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Recently, much progress has been made in our understanding of how plants use phytohormones to integrate these environmental signals with endogenous growth and developmental programmes Zhu, ; Halliday et al. It has been proposed that abscisic acid ABA , auxin, and cytokinins CKs act to coordinate demand and acquisition of nitrogen Signora et al.

Regulation of nitrogen acquisition involves modulation of nitrate uptake systems, and proliferation of lateral roots Zhang and Forde, ; Forde and Walch-Liu, In general, nitrate uptake systems consist of low- and high-affinity nitrate transporters encoded by NRT1 and NRT2 family genes, respectively Miller et al.

The expression of NRT genes is regulated by numerous signals. For instance, AtNRT2. Lateral root outgrowth is a complex developmental process regulated by various signals, including nitrate, nitrogen assimilation products, ABA, auxin, and CKs reviewed in Forde, ; Walch-Liu et al. Here, the roles of phytohormones in the regulation of nitrate uptake systems and lateral root proliferation in response to changes in nitrogen availability are discussed, together with new data and recent progress.

CKs are a class of phytohormones implicated in many aspects of plant growth and development Mok and Mok, ; Sakakibara, , including nitrogen signalling. A CKā€”nitrogen link has been indicated by findings that nitrogen supply and CK content are closely correlated in barley Samuelson and Larsson, , tobacco Singh et al. Nitrogen supplementation causes an increase in CK content in xylem sap also in roots and shoots of maize Takei et al. A similar association has been reported in Arabidopsis Takei et al.

Furthermore, it was detected that Arabidopsis seedlings grown on high concentrations of nitrate HN, 10 mM contain higher levels of CKs than those grown on low nitrate LN, 0. Recent evidence consistently indicates that CKs also act as a local signal or as a shoot-to-root long-distance signal Miyawaki et al. Because the role of CKs as a root-to-shoot long-distance signal has been well reviewed Sakakibara et al.

Shoots and roots were harvested separately, and phytohormone analyses were conducted as described by Kojima et al.

Hormone Metabolism and Signaling in Plants - 1st Edition

The full list of phytohormonal compounds and the quantification results are provided in Supplementary Table S1 at JXB online. Until the identification of genes encoding adenosine phosphate-isopentenyltransferase IPT , which catalyses the initial step of CK biosynthesis, it was believed that CKs are synthesized in roots Letham, In Arabidopsis , IPT is encoded by seven genes that are differentially expressed in various tissues, indicating that CK production is not confined to roots Miyawaki et al.

Among these seven genes, AtIPT3 is nitrate inducible. Interestingly, nitrate-inducible expression of AtIPT3 was also observed in detached shoots Miyawaki et al. Similarly, nitrogen supplementation induces CK accumulation in detached sunflower and tobacco leaves Salama and Wareing, ; Singh et al.

AtIPT3 is expressed in phloem throughout the plant Miyawaki et al. However, in shoots, NRT1.

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Therefore, whether or not NRT1. Thus, either the iP- or cZ-type CKs, or possibly both, could be the shoot-to-root long-distance signal in Arabidopsis. Grafting experiments using a higher order atipt mutant atipt1;3;5;7 have provided unequivocal evidence that iP-type CKs are translocated from the shoot to the root Matsumoto-Kitano et al. The atipt1;3;5;7 mutant is characterized by extremely low iP- and tZ-type cytokinin levels, retarded shoot growth, and enhanced lateral root outgrowth. When a wild-type shoot was grafted onto the atipt1;3;5;7 mutant root, the normal growth phenotype and levels of iP-type CKs were restored in the mutant root, indicating that iP-type CKs translocated from the shoot are biologically functional Matsumoto-Kitano et al.

Notably, the expression of AtIPT3 is also regulated by iron, phosphate, and sulphate availability, both in shoots and in roots Hirose et al. It could be that AtIPT3 functions as an integrator of nutrient availability signals. CKs produced locally within the root, or translocated from the shoot may signal that there is sufficient nitrogen present. In this regard, one of the proposed roles of CKs is negative regulation of nitrogen uptake-related genes.

