Where is auxin synthesized

They showed that low R:FR perception in the shoot inhibits lateral root emergence. Adventitious roots are post embryonic roots and auxin has been known to regulate their formation Veloccia et al. Veloccia et al. Auxin plays a critical role in root hair development Knox et al. Auxin treatment promotes root hair elongation and mutations in AUX1 gene results in shorter root hairs that can be restored to wild type levels by exogenous auxin.

Interestingly, AUX1 is expressed in non-hair epidermal cells but not in the hair cells Jones et al. Computer simulation showed that expression of AUX1 in the non-hair cells can still result in over fold accumulation of auxin in hair cells and thus Jones et al.

PIN2 which is expressed in both root hair and non-hair cells can facilitate auxin efflux out of the non-hair cells and into the apoplast and despite no AUX1 in the hair cells, these root hair cells can still maintain high auxin concentration. Root hair cell polarity has also been shown to be regulated by auxin Grebe et al. Root hairs are formed on the basal side of the hair cells and auxin treatment results in more basal position of root hairs Grebe et al.

Recently, Dindas et al. Membrane depolarization is one of the earliest auxin responses in a cell. Using an electrophysiological approach and measuring membrane potential using intracellular mini-electrodes, Dindas et al. They also showed that IAA influx is coupled with changes in cytosolic calcium and calcium influx is impaired in aux1 mutants. This suggests that early events in auxin signaling are non-genomic. This led Dindas et al. Auxin mediated root hair elongation is a key adaptive response to low P Bates and Lynch, ; Lynch, Recently, Giri et al.

Bhosale et al. They showed that auxin homeostasis Porco et al. They also showed that under low P, there is accumulation of auxin in the root apex through induction of TAA1, a key enzyme in auxin biosynthesis. AUX1 then facilitates the movement of this auxin in a shootward direction into the root hair zone where it facilitates root hair elongation.

In this study, they not only showed that both taa1 and aux1 mutants have defects in root hair elongation under low P but also mapped the tissues required for root hair elongation under low P. They showed that expression of AUX1 in lateral root cap and epidermal cells files is sufficient to rescue low P mediated root hair elongation defect in aux1 mutants. But what happens when auxin has reached the root hair zone?

What are the down-stream components mediating this low P mediated root hair elongation response? To investigate this, Bhosale et al. Next, they asked the question, what are the targets for ARF19? A close examination of the RSL2 and RSL4 promoters revealed several auxin response elements suggesting a possible mechanism for auxin mediated root hair elongation. Based on these results, Bhosale et al. Recently, Kasprzewska et al.

AUX1 expression is more confined to the leaf margins. In contrast, LAX2 expression is excluded from the margins and is localized more towards the center of the leaf primordia and gradually gets confined to the leaf vasculature. LAX1 expression appears to be most dynamic and is mainly seen in the leaf tip and the flanks in the young leaf primordia. Later, new LAX1 expression sites are seen at the leaf margins proximal to the original sites which Kasprzewska et al.

Kasprzewska et al. More recently, Moreno-Piovano et al. They showed that lax2 mutants have increased xylem length and number of xylem cell rows which can be restored by expression of LAX2 suggesting that auxin homeostasis regulates leaf venation patterning. Female gametophyte megagametophyte development begins with mega spore mother cell undergoing meiotic division and giving rise to four haploid cells. One of the haploid cell becomes a functional megaspore and undergoes three rounds of mitosis to produce seven-celled eight-nucleate highly polarized megagametophyte comprised of two synergid cells, one egg cell, one central cell and three antipodal cells.

Auxin efflux carrier PIN1 was previously shown to play a role in regulating female gametophyte development Ceccato et al. Recently, Panoli et al. They revealed that while AUX1 is primarily localized in the synergids and egg cell membranes, LAX1 is seen localized in the sporophytic tissues of nucellus surrounding the micropylar pole of embryo sac. Panoli et al. Genetic and pharmacological studies show that auxin is crucial for embryo development Hardtke and Berleth, ; Bhatia et al.

More recently, Robert et al. They showed that AUX1 is specifically expressed in the cell embryo stage and later in the provascular cells. LAX2 is also expressed in the perivascular cells from cell embryo stage onward and is also expressed in the hypophysis and the uppermost suspensor cell. In contrast, LAX1 is expressed very early on from the one-cell stage in the apical cell and from cell stage, its expression is more pronounced in the upper tier cells and by heart stage embryo, LAX1 expression is confined to the cotyledon tips.

