Natural Auxin Research Articles

Using synergistic exogenous phytohormones to enhance somatic embryogenesis from leaf explants of a Eucalyptus grandis clone

Somatic embryogenesis (SE) in Eucalyptus spp. has been limited to germinated seeds, flowers, lignotubers or zygotic embryos. The low yield of somatic embryos from leaf explants has hampered progress, even though leaves offer a more viable source of clonal explants from superior selected genotypes. It was hypothesised that SE from leaf explants could be enhanced through pairing of synergistic exogenous plant growth regulators, such as natural auxins with natural cytokinins. Callus and embryo induction using 2,4-dichlorophenoxyacetic acid (2,4-D), α-naphthalene acetic acid (NAA), indole acetic acid (IAA), and indole butyric acid (IBA), each at either 1.0 or 3.0 mg L−1, indicated that IAA and IBA favoured significantly higher numbers of embryos compared with 2,4-D or NAA. Hence, IAA and IBA were used for subsequent experiments, combining them (at 1.0 mg L−1) with either the synthetic cytokinin, kinetin, or the natural cytokinin, trans-zeatin, both at 0.1 mg L−1. The combination of trans-zeatin and either IAA or IBA resulted in a significant increase in SE (e.g. 86 ± 17.2% and 23 ± 3.2% for IAA with trans-zeatin and kinetin, respectively), compared with kinetin, or with these auxins alone. Embryo maturation and plantlet regeneration was highest in those calli that were induced with IAA and trans-zeatin, indicating that maturation was dependent on auxin depletion, based on the stability of the analogue used for induction. For the E. grandis clone under study, the use of synergistic plant growth regulators significantly enhanced SE from leaf explants, thus presenting the opportunity to benefit from the advantages that SE offers over conventional organogenesis.

Gravitropism and Lateral Root Emergence are Dependent on the Trans-Golgi Network Protein TNO1.

The trans-Golgi network (TGN) is a dynamic organelle that functions as a relay station for receiving endocytosed cargo, directing secretory cargo, and trafficking to the vacuole. TGN-localized SYP41-interacting protein (TNO1) is a large, TGN-localized, coiled-coil protein that associates with the membrane fusion protein SYP41, a target SNARE, and is required for efficient protein trafficking to the vacuole. Here, we show that a tno1 mutant has auxin transport-related defects. Mutant roots have delayed lateral root emergence, decreased gravitropic bending of plant organs and increased sensitivity to the auxin analog 2,4-dichlorophenoxyacetic acid and the natural auxin 3-indoleacetic acid. Auxin asymmetry at the tips of elongating stage II lateral roots was reduced in the tno1 mutant, suggesting a role for TNO1 in cellular auxin transport during lateral root emergence. During gravistimulation, tno1 roots exhibited delayed auxin transport from the columella to the basal epidermal cells. Endocytosis to the TGN was unaffected in the mutant, indicating that bulk endocytic defects are not responsible for the observed phenotypes. Together these studies demonstrate a role for TNO1 in mediating auxin responses during root development and gravistimulation, potentially through trafficking of auxin transport proteins.

Development of 4-methoxy-7-nitroindolinyl (MNI)-caged auxins which are extremely stable in planta.

Phytohormone auxin is a master regulator in plant growth and development. Regulation of cellular auxin level plays a central role in plant development. Auxin polar transport system modulates an auxin gradient that determines plant developmental process in response to environmental conditions and developmental programs. Photolabile caged auxins allow optical control of artificial auxin gradients at cellular resolution. Especially, two-photon uncaging system achieves high spatiotemporal control of photolysis reaction at two-photon cross-section. However, the development of caged versions of auxin has been limited by the instability of the caged auxins to higher plant metabolic activities. Here, we describe the synthesis and application of highly stable caged auxins, 4-methoxy-7-nitroindolinyl (MNI)-caged auxins. Natural auxin, indole 3-acetic acid, and two synthetic auxins, 1-NAA and 2,4-D were caged by MNI caging group. MNI-caged auxins showed a high stability in planta and a rapid release the original auxin when photolyzed. We demonstrated that optical control of auxin-responsive gene expression and auxin-related physiological responses by using MNI-caged auxins. We anticipate that MNI-caged auxins will be an effective tool for high-resolution control of endogenous auxin level.

Distinct Characteristics of Indole-3-Acetic Acid and Phenylacetic Acid, Two Common Auxins in Plants.

