段俊枝 楊翠蘋 王楠 馮麗麗 燕照玲 齊紅志 陳海燕 張會芳 卓文飛 齊學禮
摘要:土壤鹽漬化是抑制植物生長發(fā)育、降低作物產(chǎn)量的主要環(huán)境脅迫之一。利用基因工程技術培育耐鹽植物以提高植物耐鹽性是促進植物生長、提高作物產(chǎn)量的有效途徑。植物特異轉錄因子WRKY可以調控植物生長發(fā)育、響應多種脅迫(如鹽、干旱、病原菌等),在抵御鹽脅迫過程中具有重要作用。本文闡述了WRKY轉錄因子的基本結構,綜述了來自各種植物(糧食、經(jīng)濟、園藝作物及其他植物)的WRKY轉錄因子在模式植物(擬南芥、煙草)、糧食作物(水稻、玉米)、經(jīng)濟作物(大豆、棉花)、園藝作物(番茄、茄子、菊花、蘋果)及其他植物(柳樹、楊樹)耐鹽基因工程中的應用進展,分析了該領域目前存在的問題(轉單個WRKY基因對植物耐鹽性的提高程度有限,大部分轉WRKY基因植株僅僅提高了營養(yǎng)生長期耐鹽性鑒定,組成型超表達WRKY基因會引起轉基因植株生長緩慢、花期推遲甚至產(chǎn)量降低等不良后果等),并提出建議,以期為WRKY轉錄因子在植物耐鹽遺傳改良及育種中的應用提供參考依據(jù)。
關鍵詞:植物;WRKY轉錄因子;耐鹽;基因工程
中圖分類號:S188 文獻標志碼:A
文章編號:1002-1302(2023)05-0071-10
土壤鹽漬化是一個世界性問題,尤其在干旱、半干旱地區(qū),我國土壤鹽漬化問題十分嚴重。隨著氣候變化及不良灌溉方式等的影響,土壤鹽漬化越來越嚴重[1]。土壤鹽漬化是植物生長發(fā)育過程中遭遇的主要環(huán)境脅迫之一,嚴重抑制植物生長,降低作物產(chǎn)量[2]。因此,培育耐鹽植物品種對保障農(nóng)業(yè)生產(chǎn)及國家糧食安全具有重要意義。利用傳統(tǒng)的育種方法培育耐鹽品種進展緩慢,利用分子生物技術挖掘耐鹽基因,通過基因工程技術提高植物耐鹽性是一條行之有效的途徑。目前,已經(jīng)發(fā)現(xiàn)一些轉錄因子,如AP2/ERF(APETALA2/Ethylene response factor)、bZIP(basic leucine zipper)、MYB (v-myb avian myeloblastosis viral oncogene homolog)、WRKY等,會調控下游基因抵御鹽脅迫[3-11]。其中,WRKY是植物特異轉錄因子家族,其家族成員眾多,功能多樣,可以調控植物生長發(fā)育、響應多種脅迫(如鹽、干旱、病原菌等),在抵御鹽脅迫過程中具有重要作用[6-14]。本文闡述了WRKY轉錄因子的基本結構,綜述了WRKY轉錄因子在擬南芥(Arabidopsis thaliana)、煙草(Nicotiana tabacum)、小麥(Triticum aestivum L.)、玉米(Zea mays)、大豆(Glycine max)、番茄(Solanum lycopersicum)、菊花等植物耐鹽基因工程中的應用進展,分析了該領域目前存在的問題,并提出建議,以期為WRKY轉錄因子在植物耐鹽遺傳改良及育種中的應用提供參考依據(jù)。
1 WRKY轉錄因子的基本結構特征
WRKY轉錄因子都含有1~2個高度保守的能與DNA結合的WRKY結構域,該結構域包含約60個氨基酸殘基,N端為高度保守的WRKYGQK基序,C端為保守的C2H2(CX4-5CX22-23HX1H)或C2HC(C-X7-C-X23-HX1-C)類鋅指基序,這2個基序控制著WRKY轉錄因子與DNA的結合[15]。WRKY轉錄因子特異性地與下游基因啟動子區(qū)的順式作用元件W-box(TTGACC/T)結合,其結合力主要基于W、R、K、Y、C、H,但也受 W-box 周圍氨基酸的影響[16-18]。
