Sumoylation of BRI1-EMS-SUPPRESSOR 1 (BES1) by the SUMOE3 Ligase SIZ1 Negatively Regulates Brassinosteroids Signalingin Arabidopsis thalianaLirsquoe Zhang14 Qing Han14 Jiawei Xiong1 Ting Zheng1 Jifu Han1 Huanbin Zhou2 Honghui Lin1Yanhai Yin3 and Dawei Zhang11Key Laboratory of Bio-Resource and Eco-Environment of Ministry of Education College of Life Sciences Sichuan University Chengdu Sichuan PR China2State Key Laboratory for Biology of Plant Diseases and Insect Pests Institute of Plant Protection Chinese Academy of Agricultural Sciences BeijingPR China3Department of Genetics Development and Cell Biology Iowa State University Ames IA USA4These authors contributed equally to this workCorresponding author E-mail zhdaweiscueducn Fax 86 028 85415389(Received January 21 2019 Accepted June 18 2019)

Brassinosteroids (BRs) a group of plant steroid hormonesparticipate in the regulation of plant growth and develop-mental processes BR functions through the BES1BZR1family of transcription factors however the regulation ofthe BES1 activity by post-translational modifications remainslargely unknown Here we present evidence that the SUMOE3 ligase SIZ1 negatively regulates BR signaling pathway T-DNA insertion mutant siz1-2 shows BL (Brassinolide themost active BR) hypersensitivity and BRZ (Brassinazole aBR biosynthesis inhibitor) insensitivity during hypocotylelongation In addition expression of BES1-dependent BR-response genes is hyper-regulated in siz1-2 seedlings Thesiz1-2bes1-D double mutant exhibits longer hypocotyl thanbes1-D Moreover SIZ1 physically interacts with BES1 in vivoand in vitro and mediates the sumoylation of BES1 A K302Rsubstitution in BES1 blocks its sumoylation mediated by SIZ1in plants indicating that K302 is the principal site for SUMOconjugation Consistently we find that sumoylation inhibitsBES1 protein stability and activity Taken together our datashow that the sumoylation of BES1 via SIZ1 negatively regu-lates the BR signaling pathway

Keywords BR signaling Sumoylation Transcription factor

Introduction

Brassinosteroids (BRs) are a class of plant growth-promotinghormones that are structurally related to animal and insectsteroid hormones (Geuns 1978 Clouse et al 1996) BRs unlikeanimal steroids whose receptors are located in the nucleus thatcan directly bind to nuclear receptors to activate target genes(Mangelsdorf and Evans 1995) which are perceived by theplasma membrane-localized receptor kinase BRI1(BRASSINOSTEROID INSENSITIVE 1) (Li and Chory 1997Friedrichsen et al 2000) Biochemical studies have revealedthat BR binding to BRI1 activates its kinase activity (Wanget al 2001) thus initiates a signal transduction cascade to ac-tivate the downstream transcription factors BES1 and

BRASSINAZOLERESISTANT 1 (BZR1) which play crucial rolesin regulating the expression of BR-responsive genes (Yang et al2011 Guo et al 2013)

The bes1-D caused by a substitution from proline to leucinein the PEST domain of BES1 is identified as a dominant sup-pressor of a weak bri1 mutant and displays constitutive BR-response phenotype (Yin et al 2002) The bzr1-D caused bythe same method as the bes1-D has a similar phenotype tobes1-D in the dark (Wang et al 2002) Stability of BES1BZR1increased both in the bes1-D and bzr1-D mutants which leadsto constitutive BR responses indicating that the regulation ofBES1BZR1 protein stability is vitally important during BR reg-ulating plant growth and development

Several studies have shown that the stability and activity ofBES1BZR1 can be regulated by post-translational modificationsincluding phosphorylation and ubiquitination First of all BIN2(BRASSINOSTEROID-INSENSITIVE 2) a GSK3SHAGGY-likekinase phosphorylates BES1BZR1 which leads to BES1BZR1 in-stability and degradation (Li and Nam 2002 Yin et al 2002 Zhaoet al 2002) whereas PP2A (protein phosphatase 2A) dephosphor-ylates and activates BZR1 (Tang et al 2011) COP1(CONSTITUTIVE PHOTOMORPHOGENIC 1) a dark-activatedubiquitin ligase degrades the dark-dependent phosphorylated (in-active) form of BZR1 (Kim et al 2014) Moreover MAX2 a subunitof an E3 ligase degrades both phosphorylated and dephosphory-lated BES1 (Wang et al 2013) A recent study shows that SINATsubiquitin E3 ligases specifically interact with dephosphorylatedBES1 and mediate its ubiquitination and degradation (Yanget al 2017) In addition DSK2 (DOMINANT SUPPRESSOR OFKAR 2) an ubiquitin receptor protein interacts with BES1 andrecruits BES1 to the autophagy pathway for ubiquitin-mediatedBES1 degradation (Nolan et al 2017) Therefore it is important toillustrate the post-translational modifications of BES1 for under-standing the roles of BES1 in various developmental and environ-mental conditions

Sumoylation the process of SUMO covalently conjugating tosubstrate proteins is a reversible post-translational modificationsimilar to ubiquitination which consists of three biochemical

Plant Cell Physiol 60(10) 2282ndash2292 (2019) doi101093pcppcz125 Advance Access publication on 20 July 2019available online at httpsacademicoupcompcp The Author(s) 2019 Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists All rights reservedFor permissions please email journalspermissionsoupcom

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steps that include E1 SUMO activation enzyme E2 SUMO con-jugation enzyme and E3 SUMO ligase (Sampson et al 2001Wilkinson and Henley 2010) SUMO modification in plants hasbeen implicated in several basic aspects of cellular functionsincluding stress and defense responses (Kurepa et al 2003Catala et al 2007 Lee et al 2007 Miura et al 2007 Saleh et al2015) hormone responses (Lois et al 2003 Miura et al 2009Zheng et al 2012 Kim et al 2015) the regulation of flowering(Murtas et al 2003) nutrient homeostasis (Miura et al 2005 Parket al 2011) photomorphogenesis (Lin et al 2016) and so on ThePIAS (protein inhibitors of activated STATs)-type SUMO E3 ligaseSIZ1 is an SP-RING-finger protein that contains a SAP domainand a zinc-finger Miz domain which is a principle SUMO E3 ligasethat participates in all aspects of plant growth and development(Johnson and Gupta 2001 Kotaja et al 2002)

In this study we demonstrate the siz1-2 mutant displays BRhypersensitive in promoting hypocotyl elongation implicatingsumoylation in the regulation of BR signaling SIZ1 negativelyregulates the BR signaling pathway by mediating BES1 sumoyla-tion at K302 and then degrades and inhibits BES1 activityTherefore our results provide a new sight into BES1 stabilityand activity via sumoylation

