Research article
Suppression of Fe deficiency gene expression by jasmonate

https://doi.org/10.1016/j.plaphy.2011.01.025Get rights and content

Abstract

Fe deficiency genes are regulated in response to external supply of Fe as well as internal plant signals. Internal plant signals include plant hormones and systemic signals which coordinate shoot physiological requirements for Fe with local availability of Fe in roots. Induction of IRT1 and FRO2 gene expression can be used to monitor the Fe deficiency status of plant roots. Here, we investigated the role of jasmonate in the regulation of Fe deficiency responses and in the split root system. We found that jasmonate suppressed expression levels of IRT1 and FRO2 but not their inducibility in response to Fe deficiency. Analysis of the jasmonate-resistant mutant jar1-1 and pharmacological application of the lipoxygenase inhibitor ibuprofene supported an inhibitory effect of this plant hormone. Inhibition of IRT1 and FRO2 gene expression by jasmonate did not require the functional regulator FIT. By performing split root analyses we found that systemic down-regulation of Fe deficiency responses by Fe sufficiency of the shoot was not compromised by ibuprofene and in the jasmonate-insensitive mutant coi1-1. Therefore, we conclude that jasmonate acts as an inhibitor in fine-tuning Fe deficiency responses but that it is not involved in the systemic down-regulation of Fe deficiency responses in the root.

Highlights

► Jasmonate suppression of IRT1 and FRO2 gene expression but not of their inducibility in response to Fe deficiency. ► Jasmonate inhibition of IRT1 and FRO2 gene expression independent of the functional regulator FIT. ► Jasmonate action not related to systemic Fe sufficiency signalling to down-regulate Fe deficiency responses in the root.

Introduction

Plant roots are nearly constantly exposed to nutrient salts in their environment. Yet, the uptake profiles for nutrients vary diurnally and in response to various physiological and developmental stimuli. Regulation of nutrient uptake is necessary not only to ensure minimum uptake of the essential nutrients but also to avoid excessive uptake followed by potentially toxic effects. Plants sense the availability of nutrients in their local environment and their physiological diurnal and developmental needs for these nutrients [1].

Uptake of Fe can be used as a model system to study the role of plant internal signals related to nutrient regulation to investigate the underlying regulatory mechanisms. The Fe status of a wild type plant is reflected by the expression levels of marker genes. For example, increased expression of IRT1 and FRO2 in the root compared to a control condition indicates Fe deficiency [2], [3], [4]. Mutant studies showed that these two genes encode structural core components for Fe acquisition from the soil following the Fe reduction-based strategy I. FRO2 encodes the root plasmamembrane-bound ferric chelate reductase [3], while IRT1 codes for the divalent metal transporter for Fe uptake [5], [6], [7]. FRO2 and IRT1 were often found co-regulated [8]. Their expression is controlled by a bHLH transcription factor named FIT [9]. In the absence of a functional FIT protein the expression of FRO2 and IRT1 is far lower than in wild type and if not supplemented with Fe fit mutants develop a lethal leaf chlorosis [10], [11], [12]. Ethylene and nitric oxide positively affect expression of IRT1 and FRO2 suggesting that these two signals increase the sensitivity of plants for Fe uptake [13], [14], [15], [16]. Cytokinins on the other hand cause a down-regulation of the two genes [17]. Hormonal influence on Fe acquisition gene expression may serve to coordinate physiology and stress responses with necessary adaptations for altered root growth and uptake of Fe [18], [19], [20], [21]. Systemic signals controlling Fe uptake have been physiologically identified but their nature is not known. For instance, grafting of constitutive mutant Fe-deficient shoots to wild type roots can override any local Fe sufficiency sensing in the root and promote constitutive induction of Fe acquisition responses [22], [23]. On the other hand, Fe-sufficient shoots may block Fe uptake in Fe-deficient parts of split roots [4], [8], [18], [19].

Jasmonates are oxylipin-based plant hormones originating from poly-unsaturated fatty acids that act in response to developmental or environmental stimuli [24]. Environmental cues for activating the jasmonate signalling pathway include wounding, insect attack or UV light and as such jasmonate belongs to the so-called stress hormones. Jasmonate has an interesting property in that it is a systemically acting mobile plant hormone [25]. Progress has been made in identifying the jasmonate signalling pathway by thorough analysis of JA resistant and insensitive mutants [26]. Jasmonates are perceived intracellularly by a jasmonate receptor belonging to the F-box protein family that upon binding to the plant hormone targets a repressor of the jasmonate response pathway for degradation [27], [28], [29]. The activated jasmonate form most efficiently bound by the jasmonate receptor was found to be an Ile-conjugated derivative of jasmonic acid (JA-Ile, namely (+)-7-iso-JA-Ile) [30], [31]. Isoleucine conjugation to jasmonate is catalyzed by an enzyme encoded by the JAR1 gene [32], [33].

Here, we tested whether the plant hormone jasmonate had any effect on the regulation of Fe uptake responses, and whether jasmonate might be a candidate for a systemic signal involved in Fe deficiency regulation.

Section snippets

Gene expression analysis of Fe deficiency genes in response to jasmonate treatment

In search for mobile plant signalling compounds that influence the regulation of Fe acquisition and may represent candidates as systemic signals we tested the effect of jasmonate on the regulation of the IRT1 promoter using transgenic pIRT1::GUS plants [7]. We exposed two week-old Arabidopsis seedlings for 3 days to + or − Fe in the presence of 0 or 100 μM jasmonate, respectively. We found that in the absence of jasmonate GUS activity was induced four times in the root upon − Fe treatment (

Discussion

Here, we demonstrated that the mobile plant hormone jasmonate affected Fe deficiency gene expression. FRO2 and IRT1 were negatively regulated by jasmonate in a manner that was dependent on jasmonate-Ile signalling. Negative regulation of FRO2 and IRT1 by jasmonate did not require the FIT protein which is a central regulator of Fe deficiency responses although the FIT gene was partially found repressed by jasmonate. The role of jasmonate was not found to be related to systemic signalling of Fe

Plant material

The Arabidopsis thaliana accession used was Col-0. fit-3 loss of function mutant (hereafter named fit mutant) was verified due to the strong leaf chlorosis [9], [11]. The jar1-1 and coi1-1 mutants were obtained from the European Arabidopsis Stock Center, and the phenotype was verified by an in vitro jasmonate response root growth assay [32]. pIRT1::GUS plants were obtained from C. Curie [7].

Plant growth conditions

Arabidopsis seeds were surface-sterilized with 6% NaOCl, 0.1% Triton-X for 10 min, and washed 5 times

Acknowledgements

Funding by the Deutsche Forschungsgemeinschaft is greatly acknowledged. We thank Angelika Anna for assistance in plant growth.

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