根據(jù)WRKY 結構域的數(shù)量和鋅指基序的特點,WRKY 轉錄因子可分為3 類[19-21]。第Ⅰ類WRKY轉錄因子含有2個WRKY結構域,每個WRKY結構域具有不同的功能,第Ⅱ和第Ⅲ類WRKY轉錄因子都只含有1個WRKY結構域;第Ⅰ和第Ⅱ類WRKY轉錄因子都含有C2H2基序,第Ⅲ類WRKY轉錄因子含有C2HC基序;另外,第Ⅱ類WRKY轉錄因子根據(jù)發(fā)育進程,又可進一步分為a~e 5個亞類[19-21]。
2 WRKY轉錄因子在植物耐鹽基因工程中的應用進展
目前,已經(jīng)從眾多植物中分離得到WRKY基因,其中很多WRKY基因可以調控植物的耐鹽性,對這些基因進行超表達或者沉默表達提高了模式植物(擬南芥、煙草)、糧食作物(水稻、玉米)、經(jīng)濟作物(大豆、棉花)、園藝作物(番茄、茄子、菊花、蘋果)及其他植物(柳樹、楊樹)等的耐鹽性,有的提高了營養(yǎng)生長期的耐鹽性,有的提高了生殖生長期的耐鹽性,甚至產(chǎn)量,這為植物耐鹽遺傳改良及育種提供了重要的基因資源。
2.1 擬南芥耐鹽基因工程
2.1.1 糧食作物WRKY基因
小麥TaWRKY79基因受鹽和ABA(abscisic acid)誘導表達,超表達該基因降低了擬南芥植株對ABA的敏感性,提高了擬南芥在鹽脅迫條件下的主根長和ABA相關基因[ABA1、ABA2、ABI1(abscisic acid-insensitive 1)、ABI5]的表達量,進而提高了耐鹽性[6]??梢奣aWRKY79通過ABA依賴途徑調控擬南芥的耐鹽性。類似的TaWRKY93基因也受鹽和ABA誘導表達,超表達該基因擬南芥植株在鹽脅迫條件下的主根長、側根數(shù)增加,葉片脯氨酸含量、相對含水量及脅迫相關基因[ABF3(ABA responsive element binding factors 3)、ABI1、ABI2、ABI3、DREB2A(dehydration responsive element binding factors 2A)、RD19A(responsive to dehydration 1A)、ICE1(inducer of CBF expression 1)、RD21、P5CS(Δ1-pyrroline-5-carboxylate synthetase)]的表達量提高,電解質滲透率降低,進而耐鹽性提高[7]。表明TaWRKY93通過提高滲透調節(jié)能力、保持細胞膜穩(wěn)定、上調脅迫相關基因表達量來提高擬南芥的耐鹽性。另外,超表達TaWRKY2基因、TaWRKY19基因均提高了擬南芥植株在鹽脅迫條件下的存活率、抽薹率和株高[8]。這主要歸因于超表達TaWRKY2、TaWRKY19基因提高了擬南芥植株的可溶性糖含量和一些脅迫響應基因[RD29B、DREB2A、RD29A、COR6.6(cold regulated 6.6)]基因的表達量,降低了MDA含量和電解質滲透率。其中,TaWRKY2可與RD29B基因啟動子結合,TaWRKY19可與DREB2A、COR6.6基因啟動子結合。綜上,TaWRKY2、TaWRKY19通過直接或間接調控脅迫相關基因的表達量來提高擬南芥的抗逆性。此外,超表達TaWRKY49、TaWRKY92、TaWRKY112、TaWRKY142[9]和TaWRKY75-A[10]基因也均提高了擬南芥植株的耐鹽性。
除了小麥外,在其他糧食作物如苦蕎(Fagopyrum tataricum)[22]、水稻(Oryza sativa)[23]、甘薯[Ipomoea batatas (L.) Lam.][24]、玉米[25]、高粱(Sorghum bicolor)[26]中也發(fā)現(xiàn)了調控耐鹽性的WRKY基因。研究發(fā)現(xiàn),超表達苦蕎FtWRKY46基因提高了擬南芥在鹽脅迫條件下的發(fā)芽率、根長、葉綠素含量、脯氨酸含量和SOD(superoxide dismutase)、POD(peroxidase)、CAT(catalase)活性及脅迫相關基因[RD29A、DREB2B、RD29B、RD2、RAB18(responsive to ABA? 18)、SOS1(salt overly sensitive 1)、SOS2、SOS3、NHX1(Na+/H+antiporter 1)]的表達量,降低了O-2·、H2O2、MDA含量,進而提高了耐鹽性[22]。