Results

The SUMO E3 ligase SIZ1 negatively regulates BRsignaling

In order to find out whether SIZ1 participates in the BR signalingpathway and how it functions in BR responses the T-DNA inser-tion allele of SIZ1 siz1-2siz-3 was used Although the siz1-2siz-3seedlings displayed shorter hypocotyl it was more sensitive to BLand less sensitive to BRZ compared with wild type (WT) in hypo-cotyl elongation assays (Fig 1AndashD Supplementary Fig S1) Theseresults demonstrate that SIZ1 may negatively regulate BR signaling

Then transcriptional levels of BR-responsive genes were de-tected in siz1-2 and WT seedlings to determine whether SIZ1 isindeed a negative regulator in the BR signaling pathway Wechose several BR-regulated marker genes to test (Yu et al 2011)These genes were regulated in siz1-2 seedlings (Fig 1E) It im-plicates that SIZ1 is a negative regulator of BR signaling

Genetic interaction between SIZ1 and BES1

Genetic interaction between BES1 and SIZ1 was analyzed by cross-ing bes1-D and siz1-2 to create the double mutant siz1-2bes1-DAlthough the single mutant siz1-2 displayed shorter hypocotyl thedouble mutant siz1-2bes1-D showed enhanced hypocotyl elong-ation compared with bes1-D (Fig 2A B Supplementary Fig S2) Inaddition BR-induced or repressed genes were upregulated ordownregulated in siz1-2bes1-D relative to that in bes1-D seedlings(Fig 2C) These results provide a further evidence that SIZ1 playsas a negative regulator in BES1 functions on hypocotyl elongationand regulation of BR-responsive genes

SIZ1 interacts with BES1 in vivo and in vitro

To determine the mechanism how SIZ1 affects BES1 functionswe detected the interaction between SIZ1 and BES1 First we

performed the histidine pull-down assays As shown full-length of MBP-BES1 can be pulled down by His-SIZ1 but theMBP cannot be (Fig 3A) Then truncated MBP-BES1 proteinswere used to pull-down assays to map the regions of BES1 thatinteract with SIZ1 we found that the fragments lacking theamino acid 1ndash218 also were pulled down by His-SIZ1 but frag-ment lacking the amino acid 219ndash288 abolished the inter-action with His-SIZ1 (Fig 3B C) indicating that the regionof amino acid 219ndash288 in BES1 which contains the PESTdomain contributes to its interaction with SIZ1 Then we con-ducted yeast two-hybrid (Y2H) assay with full-length BES1fused to the GAL4 DNA-binding domain (BD-BES1) and SIZ1fused to the GAL4 activation domain (AD-SIZ1) The yeaststrain AH109 harboring both BD-BES1 and AD-SIZ1 plasmidssurvived on medium lacking tryptophan leucine histidine andadenine whereas no yeast cells cotransformed with negativecontrol plasmids (AD+BD or AD-SIZ1+BD or AD+BD-BES1)were recovered (Fig 3D) suggesting the interaction of SIZ1with BES1 in vitro

To further confirm this result we conducted a bimolecularfluorescence complementation (BiFC) assay The YFP fluores-cence signals were detected in the nucleus of Nicotianabenthamiana epidermal cells that coexpressed BES1-cYFP andSIZ1-nYFP (BES1-cYFP+SIZ1-nYFP) but YFP signals were notobserved in BES1-cYFP+nYFP or cYFP+SIZ1-nYFP controls(Fig 3E) Consistently SIZ1-GFP was coimmunoprecipitatedwith FLAG-HA-BES1 but not with empty FLAG-HA vector(Fig 3F) These results indicate that SIZ1 physically interactswith BES1 in plants

We also detected the interaction between SUMO1 and BES1 invitro and in vivo The pull-down assay showed that SUMO1 couldinteract with BES1 and the region of amino acid 219ndash288 in BES1contributed to its interaction with SUMO1 (Supplementary FigS3A) But mutation the SIM1SIM2 in BES1 did not affectBES1 interaction with SUMO1 (Supplementary Fig S3B C)Consistently SUMO1 interacted with BES1 in the BiFC assay(Supplementary Fig S3D) These results showing that SUMO1interacts with BES1 in vitro and in vivo Furthermore some keycomponents of BR signaling like BRI1 and HAT1 did not interactwith SIZ1 (Supplementary Fig S4)

SIZ1 mediates sumoylation of BES1

The direct interaction between SIZ1 and BES1 suggested that BES1may be a SUMO substrate There are three potential sumoylationsites (K88 K205 and K302) in BES1 by SUMOplot (httpwwwabgentcomtoolsumoplot) analysis (Fig 4A) To test this hypoth-esis sumoylation assay was conducted to determine if SUMO-modified BES1 as described previously (Miura et al 2005 Miuraet al 2007) After the reaction containing SUMO E1 (His-SAE1band His-SAE2) SUMO E2 (His-SCE1) SUMO1 (His-SUMO1) SIZ1(His-SIZ1) and substrate BES1 (MBP-BES1) incubated overnight at30C higher molecular bands above original BES1 protein weredetected using anti-MBP antibodies suggesting that BES1 was asubstrate of SUMO1 (Fig 4B) The sumoylated BES1 protein couldnot be detected in the reaction without SUMO E1 and SUMO E2or SUMO1 (Fig 4B) indicating that SUMO1 modification of BES1relies on E1 and E2 The presence of modified BES1 protein in the

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reaction without SIZ1 showed that the sumoylation of BES1 wasindependent of SIZ1 in vitro (Fig 4B) To determine the sumoyla-tion sites in BES1 we substituted K88 K205 or K302 for R respect-ively and performed an in vitro sumoylation assay K302Rsubstitution apparently reduced BES1-SUMO1 conjugation butK88R or K205R substitutions did not (Fig 4C) These results sug-gested that BES1 was a SUMO1 substrate and K302 was the prob-able sumoylation site Next we generated 355-FLAG-HA-BES1overexpression plants BES1OX 355-FLAG-HA-BES1K302R transgenicplants BES1K302ROX and we also got siz1-2BES1OX by crossingsiz1-2 and BES1OX then sumoylation of BES1 was detected Firstwe immunoprecipitated FLAG-HA-BES1 or FLAG-HA-BES1K302R

using anti-HA beads in BES1OX siz1-2BES1OX or BES1K302ROX

anti-SUMO1 antibody was used to detect sumoylated BES1Sumoylated BES1 was detected in BES1OX but not in siz1-2BES1OX and BES1K302ROX (Fig 4D) We also found that the sumoy-lation status of BES1 did not change after the BL treatment (Fig 4E)These results suggest that SIZ1 mediates the sumoylation of BES1 atK302 residue in plants and the sumoylation level of BES1 is un-affected by BL

SIZ1 destabilizes BES1

Sumoylation plays crucial roles on regulating protein stabilityand functional activity (Geiss-Friedlander and Melchior 2007)As SIZ1 mediates sumoylation of BES1 WT (Col-0) and siz1-2seedlings were used to investigate the influence of sumoylation

