatiotemporally orchestrate concentration gradients that drive plant growth and development. Metabolic inactivation of bioactive IAA is known to participate in the modulation of IAA maxima and minima. IAA can be irreversibly inactivated by oxidation and conjugation to aspartate and glutamate. Usually overlooked because of its reversible nature, the most abundant inactive IAA form is the IAA-glucose (IAA-glc) conjugate. Glycosylation of IAA in Arabidopsis thaliana is reported to be carried out by UDP-glycosyltransferase 84B1 (UGT84B1), while UGT74D1 has been implicated in the glycosylation of the irreversibly formed IAA catabolite oxIAA. Here we demonstrated that both UGT84B1 and UGT74D1 modulate IAA levels throughout plant development by dual IAA and oxIAA glycosylation. Moreover, we identified a novel UGT subfamily whose members redundantly mediate the glycosylation of oxIAA and modulate skotomorphogenic growth.

Auxin Metabolism in Plants

The major natural auxin in plants, indole-3-acetic acid (IAA), orchestrates a plethora of developmental responses that largely depend on the formation of auxin concentration gradients within plant tissues. Together with inter- and intracellular transport, IAA metabolism-which comprises biosynthesis, conjugation, and degradation-modulates auxin gradients and is therefore critical for plant growth. It is now very well established that IAA is mainly produced from Trp and that the IPyA pathway is a major and universally conserved biosynthetic route in plants, while other redundant pathways operate in parallel. Recent findings have shown that metabolic inactivation of IAA is also redundantly performed by oxidation and conjugation processes. An exquisite spatiotemporal expression of the genes for auxin synthesis and inactivation have been shown to drive several plant developmental processes. Moreover, a group of transcription factors and epigenetic regulators controlling the expression of auxin metabolic genes have been identified in past years, which are illuminating the road to understanding the molecular mechanisms behind the coordinated responses of local auxin metabolism to specific cues. Besides transcriptional regulation, subcellular compartmentalization of the IAA metabolism and posttranslational modifications of the metabolic enzymes are emerging as important contributors to IAA homeostasis. In this review, we summarize the current knowledge on (1) the pathways for IAA biosynthesis and inactivation in plants, (2) the influence of spatiotemporally regulated IAA metabolism on auxin-mediated responses, and (3) the regulatory mechanisms that modulate IAA levels in response to external and internal cues during plant development.

Rapid mapping of the IAA in leaves of Arabidopsis thaliana using a simple paper-based electroanalytical device coupled with microsampling

To deeply investigate the pivotal roles of Auxin (mainly indole-3-acetic acid, IAA), it is essential to obtain the contents of IAA in different locations of plants. It is still a challenge to quantify the levels of IAA in different sites of Arabidopsis thaliana leaves because of the small sizes. In this study, a simple paper-based electroanalytical device coupled with microsampling was used to differentiate the IAA amounts in different locations of Arabidopsis thaliana leaves. For the micro real sampling, the different areas of the thaliana leaves were retrieved by the Harris Uni-Core TM Miltex® with diameters: 1.0, 1.5, 2.5, 3.5, and 4.0 mm. The results showed that the contents of IAA can be detected from circle samples with the diameter from 1.0 to 4.0 mm. With 1.5 mm diameter sampling, the levels of IAA could be obtained in different sites of cotyledon and the first true leaf of Arabidopsis thaliana at the seedling stage. Our results suggested that the highest IAA levels were in the near petiole and lowest IAA levels in the leaf tip, which roughly agreed with those in tobacco leaves based on HPLC-MS reported before. In addition, the microsampling has a minor impact on the growth of Arabidopsis thaliana in the following especially for circle samples with the diameter 1.5 mm. This study revealed the potential application of microsampling coupled with a simple paper-based electroanalytical device for the mapping study of IAA in small plants or small tissue samples.

To deeply investigate the pivotal roles of Auxin (mainly indole-3-acetic acid, IAA), it is essential to obtain the contents of IAA in different locations of plants.

Screening and optimization of indole-3-acetic acid production by Rhizobium sp. strain using response surface methodology

BACKGROUND

The production of indole-3-acetic acid (IAA) is an essential tool for rhizobacteria to stimulate and facilitate plant growth. For this, eighty rhizobial bacteria isolated from root nodules of Acacia cyanophylla grown in different regions of Morocco were firstly screened for their ability to produce IAA. Then, IAA production by a combination of isolates and the inoculation effect on the germination of Acacia cyanophylla seeds was studied using the best performing isolates in terms of IAA production. The best IAA producer bacterial isolate (I69) was selected to optimize IAA production using response surface methodology based on the central composite design.

RESULTS

Results showed that the majority of tested isolates were able to produce IAA with a relatively higher concentration of 135 μg/ml for the isolate I69, followed by isolates I22 and I75 with respective concentrations of 116 μg/ml and 105 μg/ml IAA. The IAA production and the seed germination rate were relatively increased by the synergistic effect of I69 and I22. Later, response surface methodology was used to determine optimal operating conditions leading to IAA production optimization. Thus, an incubation temperature of 36 °C, a pH of 6.5, an incubation time of 1 day, and respective tryptophan and NaCl concentrations of 1 g/l and 0.1 g/l were optimal parameters leading to 166 μg/ml IAA which was the maximal produced concentration.

CONCLUSION

The present study highlighted that IAA-producing rhizobacteria could be harnessed to improve plant growth. Furthermore, their production can be easily controlled using response surface methodology, which represents a very useful tool for optimization.

Conifers exhibit a characteristic inactivation of auxin to maintain tissue homeostasis

Dynamic regulation of the concentration of the natural auxin (IAA) is essential to coordinate most of the physiological and developmental processes and responses to environmental changes. Oxidation of IAA is a major pathway to control auxin concentrations in angiosperms and, along with IAA conjugation, to respond to perturbation of IAA homeostasis. However, these regulatory mechanisms remain poorly investigated in conifers. To reduce this knowledge gap, we investigated the different contributions of the IAA inactivation pathways in conifers. MS-based quantification of IAA metabolites under steady-state conditions and after perturbation was investigated to evaluate IAA homeostasis in conifers. Putative Picea abies GH3 genes (PaGH3) were identified based on a comprehensive phylogenetic analysis including angiosperms and basal land plants. Auxin-inducible PaGH3 genes were identified by expression analysis and their IAA-conjugating activity was explored. Compared to Arabidopsis, oxidative and conjugative pathways differentially contribute to reduce IAA concentrations in conifers. We demonstrated that the oxidation pathway plays a marginal role in controlling IAA homeostasis in spruce. By contrast, an excess of IAA rapidly activates GH3-mediated irreversible conjugation pathways. Taken together, these data indicate that a diversification of IAA inactivation mechanisms evolved specifically in conifers.

Two new alkaloids isolated from traditional Chinese medicine Binglang the fruit of Areca catechu

Binglang, the fruit of Areca catechu L, has a long history as an important Chinese herbal medicine. Two new alkaloids (1 and 2), along with forty-one known compounds (3-43) were isolated from the dried fruit of Areca catechu L. The structures were elucidated on basis of the IR, UV, MS and 1D, 2D NMR spectroscopic data. Compounds 26 and 33 showed weak cytotoxicity against human gastric cancer cell line (BGC-823) with IC₅₀ of 15.91 μM and 20.13 μM, respectively.

