Photoreceptor-mediated regulation of the COP1/SPA E3 ubiquitin ligase

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Article

Reference

Photoreceptor-mediated regulation of the COP1/SPA E3 ubiquitin ligase

PODOLEC, Roman, ULM, Roman

Abstract

Plants have evolved specific photoreceptors that capture informational cues from sunlight.

The phytochrome, cryptochrome, and UVR8 photoreceptors perceive red/far-red, blue/UV-A, and UV-B light, respectively, and control overlapping photomorphogenic responses important for plant growth and development. A major repressor of such photomorphogenic responses is the E3 ubiquitin ligase formed by CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) and SUPPRESSOR OF PHYA-105 (SPA) proteins, which acts by regulating the stability of photomorphogenesis-promoting transcription factors. The direct interaction of light-activated photoreceptors with the COP1/SPA complex represses its activity via nuclear exclusion of COP1, disruption of the COP1-SPA interaction, and/or SPA protein degradation. This process enables plants to integrate different light signals at the level of the COP1/SPA complex to enact appropriate photomorphogenic responses according to the light environment.

PODOLEC, Roman, ULM, Roman. Photoreceptor-mediated regulation of the COP1/SPA E3 ubiquitin ligase. Current Opinion in Plant Biology , 2018, vol. 45, p. 18-25

DOI : 10.1016/j.pbi.2018.04.018 PMID : 29775763

Available at:

http://archive-ouverte.unige.ch/unige:104620

Disclaimer: layout of this document may differ from the published version.

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Photoreceptor-mediated regulation of the COP1/SPA E3 ubiquitin ligase

Roman Podolec

^1,2

and Roman Ulm

^1,2

Plantshaveevolvedspecificphotoreceptorsthatcapture informationalcuesfromsunlight.Thephytochrome,

cryptochrome,andUVR8photoreceptorsperceivered/far-red, blue/UV-A,andUV-Blight,respectively,andcontrol

overlappingphotomorphogenicresponsesimportantforplant growthanddevelopment.Amajorrepressorofsuch

photomorphogenicresponsesistheE3ubiquitinligaseformed byCONSTITUTIVELYPHOTOMORPHOGENIC1(COP1)and SUPPRESSOROFPHYA-105(SPA)proteins,whichactsby regulatingthestabilityofphotomorphogenesis-promoting transcriptionfactors.Thedirectinteractionoflight-activated photoreceptorswiththeCOP1/SPAcomplexrepressesits activityvianuclearexclusionofCOP1,disruptionoftheCOP1- SPAinteraction,and/orSPAproteindegradation.Thisprocess enablesplantstointegratedifferentlightsignalsatthelevelof theCOP1/SPAcomplextoenactappropriate

photomorphogenicresponsesaccordingtothelight environment.

Addresses

1DepartmentofBotanyandPlantBiology,SectionofBiology,Facultyof Sciences,UniversityofGeneva,CH-1211Geneva4,Switzerland

2InstituteofGeneticsandGenomicsofGeneva(iGE3),Universityof Geneva,Geneva,Switzerland

Correspondingauthor:

CurrentOpinioninPlantBiology2018,45:18–25

ThisreviewcomesfromathemedissueonCellsignalingandgene regulation

EditedbyJorgeCasalandJavierPalatnik

https://doi.org/10.1016/j.pbi.2018.04.018

1369-5266/ã2018TheAuthors.PublishedbyElsevierLtd.Thisisan openaccessarticleundertheCCBY-NC-NDlicense(http://creative- commons.org/licenses/by-nc-nd/4.0/).

Introduction

Assessile organisms, plants adapttheir development to the prevailing growth conditions. Sunlight is not only crucialastheultimatesourceofenergyfuelingphotosyn- thesisbutitalsoprovidesimportantinformationaboutthe environment,influencingplantgrowthanddevelopment throughout the life cycle [1]. For example, during de- etiolation,plantsswitchfromskotomorphogenesis(growth in darkness) to photomorphogenesis (growth in light).

