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regulation of auxin biosynthesis in Arabidopsis roots.

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Interactions between plant hormones and heavy metals …

TAA genes and YUCs were previously placed in two independent auxin biosynthesis pathways (; ; ). However, some yuc mutant combinations display phenotypes similar to those observed in the taa mutant combinations. The phenotypic similarities between yuc and taa mutants suggest that TA As and YUCs may participate in the same auxin biosynthesis pathway (; ; ; ). Both wei8 tar1 tar2 triple mutants and yuc1 yuc4 yuc10yuc11 quadruple mutants develop embryos without the basal part (, ; ). Similar vascular and floral defects have also been observed in wei8 tar2 and yuc1 yuc4 double mutants () (, ; ). In fact, yuc mutants display all of the characteristic phenotypes of the taa mutants (). For example, yucQ mutants are resistant to ACC and NPA in root elongation, a characteristic phenotype of taa1 mutants (). The yuc1 yuc4 double mutants have altered shade avoidance responses, a phenotype that is also observed in taa1 mutants (). Interestingly, Arabidopsis plants appear to use different sets of YUC genes for auxin biosynthesis in the shoot and in the root despite using the same set of TAA genes in both shoots and roots ().

To cause growth in the required domains, auxins must of necessity be active preferentially in them

Cytokinins Are Synthesized also in the Aerial Part of the Plant. In an attempt to elucidate the site of cytokinin biosynthesis, Arabidopsis roots and shoots were separated and incubated in liquid growth medium enriched with 30% 2H2O. The in vivo deuterium labeling resulted in an isotopomeric cluster for root and shoot clearly demonstrating the capacity to synthesize ZMP in both types of tissues (). This observation, that the aerial part of the plant is a significant source of cytokinin synthesis, triggered a second experiment where we used tobacco to elucidate the localization of this source. We analyzed the pool sizes and synthesis rates of ZR and ZMP in detached leaves of different sizes: large leaves where expansion has ended, leaves in expansion phase, and small leaves with active cell division and only limited expansion growth. The results in clearly show that young developing leaves with active cell division have the highest capacity to synthesize ZMP and ZR, which also is reflected in the endogenous pool sizes. Both data from experiments with Arabidopsis and tobacco show the importance of aerial tissues for cytokinin biosynthesis and clearly establish developing leaves with active cell division as major sites for biosynthesis of Z type cytokinins.

that roots of young Arabidopsis plants ..

The concept of auxin to cytokinin concentration ratios controlling plant development has been around for many years, despite investigations demonstrating that auxins are known to regulate cytokinin pool sizes and vice versa (, ). Most of these studies are based on transgenic systems with constitutive overproduction in which normal regulatory mechanisms of the hormones are often overruled. We therefore decided to study the direct effect of auxin and cytokinin on each other's biosynthetic rate to investigate the timing of potential cross talk. For our studies, we made use of NAA with experimental conditions that is standard in auxin studies related to plant development (, ). NAA is the only auxin that easily penetrates the plasma membrane without the need for active uptake, thereby ensuring a rapid increase of NAA level inside the cells. This is not as straightforward for treatment with cytokinins, due to problems related to control of exact uptake. Thus for this hormone, we used an Arabidopsis line carrying the bacterial ipt gene under an inducible promoter. We were able to demonstrate that auxin rapidly suppresses both the pool size and synthesis rate of cytokinins. During incubation with NAA for 24 h, ZMP, ZR, and Z-9-glucoside levels were reduced, and in vivo deuterium incorporation experiments showed that application of the auxin rapidly reduced deuterium incorporation into ZMP and ZR already after 6 h ().

Auxin Regulates Cytokinin Biosynthesis via the iPMP-Independent Pathway. Arabidopsis has two major biosynthesis pathways for synthesis of cytokinins, the iPMP-dependent pathway and the alternative iPMP-independent pathway () (). By addition of metyrapone, an inhibitor of isopentenylhydroxylase, the two pathways were uncoupled, and biosynthesis in both pathways was measured in an independent manner. We incubated Arabidopsis seedlings for 12 h with and without 25 μM NAA together with 2 mM metyrapone and 30% 2H2O. Analysis of isotopomer clusters as tracer/tracee ratios revealed that the biosynthetic rate of ZMP, in contrast to iPMP, was significantly reduced after auxin treatment (one-tailed t test; P > 0.05) (). These data thereby demonstrate that NAA represses the synthesis of ZMP independent of the iPMP pathway. This finding was supported by the data in , showing reduced levels of Z type cytokinins, whereas the levels of iP-type cytokinins are less affected.