Indole-3-acetylaspartate and indole-3-acetylglutamate, the IAA-amide conjugates in the diploid strawberry achene, are hydrolyzed in growing seedlings

Indole-3-acetylaspartate and indole-3-acetylglutamate are the stored auxin amino acid conjugates of the achene of the diploid strawberry and serve as sources of auxin during seedling growth. The edible part of the strawberry, a pseudocarp, has long been known to enlarge in response to auxin produced by the developing achenes, the botanical true fruit. Auxin homeostasis involves a complex interaction between biosynthesis, conjugate formation and hydrolysis, catabolism and transport. Strawberry tissues are capable of synthesizing auxin conjugates, and transcriptome data support the expression of genes involved in IAA conjugate formation and hydrolysis throughout embryo development and subsequent seedling growth. Using a highly sensitive and selective mass spectrometric method, we identified all the low molecular weight indole-auxin amino acid conjugates in achenes of F. vesca as consisting of indole-3-acetylaspartate (IAasp) and indole-3-acetylglutamate (IAglu). In contrast to what has been proposed to occur in Arabidopsis, we determined that IAasp and IAglu are hydrolyzed by seedlings to provide a source of free IAA for growth.

Whole Irradiated Plant Leaves Showed Faster Photosynthetic Induction Than Individually Irradiated Leaves via Improved Stomatal Opening

Rapid photosynthetic induction is crucial for plants under fluctuating light conditions in a crop canopy as well as in an understory. Most previous studies have focused on photosynthetic induction responses in a single leaf, whereas the systemic responses of the whole plant have not been considered. In a natural environment, however, both single leaves and whole plants are exposed to sunlight, since the light environment is not uniform even within a given plant. In the present study, we examined whether there is any difference between the photosynthetic induction response of a leaf of a whole irradiated plant and an individually irradiated leaf in Arabidopsis thaliana to consider photosynthetic induction as the response of a whole plant. We used two methods, the visualization of photosynthesis and direct measurements of gas-exchange and Chl fluorescence, to demonstrate that whole irradiated plant promoted its photosynthetic induction via improved stomatal opening compared with individually irradiated leaf. Furthermore, using two Arabidopsis knockout mutants of abscisic acid transporter, abcg25 and abcg40, the present study suggests that abscisic acid could be involved in this systemic response for stomatal opening, allowing plants to optimize the use of light energy at minimal cost in plants in a dynamic light environment.

Co-culture of Bacillus amyloliquefaciens ACCC11060 and Trichoderma asperellum GDFS1009 enhanced pathogen-inhibition and amino acid yield

Background

Bacillus spp. are a genus of biocontrol bacteria widely used for antibiosis, while Trichoderma spp. are biocontrol fungi that are abundantly explored. In this study, a liquid co-cultivation of these two organisms was tried firstly.

Results and discussion

Through liquid chromatography-mass spectrometry/mass spectrometry (LC–MS/MS), it was discovered that with an inoculation in the ratio of 1.9:1, the antimicrobial effect of the co-cultured fermentation liquor of Bacillus amyloliquefaciens ACCC11060 and Trichoderma asperellum GDFS1009 was found to be significantly higher than that of pure-cultivation. A raise in the synthesis of antimicrobial substances contributed to this significant increase. Additionally, a co-culture with the inoculation of the two organisms in the ratio of 1:1 was found to enhance the production of specific amino acids. This technique could be further explored for either a large scale production of amino acids or could serve as a theoretical base for the generation of certain rare amino acids.

Conclusions

This work clearly demonstrated that co-cultivation of B. amyloliquefaciens ACCC11060 and T. asperellum GDFS1009 could produce more specific biocontrol substances and amino acids.

Electronic supplementary material

The online version of this article (10.1186/s12934-018-1004-x) contains supplementary material, which is available to authorized users.

Spatio-temporal profiling of abscisic acid, indoleacetic acid and jasmonic acid in single rice seed during seed germination

Abscisic acid (ABA), indoleacetic acid (IAA) and jasmonic acid (JA) are plant hormones that were reported to play indispensable roles during seed germination. However, the interactions between these plant hormones during rice seed germination have still not been explored clearly. A sensitive method for determination of these plant hormones would be beneficial for the exploration of such interactions. Herein, we present a liquid chromatography coupled with mass spectrometry (LC-MS) method for the quantification of ABA, IAA and JA in a single tissue of rice seed to investigate the spatio-temporal distribution of these plant hormones during rice seed germination. To this end, an in silico strategy was developed in order to select a derivatization reagent with an ideal sensitivity of MS detection. This strategy was confirmed with experimental studies on three reagents N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide (EDC), N,N-dimethylethylenediamine (DMED), and N-(acridin-9-yl)-2-bromoacetamide (AYBA) and their formic acid derivatives. Our results from the in silico and LC-MS experiments show that AYBA is a good derivatization reagent for ABA, IAA and JA due to its reasonable ionization efficiency in electrospray ionization mass spectrometry (ESI-MS) and excellent hydrophobicity. Finally, a sensitive LC-MS method upon AYBA was established for the determination of ABA, IAA and JA in germinated seeds. Good linearities for ABA, IAA, and JA were obtained with correlation coefficients greater than 0.99. The limits of detection (LODs) were in the range of 0.14-0.16 pg mL^-1. The method exhibits good precisions with RSD 1.5%-13.8% (intra-day) and 1.2%-7.3% (inter-day) and acceptable recoveries (88.6%-102.9%, n = 6). Finally, the method was successfully employed in the spatio-temporal profiling of ABA, IAA and JA in a single tissue of rice seed during rice seed germination.

Combined transcriptome and translatome analyses reveal a role for tryptophan-dependent auxin biosynthesis in the control of DOG1-dependent seed dormancy

The importance of translational regulation during Arabidopsis seed germination has been shown previously. Here the role of transcriptional and translational regulation during seed imbibition of the very dormant DELAY OF GERMINATION 1 (DOG1) near-isogenic line was investigated. Polysome profiling was performed on dormant and after-ripened seeds imbibed for 6 and 24 h in water and in the transcription inhibitor cordycepin. Transcriptome and translatome changes were investigated. Ribosomal profiles of after-ripened seeds imbibed in cordycepin mimic those of dormant seeds. The polysome occupancy of mRNA species is not affected by germination inhibition, either as a result of seed dormancy or as a result of cordycepin treatment, indicating the importance of the regulation of transcript abundance. The expression of auxin metabolism genes is discriminative during the imbibition of after-ripened and dormant seeds, which is confirmed by altered concentrations of indole-3-acetic acid conjugates and precursors.