Thistransitionisindependentoftheprocessofphotosyn- thesis. Photomorphogenesis at the seedling stage is

characterized by both changes in seedling morphology (incl.ashort hypocotyland opengreen cotyledons) and avastreprogramming of gene expression that allowthe planttoaccommodatetoaphotosyntheticlifestyle.Other photomorphogenic responses include germination, pho- totropism,shadeavoidance,leafandrosettemorphogene- sis,chloroplastmovement,andfloweringtransition[1].

Photomorphogenic responses are initiated by several classesofphotoreceptors,namelyphytochromes(absorb- ing red/far-red; phyA to phyE in Arabidopsis), cryptochromes(blue/UV-A;cry1andcry2),phototropins(blue/

UV-A;phot1andphot2),Zeitlupefamilymembers(blue/

UV-A;ztl,fkf1, andlkp2),andtheUV-Bphotoreceptor UVRESISTANCELOCUS8(UVR8)[2].Severalpho- toreceptormechanismsofactionhavebeendescribed[3–

7].ForphototropinsandZeitlupefamilymembers,light perceptionbythephotoreceptorchromophoreleadstoa changein intrinsic serine/threoninekinase andE3 ubiquitin ligase activity, respectively [5,6]. However, for phytochromes,cryptochromes,and UVR8,lightpercep- tionprimarilyresultsinprotein-proteininteractionswith keytargets[3,4,7].Thesekeyinteractionpartnersinclude a number of transcription factors [8–12] as well as the COP1/SPAE3ubiquitinligase[13–18].Inthisreview,we willdescribetheCOP1/SPAcomplexand itsactivityin darkness, then present the latest findings on how the phytochrome,cryptochrome, and UVR8 photoreceptors interactwithandregulate theCOP1/SPAcomplex.

TheCOP1/SPA E3ubiquitinligase is a key repressoroflight responses

TheCOP1/SPAE3ubiquitinligaseisacrucialrepressor ofphotomorphogenesis[19,20](Figure1).InArabidopsis, COP1andSPA1–SPA4 encodeWD40 repeat-containing proteins [21]. COP1 consists of an N-terminal Really Interesting New Gene (RING) domain, a central coiled-coil(CC)domain,andaC-terminalWD40repeat domain,whereasSPAfamilyproteinsarecomposedofa kinase-like domain, a CC domain, and aWD40 repeat domain [22,23]. Although COP1 shows E3 ubiquitin ligaseactivityinvitro[24,25],itsinvivoactivitydepends onSPA accessoryproteins[26,27].

cop1 null mutants are seedling lethal, but weak cop1 allelesexistthatareassociatedwithstrongphotomorpho- genesis even in complete darkness[28]. The four SPA proteins show somefunctional redundancy, thus a con- stitutive photomorphogenic phenotype is particularly evident in higher order mutants, such as the spa1234

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quadruplemutant[26](Figure1a).However,incontrast to cop1nullmutants, thespa1234nullmutantis viable, althoughwithanextreme dwarfphenotype[27].

COP1 and SPAproteinsdirectly interactandform tetra- meric complexes composed of two COP1 and two SPA proteins,withthepotentialofallcombinationsofthefour SPAproteins[29–31].TheCOP1/SPAcomplexisthought to act as asubstrate adaptor for CULLIN4-DAMAGED DNA BINDINGPROTEIN1(CUL4-DDB1)-basedE3 ubiquitin ligasecomplexes[32]. Indeed,both COP1 and SPAproteinscontainaWDxRmotifassociatedwithDDB1 bindingintheirWD40repeats[32].COP1homodimeriza- tionandCOP1interactionwithSPAproteinsarerequired fornuclearE3ubiquitinligaseactivityinvivo[26,31,33].

AlthoughitisclearthattheSPAproteinsarerequiredfor

COP1activityinvivo,thefunctionofSPAproteinsinthe COP1/SPA complex in darkness remainsunknown [19].