The phytohormone auxin plays a central role in many aspects of plant growth and development. IAA is the most studied natural auxin that possesses the property of polar transport in plants. Phenylacetic acid (PAA) has also been recognized as a natural auxin for >40 years, but its role in plant growth and development remains unclear. In this study, we show that IAA and PAA have overlapping regulatory roles but distinct transport characteristics as auxins in plants. PAA is widely distributed in vascular and non-vascular plants. Although the biological activities of PAA are lower than those of IAA, the endogenous levels of PAA are much higher than those of IAA in various plant tissues in Arabidopsis. PAA and IAA can regulate the same set of auxin-responsive genes through the TIR1/AFB pathway in Arabidopsis. IAA actively forms concentration gradients in maize coleoptiles in response to gravitropic stimulation, whereas PAA does not, indicating that PAA is not actively transported in a polar manner. The induction of the YUCCA (YUC) genes increases PAA metabolite levels in Arabidopsis, indicating that YUC flavin-containing monooxygenases may play a role in PAA biosynthesis. Our results provide new insights into the regulation of plant growth and development by different types of auxins.

Auxin Binding Protein 1: A Red Herring After All?

The natural auxin indole-3-acetic acid is the first hormone identified in plants, and since it plays such a central role in plant growth and development, auxin has been the subject of intensive studies. A central question has been how the auxin signal is perceived by plant cells. The earliest experiments showed the presence of auxin binding particles at the plasma membrane (PM) and in the endoplasmic reticulum (ER) (Hertel et al., 1972). Screens for PM-localized auxin binding activities have led to the photo-affinity labeling and purification of Auxin Binding Protein 1 (ABP1) from maize coleoptile cells (Lobler and Klambt, 1985).

The studies on the toxicity mechanism of environmentally hazardous natural (IAA) and synthetic (NAA) auxin – The experiments on model Arabidopsis thaliana and rat liver plasma membranes

This paper concerns the studies towards membrane-damage effect of two auxins: indole-3-acetic acid – IAA and 1-naphthaleneacetic acid – NAA on plant (Arabidopsis thaliana) and animal (rat liver) model membranes. The foregoing auxins are plant growth regulators widely used in agriculture to control the quality of the crop. However, their accumulation in the environment makes them hazardous for the living organisms. The aim of our investigations was to compare the effect of natural (IAA) vs. synthetic (NAA) auxin on the organization of plant and animal model membranes and find a possible correlation between membrane-disturbing effect of these compounds and their toxicity. The collected data evidenced that auxins cause destabilization of membranes, decrease their condensation and weakens interactions of molecules. The alterations in the morphology of model systems were also noticed. The foregoing effects of auxins are concentration-dependent and additionally NAA was found to act on animal vs. plant membranes more selectively than IAA. Interestingly, both IAA and NAA induce the strongest disordering in model lipid system at the concentration, which is frequently reported as toxic to animal and plants. Based on the above findings it was proposed that membrane-damage effect induced by IAA and NAA may be important from the point of view of the mechanism of toxicity of these compounds and cannot be ignored in further investigations in this area.

옥수수(Zea mays) 원뿌리의 통기조직 발달에 미치는 에틸렌 전구체, 옥신, 메틸자스몬산의 효과

We have investigated the effects of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC), indole-3-acetic acid (IAA) and methyl jasmonate (MeJA) on the development of aerenchyma in the primary root of maize (Zea mays). Because plant hormones affected the longitudinal organization of the primary root, we need an indicator to direct the positions for comparison between control and hormone-treated roots. Therefore, the zones of the maize primary root were categorized as PR25, PR50 and PR75, where each value indicates the relative position between the root tip (PR0) and the base (PR100). Aerenchyma was not observed at PR25 and PR50 and rarely found at PR75 in the cortex of control roots. The aerenchymal area at PR75 increased in the presence of the ethylene precursor ACC or a natural auxin IAA. On the other hand, MeJA differentially acted on non-submerged and submerged roots. Exogenously applied MeJA suppressed the aerenchyma formation in non-submerged roots. When the primary root was submerged, aerenchymal area expanded prominently. The submergence-induced aerenchyma formation was amplified with MeJA. Lateral root primordia have been known to inhibit aerenchymal death of surrounding cells. All the three hormones stimulating aerenchyma formation as described above did not restore the inhibition caused by lateral root primordia, suggesting that the inhibitory step regulated by lateral root primordia can be located after hormonal signaling steps.