Molecular Cloning and Characterization of Four Genes Encoding Ethylene Receptors Associated with Pineapple (Ananas comosus L.) Flowering

Exogenous ethylene, or ethephon, has been widely used to induce pineapple flowering, but the molecular mechanism behind ethephon induction is still unclear. In this study, we cloned four genes encoding ethylene receptors (designated AcERS1a, AcERS1b, AcETR2a, and AcETR2b). The 5' flanking sequences of these four genes were also cloned by self-formed adaptor PCR and SiteFinding-PCR, and a group of putative cis-acting elements was identified. Phylogenetic tree analysis indicated that AcERS1a, AcERS1b, AcETR2a, and AcETR2b belonged to the plant ERS1s and ETR2/EIN4-like groups. Quantitative real-time PCR showed that AcETR2a and AcETR2b (subfamily 2) were more sensitive to ethylene treatment compared with AcERS1a and AcERS1b (subfamily 1). The relative expression of AcERS1b, AcETR2a, and AcETR2b was significantly increased during the earlier period of pineapple inflorescence formation, especially at 1-9 days after ethylene treatment (DAET), whereas AcERS1a expression changed less than these three genes. In situ hybridization results showed that bract primordia (BP) and flower primordia (FP) appeared at 9 and 21 DAET, respectively, and flowers were formed at 37 DAET. AcERS1a, AcERS1b, AcETR2a, and AcETR2b were mainly expressed in the shoot apex at 1-4 DAET; thereafter, with the appearance of BP and FP, higher expression of these genes was found in these new structures. Finally, at 37 DAET, the expression of these genes was mainly focused in the flower but was also low in other structures. These findings indicate that these four ethylene receptor genes, especially AcERS1b, AcETR2a, and AcETR2b, play important roles during pineapple flowering induced by exogenous ethephon.

Auxin and Tryptophan Homeostasis Are Facilitated by the ISS1/VAS1 Aromatic Aminotransferase in Arabidopsis

Indole-3-acetic acid (IAA) plays a critical role in regulating numerous aspects of plant growth and development. While there is much genetic support for tryptophan-dependent (Trp-D) IAA synthesis pathways, there is little genetic evidence for tryptophan-independent (Trp-I) IAA synthesis pathways. Using Arabidopsis, we identified two mutant alleles of ISS1 ( I: ndole S: evere S: ensitive) that display indole-dependent IAA overproduction phenotypes including leaf epinasty and adventitious rooting. Stable isotope labeling showed that iss1, but not WT, uses primarily Trp-I IAA synthesis when grown on indole-supplemented medium. In contrast, both iss1 and WT use primarily Trp-D IAA synthesis when grown on unsupplemented medium. iss1 seedlings produce 8-fold higher levels of IAA when grown on indole and surprisingly have a 174-fold increase in Trp. These findings indicate that the iss1 mutant's increase in Trp-I IAA synthesis is due to a loss of Trp catabolism. ISS1 was identified as At1g80360, a predicted aromatic aminotransferase, and in vitro and in vivo analysis confirmed this activity. At1g80360 was previously shown to primarily carry out the conversion of indole-3-pyruvic acid to Trp as an IAA homeostatic mechanism in young seedlings. Our results suggest that in addition to this activity, in more mature plants ISS1 has a role in Trp catabolism and possibly in the metabolism of other aromatic amino acids. We postulate that this loss of Trp catabolism impacts the use of Trp-D and/or Trp-I IAA synthesis pathways.

A rare SNP mutation in Brachytic2 moderately reduces plant height and increases yield potential in maize

Plant height has long been an important agronomic trait in maize breeding. Many plant height QTLs have been reported, but few of these have been cloned. In this study, a major plant height QTL, qph1, was mapped to a 1.6kb interval in Brachytic2 (Br2) coding sequence on maize chromosome 1. A naturally occurring rare SNP in qph1, which resulted in an amino acid substitution, was validated as the causative mutation. QPH1 protein is located in the plasma membrane and polar auxin transport is impaired in the short near-isogenic line RIL88(qph1). Allelism testing showed that the SNP variant in qph1 reduces longitudinal cell number and decreases plant height by 20% in RIL88(qph1) compared to RIL88(QPH1), and is milder than known br2 mutant alleles. The effect of qph1 on plant height is significant and has no or a slight influence on yield in four F2 backgrounds and in six pairs of single-cross hybrids. Moreover, qph1 could reduce plant height when heterozygous, allowing it to be easily employed in maize breeding. Thus, a less-severe allele of a known dwarf mutant explains part of the quantitative variation for plant height and has great potential in maize improvement.

Quantitative evaluation of IAA conjugate pools in Arabidopsis thaliana

This work has demonstrated that the major method of estimating the amount of unknown IAA conjugates-base hydrolysis-can be significantly complicated by chemical artifacts such as glucobrassicin or protein degradation. The concept of 'bound auxin' traces its origin back to more than 80 years ago and has driven research on the sources and forms of these plant hormones since. Indeed, analytical studies have demonstrated that the majority of cellular auxin is conjugated to simple sugars, cyclitols, glycans, amino acids, and other biomolecules. A number of studies have confirmed the enzymatic systems responsible for the synthesis and hydrolysis of a number of such conjugates in Arabidopsis thaliana and some of these compounds have been identified in situ. However, the amount of indole-3-acetic acid (IAA) released upon treating Arabidopsis tissue extracts with base, a commonly employed technique for estimating the amount of IAA conjugates, greatly exceeded the summation of all the IAA conjugates known individually to be present in Arabidopsis. This discrepancy has remained as an unsolved question. In this study, however, we found that a significant portion of the IAA found after base treatment could be attributed to chemical conversions other than conjugate hydrolysis. Specifically, we showed that glucobrassicin conversion, previously thought to occur at insignificant levels, actually accounted for the majority of solvent soluble IAA released and that proteinaceous tryptophan degradation accounted for a large portion of solvent insoluble IAA. These studies clearly demonstrated the limits associated with using a harsh technique like base hydrolysis in determining IAA conjugates and support using more direct approaches such as mass spectrometry-based strategies for unambiguous characterizations of the total complement of IAA conjugates in new plant materials under study.

Plant hormone-assisted early family selection in Pinus densiflora via a retrospective approach

In an even-aged pine forest trees can vary considerably in stem size. We examined the basis for this anomaly using a retrospective approach. Twelve open-pollinated families of Pinus densiflora Sieb. et Zucc. were deliberately chosen for their variation in stem volumes at age 32 years. Seedlings obtained from these families were grown to age 6 months under optimal nursery conditions. Endogenous levels of growth hormones (auxin [IAA] and gibberellins [GAs]) and expression of the GA biosynthesis gene, PdGA20ox1, all assessed at age 3 months, were significantly correlated, across family, with seedling stem and/or shoot dry biomass at age 6 months. Retrospective comparisons of seedling growth, seedling stem tissue GA(20) and seedling stem expression levels of PdGA20ox1 were then made, across family, with tree stem growth at age 32 years. Age 6 months length and shoot dry biomass at age 6 months showed positive and significant Pearson's correlations with age 32 years tree stem diameters and a tree stem volume index, as did seedling stem tissue GA(20). Even seedling stem PdGA20ox1 expression levels were positively and near significantly (P = 0.088) correlated with age 32 years tree stem diameters. Auxin and GAs control nursery growth of seedlings at the family level, and this control also extends, for GAs at least, to field growth of older trees. We propose that family differences in PdGA20ox1 gene expression, and thus endogenous GA levels, may explain much of the natural variation seen for tree stem size in even-aged pine forests. If our hypothesis is correct, then the heritable components of variation in tree stem growth capacity should be predictable by hormonal and gene expression profiling. Such profiling, combined with the measurement of seedling phenotypic growth characters, could have the potential to accelerate the early selection of those conifer families that possess traits for inherently rapid stem wood growth.