Fig 1 The SUMO E3 ligase SIZ1 negatively regulates BR signaling (A) A representative example of the phenotype of 7-day-old light-grown WT(Col-0) and siz1-2 seedlings in the presence of different concentrations of BL Scale bar 15 mm (B) The hypocotyl lengths of 7-day-old light-grown seedlings in the presence of different concentrations of BL Data are the mean plusmn SD of 10ndash20 seedlings The experiments were repeatedthree times with similar results Significant differences were based on Studentrsquos t-test (Plt 005 Plt 001) which is applied to all otherexperiments in this study (C) A representative example of the phenotype of 7-day-old dark-grown WT (Col-0) and siz1-2 seedlings in thepresence of different concentrations of BRZ Scale bar 10 mm (D) The hypocotyl lengths of 7-day-old dark-grown seedlings in the presence ofdifferent concentrations of BRZ Data are the mean plusmn SD of 10ndash20 seedlings The experiments were repeated three times with similar results (E)The relative expression of BR-responsive genes was determined by quantitative RT-PCR analysis Ten-day-old WT (Col-0) and siz1-2 seedlingswere used for this assay Data are mean plusmn SD (n = 3) from one representative experiment Relative expression was normalized to that of ActinThree independent experiments were performed with similar relative trends

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on BES1 stability Although the transcriptional level of BES1 wassame in siz1-2 as in WT (Fig 5B) BES1 protein accumulatedmore in siz1-2 seedlings than in WT seedlings (Fig 5A)Unphosphorylated BES1 accumulates in the nucleus in re-sponse to BL (Yin et al 2002) then we checked BES1 levels inWT and siz1-2 seedlings in the presence of BL The abundanceof unphosphorylated BES1 was greater in siz1-2 seedlings thanin WT seedlings (Fig 5A the top panel) We treated WT andsiz1-2 seedlings with the protein synthesis inhibitor cyclohex-imide (CHX) and the result showed that BES1 proteins aremore stable and much abundant whatever withwithout BLin siz1-2 seedlings than in WT seedlings (Fig 5A the secondand the last panels) These results indicate that the sumoylationof BES1 mediated by SIZ1 promotes instability and degradationof BES1

To give more evidence we compared BES1 protein levels inthe WT (Col-0) and siz1-2 seedlings using a cell-free degradationassay Recombinant MBP-BES1 protein was incubated in thetotal protein extracts from 10-day-old WT (Col-0) or siz1-2seedlings withwithout the carbobenzoxyl-leucinyl-leucinyl-leucinal (MG132) MBP-BES1 degraded after indicated timeboth in WT and siz1-2 seedlings when incubated withoutMG132 but with a slower degradation rate in siz1-2 than inWT seedlings (Fig 5C the first and the second panels) Whenadded the MG132 a 26S proteasome inhibitor to the incuba-tion the degradation of BES1 was apparently inhibited both inWT and siz1-2 seedlings (Fig 5C the third and the last panels)These data indicate that the SIZ1-mediated BES1 sumoylation

promotes its proteasome-dependent degradation Then we ex-tracted total proteins from BES1OX and BES1K302ROX seedlingsWestern blot analysis detected the degradation of FLAG-HA-BES1 proteins the results showed that FLAG-HA-BES1K302R pro-tein had a slower degradation rate relative to FLAG-HA-BES1protein (Supplementary Fig S5) implicating that sumoylationpromotes BES1 degradation

Sumoylation inhibits BES1 activity

In order to determine whether sumoylation affects BES1 activ-ity we performed luciferase (LUC) reporter transactivationassays in Arabidopsis protoplasts We selected two BR-re-pressed gene promoters (DWF4P and At2g45210P) and oneBR-induced gene promoter (SAUR-AC1P) and fused with LUCgene to generate promoter-LUC reporter constructs These re-porter constructs were coexpressed with empty vector (FLAG-HA) WT BES1 or BES1K302R in Col-0 protoplasts treated withMG132 and the reporter gene expression was used to evaluateBES1 transcriptional activity While BES1 repressed the expres-sion of DWF4P-LUC and At2g45210P-LUC reporter genes thereporter genes expression was further reduced by BES1K302R

(Fig 6B C) Consistently BES1 induced SAUR-AC1P-LUC re-porter gene expression when BES1K302R expressed the geneexpression was induced more than BES1 (Fig 6A) These resultsshow that sumoylation inhibits BES1 transcriptional activity

To further test the effect of sumoylation on the bindingability of BES1 in vivo we performed chromatin immunopreci-pitation (ChIP) assays We immunoprecipitated BES1 protein

Fig 2 Genetic interaction between SIZ1 and BES1 (A) A representative example of the phenotype of 7-day-old WT (Col-0) siz1-2 bes1-D andsiz1-2bes1-D seedlings siz1-2 enhanced the hypocotyl length of bes1-D Scale bar 15 mm (B) The hypocotyl lengths of WT (Col-0) siz1-2 bes1-Dand siz1-2bes1-D seedlings 7 d after sowing Data are the mean plusmn SD of 10ndash20 seedlings The experiments were repeated three times with similarresults (C) Expression levels of BR-responsive genes as determined by quantitative RT-PCR analysis Data are mean plusmn SD (n = 3) from onerepresentative date set of three independent experiments Relative expression was normalized to that of Actin (P lt 005 P lt 001)

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from Col-0 and siz1-2 seedlings treated withwithout MG132with anti-BES1 antibody Three BES1 direct targets were de-tected (SAUR-AC1 DWF4 and At2g45210) While BES1 boundto all the three gene promoters in Col-0 plants the bindingactivity was significantly enhanced in siz1-2 whatever withwithout MG132 (Fig 6DndashF) indicating that sumoylationmediated by SIZ1 negatively regulates the BES1-binding activity

Discussion

In this study we found a new mechanism that regulates BES1stability and activity by sumoylation First SIZ1 is a negativeregulator in the BR signaling pathway in which siz1-2 showedBL-sensitive and BRZ-insensitive phenotype and enhanced BR-responsive gene expression Second SIZ1 interacts with BES1 in