S01 Action mechanism of phenylacetic acid as a natural auxin in Arabidopsis

S01 Action mechanism of phenylacetic acid as a natural auxin in Arabidopsis

Synthesis, crystal structure and biological activity of 2-hydroxyethylammonium salt of p-aminobenzoic acid.

p-Aminobenzoic acid (pABA) plays important roles in a wide variety of metabolic processes. Herein we report the synthesis, theoretical calculations, crystallographic investigation, and in vitro determination of the biological activity and phytotoxicity of the pABA salt, 2-hydroxyethylammonium p-aminobenzoate (HEA-pABA). The ability of neutral and anionic forms of pABA to interact with TIR1 pocket was investigated by calculation of molecular electrostatic potential maps on the accessible surface area, docking experiments, Molecular Dynamics and Quantum Mechanics/Molecular Mechanics calculations. The docking study of the folate precursor pABA, its anionic form and natural auxin (indole-3-acetic acid, IAA) with the auxin receptor TIR1 revealed a similar binding mode in the active site. The phytotoxic evaluation of HEA-pABA, pABA and 2-hydroxyethylamine (HEA) was performed on the model plant Arabidopsis thaliana ecotype Col 0 at five different concentrations. HEA-pABA and pABA acted as potential auxin-like regulators of root development in Arabidopsis thaliana (0.1 and 0.2 mM) and displayed an agravitropic root response at high concentration (2 mM). This study suggests that HEA-pABA and pABA might be considered as potential new regulators of plant growth.

Natural vs synthetic auxin: Studies on the interactions between plant hormones and biological membrane lipids

Analysis of the interactions between two representatives of plant hormones: synthetic (1-naphthaleneacetic acid, NAA) as well as natural (indole-3-acetic acid, IAA) and phospholipids occurring in biological membrane of both plant and animal cells was the subject of present studies. The aim of undertaken experiments was to elucidate the problem of direct influence of these plant growth regulators on phosphatidylcholines (PCs) and phosphatidylethanolamines (PEs) in monolayers at the air/water solution interface. The studied phospholipids differ not only as regards the structure of polar head-groups but also in the length of hydrophobic chains as well as their saturation degree. These differences result also in the main properties and functions of these phospholipids in biomembranes. The analysis of the results was based on the characteristics of the surface pressure (π) – area (A) isotherms registered for monolayers spread on the subphase containing plant hormone and as a reference on the surface of pure water. Moreover, as a complementary technique, Brewster angle microscopy was applied for the direct visualization of the investigated surface films. The obtained results revealed that auxins effectively influence phospholipids monolayers, regardless of the lipid structure, at the concentration of 10−4M. It was found that for this concentration, the influence of auxins was visibly larger in the case of PCs as compared to PEs. On the other hand, in the case of auxins solution of ≤10−5M, the observed trend was opposite. Generally, our studies showed that the natural plant hormone (IAA) interacts with the investigated lipid monolayers stronger than its synthetic derivative (NAA). The reason of these differences connects with the steric properties of both auxins; namely, the naphthalene ring of NAA molecule occupies larger space than the indole system of IAA. Therefore molecules of the latter compound penetrate easier into the region of phospholipids׳ polar head-groups. Moreover, the NH group of the indole moiety is capable of hydrogen bond formation with the acceptor groups in the polar fragment of lipid molecules. We proved also that among the investigated phospholipids, the highest susceptibility toward auxin influence show these lipids, for which during compression, surface film increases the degree of condensation.

Auxin sensitivities of all Arabidopsis Aux/IAAs for degradation in the presence of every TIR1/AFB.

Auxin plays a key role in regulation of almost all processes of plant growth and development. Different physiological processes are regulated by different ranges of auxin concentrations; however, the underlying mechanisms creating these differences are largely unknown. The first step of auxin signaling is auxin-dependent interaction of an auxin receptor with transcriptional co-repressors (Aux/IAA), which leads to Aux/IAA degradation. Arabidopsis has six homologous auxin receptors (TIR1 and five AFBs), 29 Aux/IAA proteins and two types of active auxins, IAA and phenylacetic acid (PAA). Therefore, a large number of possible combinations between these three factors may contribute to the creation of complex auxin responses. Using a yeast heterologous reconstitution system, we investigated auxin-dependent degradation of all Arabidopsis Aux/IAAs in combination with every TIR or AFB receptor component. We found that TIR1 and AFB2 were effective in mediating Aux/IAA degradation. We confirmed that the Aux/IAA domain II, which binds TIR1, is essential for degradation. IAA and other natural auxins, 4-chloroindole-3-acetic acid (4-Cl-IAA) and PAA, induced Aux/IAA degradation; and IAA and 4-Cl-IAA had higher activity than PAA. Effective auxin concentrations for Aux/IAA degradation depended on both Aux/IAAs and TIR1 or AFB2 receptors, which is consistent with the Aux/IAA-TIR1/AFB co-receptor concept.