Notwithstanding this, the COP1/SPA complex targets a broad range of photomorphogenesis-promoting transcrip- tionfactorsforubiquitinationanddegradationthroughthe 26Sproteasomepathway[34–43](Figure1b).Interestingly, several targets contain a conserved valine-proline (VP) containingdomain that mediatesdirect bindingwiththe COP1 WD40 domain [34,35,44,45,46]. By targeting a widerangeofphotomorphogenesis-promotingtranscription factors, the COP1/SPA complex broadly regulates light- dependent developmentalresponses,including de-etiola- tionandphotoperiodicflowering[19,20,47].

Itisof interest tonotethat COP1 isconservedbetween plantsandanimals,whereasSPAproteinsarespecifictothe

RegulationofCOP1bylightPodolecandUlm 19

Figure1

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COP1 SPA

Photomorphogenesis Flowering

Modulation of shade response HY5 LAF1

HFR1 Photoreceptor

CO

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mutants

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DARK LIGHT

Current Opinion in Plant Biology

RegulationofphotomorphogenesisbyphotoreceptorsandtheCOP1/SPAcomplex.(a)Light-grownwild-typeseedlingsundergo

photomorphogenesis(opengreencotyledons,shorthypocotyl)asopposedtodark-grownseedlingsthatundergoskotomorphogenesis(closed cotyledons,apicalhook,elongatedhypocotyl).Photoreceptormutantsgrowninlightresembledark-grownwild-typeseedlings,whereasdark- growncop1andspa1234mutantsexhibitconstitutivephotomorphogenesis.(b)Generalmodel:Light-activatedphotoreceptorsinactivatethe COP1/SPAcomplexthroughvariousmechanisms,leadingtorepressionofitsE3ubiquitinligaseactivityandstabilizationofphotomorphogenesis- promotingfactors.Indarkness,photoreceptorsareinactiveandphotomorphogenesis-promotingfactorsareubiquitinatedbytheCOP1/SPA complexanddegradedbythe26Sproteasome.

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greenlineage[48].Asthenon-plantCOP1orthologsarenot regulatedbylight[49],ithasbeenhypothesizedthatthe evolution of SPA proteins was crucial for photoreceptor- mediatedlightcontrolofCOP1/SPAcomplexactivity(see below)[48,50].However,itremainsunknownhowthein vivoE3ligaseactivityofplantCOP1evolvedtorequirethe presence of SPA proteins. For a more comprehensive descriptionofthevariousrolesoftheCOP1/SPAcomplex and its regulation in darkness, we refer the reader to excellentrecentreviews[19,20,47,51].

Severalphotoreceptorsregulateactivityofthe COP1/SPAcomplex viadifferent mechanisms ActivityoftheCOP1/SPAcomplexisrequiredforseed- lingetiolationindarkness,whichissubsequentlyinhib- ited by action of the phytochrome, cryptochrome, and

UVR8 photoreceptors in light [4,19] (Figure 1b). This inhibition leadsto areleaseof the COP1/SPAcomplex repressiveactivityandthusactivatesphotomorphogenic responses. Although the different photoreceptors simi- larlyregulate activityof theCOP1/SPAcomplex, there arecleardistinct mechanisms.Theseregulatorymecha- nismsinclude nucleo-cytosolic partitioning,degradation oftheaccessorySPAproteins,andphysicalseparationof COP1fromSPA proteinsor of theCOP1/SPAcomplex fromCUL4-DDB1(Figure2).

Light-regulatednuclearexclusionofCOP1

TheregulationofCOP1sub-cellularlocalizationwasthe firstproposed mechanism of light-dependent control of COP1 activity [52] (Figure 2a). In the dark, COP1 is predominantly present in the nucleus, where nuclear

Figure2

Nuclear exclusion SPA degradation

Inhibition of COP1 dimerization COP1-SPA dissociation

(a) (b) (c)

(d) (e) (f)

