Hairy root transformation of Brassica rapa with bacterial halogenase genes and regeneration to adult plants to modify production of indolic compounds

During the last years halogenated compounds have drawn a lot of attention. Metabolites with one or more halogen atoms are often more active than their non-halogenated derivatives like indole-3-acetic acid (IAA) and 4-Cl-IAA. Within this work, bacterial flavin-dependent tryptophan halogenase genes were inserted into Brassica rapa ssp. pekinensis (Chinese cabbage) with the aim to produce novel halogenated indole compounds. It was investigated which tryptophan-derived indole metabolites, such as indole glucosinolates or potential degradation products can be synthesized by the transgenic root cultures. In vivo and in vitro activity of halogenases heterologously produced was shown and the production of chlorinated tryptophan in transgenic root lines was confirmed. Furthermore, chlorinated indole-3-acetonitrile (Cl-IAN) was detected. Other tryptophan-derived indole metabolites, such as IAA or indole glucosinolates were not found in the transgenic roots in a chlorinated form. The influence of altered growth conditions on the amount of produced chlorinated compounds was evaluated. We found an increase in Cl-IAN production at low temperatures (8 °C), but otherwise no significant changes were observed. Furthermore, we were able to regenerate the wild type and transgenic root cultures to adult plants, of which the latter still produced chlorinated metabolites. Therefore, we conclude that the genetic information had been stably integrated. The transgenic plants showed a slightly altered phenotype compared to plants grown from seeds since they also still expressed the rol genes. By this approach we were able to generate various stably transformed plant materials from which it was possible to isolate chlorinated tryptophan and Cl-IAN.

Members of the GH3 Family of Proteins Conjugate 2,4-D and Dicamba with Aspartate and Glutamate

Auxin homeostasis is a highly regulated process that must be maintained to allow auxin to exert critical growth and developmental controls. Auxin conjugase and hydrolase family proteins play important roles in auxin homeostasis through means of storage, activation, inactivation, response inhibition and degradation of auxins in plants. We systematically evaluated 60 GRETCHEN HAGEN3 (GH3) proteins from diverse plant species for amino acid conjugation activity with the known substrates jasmonic acid (JA), IAA and 4-hydroxybenzoate (4-HBA). While our results largely confirm that Group II conjugases prefer IAA, we observed no clear substrate preference among Group III proteins, and only three of 11 Group I proteins showed the expected preference for JA, indicating that sequence similarity does not always predict substrate specificity. Such a sequence-substrate relationship held true when sequence similarity at the acyl acid-binding site was used for grouping. Several GH3 proteins could catalyze formation of the potentially degradation-destined aspartate (Asp) and glutamate (Glu) conjugates of IAA and the synthetic auxins 2,4-D and dicamba. We found that 2,4-D-Asp/Glu conjugates, but not dicamba and IAA conjugates, were hydrolyzed in Arabidopsis and soybean by AtILL5- and AtIAR3-like amidohydrolases, releasing free 2,4-D in plant cells when conjugates were exogenously applied to seedlings. Dicamba-Asp or dicamba-Glu conjugates were not hydrolyzed in vivo in infiltrated plants nor in vitro with recombinant amidohydrolases. These findings could open the door for exploration of a dicamba herbicide tolerance strategy through conjugation.

Short-term salt stress in Brassica rapa seedlings causes alterations in auxin metabolism

Salinity is one of major abiotic stresses affecting Brassica crop production. Here we present investigations into the physiological, biochemical, and hormonal components of the short-term salinity stress response in Chinese cabbage seedlings, with particular emphasis on the biosynthesis and metabolism of auxin indole-3-acetic acid (IAA). Upon salinity treatments (50-200 mM NaCl) IAA level was elevated in a dose dependent manner reaching 1.6-fold increase at the most severe salt treatment in comparison to the control. IAA precursor profiling suggested that salinity activated the indole-3-acetamide and indole-3-acetaldoxime biosynthetic pathways while suppressing the indole-3-pyruvic acid pathway. Levels of the IAA catabolites 2-oxoindole-3-acetic acid and indole-3-acetic acid-aspartate increased 1.7- and 2.0-fold, respectively, under the most severe treatment, in parallel with those of IAA. Conversely, levels of the ester conjugate indole-3-acetyl-1-O-ß-d-glucose and its catabolite 2-oxoindole-3-acetyl-1-O-ß-d-glucose decreased 2.5- and 7.0-fold, respectively. The concentrations of stress hormones including jasmonic acid and jasmonoyl-isoleucine (JA and JA-Ile), salicylic acid (SA) and abscisic acid (ABA) confirmed the stress induced by salt treatment: levels of JA and JA-Ile increased strongly under the mildest treatment, ABA only increased under the most severe treatment, and SA levels decreased dose-dependently. These hormonal changes were related to the observed changes in biochemical stress markers upon salt treatments: reductions in seedling fresh weight and root growth, decreased photosynthesis rate, increased levels of reactive oxygen species, and elevated proline content and the Na⁺/K⁺ ratio. Correlations among auxin profile and biochemical stress markers were discussed based on Pearson's coefficients and principal component analysis (PCA).

