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Nutrient- and other stress-responsive microRNAs inplants: Role for thiol-based redox signalingSanjib Kumar Pandaa & Ramanjulu Sunkaraa Department of Biochemistry and Molecular Biology; Oklahoma State University; Stillwater,OK USAAccepted author version posted online: 27 Apr 2015.

To cite this article: Sanjib Kumar Panda & Ramanjulu Sunkar (2015) Nutrient- and other stress-responsive microRNAs inplants: Role for thiol-based redox signaling, Plant Signaling & Behavior, 10:4, e1010916

To link to this article: http://dx.doi.org/10.1080/15592324.2015.1010916

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Nutrient- and other stress-responsive microRNAs in plants:Role for thiol-based redox signaling

Sanjib Kumar Panda and Ramanjulu Sunkar*Department of Biochemistry and Molecular Biology; Oklahoma State University; Stillwater, OK USA

Now it is well recognized that themicroRNA (miRNA) expression isaltered in response to internal (develop-mental or hormonal) or external stimulisuch as biotic and abiotic stresses inplants. It is also known that several miR-NAs are induced in response to deficiencyof specific nutrients within the plant orin the external sources, i.e., soil/nutrientmedia. For instance, P-deprivation indu-ces miR399, S-deprivation inducesmiR395 and Cu-deprivation inducesmiR398, miR397 and miR408 in severalplant species. Although the transcriptionfactors (PHR1, SLIM1 and SPB7) thatregulate these nutrient-deprivationinducible miRNAs have been identifiedbut the upstream biochemical pathwaythat activates them is relatively unknown.In a recent study, we demonstrated forthe first time that redox signaling plays acritical role in S-deprivation-induciblemiR395 expression in Arabidopsis. Inthis short review, we draw additionalhypotheses for the involvement of redoxsignaling and/or reactive oxygen species(ROS) signaling in inducing other nutri-ent or stress-responsive miRNAs inplants.

Being sessile organisms, plants mustadjust their metabolism and growth tochanging environment fairly quickly, assuch, stress sensing and signaling are mostcritical steps in this process.1 One of theearly responses to stress also includechanges in redox (reduction-oxidation)homeostasis (redox imbalance). Stress-induced redox imbalances initiate com-pensatory responses via redox signalingnetwork either to readjust redox homeo-stasis or to repair oxidative damage as wellas in modulating gene expression

associated with acclimation to stress.2 Oneimportant mechanism of redox-dependentsignaling is based on the reversible oxida-tion and reduction of crucial Cys residues,which is a well-conserved feature amongliving organisms.3 In this thiol-based sig-naling network, redox cues convert thecysteine thiols to disulfide, sulfenic, sul-finic and sulfonic acid, nitrosothiol or glu-tathionylated forms.4

In a stricter sense thiol redox regulationand ROS signaling differ despite theirintimate linkage through thiol peroxidasesand the Halliwell-Asada-cycle.5 Distin-guishing these 2 is challenging and, there-fore, these terminologies have been usedrather vaguely and interchangeably in theliterature. Intracellular thiol redox statusregulates a variety of cellular and molecu-lar events such as the activity of proteins,signal transduction, transcription and sev-eral other cellular functions. On the otherhand, besides inducing a shift in redoxbalance, oxidative stress can directly affectsenzyme activities, signaling, gene expres-sion, lipid peroxidation, and even celldeath. Thus, often the effects of redoxchanges overlap with those of oxidativestress, therefore, it is hard to distinguishwhether the observed changes in cellularresponses are due to ROS signaling (oxi-dative stress) or due to stress-inducedredox imbalance or both6.

In a cell, energy supply and consump-tion maintains the redox equilibrium andany imbalance between these processescould lead to redox imbalance. Undernormal conditions, feedback regulationbetween upstream and downstream activi-ties in a biochemical pathway adjusts thisredox equilibrium, but such maintenancewill often fail under stress conditionsresulting in more reduced or oxidized

Keywords: acclimation, arabidopsis,micrornas, oxidative stress, redox signaling

Sanjib Kumar Panda and Ramanjulu Sunkar*Correspondence to: Ramanjulu Sunkar; Email:ramanjulu.sunkar@okstate.edu

Submitted: 11/27/2014

Revised: 12/22/2014

Accepted: 01/05/2015

http://dx.doi.org/10.1080/15592324.2015.1010916

This is an Open Access article distributed under theterms of the Creative Commons Attribution-Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/), which permits unrestrictednon-commercial use, distribution, and reproductionin any medium, provided the original work is prop-erly cited. The moral rights of the named author(s)have been asserted.

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states of the NADP/NADPH.7 Thealtered NADPH redox milieu will feedinto the thiol protein redox network viaNADPH-dependent thioredoxin reduc-tases (NTRs) and NADPH-dependentglutathione reductases, which subse-quently modify the redox state of the tar-get proteins.4 Thus far, more than 400thiol proteins are known in plants that caninteract with thioredoxin or glutaredoxinor undergo major conformational changesupon shifting from a reduced to oxidizedstate.4 The target proteins vary widely intheir functions ranging from metabolismto signaling to translation andtranscription7.