The repression was observed under both HN Fig. On the other hand, no such consistent repression was observed with other phytohormones, implying that the effect is CK specific Fig. Amongst the root-type genes, AtNRT2. These results support the hypothesis that CKs act as a satiety signal of nitrogen to inhibit nitrate uptake in the root. Similarly, it has been reported that CKs negatively regulate other nutrient acquisition-related genes in Arabidopsis , such as high-affinity phosphate transporter genes Pht1;2 and Pht1;4 ; Martin et al.

Similar regulation was also reported in rice Hirose et al.


The cre1 ahk3 double mutant Higuchi et al. In contrast, the hextuple mutant arr3,4,5,6,8,9 ; To et al. It has been reported that CK-repressive nutrient acquisition-related genes are largely expressed in root epidermal or cortical cells. Thus, it is possible that CKs specifically target those cell types. However, this assumption may be an oversimplification, because it was found that AtNRT1. The effect of phytohormones on the expression of nitrogen uptake-related genes in Arabidopsis.

The effect of phytohormones on the expression profiles of AtNRT genes in roots. A Transcript level of each AtNRT in phytohormone-treated seedling roots grown on high nitrate plates HN, 10 mM relative to that in non-treated seedlings. Wild-type seedlings were grown vertically on HN plates modified from Fujiwara et al. Each experiment was performed twice and mean values are shown on a log 2 scale. Asterisks indicate that the values are of no statistical significance because the expression level in one of the samples was below the detection limit.

A colour code is used to visualize data, and samples that were expressed below the detection limit are shown in grey ND. Seedlings were grown for 8 d on HN plates, and then treated on LN plates supplemented with trans -zeatin for 3 d, as indicated. All quantifications were standardized with AtACT8 transcript levels as an internal standard. Relative values indicate comparison with the transcript level of dimethylsulphoxide DMSO -treated roots. Error bars represent standard deviations of three technical replicates. The experiment was performed twice with similar results.

Schematic representation of the interaction between nitrogen and phytohormones abscisic acid, auxin, and cytokinin in the regulation of nitrogen acquisition. Arrows and blunted lines designate positive and inhibitory interactions, respectively.

Solid lines represent defined interactions; dashed lines indicate presumed interactions. Recent studies have assigned physiological functions to the shoot-type genes AtNRT2.

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  4. Considering the evidence that transcript abundance of these shoot-type genes in roots is much lower than that of root-type genes data not shown , it is unlikely that shoot-type AtNRT genes play a major role in nitrate uptake. Although clarification of the function of shoot-type AtNRT genes in roots awaits further analysis, a plausible explanation is that shoot-type AtNRT genes are similarly induced in shoots by CKs and enhance nitrate distribution and translocation.

    CKs also have a regulatory role in root architecture development. Many reports describe the inhibitory effects of exogenous CKs on lateral root formation Wightman et al. In contrast, mutants and transgenic plants with reduced CK levels, such as higher order atipt mutants Miyawaki et al. CKs act at both initiation and organization of the lateral root primordium LRP , most probably through perturbation of the auxin gradient, to inhibit LRP formation Laplaze et al.

    In general, a uniformly high nitrogen supply suppresses root branching, while nitrogen limitation accelerates root growth and root branching Linkohr et al. Given that nitrogen status and CK content are closely correlated Table 1 , and Supplementary Table S1 at JXB online , it is very likely that CKs have a role in regulating root architecture in response to nitrogen availability.

    However, there is as yet no solid evidence to support this. Now that a number of CK signalling and biosynthesis mutants are available, questions about the CK regulation of root architecture development can be answered. ABA is generally known as a stress hormone involved in abiotic and biotic stress responses. Although there is considerable evidence linking ABA levels and nitrogen status in several plant species Radin et al.

    Thus, whether changes in ABA content are relevant to nitrogen signalling is still unclear, but involvement of ABA in nitrogen signalling is becoming increasingly evident. Several reports provide genetic evidence for the involvement of ABA in lateral root development in response to high nitrate supply in Arabidopsis. Signora et al. A set of mutants identified based on their ability to produce lateral roots in the presence of ABA labi mutants shows reduced sensitivity to the inhibitory effects of high nitrate Zhang et al.