Auxin Overproduction in Shoots Cannot Rescue Auxin Deficiencies in Arabidopsis Roots

Auxin plays an essential role in root development. It has been a long-held dogma that auxin required for root development is mainly transported from shoots into roots by polarly localized auxin transporters. However, it is known that auxin is also synthesized in roots. Here we demonstrate that a group of YUCCA (YUC) genes, which encode the rate-limiting enzymes for auxin biosynthesis, plays an essential role in Arabidopsis root development. Five YUC genes (YUC3, YUC5, YUC7, YUC8 and YUC9) display distinct expression patterns during root development. Simultaneous inactivation of the five YUC genes (yucQ mutants) leads to the development of very short and agravitropic primary roots. The yucQ phenotypes are rescued by either adding 5 nM of the natural auxin, IAA, in the growth media or by expressing a YUC gene in the roots of yucQ. Interestingly, overexpression of a YUC gene in shoots in yucQ causes the characteristic auxin overproduction phenotypes in shoots; however, the root defects of yucQ are not rescued. Our data demonstrate that localized auxin biosynthesis in roots is required for normal root development and that auxin transported from shoots is not sufficient for supporting root elongation and root gravitropic responses.

SYNTHESIS AND INVESTIGATED OF THE INFLUENCE OF NEW AMINO ACIDS DERIVATIVES WITH INDOLE AND BENZOXAZOLINONE MOIETIES ON THE GROWTH OF RAPESEED (BRASSICA NAPUS L.) IN VITRO

Rapeseed (Brassica napus L.) is one of the world’s most important sources of vegetable oil and protein meal used for food and for industrial purposes. Primary and secondary metabolites stored in rapeseed depend on many factors: growth regulators, temperature, water stress, nutrients (nitrogen, phosphorus, potassium, calcium, boron, sulfur) and their amount, soil acidity, evolution change, and plant diseases. Indole-3-acetic acid (IAA) is one of the most important natural auxins. 3-[2-Oxo-1,3-benzoxazol-3(2H)-yl]propanoic acid is know as a synthetic auxin. The sodium salt of this compound, stilite-14, has been studied for 10 years while growing sugar beets in Lithuania. Natural and synthetic auxins are known to promote rapeseed root growth. New indole-3-acetic acid and 3-[2-oxo-1,3-benzoxazol-3(2H)-yl]propanoic acid derivatives containing b,g-amino acid or 3-carboxy-5-oxopyrrolidine fragments from the corresponding hydrazides were synthesized.It was determined that 2-({2-[2-(1H-indol-3-yl)acetyl]hydrazino}methyl)succinic acid (8.3 μM/l) increased the length of rapeseed roots in the MS medium by 1.6 times in comparison with rapeseed, which were grown in MS medium without growth regulators, whereas IAA decreased their length. Also, 2-({2-[2-(1H-indol-3-yl)acetyl]hydrazino}methyl)succinic acid (8.3 mM/l) increased rapeseed biomass by 2 times in comparison with control sample.It was determined that 5-oxo-1-({3-[2-oxo-1,3-benzoxazol-3(2H)-yl]propanoyl}amino)-3-pyrrolidinecarboxylic acid (6 mM/l) increased the length of rapeseed roots in the MS medium by 1.2 times in comparison with control sample, but the sodium salt of 3-(3-benzoxazolonyl)propanoic acid decreased their length at all concentrations. Also, 5-oxo-1-({3-[2-oxo-1,3-benzoxazol-3(2H)-yl]propanoyl}amino)-3-pyrrolidinecarboxylic acid (1.5 mM/l) increased rapeseed biomass, but the sodium salt of 3-(3-benzoxazolonyl)propanoic acid (1.5 mM/l) decreased the biomass by 39.1 % in comparison with the control sample. DOI: http://dx.doi.org/10.5755/j01.ct.64.2.6022

Insight into the mode of action of 2,4‐dichlorophenoxyacetic acid (2,4‐D) as an herbicide

2,4-Dichlorophenoxyacetic acid (2,4-D) was the first synthetic herbicide to be commercially developed and has commonly been used as a broadleaf herbicide for over 60 years. It is a selective herbicide that kills dicots without affecting monocots and mimics natural auxin at the molecular level. Physiological responses of dicots sensitive to auxinic herbicides include abnormal growth, senescence, and plant death. The identification of auxin receptors, auxin transport carriers, transcription factors response to auxin, and cross-talk among phytohormones have shed light on the molecular action mode of 2,4-D as a herbicide. Here, the molecular action mode of 2,4-D is highlighted according to the latest findings, emphasizing the physiological process, perception, and signal transduction under herbicide treatment.

