Where is auxin produced in plants

There is no clear answer to this question yet, even though the presence of auxin and auxin response have been reported in algae [ 18 ]. Without genomic, genetic, and biochemical investigations, it is not possible to tell if such responses are based on conserved mechanisms.

However, auxin response is clearly ubiquitous in all flowering plant species investigated, with Arabidopsis , maize, and rice being focal species. Recently, it has been found that a very similar auxin response pathway operates in the earliest diverging land plants, the liverworts and mosses. Rather strikingly, while the moss Physcomitrella patens still has some degree of genomic complexity in its NAP [ 19 ], the liverwort Marchantia polymorpha appears to have a nearly minimal set of NAP components [ 20 ].

Thus, auxin response has an ancient history but critical questions remain unanswered as to the origin of auxin response and how different sets of genes have become auxin-dependent during plant evolution. So far, only orthologs of auxin signaling that are not part of the NAP have been found in algae [ 18 ] and nothing is known about the emergence and evolution of the NAP components.

Hopefully, the steady release of genome or transcriptome [ 21 ] sequences thanks to new sequencing technologies will help to answer these questions. Many signaling molecules have been identified in the animal kingdom, as well as in prokaryotes.

These terms are often used in the literature to describe auxin, but it is fair to ask whether they are suitable. It is important to note that both hormone and morphogen concepts have very specific connotations in the animal kingdom.

In simple terms, a hormone is usually produced in a specific tissue, often transported e. Although auxin may act at low concentrations and can be transported, it is not produced in a specific tissue. Auxin may also be too pleiotropic to be considered a hormone.

Indeed, the number of genes affected by auxin is high and varied, depending on the tissue, and the developmental processes in which it is involved are diverse, as are the ways auxin affects these processes.

Thus, it is not possible to attribute a specific function to auxin. In fact, auxin rather appears to be a signal that triggers a pre-set system than a hormone with a specific function [ 22 ].

The same issues arise when considering auxin as a morphogen. A morphogen is a compound that can modify cell specification pattern formation in a concentration-dependent manner, with different concentrations leading to different outputs. Auxin response can be found in what could be considered gradients [ 3 , 23 ] and this has led to speculation that auxin may act in a concentration-dependent manner to instruct cells along those gradients.

While attractive, the evidence supporting a morphogen-like action is limited. If auxin concentration alone can instruct cells, then the capacity to respond should be uniform. This is clearly not the case, as all components of the NAP vary in their expression patterns and can, therefore, confer distinct downstream effects on cells. Furthermore, while dose-dependent action is conceivable, it has not yet been shown that cells can respond in unique ways to specific concentrations.

Finally, the auxin pathway is subject to intense feedback regulation or cross-talk with other signaling pathways, which makes linking concentrations to output problematic. To conclude, the precise definition of auxin is still complicated due to its pleiotropic effects. Or maybe we should not care about the terminology and appreciate the molecule for the plant growth substance it is. Because of its potent impact on cell division, cell growth, and differentiation, auxin is very commonly used for artificially controlling plant growth.

The most common use of auxin in our daily life is in growing plants from cuttings. Gardeners often use a powder to stimulate root proliferation; this is essentially auxin at low concentration.

As ever, the dose makes the poison and, at high concentrations, synthetic auxins like 2,4-D are used as herbicides to which dicotyledonous plants are much more sensitive than monocotyledonous plants. As auxin induces cell division at physiological concentrations, it can be used in a balanced cocktail with another growth regulator, cytokinin, to promote cell proliferation in cell culture or in vitro propagation.

This knowledge allowed, for instance, the emergence of low cost orchids and virus-free potatoes. Detailed knowledge of the mechanism of action of auxin has recently also led to powerful new technology in non-plant laboratories.

Finally, considering the key role of auxin in plant development, understanding how auxin works will help in elucidating how fundamental developmental processes are controlled. Besides the fascination in revealing how a complex living organism is wired, one can conceive ways to engineer plant development using knowledge gained from auxin-dependent processes.

But this is one idea among many that this fascinating molecule may lead us towards in the future. Abel S, Theologis A. Odyssey of auxin. Cold Spring Harb Perspect Biol. Sites and regulation of auxin biosynthesis in Arabidopsis roots. Plant Cell. An auxin gradient and maximum in the Arabidopsis root apex shown by high-resolution cell-specific analysis of IAA distribution and synthesis.

Auxin transport routes in plant development. Article PubMed Google Scholar. Superroot, a recessive mutation in Arabidopsis, confers auxin overproduction. TAA1-mediated auxin biosynthesis is essential for hormone crosstalk and plant development. Requirement of the Auxin polar transport system in early stages of Arabidopsis floral bud formation. Perrot-Rechenmann C. Cellular responses to auxin: division versus expansion.

Weijers D, Wagner D. Transcriptional responses to the auxin hormone. Annu Rev Plant Biol. Recapitulation of the forward nuclear auxin response pathway in yeast. Up in the air: untethered factors of auxin response. Google Scholar. Auxin perception: in the IAA of the beholder. Physiol Plant. It is possible that these functions of IAA explain the mouse phenotype of the mutant. It is also possible that the phenotype of the ipdC mutant in the mouse may be—at least partially—independent of IAA and rather dictated by other functions of IpdC.

For example, while indolpyruvate decarboxylases are generally known to have high affinity for indolpyruvate, they can also use phenylpyruvate, pyruvate and benzoylformate as substrates.