    Identification of the labi genes would provide a breakthrough toward understanding the mechanisms underlying this inhibition effect. A recent study in a Medicago truncatula latd mutant provides another line of evidence for a link between ABA and nitrogen signalling Yendrek et al. The latd mutant is characterized by severe defects in root meristem maintenance and root growth, which is rescued by exogenous ABA application Bright et al.

    Auxin has long been a candidate for mediating nitrogen signals from shoot to root because auxin is transported basipetally, and enhances lateral root initiation and development Forde, ; Fukaki and Tasaka, Auxin contents in phloem sap and roots are lower in maize supplied with high doses of nitrate, and this reduction is correlated with reduced root growth Tian et al. Similarly, transfer from high nitrate media to low nitrate media has been shown to increase auxin contents in roots, which is followed by lateral root outgrowth in Arabidopsis Walch-Liu et al.

    Furthermore, it has been confirmed that Arabidopsis seedlings grown in LN conditions contain higher levels of root auxin than seedlings grown in HN conditions Table 1 , and Supplementary Table S1 at JXB online , indicating that dicots and monocots share a common system to regulate auxin levels in the root depending on the nitrogen status of the plant. However, it was also reported that the inhibitory effect of high nitrate on lateral root growth is not alleviated by exogenous application of auxin, indicating that the auxin content is not the only factor regulating lateral root development Zhang et al.

    Recent progress in understanding auxin action indicates that, in addition to auxin levels in the tissue, a concentration gradient and the differential sensitivity of various cell types are the driving force of auxin-regulated growth and development Overvoorde et al. The auxin gradient is established by cell-to-cell polar transport, and differential sensitivity is accomplished by modulation of signalling components Overvoorde et al. Several lines of recent evidence suggest that nitrogen signalling is mediated by these same, or similar mechanisms Gifford et al.

    Using a cell sorting technique, Gifford et al. Although transcription of ARF8 is not regulated by nitrogen, miRa levels are under the control of glutamine or some downstream metabolite Gifford et al. Hence, in the presence of glutamine or a downstream metabolite, miRa levels in pericycle cells are down-regulated, thus permitting ARF8 transcript to accumulate in the cell. Recently, the auxin receptor gene AFB3 was found to be nitrate inducible Vidal et al.

    Biosynthesis, Signal Transduction, Action!

    The induction was also observed in a nitrate reductase NR -null mutant, suggesting that it is triggered by nitrate itself. Analysis of the afb3 mutant revealed that the mutant is insensitive to nitrate in the regulation of primary root growth and lateral root density, indicating that nitrate signal regulates root architecture through AFB3, possibly by modulating auxin signalling Vidal et al. Surprisingly, Krouk et al. Detailed analyses of an nrt1. Data obtained from transport assays in heterologous systems and in planta were consistent with this hypothesis.

    Furthermore, nitrate was shown to be an inhibitor of the auxin transport activity Krouk et al. Localization studies demonstrated that NRT1. Thus, apparently under low nitrate conditions, NRT1. In contrast, under high nitrate conditions the auxin transport activity of NRT1. It has long been proposed that there is an interaction between nitrogen signalling and phytohormone activity. Recent genetic studies have provided compelling evidence that ABA, auxin, and CKs are involved in nitrogen signalling Fig.

    Although it has been reported that there is a shared signalling pathway for BR and auxin Goda et al. Because BR promotes lateral root growth and development Mori et al. However, both the mechanism and physiological relevance of this regulation remain to be elucidated. Given the apparent involvement of multiple phytohormones in nitrogen signalling, one future challenge will be to understand how phytohormones interact to convey the nitrogen signal.

    A growing body of evidence shows that phytohormones interact not only with nitrogen but also with other nutrients. For instance, CKs interact with iron Seguela et al. Although this is an intriguing hypothesis, many questions remain to be answered. How is the nutritional status sensed and translated into phytohormone signals?

    In which cell, tissue, or organ does the sensing and translation occur? What is the nature of the nutrient-specific signal? How are the phytohormone and nutrient-specific signals transported to target sites? Where and how are the phytohormone signal and nutrient-specific signal perceived, and integrated to bring about a characteristic response to each nutrient? We would like to thank Dr T. Kakimoto for cre ahk , and Dr J. Kieber for arr3,4,5,6,8,9 mutant seeds. 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.

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