Fig 3 SIZ1 interacts with BES1 in vivo and in vitro (A) SIZ1 interacts with BES1 in vitro pull-down assays His-SIZ1 pulled down MBP-BES1 butnot MBP MBP and MBP-BES1 proteins were detected using anti-MBP antibodies His-SIZ1 proteins were detected with anti-His antibodieswhich were used as equal loading MBP and MBP-BES1 (Input) were showed (B) The region of amino acid 219ndash288 of BES1 is required for itsinteraction with SIZ1 Letters a b c d and e represented different truncated MBP-BES1 proteins [a MBP-BES1 (1ndash335) b MBP-BES1 (54ndash335) cMBP-BES1 (129ndash335) d MBP-BES1 (219ndash335) e MBP-BES1 (289ndash335)] a b c and d fragments all can be pulled down by His-SIZ1 but fragmente and MBP cannot be MBP and all truncated MBP-BES1 proteins were detected with anti-MBP antibodies His-SIZ1 proteins were detected withanti-His antibodies which were used as equal loading (C) Schematic diagram of various truncated BES1s Numbers indicate the amino acidpositions of these BES1 variants (D) Y2H assay for interaction between SIZ1 and BES1 Although all yeast cells survived on medium lackingtryptophan and leucine (-LW) only cotransformed with both BD-BES1 and AD-SIZ1 plasmid cells survived on medium lacking tryptophanleucine histidine and adenine (-LWHA) The yeast cells cotransformed AD with BD AD-SIZ1 with BD and AD with BD-BES1 were functioned asnegative controls (E) SIZ1 interacts with BES1 in the BiFC assays Coexpression of BES1-cYFP with SIZ1-nYFP in tobacco leaves led to thereconstitution of YFP signal in the nucleus Coexpression of BES1-cYFP with nYFP and cYFP with SIZ1-nYFP were used as negative controls Foreach panel YFP bright field (Bright) and merged images (Merge) were shown Scale bars 20 mm (F) Coimmunoprecipitation analysis showingthat SIZ1-GFP is associated with FLAG-HA-BES1 SIZ1-GFP and FLAG-HA-BES1 were transiently coexpressed in Col-0 protoplastsImmunoprecipitated FLAG-HA-BES1 was detected with anti-HA antibody and coimmunoprecipitated SIZ1-GFP was detected with anti-GFPantibody Empty FLAG-HA vector (FLAG-HA) was used as a negative control

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vivo and in vitro and SIZ1 possesses SUMO E3 ligase activitywhich mediates SUMO1 conjugation to BES1 at K302 residue inplant Finally SIZ1-mediated BES1 sumoylation promotes BES1degradation and inhibits BES1 activity

Loss-of-function siz1-2 showed dwarf phenotype includingshorter hypocotyl (Figs 1A 2A) consistent with previous stu-dies (Lin et al 2016) common in plants defective in GA auxinor BR Plant growth-promoting hormones defection in SIZ1mutant (Catala et al 2007) might result in the dwarf pheno-type of siz1-2 plants In addition DELLA accumulation becauseof the SLY1 sumoylation deduction in siz1-2 (Kim et al 2015)also contributes to its dwarfism When applied to appropriateconcentration of BR the hypocotyl of siz1-2 seedlings evenelongated more compared with the WT seedlings (Fig 1AB) which is the contribution of amplified BR pathway

In the study we found SIZ1 binds to the amino acid 219ndash288region of BES1 (Fig 3B C) which contains PEST domain [poly-peptide sequences enriched in proline (P) glutamic acid (E)serine (S) and threonine (T)] PEST regions serve as proteolyticsignals and are responsive for protein degradation (Rechsteinerand Rogers 1996) SIZ1 binding to the PEST domain in BES1could provide an explanation why BES1 is degraded by SIZ1-mediated sumoylation SINAT-mediated BES1 degradation and

COP1-mediated BZR1 degradation may differ from SIZ1-mediated BES1 degradation since SINATs broadly regulatedephosphorylated BES1 degradation and COP1 regulates phos-phorylated BES1 degradation (Kim et al 2014 Yang et al 2017)BL treatment did not affect BES1 sumoylation (Fig 4E) this isto say phosphorylated and dephosphorylated BES1 all could besumoylated Previous reports showed that phosphorylation andsumoylation work together in regulating protein activity andfunctions (Saleh et al 2015) Our study finds that phosphoryl-ation has no effect on BES1 sumoylation (Fig 4E) and sumoy-lation did not affect BES1 phosphorylation as phosphorylationstatus of BES1 is almost the same in the BES1OX andBES1K302ROX (Supplementary Figs S5 S6) It suggests other en-vironmental or developmental signals maybe cross talk with BRsignaling to modulate plant growth and development by reg-ulating BES1 sumoylation

We compared the BR response of Col-0 BES1OX andBES1K302ROX lines in hypocotyl elongation and found therewas no difference in the control condition However BL pro-moted hypocotyl elongation was more sensitive in BES1K302ROXthan in BES1OX (Supplementary Fig S7) Actually overexpres-sion of WT BES1 did not cause any visible phenotypes in mostof the transgenic lines (Yin et al 2002) and BES1K302ROX lines

Fig 4 SIZ1 mediates the sumoylation of BES1 (A) Amino acids sequence of BES1 The red amino acids showed predicted sumoylation sites inBES1 (B C) In vitro sumoylation assays of BES1 BES1 proteins were modified by SUMO1 in the presence of SUMO E1 E2 SUMO1 (B) K302Rsubstitution reduced the sumoylation of BES1 (C) Nonsumoylated or sumoylated BES1 proteins were detected using anti-MBP antibodiesArrows indicate the nonsumoylated form of BES1 proteins lines indicate sumoylated BES1 proteins (D) Sumoylation of BES1 in the plant Totalproteins were extracted from 10-day-old BES1OX siz1-2 BES1OX and BES1K302ROX transgenic plants Anti-HA beads were used to immunopre-cipitate 355-FLAG-HA-BES1 or 355-FLAG-HA-BES1K302R proteins in BES1OX siz1-2BES1OX or BES1K302ROX seedlings and anti-BES1 antibodieswere used to detect immunoprecipitated FLAG-HA-BES1 or FLAG-HA-BES1K302R proteins and anti-SUMO1 antibodies were used to determinesumoylated BES1 proteins (E) The sumoylation level of BES1 was unaffected by BL After growth on 12 MS plates for 10 d 35S-FLAG-HA-BES1overexpression transgenic plants BES1OX were treated with 1 mM BL (+) or mock solvent () for 2 h The sumoylation level of BES1 wasdetermined as described in (D)

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Fig 6 Sumoylation inhibits BES1 activity (AndashC) LUC reporter transactivation assays showed sumoylation inhibits BES1 transcriptional activityThe vector (FLAG-HA) BES1 (FLAG-HA-BES1) or BES1K302R (FLAG-HA-BES1K302R) was coexpressed with the promoter-LUC reporter constructin Col-0 protoplasts treated with 30mM MG132 for 3 h The average and SDs were from three biological repeats (DndashF) ChIP assay showedBES1 binding activity was enhanced in siz1-2 compared with Col-0 whatever withwithout MG132 ChIP with anti-BES1 antibodies andthe products were used for qPCR assays TA3 was used as the internal control Data are shown as mean plusmn SD of three independent experiments(P lt 005 P lt 001)