No LAX3 expression is reported in the embryo. To get a better understanding of the role of auxin influx carriers in embryo development, Robert et al. They uncovered patterning defects in the upper pole in the aux1lax1 double mutants at a low frequency and this was considerably more pronounced both in the severity and the frequency in the aux1lax1lax2 triple mutants. This led Robert et al.

More recently, Liu et al. AUX1 is localized to the apical face of the cell in the embryo central vascular cells and the protophloem cells. Liu et al. They also showed that ropgef1 mutants also have altered accumulation of PIN2 and PIN7 and cannot establish asymmetric auxin gradient in gravistimulated roots and have embryo defects as well as cotyledon vein breaks and altered root gravitropic response.

Hochholdinger et al. Figure 2 The phylogeny of AUX1 homologs of selected plant species. This tree was generated using interactive phylogenetic module from Plaza 4. Some of these homologs have been previously characterised discussed in the main text. Figure 3 Arabidopsis AUX1 homoliogs play crucial roles in plant development across severals species.

Arabidopsis auxin influx carrier AUX1 homologs regulate aspects of plant development such as root gravitropism; root architecture e. Recently, Huang et al. Inflorescence architecture is an important agronomic trait as it influences grain yield. Using a forward genetic approach, Huang et al. These mutants spp and spp also show decreased plant height, reduced inflorescence branching and spikelet numbers and increased panicle length compared to the control plants.

In maize, auxin synthesis, transport and signaling have been previously linked to inflorescence architecture variation, including branching pattern changes Gallavotti et al. Auxin efflux carrier ZmPIN1 has been also implicated in maize inflorescence development Skirpan et al. Now Huang et al. Interestingly, Huang et al.

Additionally, Huang et al. But unlike ataux1 , spp-1 and spp mutants have no lateral root defects. Zhao et al. Mutations in OsAUX1 result in reduced lateral root initiation events whereas OsAUX1 overexpression plants exhibit increased lateral root initiation events. Transcript levels of several auxin signaling and cell cycle genes are significantly downregulated in osaux1 , further highlighting the importance of OsAUX1 in regulating lateral root development in rice.

Cd stress induces the production of reactive oxygen species, which trigger cell death in plants. Auxin signaling is known to be involved in activating Cd-induced morphogenic defense responses in wheat, barley and Arabidopsis Tamas et al.

Yu et al. Reporter analysis showed that OsAUX1 is distinctly induced under Cd stress in primary roots, lateral roots and root hairs and osaux1 mutants are more sensitive to Cd stress.

Cd contents in the osaux1 mutant were not altered, but reactive oxygen species-mediated damage was enhanced, further increasing the sensitivity of the mutant to Cd stress.

Taken together, their results indicated that OsAUX1 plays an important role in mediating plant responses to Cd stress. Giri et al. Using direct auxin quantification by mass spectrometry as well as auxin reporter-based approaches, they showed that low P results in increased auxin accumulation in the root apex in OsAUX1 dependent fashion.

Although mechanistic details are not yet fully understood in rice, it is tempting to speculate that it is similar to Arabidopsis Bhosale et al. Further, using an elegant split root experiments, by exposing half of the crown roots from the same plants to low P and the other half to high P, Giri et al. More recently, Wang et al.

OsAUX3 is expressed in primary roots, lateral roots and in the root hairs. Mutations in OsAUX3 result in shorter primary roots, decreased lateral root density, and longer root hairs compared to control. Recently, van der Schuren et al. Thus, possibly BdAUX1 have the combined functions in these tissues. Bdaux1 mutant roots are agravitropic and show longer root phenotypes due to increased mature cell lengths possibly due to higher free auxin content.

Additionally, Bdaux1 mutants are also significantly thinner due to reduced cell file numbers in every tissue except xylem and phloem and due to smaller cell sizes radially. Bdaux1 mutants also show reduced root hair length and density. More recently, Roy et al.

They showed that MtLAX2 is auxin inducible and is expressed in the nodule primordia, vasculature of developing nodules and at the apex of matured nodules. Upon Rhizobium infection, mtlax2 mutants have fewer nodules and reduced DR5 activity at the infection sites clearly implicating the role of MtLAX2 in nodule development.

This led Roy et al. Modeling studies on auxin transport. Computer simulations and modelling approaches in the past decade have proven very useful in getting better understanding of the role of auxin transport in regulating auxin mediated developmental processes in Arabidopsis and how auxin fluxes are established and maintained Swarup et al.