Nucleus Cytosol

HY5 COP1

SPACOP1 phyA-, phyB-, cry1- SPA

dependent

SPA- dependent COP1

COP1

HY5

phyA-dependent COP1-dependent COP1

SPA2COP1 SPA2

COP1COP1

Photomorphogenesis HY5

HY5 HY5

Sequestering into inactive complexes

COP1 SPACOP1

SPA

Photomorphogenesis HY5 CUL4

DDB1

UVR8

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SPA CUL4

DDB1 UVR8UVR8

HY5 HY5 HY5

C C C C COP111

C COP111 D

D DDB11

U U U U UVR888 U U UV 8R888

COP1 SPACOP1

SPA

Flowering COP1

SPA

COP1 SPA fkf1 fkf1 fkf1

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CO CO CO cry2-dependent inhibition of COP1/SPA activity

COP1 SPA

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SPA COP1

cry2 cry2

CO

CO CO CO

C C C COP111

C COP11

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SPA

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SPA cry1

COP1 SPA phyA/B cry1

phyA/B

HY5 HY5 HY5 HY5

COP1 SPA cry1

Current Opinion in Plant Biology

MechanismsofphotoreceptorcontroloverCOP1/SPAE3ligaseactivity.(a)PhytochromesandcryptochromespromoterelocationofCOP1tothe cytosol,separatingitfromitsnuclearsubstrates.WhetherSPAproteinsarealsorelocatedtothecytosolinresponsetolightremainsunknown.(b) LightinducesthephyA-dependentandCOP1-dependentdegradationofSPA2,leadingtoadecreaseinCOP1E3ligaseactivity.(c)cry1and phyA/Binteractlight-specificallywithSPAproteins.ThisinteractionleadstophysicalseparationofCOP1andSPAproteins,leadingtoadecrease inCOP1/SPAE3ligaseactivity.CryptochromesandphytochromesalsodirectlyinteractwithCOP1.NotethatitisnotknownwhethertheCOP1- SPAdissociationalsodisruptstheCOP1-COP1andSPA-SPAinteractions.(d)cry2interactsbluelight-dependentlywithCOP1andSPAproteins, leadingtoanincreaseinCOP1-SPAaffinitybutadecreaseinE3ligaseactivity,promotingfloweringunderlongdays.(e)UV-B-dependent interactionofUVR8withCOP1inducesachangeincomplexorganizationfromaCUL4-DDB1-COP1/SPAcomplexthatubiquinatestarget substratestoasubstrate-stabilizingUVR8-COP1/SPAcomplex.(f)fkf1interactswithCOP1underbluelightandinhibitsCOP1homodimerization, whereasCOP1-SPAheterodimerizationisunaffected.ThisinactivatestheCOP1/SPAcomplexandpromotesflowering.(a)–(f)HY5andCO exemplifyCOP1/SPAsubstratespromotingseedlingphotomorphogenesisandflowering,respectively.

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transcriptionfactorsaretargetedforubiquitination.Light inducesCOP1relocationtothecytosol,separatingitfrom itsnuclearsubstrates[20,52].ItwasproposedthatCOP1 relocationwasrelativelyslow(12–24h)basedonstudies with GUS-COP1 fusions [52]. However, this view was recentlychallengedbytheobservationthataYFP-COP1 fusion was ableto relocatemuchfaster (2.5h half-life), suggesting this mechanism couldpossibly contributeto theratherrapidstabilizationofCOP1targetsunderlight [53,54].Conversely,shadepromotesthere-accumulation ofCOP1inthenucleustoexertitsphotomorphogenesis- repressingactivity[55].