Effect of root length on epicotyl dormancy release in seeds of Paeonia ludlowii, Tibetan peony

BACKGROUND AND AIMS

Epicotyl dormancy break in seeds that have deep simple epicotyl morphophysiological dormancy (MPD) requires radicle emergence and even a certain root length in some species. However, the mechanisms by which root length affects epicotyl dormancy break are not clear at present. This study aims to explore the relationship between root length and epicotyl dormancy release in radicle-emerged seeds of Tibetan peony, Paeonia ludlowii, with discussion of the possible mechanisms.

METHODS

Radicle-emerged seeds (radicle length 1.5, 3.0, 4.5 and 6.0 cm) were incubated at 5, 10 and 15 °C. During the stratification, some seeds were transferred to 15 °C and monitored for epicotyl-plumule growth. Hormone content was determined by ELISA, and the role of hormones in epicotyl dormancy release was tested by exogenous hormone and embryo culture.

KEY RESULTS

Cold stratification did not break the epicotyl dormancy until the root length was ≥6 cm. The indole-3-actic acid (IAA) and GA3 contents of seeds having 6 cm roots were significantly higher than those of seeds with other root lengths, but the abscisic acid (ABA) content was lowest among radicle-emerged seeds. GA3 (400 mg L(-1)) could break epicotyl dormancy of all radicle-emerged seeds, while IAA (200 mg L(-1)) had little or no effect. When grown on MS medium, radicles of naked embryos grew and cotyledons turned green, but epicotyls did not elongate. Naked embryos developed into seedlings on a mixed medium of MS + 100 mg L(-1) GA3.

CONCLUSIONS

A root length of ≥6.0 cm is necessary for epicotyl dormancy release by cold stratification. The underlying reason for root length affecting epicotyl dormancy release is a difference in the GA3/ABA ratio in the epicotyl within radicle-emerged seeds, which is mainly as a result of a difference in ABA accumulation before cold stratification.

Theoretical and experimental evidence indicates that there is no detectable auxin gradient in the angiosperm female gametophyte

The plant life cycle alternates between a diploid sporophytic and a haploid gametophytic generation. The female gametophyte (FG) of flowering plants is typically formed through three syncytial mitoses, followed by cellularisation that forms seven cells belonging to four cell types. The specification of cell fates in the FG has been suggested to depend on positional information provided by an intrinsic auxin concentration gradient. The goal of this study was to develop mathematical models that explain the formation of this gradient in a syncytium. Two factors were proposed to contribute to the maintenance of the auxin gradient in Arabidopsis FGs: polar influx at early stages and localised auxin synthesis at later stages. However, no gradient could be generated using classical, one-dimensional theoretical models under these assumptions. Thus, we tested other hypotheses, including spatial confinement by the large central vacuole, background efflux and localised degradation, and investigated the robustness of cell specification under different parameters and assumptions. None of the models led to the generation of an auxin gradient that was steep enough to allow sufficiently robust patterning. This led us to re-examine the response to an auxin gradient in developing FGs using various auxin reporters, including a novel degron-based reporter system. In agreement with the predictions of our models, auxin responses were not detectable within the FG of Arabidopsis or maize, suggesting that the effects of manipulating auxin production and response on cell fate determination might be indirect.

Auxin: simply complicated

Auxin is a plant hormone involved in an extraordinarily broad variety of biological mechanisms. These range from basic cellular processes, such as endocytosis, cell polarity, and cell cycle control over localized responses such as cell elongation and differential growth, to macroscopic phenomena such as embryogenesis, tissue patterning, and de novo formation of organs. Even though the history of auxin research reaches back more than a hundred years, we are still far from a comprehensive understanding of how auxin governs such a wide range of responses. Some answers to this question may lie in the auxin molecule itself. Naturally occurring auxin-like substances have been found and they may play roles in specific developmental and cellular processes. The molecular mode of auxin action can be further explored by the utilization of synthetic auxin-like molecules. A second area is the perception of auxin, where we know of three seemingly independent receptors and signalling systems, some better understood than others, but each of them probably involved in distinct physiological processes. Lastly, auxin is actively modified, metabolized, and intracellularly compartmentalized, which can have a great impact on its availability and activity. In this review, we will give an overview of these rather recent and emerging areas of auxin research and try to formulate some of the open questions. Without doubt, the manifold facets of auxin biology will not cease to amaze us for a long time to come.

Usual and unusual development of the dicot leaf: involvement of transcription factors and hormones

Morphological diversity exhibited by higher plants is essentially related to the tremendous variation of leaf shape. With few exceptions, leaf primordia are initiated postembryonically at the flanks of a group of undifferentiated and proliferative cells within the shoot apical meristem (SAM) in characteristic position for the species and in a regular phyllotactic sequence. Auxin is critical for this process, because genes involved in auxin biosynthesis, transport, and signaling are required for leaf initiation. Down-regulation of transcription factors (TFs) and cytokinins are also involved in the light-dependent leaf initiation pathway. Furthermore, mechanical stresses in SAM determine the direction of cell division and profoundly influence leaf initiation suggesting a link between physical forces, gene regulatory networks and biochemical gradients. After the leaf is initiated, its further growth depends on cell division and cell expansion. Temporal and spatial regulation of these processes determines the size and the shape of the leaf, as well as the internal structure. A complex array of intrinsic signals, including phytohormones and TFs control the appropriate cell proliferation and differentiation to elaborate the final shape and complexity of the leaf. Here, we highlight the main determinants involved in leaf initiation, epidermal patterning, and elaboration of lamina shape to generate small marginal serrations, more deep lobes or a dissected compound leaf. We also outline recent advances in our knowledge of regulatory networks involved with the unusual pattern of leaf development in epiphyllous plants as well as leaf morphology aberrations, such as galls after pathogenic attacks of pests.

Divide Et Impera--cellular auxin compartmentalization

The phytohormone auxin is an essential regulator for plant growth and development. Decades of intensive research revealed the mutual importance of auxin metabolism and intercellular cell-to-cell transport for the regulation of spatiotemporal auxin distribution. Just recently, intracellular putative auxin carriers, such as the PIN-FORMED (PIN)5/PIN8 and the PIN-LIKES (PILS)2/PILS5 were discovered at the endoplasmic reticulum (ER) and seem to limit nuclear auxin signaling via an auxin sequestration mechanism. Moreover, these auxin carriers at the ER might provide a link between auxin compartmentalization and auxin conjugation-based metabolism. Here we review the recent findings on auxin compartmentalization at the ER and discuss its potential contribution to cellular auxin homeostasis and its importance for plant development.

Lens culinaris Medik. seed proteome: analysis to identify landrace markers

Unlike modern cultivars selected for their growth performances in specific environmental conditions, local landraces have a high genetic variability that is an important resource for plant breeding. Consequent to their high adaptation to different environmental conditions, these landraces may have evolved adaptive gene complexes To promote the survival of endangered lentil landraces, we previously investigated the genetic relationship between two ancient landraces cultivated in the Molise region (Capracotta and Conca Casale, south-central Italy) and widely spread commercial varieties using an integrated approach consisting of morphological, DNA and protein characterization. In the present study, we used a proteomic approach to compare the mature seed proteomes of the Capracotta and Conca Casale lentil landraces. Multivariate analysis of 145 differentially expressed protein spots demonstrated that 52 proteins are required to discriminate among the two landraces. Therefore, these 52 proteins can be considered "landrace markers". The results of this study show that the combination of proteomics and multivariate analysis can be used to identify physiological and/or environmental markers, and is thus a powerful tool that complements the analysis of biodiversity in plant ecotypes.