Fig 5 SIZ1 destabilizes BES1 (A) BES1 protein levels in WT (Col-0) and siz1-2 plants The WT and siz1-2 seedlings were grown on 12 MS platesfor 10 d After 10 d they were treated with BL (12 MS+BL+EtOH) CHX (12 MS+DMSO+CHX) or CHX+BL (12 MS+BL+CHX) for theindicated time respectively The abundance of BES1 was greater in siz1-2 plants than in WT plants BES1 proteins were detected with anti-BES1antibodies Actin was used for equal loading (B) BES1 transcript abundance determined by quantitative RT-PCR in Col-0 and siz1-2 seedlings (C)Cell-free degradation assay showed the delayed degradation rate of MBP-BES1 in siz1-2 relative to that in WT seedlings and MG132 inhibited thedegradation of MBP-BES1 both in Col-0 and in siz1-2 plants MBP-BES1 protein was detected with anti-MBP antibodies (D) Quantification ofMBP-BES1 protein levels in (C) using Image J software (httpsimagejnihgovij) MBP-BES1 protein levels at 0 min were defined as lsquo10rsquo

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also did not show visible phenotypes (Supplementary Fig S7Athe top panel) We thought there were four forms of BES1 inplants including phosphorylatedsumoylated form (the weak-est activity) phosphorylateddesumoylated form depho-sphorylatedsumoylated form and dephosphorylateddesumoylated form of BES1 (the strongest activity) In the con-trol condition BES1K302ROX and BES1OX transgenic plantsshowed similar hypocotyl length due to almost equal activityof phosphorylateddesumoylated form BES1 in BES1K302ROXand phosphorylatedsumoylated BES1 in BES1OX (Fig 4DSupplementary Fig S7) When applied appropriate BL theforms of BES1 in BES1K302ROX and BES1OX all existed in thedephosphorylated state but dephosphorylateddesumoylatedBES1 in BES1K302ROX was more active than dephosphorylatedsumoylated BES1 in BES1OX (Supplementary Fig S7) In otherwords desumoylation-activated BES1 activity is dependent ondephosphorylation of BES1 When BES1 is phosphorylatedwhatever BES1 is sumoylated or desumoylated the activity ofBES1 is inhibited once BES1 is dephosphorylated the activity ofBES1 is further activated by desumoylation So BR signalingpathway regulates BR-response genes expression almost bydephosphorylating BES1BZR1 And other environmental anddevelopmental signals maybe cross talk with BR signaling byBES1 sumoylation Different modifications of the BES1 is usefulfor plants adaption to different growth and developmentconditions

SUMO can covalently conjugate to target proteins mainlythrough a SUMO consensus motif [(KxED a large hydro-phobic amino acid residue K the acceptor lysine x any aminoacid ED glutamate or aspartate)] in substrate proteins inaddition to covalent conjugation to target proteins SUMOcan also noncovalently attach to proteins via SIMs (SUMO-interacting motifs Kerscher 2007 Wilkinson and Henley2010) NPR1 interacts with SUMO3 via a SIM in NPR1 andthis SUMO3-NPR1 interaction is required for NPR1 sumoyla-tion NIb-SUMO3 interaction leads to covalent conjugation ofSUMO3 to NIb and the interaction is essential for NIb sumoy-lation (Saleh et al 2015 Cheng et al 2017) In our study SUMO1covalently conjugated to BES1 through a sumoylation motifwhich is a SIZ1-dependent process (Fig 4D) and BES1 alsodirectly interacted with SUMO1 (Supplementary Fig S3)but we failed to find the typical SIMs in BES1 SIZ1 hasbeen shown to modulate gene expression in response to BR(Catala et al 2007) in this study we found SIZ1 and SUMO1expression was repressed in bes1-D and induced in bri1-5(Supplementary Fig S8)

This discovery provides an insight into a post-translationalregulatory mechanism of BES1 The broadly researched regula-tion of BES1 activity is mainly through phosphorylation anddephosphorylation (Yin et al 2002 Yang et al 2011) In thisstudy we found sumoylation inhibited BES1 transcriptional ac-tivity and DNA-binding capacities (Fig 6) which showed thatthere were other post-translational modifications such assumoylation regulated BES1 activity Diverse regulatory mech-anisms may function to regulate BR-responsive gene expressionaccurately by modulating BES1 activity during plant growth anddevelopment

Materials and Methods

Plant materials and growth condition

Arabidopsis thaliana ecotype Columbia (Col-0) was used as the WT control T-

DNA insertion mutants siz1-2 (At5g60410 SALK_065397) and siz1-3

(At5g60410 SALK_034008) were obtained from ABRC (Arabidopsis

Biological Resource Center) The homozygous double mutant siz1-2bes1-D

was obtained by crossing siz1-2 to bes1-D 35S-FLAG-HA-BES1 overexpression

transgenic plants BES1OX and 355-FLAG-HA-BES1K302R transgenic plants

BES1K302ROX were obtained by Agrobacterium-mediated floral transformation

siz1-2BES1OX plants were obtained by crossing siz1-2 and BES1OX4 All of the

plants were grown on half-strength MS (12 MS) plates andor in soil under

long-day conditions (16-h light8-h dark) at 22C

Hypocotyl elongation assays

The hypocotyl elongation assays were carried out as previously described (Ye

et al 2017) In brief seeds were sterilized with 70 ethanol and 01 Triton X-

100 for 20 min washed three times with 100 ethanol and dried on the filter

papers The dried seeds were sowed onto 12 MS plates with 06 agar powder

and different concentrations of BL or BRZ All the plates with seeds were placed

at 4C for 2 d For BRZ response experiments the plates were wrapped with

three layers of aluminum foil after exposing to light for 10 h and incubated

under long-day conditions (16-h light8-h dark) at 22C for 7 d for BL response

experiments the plates with seeds were directly incubated under long-day

conditions (16-h light8-h dark) at 22C for 7 d After 7 d hypocotyls were

measured using Image J software (httpsimagejnihgovij) About 10ndash20

seedlings were measured

Plasmid constructs

For FLAG-HA-tagged transgenic plants BES1 or BES1K302R coding region se-

quence was cloned from WT and fused with FLAG-HA tag into the pCM1307

vector For the BiFC assay the constructs of the N- or C-termini of YFP used

have been described previously (Yu et al 2008) The coding regions of BES1 and

SIZ1 or BES1 and SUMO1 were inserted into the YFP-C and YFP-N construct

respectively For yeast two-hybrid assay the coding region of BES1 was cloned

into pGBKT7 containing a binding domain (BD) whereas SIZ1 was cloned into

pGADT7 containing an activation domain (AD) For recombinant protein puri-

fication and His pull-down assay SIZ1 or SUMO1 coding region was cloned into

pET-28a vector whereas BES1 truncated BES1 fragments and different

mutated BES1 were incorporated into pETMAL vector respectively For Co-IP

assays in the Arabidopsis protoplasts BES1 coding region was cloned into

pCM1307 2X35SPTL vector fused with FLAG-HA tag SIZ1-GFP construct was

kindly provided by HZ

Transgenic plants

The construct of FLAG-HA-BES1 or FLAG-HA-BES1K302R driven by 35S pro-

moter was transformed into Agrobacterium tumefaciens (strain GV3101)