綜上,F(xiàn)tWRKY46通過調控活性氧清除系統(tǒng)和脅迫相關基因的表達量來提高擬南芥的耐鹽性。水稻OsWRKY42基因受鹽、細胞壁降解酶誘導表達,超表達該基因提高了擬南芥植株在鹽脅迫條件下的胼胝質和花青素含量,降低了一些細胞壁損傷和鹽脅迫誘導的茉莉酸合成及響應基因[AOC3(allene oxide cyclase3)、LOX2(lipoxygenase-2)、COI-1(coronatine insensitive-1)、JAZ1(jasmonate ZIM motif 1)、JAZ10、ERF]的表達量,最終提高了耐鹽性[23]。說明OsWRKY42通過負調控茉莉酸介導的脅迫響應來提高擬南芥的耐鹽性。甘薯IbWRKY2基因受鹽、干旱、ABA誘導表達,超表達該基因提高了擬南芥植株的耐鹽性和抗旱性[24]。這主要歸因于超表達IbWRKY2基因提高了擬南芥葉片ABA含量、脯氨酸含量、SOD活性及ABA信號轉導途徑基因[ZEP(zeaxanthine poxidase)、NCED(9-cis-epoxycarotenoid dioxygenase)]、脯氨酸合成途徑基因[P5CR(pyrroline-5-carboxylate reductase)]、活性氧清除系統(tǒng)基因[CAT、APX(ascorbate peroxidase)、POD、GPX(glutathione peroxidase)、DHAR(dehydroascorbate reductase)]的表達量,降低了MDA和H2O2 含量;另外,IbWRKY2可與IbVQ4(含有VQmotif的蛋白)相互作用,而IbVQ4基因受鹽和干旱誘導表達。綜上,IbWRKY2通過激活ABA信號途徑、活性氧清除系統(tǒng)及與IbVQ4互作來提高擬南芥的抗逆性。此外,玉米ZmWRKY17[25]基因、甜高粱SbWRKY50基因[26]均受鹽誘導表達,但這2個基因負調控鹽脅迫響應。在鹽脅迫條件下,超表達ZmWRKY17基因擬南芥植子葉綠化率及RAB18、RD29B、DREB1F、KIN1(kinase 1)、NAC019[NAM(No apical meristem)、ATAF1(Arabidopsis transcription activation factor 1)、ATAF2、CUC2(Cup-shaped cotyledon 2) 019]基因的表達量降低,根系生長緩慢,相對電解質滲透率、MDA含量提高,且超表達ZmWRKY17降低了擬南芥植株對ABA的敏感性[25];超表達SbWRKY50基因擬南芥的發(fā)芽率、根長、生物量和鉀離子含量顯著降低,O-2·、H2O2 含量、鈉離子含量提高,且SbWRKY50與SOS1和HKT1(high-affinity K transporter 1)基因啟動子區(qū)結合,進而調控其表達[26]。綜上,ZmWRKY17通過ABA信號通路負調控擬南芥的耐鹽性,SbWRKY50通過調控離子平衡來負調控擬南芥的耐鹽性,可以通過沉默這些基因表達的方式來提高植物的耐鹽性。
2.1.2 經(jīng)濟作物WRKY基因