Increased understanding of the auxin transport proteins and their sub-cellular localization have helped refine previous auxin-transport models and improved our understanding of how changes at cellular level regulate organ-scale auxin patterns. For example, Band et al. They concluded that both auxin influx and efflux carriers are required to create a pattern of auxin distribution in the root tip. More recently, Moore et al. Their model predicts that the localization of influx carriers can either get more polar when auxin efflux carrier levels are changed or modulate efflux carrier level and polarity to maintain the auxin patterns.

In the past two decades, there has been a significant increase in our understanding of molecular basis of auxin transport and roles of auxin transporters in plant development. Particularity, there has been a better understanding of auxin influx carriers and how they play crucial roles in almost all aspects of plant growth and development. More advanced computer models and high-resolution imaging and segmentation approaches have proved crucial in providing better understanding of auxin influx carriers in pattern formation especially how changes at the cellular scale affect organ-scale auxin patterns.

Root development is very plastic and respond to their environment. Recently it has been shown that chromium inhibits primary root growth by regulating cell cycle genes. Chromium toxicity can cause major damage to crop yield. Genetic and physiological studies show a role for AUX1 in chromium mediated inhibition of root growth Wakeel et al.

Similarly, as stated above, AUX1 has also been implicated in Al and Cd mediated inhibition of root growth. Similarly, low P mediated root hair elongation response is mediated via AUX1 but early events are not well understood as to how low P status is sensed by the plants. Further understanding of the early events will be crucial for our understanding of the root growth and development in changing environment and may help develop predictive models for future crop improvement programmes. Alternative splicing is another key area that has not been explored much in the plants but may be crucial for better understanding of the role of alternatively spliced transcripts in regulating and shaping plant development.

It appears that alternative splicing in plants is more common than previously appreciated Li et al. With the advancement in sequencing technology, longer reads and single cell transcriptome, it is possible now to get a much better view of the cellular transcriptome and what if any is the role of alternative spliced transcripts in regulating auxin transport.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Agami, R. Exogenous treatment with indoleacetic acid and salicylic acid alleviates cadmium toxicity in wheat seedlings. Abbas, M. Differential growth at the apical hook: all roads lead to auxin. Plant Sci. Aloni, R. Role of cytokinin and auxin in shaping root architecture: regulating vascular differentiation, lateral root initiation, root apical dominance and root gravitropism.

Ann Bot Lond. Bainbridge, K. Auxin influx carriers stabilize phyllotactic patterning. Genes Dev. Band, L. Systems analysis of auxin transport in the Arabidopsis root apex. Plant Cell 26, — Root gravitropism is regulated by a transient lateral auxin gradient controlled by a novel tipping point mechanism. Barbez, E. A novel putative auxin carrier family regulates intracellular auxin homeostasis in plants.

Nature , — Bates, T. Stimulation of root hair elongation in Arabidopsis thaliana by low Pi availability. Plant Cell Environ. Local, efflux-dependent auxin gradients as a common module for plant organ formation. Bennett, M. Arabidopsis AUX1 gene: a permease-like regulator of root gravitropism.

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Blakeslee, J. Plant Cell 19, — Blilou, I. The PIN auxin efflux facilitator network controls growth and patterning in Arabidopsis roots. Nature , 39— Bosco, C. The endoplasmic reticulum localized PIN8 is a pollen specific auxin carrier involved in intracellular auxin homeostasis.

Plant J. Brady, S. Bridge, L. Math Biosci. Brunoud, G. A novel sensor to map auxin response and distribution at high spatio-temporal resolution.

Carrera, E. Plant Physiol. Casanova, J. Camalexin is a characteristic phytoalexin of Arabidopsis and is synthesized only under inducing conditions such as infection by plant pathogens. CYP79B2 is induced in response to infection by pathogens Hull et al. On the other hand, the transgenic Arabidopsis lines overexpressing the CYP79B2 gene have significantly elevated levels of indole glucosinolates and IAN Mikkelsen et al.

These findings indicate that CYP79B is involved in indole glucosinolate and camalexin biosynthesis Mikkelsen et al. IAN has also been proposed as an intermediate in IAA biosynthesis via indole glucosinolate metabolism. It was thought that IAN was an enzymatic breakdown product of indole glucosinolate, induced upon tissue damage Halkier and Gershenzon, However, Nafisi et al.