COP1 nuclear localization is dependent on a bipartite nuclear localization signal in the regionbetween theCC and WD40 domains [56,57], and SPA proteins are not required for nuclear localization of COP1 in darkness [50].CytosoliclocalizationofCOP1inresponsetolight involves acytosolic localization signal in the CC domain [56,57]. Phytochrome and cryptochrome photoreceptors mediate COP1 nuclear exclusion [58,59]; however, how exactlyphotoreceptorsignalingregulatesCOP1sub-cellu- lar localizationremainsunknown.Interestingly,theinter- play between different photoreceptors is complex, as is evident whenCOP1 nuclearexclusion isanalyzedunder monochromatic conditions. For example, whereas phyA- mediatednuclearexclusionofCOP1underfar-redlightis dependentonSPAproteins,thisresponseisSPAindepen- dentunderbluelightdownstreamofcry1[50].Moreover, phyBandredlightwerefoundtobelesspotentininducing COP1nuclearexclusion[50].Notwithstandingtheexact contributionofindividualphotoreceptors,SPAproteinsare necessary for COP1 nucleo-cytosolic partitioning in response to white light [50]. However, it is clear that nuclear COP1isinactiveintheabsenceof SPAproteins, both in light as well as in darkness [26,27]. Thus, SPA proteinsplayadualroleinthattheyarerequiredforCOP1 functionandalsoenableCOP1light-responsiveness.How- ever,itispresentlynotknownwhetherSPAproteinsshow light-dependent nucleo-cytosolic partitioning themselves, orwhetheronlyCOP1 isaffected.

Interestingly,supplementalUV-Bcounteractsthenuclear exclusionofCOP1inwhitelightandcausesnuclearCOP1 accumulation[60],althoughitisnotknownwhetherUV-B affectsCOP1nuclear-cytosolicshuttlingperse.Thefind- ingthatnuclearCOP1accumulationunderUV-Bisasso- ciated with elevated COP1 levels [60,61] suggests that UVR8maynotaffectCOP1sub-cellularlocalizationbut maycauseinhibitionofCOP1autoubiquitinationandthus affect nuclearaccumulationindirectly.

Photoreceptor-mediatedregulationofCOP1andSPA proteinlevels

Given that SPA proteins are required for COP1/SPA complexactivity,targeteddestabilizationofSPAproteins is also a mechanism to control activity of the complex.

Indeed, SPA2, and to alesser extent SPA1, show light- induced, phyA-dependent, and proteasome-mediated degradation[62,63](Figure2b).The N-terminalkinase- like domain of SPA2 confers instability to the protein underlight[64].Interestingly,thephotoreceptor-induced degradationofSPA2seemsdependentonCOP1activity, suggestingcross-regulationofstabilityamongCOP1/SPA complexcomponents[29].Incontrast,theCOP1proteinis stabilized and accumulates in the nucleus after UV-B exposureinaUVR8-dependentmanner,andthisisaccom- panied by increased transcription of COP1 under UV-B [60,61,65]. However, increased nuclear accumulation of COP1 under UV-B is accompanied by increased rather than decreasedELONGATED HYPOCOTYL5 (HY5) stability, emphasizing that the COP1/SPA complex is efficientlyinactivatedbyUV-B-activatedUVR8[13,60].

Photoreceptor-mediatedregulationoftheCOP1-SPA interaction

Alongside the physical separation of COP1 from sub- strates,photoreceptor-mediateddisruptionofthecrucial COP1-SPA interaction inactivates COP1/SPA ubiquitin ligase activity and results in stabilization of photomorphogenesis-promoting transcription factors (Figure 2c).

This mechanism contributes to bothcryptochrome and phytochromesignaling.

Indeed,bluelight-dependentinteractionofcry1andcry2 withSPAproteinsregulatestheactivityoftheCOP1/SPA complex [16–18,66].The cry1C-terminal CCTdomain interacts with the SPA1 C-terminal region specifically underbluelight[16,17].Mechanistically,thebluelight- dependent cry1-SPA1 interaction leads to a reduced SPA1 binding to COP1, thereby inactivating the COP1/SPAubiquitinligaseandstabilizing,forexample, the HY5 transcription factor to promote photomorphogenesis[16,17].Forthebluelight-dependentinteraction betweencry2andSPA1,itistheN-terminalPHRdomain ofcry2thatinteractswiththeN-terminaldomainofSPA1 [18,67],which converselycausestheCOP1-SPA1 inter- actiontobestrengthenedratherthandissociated.Never- theless, cry2 interaction with the COP1/SPA complex increases thestabilityof theCOP1/SPAcomplextarget CONSTANS(CO) thatis involvedin theregulationof photoperiodicfloweringunderlongdays[18](Figure2d).