Cellular auxin homeostasis: gatekeeping is housekeeping

The phytohormone auxin is essential for plant development and contributes to nearly every aspect of the plant life cycle. The spatio-temporal distribution of auxin depends on a complex interplay between auxin metabolism and cell-to-cell auxin transport. Auxin metabolism and transport are both crucial for plant development; however, it largely remains to be seen how these processes are integrated to ensure defined cellular auxin levels or even gradients within tissues or organs. In this review, we provide a glance at very diverse topics of auxin biology, such as biosynthesis, conjugation, oxidation, and transport of auxin. This broad, but certainly superficial, overview highlights the mutual importance of auxin metabolism and transport. Moreover, it allows pinpointing how auxin metabolism and transport get integrated to jointly regulate cellular auxin homeostasis. Even though these processes have been so far only separately studied, we assume that the phytohormonal crosstalk integrates and coordinates auxin metabolism and transport. Besides the integrative power of the global hormone signaling, we additionally introduce the hypothetical concept considering auxin transport components as gatekeepers for auxin responses.

Molecular cloning and characterization of the genes encoding an auxin efflux carrier and the auxin influx carriers associated with the adventitious root formation in mango (Mangifera indica L.) cotyledon segments

Polar auxin transport (PAT) plays an important role in the adventitious root formation of mango cotyledon segments, but the molecular mechanism remains unclear. In this study, we cloned a gene encoding an auxin efflux carrier (designated as MiPIN1), and we cloned four genes encoding auxin influx carriers (designated as MiAUX1, MiAUX2, MiAUX3 and MiAUX4). The results of a phylogenetic tree analysis indicated that MiPIN1 and the MiAUXs belong to plant PIN and AUXs/LAXs groups. Quantitative real-time PCR indicated that the expression of MiPIN1 and the MiAUXs was lowest at 0 days but sharply increased on and after day 4. During the root formation in the mango cotyledon segments, the MiPIN1 expression in the distal cut surface (DCS) was always higher than the expression in the proximal cut surface (PCS) whereas the expression of the MiAUXs in the PCS was usually higher than in the DCS. This expression pattern might be result in the PAT from the DCS to the PCS, which is essential for the adventitious root formation in the PCS. Our previous study indicated that a pre-treatment of embryos with indole-3-butyric acid (IBA) significantly promoted adventitious rooting in PCS whereas a pre-treatment with 2,3,5-triiodobenzoic acid (TIBA) completely inhibited this rooting. In this study, however, IBA and TIBA pre-treatments slightly changed the expression of MiPIN1. In contrast, while the MiAUX3 and MiAUX4 expression levels were significantly up-regulated by the IBA pre-treatment, the expression levels were down-regulated by the TIBA pre-treatment. These findings imply that MiAUX3 and MiAUX4 are more sensitive to the IBA and TIBA treatments and that they might play important roles during adventitious root formation in mango cotyledon segments.

Diversification and Expression of the PIN, AUX/LAX, and ABCB Families of Putative Auxin Transporters in Populus

Intercellular transport of the plant hormone auxin is mediated by three families of membrane-bound protein carriers, with the PIN and ABCB families coding primarily for efflux proteins and the AUX/LAX family coding for influx proteins. In the last decade our understanding of gene and protein function for these transporters in Arabidopsis has expanded rapidly but very little is known about their role in woody plant development. Here we present a comprehensive account of all three families in the model woody species Populus, including chromosome distribution, protein structure, quantitative gene expression, and evolutionary relationships. The PIN and AUX/LAX gene families in Populus comprise 16 and 8 members respectively and show evidence for the retention of paralogs following a relatively recent whole genome duplication. There is also differential expression across tissues within many gene pairs. The ABCB family is previously undescribed in Populus and includes 20 members, showing a much deeper evolutionary history, including both tandem and whole genome duplication as well as probable gene loss. A striking number of these transporters are expressed in developing Populus stems and we suggest that evolutionary and structural relationships with known auxin transporters in Arabidopsis can point toward candidate genes for further study in Populus. This is especially important for the ABCBs, which is a large family and includes members in Arabidopsis that are able to transport other substrates in addition to auxin. Protein modeling, sequence alignment and expression data all point to ABCB1.1 as a likely auxin transport protein in Populus. Given that basipetal auxin flow through the cambial zone shapes the development of woody stems, it is important that we identify the full complement of genes involved in this process. This work should lay the foundation for studies targeting specific proteins for functional characterization and in situ localization.

Low-fluence red light increases the transport and biosynthesis of auxin

In plants, light is an important environmental signal that induces photomorphogenesis and interacts with endogenous signals, including hormones. We found that light increased polar auxin transport in dark-grown Arabidopsis (Arabidopsis thaliana) and tomato (Solanum lycopersicum) hypocotyls. In tomato, this increase was induced by low-fluence red or blue light followed by 1 d of darkness. It was reduced in phyA, phyB1, and phyB2 tomato mutants and was reversed by far-red light applied immediately after the red or blue light exposure, suggesting that phytochrome is involved in this response. We further found that the free indole-3-acetic acid (IAA) level in hypocotyl regions below the hook was increased by red light, while the level of conjugated IAA was unchanged. Analysis of IAA synthesized from [¹³C]indole or [¹³C]tryptophan (Trp) revealed that both Trp-dependent and Trp-independent IAA biosynthesis were increased by low-fluence red light in the top section (meristem, cotyledons, and hook), and the Trp-independent pathway appears to become the primary route for IAA biosynthesis after red light exposure. IAA biosynthesis in tissues below the top section was not affected by red light, suggesting that the increase of free IAA in this region was due to increased transport of IAA from above. Our study provides a comprehensive view of light effects on the transport and biosynthesis of IAA, showing that red light increases both IAA biosynthesis in the top section and polar auxin transport in hypocotyls, leading to unchanged free IAA levels in the top section and increased free IAA levels in the lower hypocotyl regions.

Polyamines, IAA and ABA during germination in two recalcitrant seeds: Araucaria angustifolia (Gymnosperm) and Ocotea odorifera (Angiosperm)

BACKGROUND AND AIMS

Plant growth regulators play an important role in seed germination. However, much of the current knowledge about their function during seed germination was obtained using orthodox seeds as model systems, and there is a paucity of information about the role of plant growth regulators during germination of recalcitrant seeds. In the present work, two endangered woody species with recalcitrant seeds, Araucaria angustifolia (Gymnosperm) and Ocotea odorifera (Angiosperm), native to the Atlantic Rain Forest, Brazil, were used to study the mobilization of polyamines (PAs), indole-acetic acid (IAA) and abscisic acid (ABA) during seed germination.

METHODS

Data were sampled from embryos of O. odorifera and embryos and megagametophytes of A. angustifolia throughout the germination process. Biochemical analyses were carried out in HPLC.