Plant growth, development and adap-tation to biotic and abiotic stresses dependon an accurate regulation of gene expres-sion both in time and space. Recently thegenome-encoded miRNAs emerged asimportant regulators of gene expressionassociated with acclimation to stressincluding nutrient deprivation as theabundances of various miRNAs are alteredunder stress.8,9 In a few cases, even thetranscription factors (PHB, SPB, andSLIM1) that bind to the promoter regionsof nutrient-responsive miRNAs have beencharacterized. However the upstream sig-naling pathway that regulates/activatesthese transcription factors is completelyunknown. In our earlier report usingmutants partially defective in redox path-ways, redox signaling was shown to beresponsible for the induction of miR395under S deprivation.10 This was the firstcharacterization of upstream biochemicalpathway(s) involved in regulation of amiRNA expression in plants.

Changes in cellular redox propertieshave been observed under a variety ofbiotic, abiotic stresses including nutrientdeprivation. It has been reported previ-ously that the reduction potential wasincreased in spinach leaves subjected to S-and P-deprivation11 and N-deprivation.12

The readjustment of redox homeostasisfails when excessively available electronsare transferred to oxygen producing reac-tive oxygen species (ROS), establishingstrong electron sinks and thus inducingoxidative stress. The thiol redox homeo-stasis is maintained by NADPH-depen-dent thioredoxin reductases (NTRa, -band -c) - thioredoxin (TRX) and

glutathione reductase - GSH-dependentglutaredoxin (GRX) systems that modifytarget proteins facilitating stress responsesin plants. Compared to wild-type miR395levels, in single (cad2: g-glutamyl cysteinesynthetase involved in GSH biosynthesis),double (ntra/ntrb) and triple (ntra/ntrb/cad2) mutants grown under S- deprivedconditions approximately 20, 40 and 45%decrease in induction of miR395 levels,respectively, was noted suggesting the

involvement of redox signaling in miR395expression under S-deprivation.10 Onlypartial decrease even in double and triplemutants suggested potential redundancybetween the TRX and GRX systems inplants. Thus, Jagadeeswaran et al.10 estab-lished a link between redox signaling andthe SLIM1 transcription factor that regu-lates miR395 expression during S-depriva-tion in Arabidopsis. It could also beshown that miR395 is induced by arsenic

Figure 1. Generalized scheme for the involvement of redox signaling mediated regulation ofmiRNA expression and subsequent effect on their target gene expression in plants.

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and copper, both of which impose oxida-tive stress.10 Expression analysis ofmiR395 under excess arsenic and copperexposure in single (cad2), double (ntra/ntrb) and triple (ntra/ntrb/cad2) mutantsshould tentatively reveal whether or notmiR395 induction under oxidative stressis due to redox or ROS signaling.

Oxidative stress resulting from S-defi-ciency was confirmed by analyzingmiR398 and the target Cu/Zn superoxidedismutase (CSD1) expression, which canserve as markers for oxidative stress.10

miR398 expression is down-regulated bythe oxidative stress (ROS accumulation)in Arabidopsis13,14 but it was unknownwhether and to which extent redox signal-ing is involved in this process. miR398levels were almost unaffected in mutantsdefective in redox signaling when com-pared with wild type miR398 levels underS-deprivation.10 Therefore, only miR395expression is compromised in mutantsdefective in redox signaling but notmiR398 levels, which suggested that redoxsignaling controls the expression ofmiR395 but not miR398 under S-depri-vation. These findings argue in favor thatthe ROS signaling (oxidative stress) andredox signaling are distinct under S-deprivation.

Salt stress, drought, ozone, hydrogenperoxide, excess levels of copper, iron,among others that generate ROS pro-duction have been shown to alter theexpression of miRNAs in plants.9 Sevennovel miRNA families that showed dif-ferential regulation under hydrogen per-oxide stress in rice have beenidentified.15 An in depth comparativeanalysis in wild-type and mutants defec-tive in redox signaling will reveal theinvolvement of redox in regulating theexpression of nutrient and stress-respon-sive miRNAs (Fig. 1). Also such studieswill help to distinguish between thiolredox and ROS-mediated regulation oreven the extent of overlap between these2 signaling pathways.

The link between oxidative stress sig-naling and induction of miRNAs has alsobeen known in animals,1618 although theinvolvement of redox singling per se hasnot been studied. Thus, this short reviewmay induce more studies to address theinvolvement of thiol-based-redox signal-ing in regulating miRNA expression inplants and animals. Such studies shouldalso consider other redox- and ROS-related cell parameters such as the redoxpair ascorbate and dehydroascorbate oroxylipins that likely interfere with thedescribed mechanisms.

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest weredisclosed.

Acknowledgments

We sincerely thank Prof. Dr. Karl-JosefDietz, University of Bielefeld, Germanyfor critical reading of the manuscript andconstructive suggestions.

Funding

Oklahoma Agricultural ExperimentStation supported the research in Sunkarlaboratory. SKP acknowledges the Indo-US Science and Technology Forum(IUSSTF) for the award of postdoctoralfellowship.

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