Next to theeffect of light-dependent interactionswith SPA proteins, cry1 and cry2 C-terminal domains also interact directly with COP1 [68,69]. Whereasthecry1–

COP1interactionisdependentonSPAproteins,thecry2- COP1interactionisnot,andbothappeartobeblue-light dependent [70]. The direct interaction of the crypto- chromeswithCOP1mayindeedfurthercontributetothe light-mediatedeffectontheCOP1–SPAinteractionand therebyCOP1/SPAcomplexactivity.

Similar to cryptochromes, the light-activated phytochromes phyA and phyB interact with SPA proteins

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[14,15]. The phytochromesinhibit COP1/SPA complex activitybydisruptingtheinteractionbetweenCOP1and SPAproteins,therebyreorganizingtheCOP1/SPAcom- plex[14,15],analogousto thecry1mechanismof action [16,17,66]. Moreover, direct interaction of the phytochromes with COP1 may contribute to the regulation ofCOP1/SPAcomplexactivity[15,71,72].

UVR8-mediatedinhibitionofCOP1,independentofSPA proteins

TheUVR8photoreceptorwasshowntodirectlyinteract with the WD40 repeat domain of COP1 in a UV-B- dependentmanner[13,65,73–75,76].Incontrasttophy- tochromes and cryptochromes, UVR8 does not interact with SPA proteins and UVR8 early signaling is not impairedinspa1234quadruplemutants[60,65].Indeed, although UV-B-activated UVR8 interacts directly with COP1,itisonlyassociatedwithSPA1indirectlythrough COP1 [65]. Moreover, the COP1–SPA interaction remainsintactuponinteractionwithUVR8[65,73].

Two separate domains of UVR8 are involved in the interaction with COP1, namely a domain of maximum 27amino acids(C27)in the UVR8C-terminus andthe UVR8b-propeller core[77,78].The twoCOP1 interaction surfaces are not accessible in the inactive UVR8 homodimer but are exposed in monomeric, active UVR8that formsupon UV-Bperception [75,77]. Inter- action of COP1 with the b-propeller domain of UVR8 allows UV-B-dependent UVR8-COP1 interaction, whereastheC-terminalUVR8domainisthoughttoaffect COP1 activity [77]. Interestingly, the C27 domain of UVR8comprisesafunctionalandconservedVP-contain- ingmotif[77]thatmimicstheCOP1-interactiondomains of several COP1 substrate proteins [34,35,44,45,46].

Thus, competition between the UVR8 VP interaction motifandthatofCOP1targetsmaypreventubiquitina- tionanddegradationofthelatterthroughthe26Sprotea- some pathway and underlie how UVR8 inactivates the COP1/SPA complex [4,13,77]. It is of note that UVR8 stability is not affected by its interaction with COP1 [13,79]. UVR8 binding to COP1 also induces physical and functional dissociation of the COP1/SPA complex corefromthe HY5-degradingCUL4-DDB1-COP1/SPA E3ligase complex,thusinactivating theCOP1/SPAE3 ligase and creating aUVR8-COP1/SPA complex which may actively promote HY5 stabilization through an unknownmechanism [51,73](Figure 2e).

Otherlight-mediated changesin COP1/SPA complexactivity

Recently, several post-translational mechanisms have been reported that regulate activity of the COP1/SPA complex. COP1/SPA complex activity is enhanced by COP1 sumoylation in darkness. Light is proposed to negatively regulate COP1 sumoylation; however, this responseisyet to be linkedto aspecific photoreceptor

[80].Moreover, COP1isregulatedvia phosphorylation by thePINOID kinase; however, the mechanistic link betweenphotoreceptorsignalingandCOP1 phosphorylation remains to be identified [81]. Alongside post- translational modifications, COP1 activity is also regu- lated by interaction with PHYTOCHROME INTER- ACTINGFACTOR1(PIF1),whichenhancesactivityof theCOP1/SPAcomplexindarkness[82].PIF1isdesta- bilizedinlightinaphytochrome-dependentprocessand COP1/SPA complex-dependent process, which poten- tiallycontributes toinactivationof theCOP1/SPAcom- plex in light [83]. Degradation of the PIF1 co-factor is reminiscent of the previously discussed COP1-depen- dentdegradationof SPA2.