KEY RESULTS

During seed germination, an increase in the (Spd + Spm) : Put ratio was recorded in embryos in both species. An increase in IAA and PA levels was also observed during seed germination in both embryos, while ABA levels showed a decrease in O. odorifera and an increase in A. angustifolia embryos throughout the period studied.

CONCLUSIONS

The (Spd + Spm) : Put ratio could be used as a marker for germination completion. The increase in IAA levels, prior to germination, could be associated with variations in PA content. The ABA mobilization observed in the embryos could represent a greater resistance to this hormone in recalcitrant seeds, in comparison to orthodox seeds, opening a new perspective for studies on the effects of this regulator in recalcitrant seeds. The gymnosperm seed, though without a connective tissue between megagametophyte and embryo, seems to be able to maintain communication between the tissues, based on the likely transport of plant growth regulators.

Integration of auxin and salt signals by the NAC transcription factor NTM2 during seed germination in Arabidopsis

Seed germination is regulated through elaborately interacting signaling networks that integrate diverse environmental cues into hormonal signaling pathways. Roles of gibberellic acid and abscisic acid in germination have been studied extensively using Arabidopsis (Arabidopsis thaliana) mutants having alterations in seed germination. Auxin has also been implicated in seed germination. However, how auxin influences germination is largely unknown. Here, we demonstrate that auxin is linked via the IAA30 gene with a salt signaling cascade mediated by the NAM-ATAF1/2-CUC2 transcription factor NTM2/Arabidopsis NAC domain-containing protein 69 (for NAC with Transmembrane Motif1) during seed germination. Germination of the NTM2-deficient ntm2-1 mutant seeds exhibited enhanced resistance to high salinity. However, the salt resistance disappeared in the ntm2-1 mutant overexpressing the IAA30 gene, which was induced by salt in a NTM2-dependent manner. Auxin exhibited no discernible effects on germination under normal growth conditions. Under high salinity, however, whereas exogenous application of auxin further suppressed the germination of control seeds, the auxin effects were reduced in the ntm2-1 mutant. Consistent with the inhibitory effects of auxin on germination, germination of YUCCA 3-overexpressing plants containing elevated levels of active auxin was more severely influenced by salt. These observations indicate that auxin delays seed germination under high salinity through cross talk with the NTM2-mediated salt signaling in Arabidopsis.

Development of sample preparation method for auxin analysis in plants by vacuum microwave-assisted extraction combined with molecularly imprinted clean-up procedure

A novel sample preparation method for auxin analysis in plant samples was developed by vacuum microwave-assisted extraction (VMAE) followed by molecularly imprinted clean-up procedure. The method was based on two steps. In the first one, conventional solvent extraction was replaced by VMAE for extraction of auxins from plant tissues. This step provided efficient extraction of 3-indole acetic acid (IAA) from plant with dramatically decreased extraction time, furthermore prevented auxins from degradation by creating a reduced oxygen environment under vacuum condition. In the second step, the raw extract of VMAE was further subjected to a clean-up procedure by magnetic molecularly imprinted polymer (MIP) beads. Owing to the high molecular recognition ability of the magnetic MIP beads for IAA and 3-indole-butyric acid (IBA), the two target auxins in plants can be selectively enriched and the interfering substance can be eliminated by dealing with a magnetic separation procedure. Both the VMAE and the molecularly imprinted clean-up conditions were investigated. The proposed sample preparation method was coupled with high-performance liquid chromatogram and fluorescence detection for determination of IAA and IBA in peas and rice. The detection limits obtained for IAA and IBA were 0.47 and 1.6 ng/mL and the relative standard deviation were 2.3% and 2.1%, respectively. The IAA contents in pea seeds, pea embryo, pea roots and rice seeds were determined. The recoveries were ranged from 70.0% to 85.6%. The proposed method was also applied to investigate the developmental profiles of IAA concentration in pea seeds and rice seeds during seed germination.

Auxin conjugates: their role for plant development and in the evolution of land plants

Auxin conjugates are thought to play important roles as storage forms for the active plant hormone indole-3-acetic acid (IAA). In its free form, IAA comprises only up to 25% of the total amount of IAA, depending on the tissue and the plant species studied. The major forms of IAA conjugate are low molecular weight ester or amide forms, but there is increasing evidence of the occurrence of peptides and proteins modified by IAA. Since the discovery of genes and enzymes involved in synthesis and hydrolysis of auxin conjugates, much knowledge has been gained on the biochemistry and function of these compounds, but there is still much to discover. For example, recent work has shown that some auxin conjugate hydrolases prefer conjugates with longer-chain auxins such as indole-3-propionic acid and indole-3-butyric acid as substrate. Also, the compartmentation of these reactions in the cell or in tissues has not been resolved in great detail. The function of auxin conjugates has been mainly elucidated by mutant analysis in genes for synthesis or hydrolysis and a possible function for conjugates inferred from these results. In the evolution of land plants auxin conjugates seem to be connected with the development of certain traits such as embryo, shoot, and vasculature. Most likely, the synthesis of auxin conjugates was developed first, since it has been already detected in moss, whereas sequences typical of auxin conjugate hydrolases were found according to database entries first in moss ferns. The implications for the regulation of auxin levels in different species will be discussed.

Approaching cellular and molecular resolution of auxin biosynthesis and metabolism

There is abundant evidence of multiple biosynthesis pathways for the major naturally occurring auxin in plants, indole-3-acetic acid (IAA), and examples of differential use of two general routes of IAA synthesis, namely Trp-dependent and Trp-independent. Although none of these pathways has been completely defined, we now have examples of specific IAA biosynthetic pathways playing a role in developmental processes by way of localized IAA synthesis, causing us to rethink the interactions between IAA synthesis, transport, and signaling. Recent work also points to some IAA biosynthesis pathways being specific to families within the plant kingdom, whereas others appear to be more ubiquitous. An important advance within the past 5 years is our ability to monitor IAA biosynthesis and metabolism at increasingly higher resolution.

A molecular phylogenomic analysis of the ILR1-like family of IAA amidohydrolase genes

The ILR1-like family of hydrolase genes was initially isolated in Arabidopsis thaliana and is thought to help regulate levels of free indole-3-acetic-acid.We have investigated how this family has evolved in dicotyledon, monocotyledon and gymnosperm species by employing the GenBank and TIGR databases to retrieve orthologous genes. The relationships among these sequences were assessed employing phylogenomic analyses to examine molecular evolution and phylogeny. The members of the ILR1-like family analysed were ILL1, ILL2, ILL3, ILL6, ILR1 and IAR3. Present evidence suggests that IAR3 has undergone the least evolution and is most conserved. This conclusion is based on IAR3 having the largest number of total interspecific orthologues, orthologous species and unique orthologues. Although less conserved than IAR3, DNA and protein sequence analyses of ILL1 and ILR1 suggest high conservation. Based on this conservation, IAR3, ILL1 and ILR1 may have had major roles in the physiological evolution of 'higher' plants. ILL3 is least conserved, with the fewest orthologous species and orthologues. The monocotyledonous orthologues for most family-members examined have evolved into two separate molecular clades from dicotyledons, indicating active evolutionary change. The monocotyledon clades are: (a) those possessing a putative endoplasmic reticulum localizing signal; and (b) those that are putative cytoplasmic hydrolases. IAR3, ILL1 and ILL6 are all highly orthologous to a gene in the gymnosperm Pinus taeda, indicating an ancient enzymatic activity. No orthologues could be detected in Chlamydomonas, moss and fern databases.