Auxin signal transduction into a quantitative change in natural auxin polar transport in pine cambium

Results of experiments performed with 6-mm high stem sections of <em>Pinus sylvestris</em> L. confirmed the hypothesis that the fusiform cells of the cambial region respond to the arrival of indole-3-acetic-acid (IAA) at the signalling concentration in the apoplastic space around their apical ends by increasing the basipetal efflux of endogenous natural auxin. Thus, the auxin signal propagation along the stem cambial region could be a chain of reactions between the axially neighbouring cells, each capable of responding and contributing to the change of auxin concentration in the apoplast which requires only transduction of the foreign auxin signal by each of the cells to increase the basipetal efflux of their own endogenous auxin. The rate of auxin signal propagation in this system is not limited by the rate of auxin molecular transport, and if it functions in conjunction with feedback inhibition, it may produce oscillations of the auxin basipetal efflux generating supracellular auxin waves. Inhibitors of the proteinaceous auxin efflux carrier associated with the plasmalemma (NPA and TIBA), although reducing total basipetal transport of the natural auxin, did not prevent the stimulated additional efflux of this phytohormone. The studied auxin-signal transduction processes seem to be intracellular but not mediated by the Ca-calmodulin complex. The natural auxin basipetal efflux increased significantly within 45 min following the period when it had been strongly reduced by successive collections to agar receivers replaced several times at the basal ends of 6-mm high stem sections in which the intact fusiform cells of the cambial region in about 90% are arranged in only one axial row. Such exhaustion of the cellular reserve of auxin did not prevent the additional auxin basipetal efflux stimulated by the IAA apical treatment. The results may suggest an effect of the auxin signal upon the supply of newly-synthesised auxin directly to the system responsible for its basipetal efflux.

Oscillation of stem polarity expression in transport of natural auxin of pine cambium

Simultaneous measurements of acropetal and basipetal efflux of natural auxin from the cambial region of a series of successive sections from <em>Pinus silvestris</em> stems revealed an unknown until now variation of polarity expression in the transport of auxin, which occurs in short cycles along the stem. It is suggested that the direction of phase synchronization of this oscillation between the neighboring cambial cells determines the fronts of the auxin waves. Modulations of the auxin-wave caused by apical application of synthetic IAA were shown to depend upon changes in the polarity expression in respect to natural auxin transport. Some evidence of a direct involvement of the exogenous auxin in the mechanism responsible for oscillations of the polarity expression was also obtained in experiments, in which <sup>14</sup>C-IAA was applied directly into the phloem of living trees of <em>Pinus taeda</em> L. Transport of labelled auxin was very slow however, and its direct involvement in polar transport (the oscillation of which was measured) extended only for a short distance from the place of aplication. Simultaneously, the natural auxin wave measured by bioassays extended to further distances. The results corroborate the hypothesis that oscillations of the polarity expression responsible for generation of the auxin wave in cambium depend primarily upon a system autonomous to cells of the cambial region, independent of the direct contribution of the molecules of auxin produced at distant sources. On the other hand, such external to cambium sources of auxin modulate the auxin-wave parameters affecting the system responsible for expression of the polarity in the regions close to their synthesis. These modulations are than propagated in the cambial zone.

Acropetal effects of IAA upon the wave-like pattern in the basipetal efflux of natural growth substances of Fraxinus excelsior L.

Basipetal efflux of natural growth substances from the cambial region of a successive series of short sections of the stem of <em>Fraxinus excelsior</em> L. was measured by Went's oat coleoptile curvature bioassay. A wave-like pattern of stimulation by natural growth substances collected after efflux of only 10 min in agar containing 0.56 µM synthetic IAA was observed, confirming to earlier findings in other species. Both inhibitory and stimulatory effects were recorded, however, the oscillations were manifested in stimulation, which is probably due to natural auxin. IAA applied for 1 h directly to the cut surface of the lateral branch, distal to the stem segment used for the measurements of natural auxin efflux, caused acropetal modulations of the wave-like pattern. The results seem to indicate that the auxin-wave system analogous to that already described for other trees operates also in the cambial region of the <em>Fraxinus excelsior</em> L. stem. Modulations of such waves caused by IAA (seen as phase shift) can be propagated acropetally at a rate excluding direct action of molecules of auxin at the site of measurement.