which was used to transform WT plants by the floral dip method (Clough

and Bent 1998) Transgenic lines were selected on 12 MS medium plus

50mgml Hygromycin B Transgene expression was analyzed by western blot-

ting and quantitative RT-PCR

Proteinndashprotein interaction assay

For the BiFC assay the BES1-cYFP SIZ1-nYFP SUMO1-nYFP cYFP and nYFP

constructs were transformed into A tumefaciens (strain GV3101) respectively

Agrobacteria were grown in LB medium plus both 50 mgml Rifampin and

50mgml spectinomycin Combinations of Agrobacterium were infiltrated

into young leaves of N benthamiana and examined for YFP signals under a

fluorescence microscope (Leica) 2 d after infiltration

Yeast two-hybrid assay was performed using the Gal4-based two-hybrid

system (Clontech) The AD-SIZ1 and BD-BES1 constructs were transformed

into yeast strain AH109 using the lithium acetate method Yeast cells were

grown on medium lacking leucine tryptophan (-LW) Transformants were

plated on to medium lacking tryptophan leucine histidine and adenine

(-LWHA) to test the interaction between SIZ1 and BES1

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For His pull-down assay BES1 truncated BES1 fragments and different mutated

BES1 fused with MBP were purified with amylose resin (NEB) SIZ1 and SUMO1

fused with His were purified with Ni-NTA agarose (Qiagen) His pull-down assays

were performed as previously described (Yin et al 2002) Ni-NTA agarose contain-

ing His-SIZ1 or His-SUMO1 were incubated with MBP MBP-BES1 truncated MBP-

BES1 proteins or mutated MBP-BES1 proteins in pull-down binding buffer [20 mM

Tris (pH 80) 150 mM NaCl 02 Triton X-100] The mixtures were rotated in a

cold room for 2 h and the agarose was washed five times with washing buffer

(50 mM NaH2PO4 300 mM NaCl 20 mM imidazole 005 Tween 20) and boiled

with 1 SDS loading buffer separated by SDS-polyacrylamide gel electrophoresis

(PAGE) and immunoblotted with anti-MBP antibodies

For Co-IP assays in the Arabidopsis protoplasts GFP-fused SIZ1 was

cotransformed with FLAG-HA-fused BES1 or empty FLAG-HA vector into

Arabidopsis protoplasts After overnight incubation total proteins were ex-

tracted from the protoplasts The protein extracts were incubated with anti-

HA affinity gel (Sigma E-6779-1ML) at 4C for 3 h Next the anti-HA affinity gel

was washed with protein extraction buffer at least four times and mixed with

SDS loading buffer for immunoblot analysis The input proteins and immuno-

precipitates were detected with either anti-HA antibodies or anti-GFP

antibodies

Gene expression analysis

Ten-day-old seedlings grown under long-day conditions were used for total

RNA extraction and qRT-PCR analysis of BR-responsive gene expression Total

RNAs were extracted using the RNAprep pure Plant Kit (from Transgene

Biotech Co Ltd of Qiagen Beijing) according to the manufacturesrsquo protocols

and 2 mg total RNAs were converted into cDNAs using M-MLV Reverse

Transcriptase Kit (Invitrogen USA) The qRT-PCR analysis was carried out

using the SYBR Premix Ex TaqTM II (TAKARA) on a BIO-RAD CFX

ConnectTM Real-Time System according to the manufacturerrsquos instruction

Three independent experiments were performed and three technical replicates

of each experiment were performed Actin was used as an internal control for

the normalization of transcript levels (Czechowski et al 2005)

DNA extraction

Four-week-old plants were used for DNA extraction Ten milligrams leaves of

Col-0 siz1-2 bes1-D and different lines of siz1-2bes1-D plants were harvested

and ground to a fine powder in liquid nitrogen and then added to 660 ml DNA

extraction buffer (1 M Tris-HCl pH 74 05 M EDTA 5 M NaCl 20 SDS) After

centrifugation at 13000 rpm for 10 min the supernatants were obtained and

added to an equal volume of isopropanol After 5 min at room temperature the

DNA extractions were collected by centrifuging at 12000 rpm for 10 min and

then washed with 70 ethanol for two times The DNA extractions were

dissolved in 40ndash60 ml water

In vitro sumoylation assay

For in vitro sumoylation assay the coding regions of SAE1b SAE2 SCE1 SIZ1

and SUMO1 were cloned into pET-28a vector whereas BES1 and mutated BES1

(BES1K88R BES1K205R and BES1K302R) were incorporated into pETMAL vector All

of the constructs (His-SAE1b His-SAE2 His-SCE1 His-SIZ1 His-SUMO1 MBP-

BES1 MBP-BES1K88R MBP-BES1K205R and MBP-BES1K302R) were transformed

into Escherichia coli BL21 (DE3) which was used to express recombinant pro-

teins His-tagged proteins were purified with Ni-NTA agarose (Qiagen) whereas

MBP-tagged proteins were purified with amylose resin (NEB) The in vitro

sumoylation assay was performed as described previously (Miura et al 2005

Miura et al 2007) In brief 50 ng of His-SAE1b 50 ng of His-SAE2 50 ng of

His-SCE1 8 mg of His-SUMO1 and 100 ng of MBP-BES1 (or MBP-BES1K88R or

MBP-BES1K205R or MBP-BES1K302R) were incubated in 30ml of reaction buffer

containing 50 mM Tris-HCl pH 74 10 mM ATP 2 mM dithiothreitol (DTT) and

5 mM MgCl2 The reactions were incubated over-night at 30C Proteins were

separated by SDS-PAGE and immunoblot analysis was performed using anti-

MBP antibodies

In vivo sumoylation assay

To determine the sumoylation status of BES1 total proteins were extracted in

a buffer composed of 100 mM Tris-Cl (pH 75) 300 mM NaCl 2 mM EDTA

(pH 80) 1 TritonX-100 and 10 Glycerol (Yang et al 2017) Then the protein

extracts were immunoprecipitated with anti-HA affinity gel (Sigma E-6779-1ML)

for 3 h in a cold room Next the anti-HA affinity gel was washed with protein

extraction buffer at least four times and the immunoprecipitated proteins were

eluted with 2 SDS loading buffer for immunoblot analysis The sumoylated form

of BES1 was identified with anti-SUMO1 antibodies (Abcam ab5316)