Induction of xylem and fiber differentiation inPopulus tremuloidesThis article is one of a selection of papers published in the Special Issue on Poplar Research in Canada

Vascular tissues are of particular importance to terrestrial plants as they allow long-distance transport within the plant and also provide support for upright growth. Nowhere are these traits more obvious than in tree species. Here we have evaluated the role of auxin transport in the differentiation of primary and secondary vascular tissues in a tree species, trembling aspen ( Populus tremuloides Michx). We found that a partial inhibition of auxin transport resulted in increased width and numbers of veins in leaves. A similar vascular overgrowth was observed during early secondary vascular differentiation of stems. This stem overgrowth consisted almost entirely of early differentiation of metaxylem and fibers. We hypothesize that the early differentiation of metaxylem and fibers results from inhibitor-induced accumulation of auxin in stems and that the differentiation of these tissues requires higher levels of auxin exposure than protoxylem. The controlled conditions used in this study also provide a framework for reverse genetics approaches to identify genes involved in vascular differentiation based on elevated expression in tissues developing vascular overgrowth.

Structural biology in plant natural product biosynthesis--architecture of enzymes from monoterpenoid indole and tropane alkaloid biosynthesis

Several cDNAs of enzymes catalyzing biosynthetic pathways of plant-derived alkaloids have recently been heterologously expressed, and the production of appropriate enzymes from ajmaline and tropane alkaloid biosynthesis in bacteria allows their crystallization. This review describes the architecture of these enzymes with and without their ligands.

Metabolism of indole-3-acetic acid in rice: identification and characterization of N-beta-D-glucopyranosyl indole-3-acetic acid and its conjugates

A search was made for conjugates of indole-3-acetic acid (IAA) in rice (Oryza sativa) using liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS) in order to elucidate unknown metabolic pathways for IAA. N-beta-d-Glucopyranosyl indole-3-acetic acid (IAA-N-Glc) was found in an alkaline hydrolysate of rice extract. A quantitative analysis of 3-week-old rice demonstrated that the total amount of IAA-N-Glc was equal to that of IAA. A LC-ESI-MS/MS-based analysis established that the major part of IAA-N-Glc was present as bound forms with aspartate and glutamate. Their levels were in good agreement with the total amount of IAA-N-Glc during the vegetative growth of rice. Further detailed analysis showed that both conjugates highly accumulated in the root. The free form of IAA-N-Glc accounted for 60% of the total in seeds but could not be detected in the vegetative tissue. An incorporation study using deuterium-labeled compounds showed that the amino acid conjugates of IAA-N-Glc were biosynthesized from IAA-amino acids. IAA-N-Glc and/or its conjugates were also found in extracts of Arabidopsis, Lotus japonicus, and maize, suggesting that N-glucosylation of indole can be the common metabolic pathway of IAA in plants.

Molecular and structural characterization of hexameric beta-D-glucosidases in wheat and rye

The wheat (Triticum aestivum) and rye (Secale cereale) beta-D-glucosidases hydrolyze hydroxamic acid-glucose conjugates, exist as different types of isozyme, and function as oligomers. In this study, three cDNAs encoding beta-D-glucosidases (TaGlu1a, TaGlu1b, and TaGlu1c) were isolated from young wheat shoots. Although the TaGlu1s share very high sequence homology, the mRNA level of Taglu1c was much lower than the other two genes in 48- and 96-h-old wheat shoots. The expression ratio of each gene was different between two wheat cultivars. Recombinant TaGlu1b expressed in Escherichia coli was electrophoretically distinct fromTaGlu1a and TaGlu1c. Furthermore, coexpression of TaGlu1a and TaGlu1b gave seven bands on a native-PAGE gel, indicating the formation of both homo- and heterohexamers. One distinctive property of the wheat and rye glucosidases is that they function as hexamers but lose activity when dissociated into smaller oligomers or monomers. The crystal structure of hexameric TaGlu1b was determined at a resolution of 1.8 A. The N-terminal region was located at the dimer-dimer interface and plays a crucial role in hexamer formation. Mutational analyses revealed that the aromatic side chain at position 378, which is located at the entrance to the catalytic center, plays an important role in substrate binding. Additionally, serine-464 and leucine-465 of TaGlu1a were shown to be critical in the relative specificity for DIMBOA-glucose (2-O-beta-D-glucopyranosyl-4-hydroxy-7-methoxy-1,4-benzoxazin-3-one) over DIBOA-glucose (7-demethoxy-DIMBOA-glucose).

Purification, crystallization and preliminary X-ray analysis of a hexameric beta-glucosidase from wheat

The wheat beta-glucosidase TaGlu1b, which is only active in a hexameric form, was tagged with 6xHis at the N-terminus, overexpressed in Escherichia coli and purified in two steps. The protein complexed with a substrate aglycone was crystallized at 293 K from a solution containing 10 mM HEPES pH 7.2, 1 M LiSO4 and 150 mM NaCl using the hanging-drop vapour-diffusion method. Diffraction data were collected to 1.7 A at the Photon Factory. The crystal belongs to space group P4(1)32, with unit-cell parameters a = b = c = 194.65 A, alpha = beta = gamma = 90 degrees. The asymmetric unit was confirmed by molecular-replacement solution to contain one monomer, giving a solvent content of 72.1%.

Auxins

Auxin is a multifactorial phytohormone that is required for cell division. Fine gradients determine points of developmental change in time and space. It is associated intimately with the axiality of plant growth, and increasing doses lead to cell expansion or inhibition of cell expansion in different tissues. From embryonic patterning to fruit dehiscence every plant process has some involvement with auxin as a hormonal signal, including responses to wounding. Moreover, synthetic auxins have widespread uses as agrochemicals, particularly as selective herbicides. Despite the importance of auxin as a plant signal the pathways of its biosynthesis are still not clear. Much more is known about auxin perception and the mechanisms through which gene transcription is regulated. One receptor has been identified, and protein crystallography data has explained its auxin-binding capacity, but this is likely to control only a subset of auxin-mediated responses. Little is known of the signal transduction intermediates. A second receptor has been nominated and may be involved in controlling auxin-mediated gene transcription. A complex set of proteins comprising signalosome and proteasome contribute to the regulation of sets of transcription factors to confer regulation by derepression. A set of auxin transport proteins has been described with associated regulatory interactors, and these account for polar auxin flow and the control of auxin movements across cells, tissues, and around the plant. The gradients these transport systems build regulate the responses of growth and differentiation, including the plant's response to gravity. These areas are described and discussed by relating the physiology of the whole plant to the details of genetic and protein activities.