08/04/2004 · National Academy of Sciences

Auxin Rapidly Regulates the Rate of Biosynthesis and the Pool Size of Cytokinins. In the first experiment, Arabidopsis seedlings were treated with 25 μM NAA for 24 h, resulting in significantly reduced pool sizes of several of the major cytokinin intermediates and end products (). To measure the auxin effect on actual synthesis rate, a second experiment was performed in which seedlings were incubated in growth medium enriched with 30% 2H2O with 0 (control), 5, and 25 μM NAA, respectively. We used a range of concentrations in this study that have previously been demonstrated to be relevant for studies with exogenous auxin in plant development (, ). Within 3 h of incubation, labeling of the ZMP pool was observed, and a significant reduction of synthesis rate was clearly visible for both ZMP and ZR already 6 h after application of 5 μM NAA (). We thus confirmed rapid repression of cytokinin biosynthesis that occurred in a clearly dose-dependent manner, as demonstrated by the differences between the 5- and 25-μM NAA treatments.

Sites of Cytokinin Biosynthesis. The apical part of Arabidopsis plants grown on soft agar was separated from roots at the hypocotyls junction, and the different plant tissues were incubated separately in one-half of MS medium containing 1.5% sucrose and 30% 2H2O for 12 h. The rate of cytokinin biosynthesis was measured according to ref. . In a second experiment, leaves of 6-wk-old tobacco plants were analyzed for pool size and synthesis rate of ZMP and ZR. The leaves analyzed represent three different developmental stages: fully expanded, ≈20-cm length; middle size, ≈13-cm length; and small developing leaves, 2H2O. After incubation of detached leaves for 24 h, samples were harvested, and ZMP and ZR biosynthesis was measured.

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  • Auxin Biosynthesis | The Arabidopsis Book

    Auxin - Wikipedia

  • 13/06/2014 · The Arabidopsis Book 12: e0173 ..

    Plant Development-Introduction


    Cytokinin - Wikipedia

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Auxin Regulation of Cytokinin Pool Sizes and Biosynthetic Rate. For quantification of pool sizes, 3-wk-old Arabidopsis plants were transferred to one-half of MS medium containing 5 or 25 μM NAA (Sigma) and 1.5% sucrose. In vivo deuterium-labeling experiments were performed by incubating 3-wk-old plants grown as described above with one-half of MS medium/1.5% sucrose/30% 2H2O/0, 5, or 25 μM NAA.

The main auxin biosynthesis pathway in Arabidopsis

One of the most long-lived models in plant science is the belief that the long-distance transport and ratio of two plant hormones, auxin and cytokinin, at the site of action control major developmental events such as apical dominance. We have used in vivo deuterium labeling and mass spectrometry to investigate the dynamics of homeostatic cross talk between the two plant hormones. Interestingly, auxin mediates a very rapid negative control of the cytokinin pool by mainly suppressing the biosynthesis via the isopentenyladenosine-5′-monophosphate-independent pathway. In contrast, the effect of cytokinin overproduction on the entire auxin pool in the plant was slower, indicating that this most likely is mediated through altered development. In addition, we were able to confirm that the lateral root meristems are likely to be the main sites of isopentenyladenosine-5′-monophosphate-dependent cytokinin synthesis, and that the aerial tissue of the plant surprisingly also was a significant source of cytokinin biosynthesis. Our demonstration of shoot-localized synthesis, together with data demonstrating that auxin imposes a very rapid regulation of cytokinin biosynthesis, illustrates that the two hormones can interact also on the metabolic level in controlling plant development, and that the aerial part of the plant has the capacity to synthesize its own cytokinin independent of long-range transport from the root system.

These effects indicate that in pea roots, auxin at ..

Environmental signals have a profound effect on auxin biosynthesis. TAA1 was identified from a screen for mutants that were defective in shade avoidance responses, a light-mediated signaling process (). Shade avoidance responses require the phytochrome photoreceptors and the Phytochrome-InteractingFactors (PIFs), which are bHLH transcription factors (). Recent studies have shown that part of the light signaling response is to modulate auxin homeostasis. Some of the auxin biosynthetic genes are direct targets of the PIFs (). Under shade conditions, auxin levels in Arabidopsis are elevated more than 50% compared to the levels in plants grown under normal white light conditions (). The increased auxin concentrations are caused by the up regulation of the expression of several YUC genes (). PIF7 (AT5G61270) is one of the key transcription factors downstream of the photoreceptor phytochrome B. Disruption of PIF7 leads to short hypocotyls and expanded cotyledons under shade, suggesting that PIF7 plays a positive role in shade avoidance response. It is reported that PIF7 binds to the promoters of YUC2,YUC5, YUC8, and YUC9 and activates the expression of the YUC genes under shade conditions ().

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