Immunoblotting analysis of BES1

Col-0 and siz1-2 Arabidopsis seedlings were grown on 12 MS medium under

long-day conditions (16-h light8-h dark) at 22C after 10 d the seedlings were

harvested and ground to a fine powder in liquid nitrogen and then added to

appropriate 2 SDS loading buffer (Martınez-Garcıa et al 1999) boiled at 95C

for 8ndash10 min After centrifugation at 12000 rpm for 1 min the supernatants

containing total proteins were separated on 12 SDS-PAGE Since BES1 and

ACTIN have similar molecular weights we separated our analyses onto two

separate blots One membrane was probed with anti-BES1 antibodies and an-

other with anti-Actin antibodies for equal loading

Protein extraction and cell-free degradation

Ten-day-old Col-0 and siz1-2 seedlings were harvested and ground into fine

powder in liquid nitrogen Total proteins were extracted in degradation buffer

[26 mM Tris-HCl pH 75 10 mM NaCl 10 mM MgCl2 4 mM phenylmethylsul-

fonyl fluoride (PMSF) 5 mM DTT and 10 mM ATP] The cell-free degradation

assay was performed as described (Wang et al 2009) In brief the total protein

extracts were adjusted to equal concentration with the degradation buffer

determined by the Bio-Rad protein assay and then 100 ng of recombinant

MBP-BES1 protein was incubated in 40 ml extracts (containing 20 mg of total

proteins) for individual assays Exogenous MG132 was added to the extracts for

the indicated time The extracts were incubated at 22C and samples were

taken at indicated times for determination of BES1 protein abundance by

immunoblots with anti-MBP antibodies The band intensity was quantified

using ImageJ software

Protoplast transformation and LUC reportertransactivation assays

The At2g45210 promoter (1021 bp including 50-UTR) DWF4 promoter (972 bp

including 50-UTR) and SAUR-AC1 promoter (697 bp including 50-UTR ) were

cloned into the pGreenII0800-LUC vector separately to generate the reporter

constructs (At2g45210P-LUC DWF4P-LUC and SAUR-AC1P-LUC) whereas the

coding regions of BES1 and BES1K302R were fused with the FLAG-HA tag into

pCM1307 vector to generate the recombinant constructs (FLAG-HA-BES1 and

FLAG-HA-BES1K302R) These constructs were used to transform Col-0

Arabidopsis protoplasts The protoplasts preparation and transformation

were performed as described previously (Yoo et al 2007) In brief well-ex-

panded leaves of 4-week-old plants were cut into 05ndash1 mm strips with a

razor blade and incubated in freshly prepared enzyme solution [20 mM MES

pH 57 15 (wv) cellulase R10 04 (wv) macerozyme R10 04 M mannitol

10 mM CaCl2 and 20 mM KCl] at room temperature in the dark for 3ndash4 h The

protoplast-containing enzyme solution was diluted with an equal volume of

W5 buffer (2 mM MES pH 57 154 mM NaCl 125 mM CaCl2 and 5 mM KCl)

before filtration through a clean 75-mm nylon mesh And then the flow-

through was centrifuged at 100g to obtain a protoplast pellet The pellet

was washed two times with W5 solution the protoplasts were resuspended

in MMG solution (4 mM MES pH 57 04 M mannitol and 15 mM MgCl2) and

kept at room temperature for PEG-mediated transformation For each trans-

formation 10ndash20 mg of plasmid DNA was added to a 2-ml microfuge tube to

which 100 ml of protoplasts were gently added Then 120ml of PEG solution was

added slowly and mixed by tapping and the mixture was incubated at room

temperature for 15 min A solution of 440 ml W5 was added to dilute the mix-

ture mixed by inverting the tubes and centrifuged at 100g for 2 min at room

temperature The supernatant was removed and the protoplasts were resus-

pended in 1 ml of WI solution (4 mM MES pH 57 05 M mannitol and 20 mM

KCl) and incubated overnight at room temperature The protoplasts were

treated with 30mM MG132 for 3 h before they were resuspended and harvested

by centrifugation at 100g for 2 min then the supernatants were removed and

the protoplasts were freezed in liquid nitrogen for 1 min

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For LUC assay 100ml of protoplast lysis buffer was added to the frozen

protoplasts and mixed with a pipette gun After 5 min incubation on ice 20 ml

of the lysate harvested by centrifugation at 1000g for 2 min and 100 ml LUC

mix were used to measure LUC activity

ChIP assay

ChIP was performed as previously described (Saleh et al 2008) In brief 15 g of

the 10-day-old Col-0 and siz1-2 plants treated withwithout 30 mM MG132 for

3 h were harvested and cross-linked with formaldehyde Chromatin was isolated

and sonicated to generate fragments with the average size of 300 bp Anti-BES1

antibodies were used to immunoprecipitate chromatin The level of precipi-

tated DNA fragments was quantified by qPCR TA3 was used as an internal

control for the normalization of transcript levels

Primers

All primers used in this article are listed in Supplementary Table S1

Supplementary Data

Supplementary data are available at PCP online

Funding

The National Natural Science Foundation of China [31570237and 31670235] the National Basic Research Program of China[973 Program (2015CB150100)] the Development Project ofTransgenic Crops of China [2016ZX08009-003ndash002] NationalKey RampD Program of China [2018YFD0201100] theFundamental Research Funds for the Central Universities[SCU2019D013 SCU2018D006]

Disclosures

The authors have no conflicts of interest to declare

References

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The Arabidopsis E3 SUMO ligase SIZ1 regulates plant growth anddrought responses Plant Cell 19 2952ndash2966

Cheng XF Xiong RY Li YZ Li FF Zhou XP and Wang AM (2017)Sumoylation of Turnip mosaic virus RNA polymerase promotes viral

infection by counteracting the host NPR1-mediated immune responsePlant Cell 29 508ndash525

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Plant J 16 735ndash743Clouse SD Langford M and McMorris TC (1996) A brassinosteroid-

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(2005) Genome-wide identification and testing of superior referencegenes for transcript normalization in Arabidopsis Plant Physiol 139

5ndash17Friedrichsen DM Joazeiro CAP Li JM Hunter T and Chory J (2000)

Brassinosteroid-insensitive-1 is a ubiquitously expressed leucine-richrepeat receptor serinethreonine kinase Plant Physiol 123 1247ndash1256

Geiss-Friedlander R and Melchior F (2007) Concepts in sumoylation adecade on Nat Rev Mol Cell Biol 8 947ndash956

Geuns JMC (1978) Steroid hormones and plant growth and develop-ment Phytochemistry 17 1ndash14

Guo HQ Li L Aluru M Aluru S and Yin YH (2013) Mechanisms andnetworks for brassinosteroid regulated gene expression Curr Opin

Plant Biol 16 545ndash553Johnson ES and Gupta AA (2001) An E3-like factor that promotes

SUMO conjugation to the yeast septins Cell 106 735ndash744Kerscher O (2007) SUMO junctionmdashwhatrsquos your function New insights

through SUMO-interacting motifs EMBO Rep 8 550ndash555Kim B Jeong YJ Corvalan C Fujioka S Cho S Park T et al (2014)

Darkness and gulliver2phyB mutation decrease the abundance ofphosphorylated BZR1 to activate brassinosteroid signaling in

Arabidopsis Plant J 77 737ndash747Kim SI Park BS Kim DY Yeu SY Song SI Song JT et al (2015) E3

SUMO ligase AtSIZ1 positively regulates SLY1-mediated GA signalingand plant development Biochem J 469 299ndash314