Roles for IBA-derived auxin in plant development

The plant hormone auxin is a central regulator of plant growth and development. Because auxin plays critical roles in cell division and cell expansion, plants use a number of cellular mechanisms to regulate auxin levels and response. Among these mechanisms is regulated input from the auxin precursor indole-3-butyric acid (IBA) toward the pool of active auxin [indole-3-acetic acid (IAA)]. In this review, we cover the mechanisms of IBA transport and conversion, and discuss specific roles for IBA-derived auxin in driving certain developmental events. We further discuss multiple open questions remaining for the IBA field.

The role of conserved Cys residues in Brassica rapa auxin amidohydrolase: Cys139 is crucial for the enzyme activity and Cys320 regulates enzyme stability

Brassica rapa auxin amidohydrolase (BrILL2) participates in the homeostasis of the plant hormones auxins by hydrolyzing the amino acid conjugates of auxins, thereby releasing the free active form of hormones. Herein, the potential role of the two conserved Cys residues of BrILL2 (at sequence positions 139 and 320) has been investigated by using interdisciplinary approaches and methods of molecular biology, biochemistry, biophysics and molecular modelling. The obtained results show that both Cys residues participate in the regulation of enzyme activity. Cys320 located in the satellite domain of the enzyme is mainly responsible for protein stability and regulation of enzyme activity through polymer formation, as has been revealed by enzyme kinetics and differential scanning calorimetry analysis of the BrILL2 wild type and mutants C320S and C139S. Cys139 positioned in the active site of the catalytic domain is involved in the coordination of one Mn(2+) ion of the bimetal center and is crucial for the enzymatic activity. Although the point mutation Cys139 to Ser causes the loss of enzyme activity, it does not affect the metal binding to the BrILL2 enzyme, as has been shown by isothermal titration calorimetry, circular dichroism spectropolarimetry and differential scanning calorimetry data. MD simulations (200 ns) revealed a different active site architecture of the BrILL2C139S mutant in comparison to the wild type enzyme. Additional possible reasons for the inactivity of the BrILL2C139S mutant have been discussed based on MD simulations and MM-PBSA free energy calculations of BrILL2 enzyme complexes (wt and C139S mutant) with IPA-Ala as a substrate.

Structural and dynamic basis of acid amido synthetase GH3.1: an investigation of substrate selectivity and major active site access channels

Auxin/IAA (Indole-3-acetic acid) plays critical roles in many aspects of plant growth and development. Gretchen Hagen 3.1 (GH3.1) enzyme from grapevine (Vitis vinifera) catalyzes the ATP-dependent conjugation of aspartate to IAA suggesting a significant way to modulate levels of cellular auxins/IAA. It is reported that VvGH3.1 prefers IAA as the substrate than 1-naphthaleneacetic acid (NAA) and benzothiazole-2-oxyacetic acid (BTOA), whereas the detailed interaction mechanism of these substrates remains unclear. In this study, based on the recently reported crystal structure of VvGH3.1 and AtGH3.12, the open form of VvGH3.1 was built. Then combined computational techniques including molecular docking, molecular dynamics (MD) simulations, Molecular Mechanics Generalized Born Surface Area and Poisson-Boltzmann Surface Area (MM-GB/PBSA) methods and active site access channel analysis were utilized to investigate the binding mechanism of three substrates, IAA, NAA and BTOA, and dynamic behaviors of GH3.1 induced by substrate binding. The predicted binding free energy is in agreement with the experimental work with an order of IAA < NAA < BTOA. Key residues interacting with three substrates and residues specifically binding to one of them were identified. The strong hydrogen bond interaction formed between IAA and Ser108, Ser339, Gln560 makes it the best substrate over NAA and BTOA. The dynamic behaviors, especially in the dominant access channels of IAA and NAA bound GH3.1, were found to be different from those in the BTOA-GH3.1 system with larger bottleneck radius and shorter length. This study provided novel insight to understand the substrate selectivity mechanism of GH3.1 and also for the rational design of novel auxin-based herbicides and growth regulators.