IAN has not been detected in rice, maize, tobacco, or pea Quittenden et al. IAN has been recognized as a phytoalexin, an inducible metabolite involved in defence responses against fungal attack, in Brassica juncea Pedras et al.

Although not widely recognized, this view has changed considerably in recent years Piotrowski, However, these three nitrilases have been found to have a strong substrate preference towards phenylpropionitrile, allylcyanide, phenylthio acetonitrile, and methylthio acetonitrile Vorwerk et al. IAN hydrolysis by these nitrilases in vitro is inefficient. The preferred substrates are either naturally occurring substrates, which may originate from glucosinolate breakdown, or close relatives of these.

AtNIT4 may represent an important detoxification mechanism in A. How should this be interpreted? ZmNIT2 showed high activity toward IAN, 3-phenylpropionitrile, allylcyanide, methylthioacetonitrile, and 4-phenylbutyronitrile, which was hydrolysed most rapidly Park et al. Phylogenetic analysis of the deduced amino acid sequences of NIT2 proteins. Nitrilases belonging to the AtNIT4 family may have a different and more general function and are apparently not associated with IAA biosynthesis Piotrowski et al.

Thus, recent works have shown that nitrilases are involved in the process of cyanide detoxification, in the catabolism of cyanogenic glycosides, and presumably in the catabolism of glucosinolates. The genetic, enzymatic, and metabolite-based evidence indicated that TAA and YUC families function in the same auxin biosynthetic pathway in Arabidopsis.

To test which compound is mainly produced from Trp in Arabidopsis , estradiol-inducible TAA1 overexpression TAA1ox plants were used in feeding experiments. These results, together with the evidence that TAA1 protein possesses Trp aminotransferase activity Tao et al. L -Kynurenine Kyn , which inhibits ethylene responses by decreasing ethylene-induced auxin biosynthesis in A.

In contrast, the yuc1 yuc2 yuc6 triple mutants had an elevated content of IPA Mashiguchi et al. Interestingly, the yuc1 yuc4 wei8 tar2 quadruple mutants did not make any hypocotyls and roots, although the juvenile plants of wei8 tar were similar to plants of yuc1 yuc4 mutants Won et al.

Cheng et al. Overexpression of the iaaM gene led to the typical auxin overproduction phenotypes in both wild-type and wei8 tar2 mutants of Arabidopsis Won et al. Together with the results showing that the iaaM gene also partially rescued the defects of wei8 tar2 phenotypes at juvenile and adult stages, the authors indicated that YUC genes and iaaM genes probably use different mechanisms for auxin biosynthesis in Arabidopsis Won et al.

Plants would be expected to share evolutionarily conserved core mechanisms for auxin biosynthesis because IAA is a fundamental substance in the plant life cycle, although different plant species may have unique strategies and modifications to optimize their metabolic pathways. In the IAM pathway, indoleacetamide hydrolase, encoded by the AMI1 gene, is widely distributed in the plant kingdom.

The IAOX pathway is a Brassicaceae species-specific pathway that may be involved in the synthesis of plant secondary metabolites, such as indole glucosinolates and the alkaloid camalexin. For the next step in advancing our understanding, it must be revealed whether or not the TAA1—YUC pathway is widely distributed in the plant kingdom.

Additionally, the gene s functioning in IAM biosynthesis must be identified. By analysing the expression of the IAM biosynthesis gene s , together with the AMI1 gene, it will be possible finally to determine how, when, and where auxin is synthesized in plants. Google Scholar. Google Preview. Oxford University Press is a department of the University of Oxford.

It furthers the University's objective of excellence in research, scholarship, and education by publishing worldwide. Sign In or Create an Account. Sign In. Advanced Search. Search Menu. Article Navigation. Close mobile search navigation Article Navigation. Volume Article Contents Abstract.

Multiple pathways postulated for auxin biosynthesis in plants. Tryptophan is synthesized in the chloroplast. The indoleacetamide pathway. The indolepyruvic acid pathway. The tryptamine pathway.

The indoleacetaldoxime pathway. Conclusions and perspectives. The pathway of auxin biosynthesis in plants. E-mail: y-mano wing. Oxford Academic. Keiichirou Nemoto. Revision received:. Select Format Select format. Permissions Icon Permissions. Abstract The plant hormone auxin, which is predominantly represented by indoleacetic acid IAA , is involved in the regulation of plant growth and development.

Auxin , auxin biosynthesis , IAA , indoleacetaldehyde , indoleacetaldoxime , indoleacetamide , indoleacetic acid , indolepyruvic acid , plant hormone. Open in new tab Download slide. Table 1. Plant genes thought to be involved in IAA biosynthesis.