陸地棉(Gossypium hirsutum)GhWRKY6-like基因受鹽、干旱、ABA誘導表達,超表達GhWRKY6-like基因擬南芥植株在鹽脅迫條件下的發(fā)芽率、根長增加,耐鹽性提高,這主要得益于超表達GhWRKY6-like基因降低了擬南芥植株H2O2、MDA含量,提高了脯氨酸含量、SOD活性、POD活性及ABA信號途徑基因[AtABF4、AtABI5、AtMYC2(v-myc avian myelocytomatosis viral oncogene homolog 2)]、滲透脅迫基因(AtSOS2、AtRD29a、AtRD29b)的表達量[27]。說明GhWRKY6-like通過清除活性氧、調控ABA信號途徑來提高擬南芥的耐鹽性。類似的,超表達GhWRKY34基因也提高了擬南芥植株的耐鹽性,這是因為超表達GhWRKY34基因降低了擬南芥植株Na+/K+,提高了葉綠素含量及一些脅迫相關基因(RD29A、ABF4、SOS1、SOS2)的表達量[28]??梢奊hWRKY34通過調控Na+/K+和脅迫相關基因的表達量來提高擬南芥的耐鹽性。另外,超表達旱地棉(Gossypium aridum L.)GarWRKY5基因提高了鹽脅迫條件下擬南芥葉片SOD活性、POD活性及活性氧清除系統(tǒng)基因[GST(glutamine S-transferases)、SOD]、茉莉酸和SA響應基因的表達量,進而提高了擬南芥的耐鹽性[29-30]。綜上,GarWRKY5通過提高活性氧清除能力來提高擬南芥的耐鹽性。
大豆GmWRKY16基因受鹽、干旱、ABA誘導表達,超表達該基因促進了擬南芥植株在鹽、干旱脅迫條件下的發(fā)芽和根系生長,降低了失水量和MDA含量,提高了葉片脯氨酸含量及AtWRKY8、KIN1、RD29A基因的表達量,進而提高了耐鹽性和抗旱性[31]。類似的,GmWRKY49基因[32-33]受鹽誘導表達,GmWRKY45基因[34]受鹽、磷饑餓誘導表達,超表達這2個基因也均提高了擬南芥植株的耐鹽性,這主要歸因于轉基因擬南芥植株可溶性糖、脯氨酸含量提高。另外,超表達GmWRKY45基因還提高了擬南芥植株對磷饑餓的耐受性[34]。此外,超表達大豆GmWRKY54基因提高了擬南芥植株中DREB2A、RD29B、ERD10(early responsive to dehydration 10)基因的表達量,增強了擬南芥的耐鹽性[35]。
2.1.3 其他植物WRKY基因
長葉紅砂(Reaumuria trigyna)RtWRKY1[36]基因、RtWRKY23[37-38]基因均受鹽、低溫、ABA誘導表達,超表達這2個基因均提高了擬南芥植株的耐鹽性。超表達RtWRKY23基因擬南芥耐鹽性的提高主要得益于葉片脯氨酸含量和GPX、POD、SOD、CAT活性及離子轉運相關基因(AtSOS1)、脯氨酸合成相關基因[AtP5CS1、AtP5CS2、AtPRODH1(proline dehydrogenase 1)、AtPRODH2]、抗氧化相關基因(AtAPX1、AtCAT1、AtSOD1)表達量提高,MDA、Na+含量和Na+/K+降低[36]??梢奟tWRKY1通過調控滲透平衡、Na+/K+平衡、抗氧化系統(tǒng)來提高擬南芥的耐鹽性。超表達RtWRKY23基因擬南芥耐鹽性的提高主要是因為超表達RtWRKY23基因提高了擬南芥根長、葉綠素含量、脯氨酸含量、POD活性、O-2·清除率及一些脅迫相關基因(AtPOD、AtPOD22、AtPOD23、AtP5CS1、AtP5CS2、AtPRODH2)表達量,降低了H2O2和MDA含量[37-38]。說明RtWRKY23通過維持活性氧平衡、滲透平衡來提高擬南芥的耐鹽性。另外,剛毛怪柳(Tamarix hispida)ThWRKY4基因受鹽、干旱、ABA誘導表達,超表達該基因提高了擬南芥在鹽脅迫條件下的SOD活性、POD活性、葉綠素含量及SOD、POD基因表達量,降低了O-2·、H2O2含量及電解質滲透率、細胞死亡率,進而提高了耐鹽性[39]。綜上,ThWRKY4通過提高活性氧清除能力來提高擬南芥的耐鹽性。 此外,超表達楊樹(Populussimonii×P. nigra)WRKY56基因也提高了擬南芥在鹽脅迫條件下的脯氨酸含量及POD、SOD活性,降低了MDA含量、脂質過氧化率,進而提高了耐鹽性[40]。