Soundwave stimulation triggers the content change of the endogenous hormone of the Chrysanthemum mature callus

Hormones have been suggested to play a prominent role in the control of callus growth. In this paper, with the method of indirect enzyme-linked immunosorbent assays (ELISA), we investigated the induction effect of soundwave on the dynamic change of endogenous indole-3-acetic acid (IAA) and abscisic acid (ABA) in vitro during the differentiation process of Chrysanthemum synchronized mature Callus. These experiments showed that groups treated by optimal soundwave (1.4 kHz, 0.095 kdb) had significantly higher IAA levels and lower ABA than that of the control, which had been implicated activation of endogenous IAA and inhibition of ABA. Through the biochemical analysis, it revealed that the increased level of IAA as well as decreased levels of ABA correlated with soundwave stimulus. High rate of IAA/ABA was favorable to development of the callus and differentiation of mature callus. We conclude that soundwave contributes to endogenous hormone as well as the control of callus growth.

IAA production during germination of Orobanche spp. seeds

Broomrapes (Orobanche spp.) are parasitic plants, whose growth and development fully depend on the nutritional connection established between the parasite and the roots of the respective host plant. Phytohormones are known to play a role in establishing the specific Orobanche-host plant interaction. The first step in the interaction is seed germination triggered by a germination stimulant secreted by the host-plant roots. We quantified indole-3-acetic acid (IAA) and abscisic acid (ABA) during the seed germination of tobacco broomrape (Orobanche ramosa) and sunflower broomrape (O. cumana). IAA was mainly released from Orobanche seeds in host-parasite interactions as compared to non-host-parasite interactions. Moreover, germinating seeds of O. ramosa released IAA as early as 24 h after the seeds were exposed to the germination stimulant, even before development of the germ tube. ABA levels remained unchanged during the germination of the parasites' seeds. The results presented here show that IAA production is probably part of a mechanism triggering germination upon the induction by the host factor, thus resulting in seed germination.

IAR4, a gene required for auxin conjugate sensitivity in Arabidopsis, encodes a pyruvate dehydrogenase E1alpha homolog

The formation and hydrolysis of indole-3-acetic acid (IAA) conjugates represent a potentially important means for plants to regulate IAA levels and thereby auxin responses. The identification and characterization of mutants defective in these processes is advancing the understanding of auxin regulation and response. Here we report the isolation and characterization of the Arabidopsis iar4 mutant, which has reduced sensitivity to several IAA-amino acid conjugates. iar4 is less sensitive to a synthetic auxin and low concentrations of an ethylene precursor but responds to free IAA and other hormones tested similarly to wild type. The gene defective in iar4 encodes a homolog of the E1alpha-subunit of mitochondrial pyruvate dehydrogenase, which converts pyruvate to acetyl-coenzyme A. We did not detect glycolysis or Krebs-cycle-related defects in the iar4 mutant, and a T-DNA insertion in the IAR4 coding sequence conferred similar phenotypes as the originally identified missense allele. In contrast, we found that disruption of the previously described mitochondrial pyruvate dehydrogenase E1alpha-subunit does not alter IAA-Ala responsiveness or confer any obvious phenotypes. It is possible that IAR4 acts in the conversion of indole-3-pyruvate to indole-3-acetyl-coenzyme A, which is a potential precursor of IAA and IAA conjugates.

The control of storage xyloglucan mobilization in cotyledons of Hymenaea courbaril

Hymenaea courbaril is a leguminous tree species from the neotropical rain forests. Its cotyledons are largely enriched with a storage cell wall polysaccharide (xyloglucan). Studies of cell wall storage polymers have been focused mostly on the mechanisms of their disassembly, whereas the control of their mobilization and the relationship between their metabolism and seedling development is not well understood. Here, we show that xyloglucan mobilization is strictly controlled by the development of first leaves of the seedling, with the start of its degradation occurring after the beginning of eophyll (first leaves) expansion. During the period of storage mobilization, an increase in the levels of xyloglucan hydrolases, starch, and free sugars were observed in the cotyledons. Xyloglucan mobilization was inhibited by shoot excision, darkness, and by treatment with the auxin-transport inhibitor N-1-naphthylphthalamic acid. Analyses of endogenous indole-3-acetic acid in the cotyledons revealed that its increase in concentration is followed by the rise in xyloglucan hydrolase activities, indicating that auxin is directly related to xyloglucan mobilization. Cotyledons detached during xyloglucan mobilization and treated with 2,4-dichlorophenoxyacetic acid showed a similar mobilization rate as in attached cotyledons. This hormonal control is probably essential for the ecophysiological performance of this species in their natural environment since it is the main factor responsible for promoting synchronism between shoot growth and reserve degradation. This is likely to increase the efficiency of carbon reserves utilization by the growing seedling in the understorey light conditions of the rain forest.

A gene encoding a germin-like protein, identified by a cDNA-AFLP approach, is specifically expressed during germination of Phaseolus vulgaris

In order to identify markers of germination in Phaseolus vulgaris L., a cDNA-amplified fragment length polymorphism (AFLP) approach was conducted on mRNAs from embryo axes and from cotyledons. Among changes observed throughout the germination process, a cDNA fragment not detected 9 h after imbibition (HAI) but present specifically in axes 24 HAI was further studied. The complete cDNA was recovered by rapid amplification of cDNA ends, then cloned and sequenced. It includes an open reading frame predicting a 206-amino-acid polypeptide of 21.8 kDa. Analysis of the nucleotide sequence and deduced amino acid sequence revealed a high homology with germin-like proteins (GLPs), and particularly with an auxin-binding protein from peach, ABP19, that belongs to the GLP family. Thus, we propose that this cDNA encodes the first GLP described in P. vulgaris, designated PvGLP1. Northern blot analysis carried out on mRNAs from seed axes showed a dramatic increase in PvGLP1 expression a few hours before radicle emergence (17 HAI). Among mature vegetative tissues, PvGLP1 expression was very weak in pods and not detected in leaves, stems or roots. Immunoblot analysis using antibodies raised against AtGER3 from Arabidopsis thaliana showed that the protein could be detected only in axes from the dry seed stage onwards, at a steady-state level. Then, PvGLP1 expression seems to be associated with the early stages of embryo axis growth. The high homology indicated with ABP19 led us to study the effect of different concentrations of indole-3-acetic acid (IAA) on PvGLP1 expression during germination. Whereas no effect was noticed at low concentrations (1, 5, 10 microM), a marked decrease in PvGLP1 mRNA level was observed in axes of seeds imbibed with 100 microM IAA. Thus, PvGLP1 gene expression is not stimulated by auxin and, moreover, it might be inhibited by high concentrations of IAA.

Points of regulation for auxin action

There have been few examples of the application of our growing knowledge of hormone action to crop improvement. In this review we discuss what is known about the critical points regulating auxin action. We examine auxin metabolism, transport, perception and signalling and identify genes and proteins that might be keys to regulation, particularly the rate-limiting steps in various pathways. Certain mutants show that substrate flow in biosynthesis can be limiting. To date there is little information available on the genes and proteins of catabolism. There have been several auxin transport proteins and some elegant transport physiology described recently, and the potential for using transport proteins to manage free indole-3-acetic acid (IAA) concentrations is discussed. Free IAA is very mobile, and so while it may be more practical to control auxin action through managing the receptor and signalling pathways, the candidate genes and proteins through which this can be done remain largely unknown. From the available evidence, it is clear that the reason for so few commercial applications arising from the control of auxin action is that knowledge is still limited.