It is, therefore, possible that IAA production is not the main, but rather incidental, role for these enzymes in some organisms Duca et al. The conservation of ipdC in many bacterial animal pathogens that behave as opportunistic plant colonists and, reversely, in epiphytic bacteria that opportunistically colonize animal tissue, provides new incentives to gain insight into the function of this plant hormone in a larger biological context.

Results of mouse experiments are described. MdM designed and conducted mouse experiments. The other 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.

We thank M. Farias for assistance with plant colonization experiments Figure 3C , M. Hoffman for animal infections. We are grateful to Dr. Bensmihen, S.

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Oberto, J. SyntTax: a web server linking synteny to prokaryotic taxonomy. BMC Bioinformatics Ongeng, D. These mutants are of particular interest because of the agronomic importance in terms of their ability to resist to lodging and to dramatically enhance the harvest index of the plant. Thus dwarfing traits are important due to the potential distribution of nutrients and energy to grain production rather than vegetative growth.

Given that br2 , which has a defect in ZmABCB1 , causes the stunting of lower internodes mostly, it raises the possibility that other brachytic mutants may arise from defects in other ABCB transporters. The br2 maize mutant shows dramatically impacted stalk architecture.

The br2 adult plant shows altered stalk height due to reduction in internode length, which is caused by the disruption of IAA transport mediated by ZmABCB1. The existence of auxin importers in plants was first demonstrated studying the Arabidopsis auxin insensitive 1 aux1 mutant, which carries defects in roots gravitropic response. AtAUX1 encodes a protein similar to fungal amino acid permeases and is expressed in columella, lateral root cap, epidermis, and stele tissues of the primary root where it acts as an auxin importer Bennett et al.

The coordinated action of these two proteins forms a lateral auxin gradient which inhibits the expansion of epidermal cells on the lower side of the root relative to the upper side, eventually causing the downward root curvature Swarup et al. AtLAX3 is expressed in the columella and stele of the primary root and it is involved in lateral root development, as Arabidopsis lax3 mutants show delayed lateral root emergence Swarup et al. No root growth—related defects or lateral root—related defects are observed in either lax1 or lax2 single mutants while aux1lax3 double mutant shows a severe reduction in the number of emerged lateral roots Swarup et al.

Northern blot experiments show expression in the tips of primary, lateral, seminal, and crown roots. In situ hybridization shows that ZmAUX1 expression is tissue-specific and confined to the endodermal and pericycle cell layers of the primary root, as well as to the epidermal cell layer Hochholdinger et al. Transcriptome analysis indicates a role for ZmAUX1 in leaf primordia differentiation, although evidence is still not conclusive Brooks et al.

The corresponding proteins present a highly conserved core region with 10 predicted transmembrane helices and their transcript levels are higher in leaves and stems rather than in roots and inflorescence tissues Shen et al.

PILS regulate intracellular auxin accumulation at the ER and thus reduce the availability for free auxin that can reach the nucleus, possibly exerting a role in auxin signaling that is comparable to that of AtPIN5 Barbez et al. The PILS family is conserved throughout the plant lineage, having representatives in several taxa including unicellular algae, where PINs have not been found yet. Forestan et al.

Expression analysis for these genes shows that they are ubiquitously expressed and differentially up-regulated in maize organs. Bootstrap values higher than 60 are indicated at each node. Auxin has a fundamental role in plant organs formation and its polar transport across cellular membranes is crucial for the correct development and response to external stimuli.

Master regulators of PAT are auxin transport proteins, which have been extensively studied in Arabidopsis but not in other species, mainly due to the difficulty to obtain loss-of-function mutants.

In monocots, only a few of these transporters have been characterized, mainly in rice and maize and most of the information available has been obtained by expression analyses without functional characterization. There are substantial divergences in development and plant structure between monocots and dicots.

Differences are present in seed, vascular system, and leaf developmental programs Tsiantis, ; Scarpella and Meijer, ; Coudert et al. The monocot root system architecture and cellular organization also differ considerably from those of dicots Hochholdinger et al.

In addition, monocots have a segmented stem as opposed to the unsegmented stem of dicots. Auxin transporter families are larger in monocots allowing for the possibility of functional redundancy, but also for neo- and sub-functionalization of specific proteins.

Monocot-specific and organ-specific proteins exist and they have a distinct role in regulating auxin driven organ development PIN9. In some cases, alterations in PAT result in interesting new traits, such as dwarfism in maize and sorghum br2 and dw3 mutants respectively, which can be exploited to generate more productive lines through breeding programs.

Moreover, many more short-statured mutants exist in maize that may have defects in auxin transport, although none of these mutants have been characterized in any detail. Interestingly, quite a few of these mutants exhibit dominant inheritance Johal, unpublished that makes them interesting in at least two ways. First, they can help side step gene redundancy problems and allow the functional exploration of additional genes.

Second, they can be used in MAGIC mutant-assisted gene identification and characterization -based enhancer suppressor screens to unveil natural variation in a trait of interest Johal et al. Even transgenic reporters for auxin activity can be used in lieu of bona fide dominant mutants for such MAGIC screens.

One such resource already exists in sorghum, where a line carrying a dw3 mutation in SbABCB1 was EMS mutagenized to produce and sequence M2 populations for both forward and reverse genetics Krothapalli et al.

These M2 populations can be screened to identify other genes in the network with the ability to suppress or enhance the dw3 phenotype. Technologies like this can be used to alter the expression and function of genes encoding auxin transporters in monocots and this may lead to important new breakthroughs in our understanding of their roles in development and response to the environment.

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