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Cell Biol 22 5222ndash5234Kurepa J Walker JM Smalle J Gosink MM Davis SJ Durham TL

et al (2003) The small ubiquitin-like modifir (SUMO) protein modifia-tion system in Arabidopsis Accumulation of SUMO1 and -2 conjugates

is increased by stress J Biol Chem 278 6862ndash6872Lee J Nam J Park HC Na G Miura K Jin JB et al (2007) Salicylic

acid-mediated innate immunity in Arabidopsis is regulated by SIZ1SUMO E3 ligase Plant J 49 79ndash90

Li JM and Chory J (1997) A putative leucine-rich repeat receptor kinaseinvolved in brassinosteroid signal transduction Cell 90 929ndash938

Li JM and Nam KH (2002) Regulation of brassinosteroid signaling by aGSK3SHAGGY-like kinase Science 295 1299ndash1301

Lin XL Niu D Hu ZL Kim DH Jin YH Cai B et al (2016) AnArabidopsis SUMO E3 ligase SIZ1 negatively regulates photomorpho-

genesis by promoting COP1 activity PLoS Genet 12 e1006016ndash

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modulates abscisic acid signaling in Arabidopsis Plant Cell 15 1347ndash1359Mangelsdorf DJ and Evans RM (1995) The RXR heterodimers and

orphan receptors Cell 83 841ndash850Martınez-Garcıa JF Monte E and Quail PH (1999) A simple rapid and

quantitative method for preparing Arabidopsis protein extracts forimmunoblot analysis Plant J 20 251ndash257

Miura K Jin JB Lee J Yoo CY Stirm V Miura T et al (2007) SIZ1-mediated sumoylation of ICE1 controls CBF3DREB1A expression and

freezing tolerance in Arabidopsis Plant Cell 19 1403ndash1414Miura K Lee J Jin JB Yoo CY Miura T and Hasegawa PM (2009)

Sumoylation of ABI5 by the Arabidopsis SUMO E3 Ligase SIZ1 negativelyregulates abscisic acid signaling Proc Natl Acad Sci USA 106 5418ndash5423

Miura K Rus A Sharkhuu A Yokoi S Karthikeyan AS RaghothamaKG et al (2005) The Arabidopsis SUMO E3 ligase SIZ1 controls phos-

phate deficiency responses Proc Natl Acad Sci USA 102 7760ndash7765Murtas G Reeves PH Fu YF Bancroft I Dean C and Coupland G

(2003) A nuclear protease required for flowering-time regulation inArabidopsis reduces the abundance of SMALL UBIQUITIN-RELATED

MODIFIER conjugates Plant Cell 15 2308ndash2319Nolan TM Brennan B Yang MR Chen JN Zhang MC Li ZH et al

(2017) Selective autophagy of BES1 mediated by DSK2 balances plantgrowth and survival Dev Cell 41 33ndash46

Park BS Song JT and Seo HS (2011) Arabidopsis nitrate reductaseactivity is stimulated by the E3 SUMO ligase AtSIZ1 Nat Commun 2

400ndash409Rechsteiner M and Rogers SW (1996) PEST sequences and regulation by

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matin immunoprecipitation (CHIP) protocol for studying histonemodifications in Arabidopsis plants Nat Protoc 3 1018ndash1025

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Saleh A Withers J Mohan R Marques J Gu YN Yan SP et al (2015)Posttranslational modifications of the master transcriptional regulator

NPR1 enable dynamic but tight control of plant immune response CellHost Microbe 18 169ndash182

Sampson DA Wang M and Matunis MJ (2001) The small ubiquitin-likemodifier-1 (SUMO-1) consensus sequence mediates ubc9 binding and

is essential for SUMO-1 modification J Biol Chem 276 21664ndash21669Tang WQ Yuan M Wang RJ Yang YH Wang CM Oses-Prieto JA et al

(2011) PP2a activates brassinosteroid-responsive gene expression and plantgrowth by dephosphorylating BZR1 Nat Cell Biol 13 124ndash131

Wang F Zhu DM Huang X Li S Gong YN Yao QF et al (2009)Biochemical insights on degradation of Arabidopsis DELLA proteins

gained from a cell-free assay system Plant Cell 21 2378ndash2390Wang Y Sun SY Zhu WJ Jia KP Yang HQ and Wang XL (2013)

StrigolactoneMAX2-induced degradation of brassinosteroid transcrip-tional effector BES1 regulates shoot branching Dev Cell 27 681ndash688

Wang ZY Nakano T Gendron J He JX Chen M Vafeados D et al(2002) Nuclear-localized BZR1 mediates brassinosteroid-induced

growth and feedback suppression of brassinosteroid biosynthesis DevCell 2 505ndash513

Wang ZY Seto H Fujioka S Yoshida S and Chory J (2001) BRI1 is acritical component of a plasma-membrane receptor for plant steroids

Nature 410 380ndash383Wilkinson KA and Henley JM (2010) Mechanisms regulation and con-

sequences of protein SUMOylation Biochem J 428 133ndash145Yang CJ Zhang C Lu YN Jin JQ and Wang XL (2011) The mechan-

isms of brassinosteroidsrsquo action from signal transduction to plant de-velopment Mol Plant 4 588ndash600

Yang MR Li CX Cai ZY Hu YM Nolan T Yu FF et al (2017) SINATE3 ligases control the light-mediated stability of the brassinosteroid-

activated transcription factor BES1 in Arabidopsis Dev Cell 41 47ndash58Ye HX Liu SZ Tang BY Chen JN Xie ZL Nolan TM et al (2017)

RD26 mediates crosstalk between drought and brassinosteroid signal-ling pathways Nat Commun 8 14573ndash14585

Yin YH Wang ZY Mora-Garcia S Li JM Yoshida S Asami T et al(2002) BES1 accumulates in the nucleus in response to brassinosteroids

to regulate gene expression and promote stem elongation Cell 109181ndash191

Yoo SD Cho YH and Sheen J (2007) Arabidopsis mesophyll proto-plasts a versatile cell system for transient gene expression analysis Nat

Protoc 2 1565ndash1572Yu XF Li L Li L Guo M Chory J and Yin YH (2008) Modulation of

brassinosteroid-regulated gene expression by Jumonji domain-contain-ing proteins ELF6 and REF6 in Arabidopsis Proc Natl Acad Sci USA

105 7618ndash7623Yu XF Li L Zola J Aluru M Ye HX Foudree A et al (2011) A

brassinosteroid transcriptional network revealed by genomewide iden-tification of BESI target genes in Arabidopsis thaliana Plant J 65 634ndash

646Zhao J Peng P Schmitz RJ Decker AD Tax FE and Li JM (2002) Two

putative BIN2 substrates are nuclear components of brassinosteroidsignaling Plant Physiol 130 1221ndash1229

Zheng Y Schumaker KS and Guo Y (2012) Sumoylation of transcrip-tion factor MYB30 by the small ubiquitin-like modifier E3 ligase SIZ1

mediates abscisic acid response in Arabidopsis thaliana Proc NatlAcad Sci USA 109 12822ndash12827

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