Influence of stress hormones on the auxin homeostasis in Brassica rapa seedlings

KEY MESSAGE : Stress hormones, particularly jasmonic acid, influenced root growth, auxin levels, and transcription of auxin amidohydrolase BrIAR3 in Brassica rapa seedlings, while auxin conjugate synthetases BrGH3.1 and BrGH3.9 were down-regulated by all treatments. The influence of stress hormones: jasmonic acid (JA), salicylic acid (SA), and abscisic acid (ABA) on 1-day-old seedlings of Chinese cabbage (Brassica rapa L. ssp. pekinensis) was investigated with particular focus on auxin levels and the regulation of reversible auxin conjugation as a mechanism of auxin homeostasis. At the physiological level, stress hormones inhibited root growth, where JA was the most prominent inhibitor with an IC50 value 3.1 μM, which is one and two orders of magnitude lower than that found for ABA and SA, respectively. JA treatment significantly increased the total auxin content, by induction of free and conjugated forms. Also, the stress hormones affected the transcription of genes involved in the process of the reversible auxin conjugation: auxin amidohydrolases BrIAR3 and BrILL2, and auxin conjugate synthetases BrGH3.1 and BrGH3.9. JA treatment increased the transcript level of BrIAR3 two-fold, while it did not affect the transcription of BrILL2. SA and ABA down-regulated the transcription of both auxin amidohydrolase genes by 30 %. Transcription of both auxin conjugate synthetases was significantly down-regulated by all treatments by 30-70 %. Among the investigated biochemical stress markers, glutathione along with protein carbonylation appeared the most affected upon treatments. The redox status of the seedlings was shifted to the more oxidized state upon JA and ABA treatments, whereas SA caused more reduced redox state in comparison to the control. The principal component analysis visualized relationship among auxin and stress parameters upon treatments. Accordingly, the role of auxin in stress response of Brassica seedlings was discussed.

Auxin biosynthesis and storage forms

The plant hormone auxin drives plant growth and morphogenesis. The levels and distribution of the active auxin indole-3-acetic acid (IAA) are tightly controlled through synthesis, inactivation, and transport. Many auxin precursors and modified auxin forms, used to regulate auxin homeostasis, have been identified; however, very little is known about the integration of multiple auxin biosynthesis and inactivation pathways. This review discusses the many ways auxin levels are regulated through biosynthesis, storage forms, and inactivation, and the potential roles modified auxins play in regulating the bioactive pool of auxin to affect plant growth and development.

Jasmonoyl-L-isoleucine hydrolase 1 (JIH1) regulates jasmonoyl-L-isoleucine levels and attenuates plant defenses against herbivores

For most plant hormones, biological activity is suppressed by reversible conjugation to sugars, amino acids and other small molecules. In contrast, the conjugation of jasmonic acid (JA) to isoleucine (Ile) is known to enhance the activity of JA. Whereas hydroxylation and carboxylation of JA-Ile permanently inactivates JA-Ilemediated signaling in plants, the alternative deactivation pathway of JA-Ile by its direct hydrolysis to JA remains unstudied. We show that Nicotiana attenuata jasmonoyl-L-isoleucine hydrolase 1 (JIH1), a close homologue of previously characterized indoleacetic acid alanine resistant 3 (IAR3) gene in Arabidopsis, hydrolyzes both JA-Ile and IAA-Ala in vitro. When the herbivory-inducible NaJIH1 gene was silenced by RNA interference, JA-Ile levels increased dramatically after simulated herbivory in irJIH1, compared with wild-type (WT) plants. When specialist (Manduca sexta) or generalist (Spodoptera littoralis) herbivores fed on irJIH1 plants they gained significantly less mass compared with those feeding on wild-type (WT) plants. The poor larval performance was strongly correlated with the higher accumulation of several JA-Ile-dependent direct defense metabolites in irJIH1 plants. In the field, irJIH1 plants attracted substantially more Geocoris predators to the experimentally attached M. sexta eggs on their leaves, compared with empty vector plants, which correlated with higher herbivory-elicited emissions of volatiles known to function as indirect defenses. We conclude that NaJIH1 encodes a new homeostatic step in JA metabolism that, together with JA and JA-Ilehydroxylation and carboxylation of JA-Ile, rapidly attenuates the JA-Ile burst, allowing plants to tailor the expression of direct and indirect defenses against herbivore attack in nature.

Mechanism and thermodynamics of ligand binding to auxin amidohydrolase

BrILL2 is catalytically the most efficient auxin amidohydrolase from Brassica rapa, playing a key role in auxin metabolism by catalyzing its release from amino acid conjugates. Auxins, with the most abundant representative indole-acetic acid ([1H-indol-3-yl]-acetic acid, IAA), are a group of plant hormones that in very small concentrations regulate ubiquitin-mediated degradation of transcription regulators. Kinetic studies on BrILL2 showed that it hydrolyzes alanine conjugates of IAA and of its larger analogues, indole-propionic acid (3-[1H-indol-3-yl]-propionic acid, IPA) and indole-butyric acid (4-[1H-indol-3-yl]-butyric acid, IBA). Structurally, BrILL2 belongs to the largest known family of metallopeptidases (M20) that share a recognizable 3D structure, characterized by two perpendicular domains. Its members have been implicated in numerous biochemical processes and have been found across all species sequenced to date. Here, molecular dynamics simulations were carried out to study structural and thermodynamic properties of ligand binding to BrILL2. A conformational change was captured in multiple copies of 10 ns long simulations, described by a rigid body movement of the two domains, and its associated key interactions between residues were examined. For the three substrates, complexes in two possible binding modes were recreated, along with a single binding mode for the putative substrate tryptophanyl-alanine (Trp-Ala), which were subsequently simulated in multiple copies of 10 ns long simulations. Thermodynamic calculations were used to assess their binding affinities and explain the selectivity toward the longer ligands. Based on the results, a possible route for the reaction is proposed.

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.