Arabidopsis thaliana Yamada et al. Arabidopsis thaliana Stepanova et al. Arabidopsis thaliana Mashiguchi et al. Ophiorrhiza pumila Yamazaki et al. Oryza sativa Ueno et al. Oryza sativa Kang et al. AtYUC1 Flavin monooxygenase-like enzyme? Arabidopsis thaliana Zhao et al. Petunia hybrida Tobena-Santamaria et al. Oryza sativa Yamamoto et al. Zea mays Gallavotti et al. Solanum lycopersicum Tivendale et al. Solanum lycopersicum Exposito-Rodriguez et al.

Pisum sativum Tivendale et al. Zea mays Park et al. Open in new tab. Table 2. Abbreviations used in the phylogenetic tree. Partial purification of an enzyme hydrolyzing indoleacetamide from rice cells. Google Scholar Crossref. Search ADS. The presence of CYP79 homologues in glucosinolate-producing plants shows evolutionary conservation of the enzymes in the conversion of amino acid to aldoxime in the biosynthesis of cyanogenic glucosides and glucosinolates.

CYP83B1, a cytochrome P at the metabolic branch point in auxin and indole glucosinolate biosynthesis in Arabidopsis. Differential regulation of an auxin-producing nitrilase gene family in Arabidopsis thaliana. Cloning and expression of an Arabidopsis nitrilase which can convert indoleacetonitrile to the plant hormone, indoleacetic acid. Molecular characterization of two cloned nitrilases from Arabidopsis thaliana : key enzymes in biosynthesis of the plant hormone indoleacetic acid.

Anthranilate synthase from Ruta graveolens. Duplicated AS alpha genes encode tryptophan-sensitive and tryptophan-insensitive isoenzymes specific to amino acid and alkaloid biosynthesis. The TR-DNA region carrying the auxin synthesis genes of the Agrobacterium rhizogenes agropine-type plasmid pRiA4: nucleotide sequence analysis and introduction into tobacco plants. Auxin biosynthesis by the YUCCA flavin monooxygenases controls the formation of floral organs and vascular tissues in Arabidopsis.

Auxin synthesized by the YUCCA flavin monooxygenases is essential for embryogenesis and leaf formation in Arabidopsis. A plant outer mitochondrial membrane protein with high amino acid sequence identity to a chloroplast protein import receptor. Agrobacterium rhizogenes inserts T-DNA into the genome of the host plant root cells.

Cloning characterization of iaaM , a virulence determinant of Pseudomonas savastanoi. Google Scholar PubMed. Biosynthesis of indoleacetic acid in tomato shoots: measurement, mass spectral identification and incorporation of 2H from 2H2O into indoleacetic acid, D - and L -tryptophan, indolepyruvate and tryptamine. Molecular cloning and sequence analysis of an Azospirillum brasilense indolepyruvate decarboxylase gene. De Luca.

Molecular cloning and analysis of cDNA encoding a plant tryptophan decarboxylase: comparison with animal dopa decarboxylases.

Indoleacetonitrile production from indole glucosinolates deters oviposition by Pieris rapae. Di Fiore. Targeting tryptophan decarboxylase to selected subcellular compartments of tobacco plants affects enzyme stability and in vivo function and leads to a lesion-mimic phenotype. Plant aromatic L -amino acid decarboxylases: evolution, biochemistry, regulation, and metabolic engineering applications. Sekiguchi lesion gene encodes a cytochrome P monooxygenase that catalyzes conversion of tryptamine to serotonin in rice.

Multiple regions of a divergent promoter control the expression of the Agrobacterium rhizogenes aux1 and aux2 plant oncogenes. Expression of Agrobacterium rhizogenes auxin biosynthesis genes in transgenic tobacco plants. Camalexin is synthesized from indoleacetaldoxime, a key branching point between primary and secondary metabolism in Arabidopsis. New insight into the biosynthesis and regulation of indole compounds in Arabidopsis thaliana. An auxin-dependent distal organizer of pattern and polarity in the Arabidopsis root.

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Sztein, A. Indoleacetic acid biosynthesis in isolated axes from germinating bean seeds: the effect of wounding on the biosynthetic pathway. Plant Growth Regul. Chen, L. YUCCA-mediated auxin biogenesis is required for cell fate transition occurring during de novo root organogenesis in Arabidopsis.

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