Averyrecentstudyhasimpliedthatfkf1photoreceptor- dependentcontroloffloweringtimeinvolvesinactivation oftheCOP1/SPAcomplex[33].fkf1interactswiththe RING domain of COP1, and this interaction inhibits COP1 dimerization,whereas COP1-SPA1 heterodimer- ization remains unaffected [33] (Figure 2f). Whether thisinhibitionofCOP1dimerizationfollowingfkf1inter- actionisuniqueorrepresentsamoregeneralmechanism ofCOP1regulationremainsto bedetermined.

Feedbackregulationofphotoreceptorsbythe COP1/SPAcomplex

The COP1/SPA complex may also provide feedback regulation of photoreceptor activities, and indeed has been implicatedin theregulation of cryptochrome and phytochromelevels[71,84,85].Ontheotherhand,UVR8 directly interacts with COP1 but there is no indication thatthis interactionleads to UVR8destabilization[13].

Interestingly,however,COP1isrequiredfor theUV-B- induced nuclear accumulation of UVR8, providing an additionalmechanismforCOP1photoreceptorsignaling feedbackregulation[86,87].

Conclusionsand futuredirections

Severalmechanisms havebeen proposedtocollectively explain the light-dependent, photoreceptor-mediated regulation of COP1/SPA E3 ligase activity. Firstly, nuclear exclusion of COP1 under light was identified.

However,itremainsunclearhowthedifferentphotore- ceptors are mechanistically involved, and whether this process is fast enough to account for thelight-induced stabilizationof COP1 substratesresulting in earlygene expressionchanges.Secondly,light-inducedseparationof COP1fromitscrucialaccessorySPAproteinsseemstobe a common mechanism for phytochrome and cryptochrome signaling [14,15,66]. However, the role that SPAproteinsplayandhowphotoreceptor-mediatedsep- arationimpairstheoverallCOP1/SPA complexremains unclear.Thirdly,asforUVR8,ithasbeensuggestedthat dissociationoftheCUL4-DDB1scaffoldfromtheCOP1/

SPA complex prevents ubiquitination of and therefore stabilizes HY5 [13,65,73]. How the UVR8-COP1/SPA

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interaction results in theproposed functional switchof COP1 from repressor to promoter of HY5 stability remains to be investigated. SPA proteins do not seem toplayadecisiveroleinthisprocess[65,73].Lastly,light- induced destabilization of accessory proteins, such as SPA2 or PIF1, can account for a decrease in COP1/

SPAcomplexactivity[62,63,82].Itispresentlyunknown if and how the mechanisms of COP1/SPA regulation through photoreceptors interact, and what the relative contributionsofthesemechanismsareinthestabilization of differentCOP1/SPAsubstrates.

The COP1/SPA complex haslikely arisen early in the evolutionofthegreenlineageasapointofconvergence throughwhichphotoreceptorscancontrolphotomorpho- genesis in line withthesurrounding light environment.

Futureresearch shouldfocusonunderstandingthepre- cise composition, localization, stability, and E3 ligase activity of the COP1/SPA complex in the presence of different photoreceptors and different light qualities.

This should further reveal the common and specific mechanisms through which photoreceptors control COP1/SPAcomplex activityandtherebyphotomorpho- genic development.

Acknowledgements

WethankSongChenforhelpfulcommentsonthemanuscript.We apologizetocolleagueswhoseworkcouldnotbediscussedduetospace restrictions.OurresearchissupportedbytheUniversityofGenevaandthe SwissNationalScienceFoundation(grantnumbers31003A_153475, CRSII3_154438andIZSAZ3_173361).RomanPodolecissupportedbyan iGE3PhDSalaryAward.

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