Cis-Cinnamic acid selective suppressors distinct from auxin inhibitors.

The activity of cis-cinnamic acid (cis-CA), one of the allelochemicals, in plants is very similar to that of indole-3-acetic acid (IAA), a natural auxin, and thus cis-CA has long been believed to be an analog of auxin. We have reported some structure-activity relationships studies by synthesizing over 250 cis-CA derivatives and estimating their inhibitory activities on root growth inhibition in lettuce. In this study, the compounds that showed low- or no-activity on root growth inhibition were recruited as candidates suppressors against cis-CA and/or auxin and tested for their activity. In the presence of cis-CA, lettuce root growth was inhibited; however, the addition of some cis-CA derivatives restored control-level root growth. Four compounds, (Z)-3-(4-isopropylphenyl)acrylic acid, (Z)-3-(3-butoxyphenyl)acrylic acid, (Z)-3-[3-(pentyloxy)phenyl]acrylic acid, and (Z)-3-(naphthalen-1-yl)acrylic acid were selected as candidates for a cis-CA selective suppressor they allowed the recovery of root growth from inhibition by cis-CA treatment without any effects on the IAA-induced effect or elongating activity by themselves. Three candidates significantly ameliorated the root shortening by the potent inhibitor derived from cis-CA. In brief, we have found some cis-CA selective suppressors which have never been reported from inactive cis-CA derivatives for root growth inhibition. cis-CA selective suppressors will play an important role in elucidating the mechanism of plant growth regulation.

Cloning and Characterization of a Candidate Auxin Plant Growth Regulator-Activated Cell Surface Hydroquinone (NADH) Oxidase

ENOX (ECTO-NOX) proteins of the external surface of the plasma membrane catalyze oxidation of both NADH and hydroquinones and protein disulfide-thiol interchange. They exhibit both prion-like and time-keeping (clock) properties. The oxidative and interchange activities alternate to generate a regular period of 24 min in length. Here we report the cloning, expression and characterization of a constitutive plant ENOX protein activated by both natural (Indole-3-acetic acid, IAA) and synthetic (2,4-dichlorophenoxyacetic acid, 2,4-D) auxin plant growth regulators with an optimum of about 1 μM, higher concentrations being less effective. The gene encoding the 213 amino acid protein (ABP20) is found in EMBL accession number U81162. Functional motifs characteristic of ENOX1 proteins, previously identified by site-directed mutagenesis, are present in the candidate auxin-activated ENOX (dNOX, ENOX5), including adenine nucleotide and copper binding motifs along with essential cysteines and a motif having homology with a previously identified auxin-binding motif. Periodicity was exhibited by both the oxidative and protein disulfide-thiol inter-change activities as is characteristic for other ENOX proteins. Activity was blocked by the ENOX2-specific quassinoid inhibitor glaucarubolone and other ENOX2 inhibitors but not by the ENOX1-specific quassinoid inhibitor simalikalactone D. Activity required both auxin and bound copper. The inactive auxin 2,3-D was without effects.

Auxin biosynthesis.

lndole-3-acetic acid (IAA), the most important natural auxin in plants, is mainly synthesized from the amino acid tryptophan (Trp). Recent genetic and biochemical studies in Arabidopsis have unambiguously established the first complete Trp-dependent auxin biosynthesis pathway. The first chemical step of auxin biosynthesis is the removal of the amino group from Trp by the TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS (TAA) family of transaminases to generate indole-3-pyruvate (IPA). IPA then undergoes oxidative decarboxylation catalyzed by the YUCCA (YUC) family of flavin monooxygenases to produce IAA. This two-step auxin biosynthesis pathway is highly conserved throughout the plant kingdom and is essential for almost all of the major developmental processes. The successful elucidation of a complete auxin biosynthesis pathway provides the necessary tools for effectively modulating auxin concentrations in plants with temporal and spatial precision. The progress in auxin biosynthesis also lays a foundation for understanding polar auxin transport and for dissecting auxin signaling mechanisms during plant development.