除了長葉紅砂、剛毛怪柳、楊樹等,在毛竹(Phyllostachys edulis)[41]、葡萄[42]、擬南芥[43-44]中也發(fā)現(xiàn)了調控耐鹽性的WRKY基因。研究發(fā)現(xiàn),超表達毛竹PeWRKY83基因提高了擬南芥植株在鹽脅迫條件下的發(fā)芽率、根長、鮮質量、脯氨酸含量、ABA含量及ABA合成基因[AtAAO3 (aldehyde oxidase 3)、AtNCED2、AtNCED3]、信號通路基因[AtABI1、AtPP2CA(serine/threonine protein phosphatases 2CA)]、響應基因(AtRD29A、AtRD29B、AtABF1) 的表達量,降低了MDA含量和電解質滲透率及對ABA的敏感性[41]。說明PeWRKY83通過調控脅迫誘導ABA合成來提高擬南芥的耐鹽性。另外,超表達葡萄(Vitis vinifera L.)VvWRKY30基因提高了擬南芥在鹽脅迫條件下的抗氧化酶(SOD、POD、CAT)活性、可溶性糖含量、脯氨酸含量及抗氧化酶編碼基因(Cu/ZnSOD、CAT1、CAT3、POD1)、糖代謝相關基因[SS1(sucrose synthetase 1)、SS4、G6DPH(glucose-6-phosphate dehydrogenase)、BAM1(β-amylase 1)、BAM4]、脯氨酸合成基因(P5CS)的表達量,降低了活性氧含量,進而提高了耐鹽性[42]。可見VvWRKY30通過調控活性氧清除和滲透調節(jié)物質積累來提高擬南芥的耐鹽性。此外,超表達AtWRKY25[43]、AtWRKY33[43]、AtWRKY30[44]基因也均提高了擬南芥植株的耐鹽性,其突變體植株反之。
2.2 煙草耐鹽基因工程
2.2.1 糧食作物、經(jīng)濟作物WRKY基因
小麥TaWRKY10基因受鹽、干旱、H2O2誘導表達,在煙草中超表達該基因提高了轉基因植株在鹽和干旱脅迫條件下的存活率,這主要歸因于超表達TaWRKY10基因提高了煙草葉片相對含水量、脯氨酸含量、可溶性糖含量及脅迫相關基因[NtSPSA(sucrose phosphate synthase)、NtGPX、NtERD10]的表達量,降低了MDA、O-2·、H2O2含量[45]。說明TaWRKY10通過調控滲透平衡、活性氧清除和脅迫相關基因的表達量來提高煙草的耐鹽性和抗旱性。類似的,超表達TaWRKY44基因也提高了煙草在鹽脅迫條件下的存活率、葉片相對含水量、可溶性糖含量、脯氨酸含量和CAT、POD、SOD活性及活性氧清除基因(NtCAT、NtAPX、NtPOD、NtSOD、NtGST)、多胺合成酶基因[NtSAMDC(S-adenosyl-L-methionine decarboxylase)、NtADC1(arginine decarboxylase? 1)]、蔗糖合成酶基因(NtSPSA)、脅迫響應蛋白基因(ERD10)、脂質轉移蛋白基因[NtLTP1(Lipid-transfer protein 1)]的表達量,降低了離子滲透率及H2O2、MDA含量[46]??梢奣aWRKY44可以調控活性氧平衡和脅迫相關基因的表達量,進而提高煙草的耐鹽性。
陸地棉GhWRKY41基因受鹽、干旱誘導表達,超表達該基因提高了煙草在鹽和干旱脅迫條件下的存活率、抗氧化酶(POD、SOD、CAT)活性及抗氧化酶編碼基因(NtPOD、NtSOD、NtCAT)的表達量,促進了氣孔關閉,降低了MDA含量,進而提高了耐鹽性和抗旱性[47]。可見GhWRKY41通過調控活性氧平衡和氣孔關閉來提高煙草的耐鹽性和抗旱性。類似的,GhWRKY39-1基因受鹽和MV(methyl viologen)誘導表達,超表達該基因提高了煙草對鹽及紋枯病、青枯病的抗性,伴隨著提高了一些病原相關基因[PR1c(pathogen-related 1c)、PR2、PR4]、活性氧清除基因(APX、CAT、GST、SOD)的表達量,降低了 O-2·、H2O2含量[48]。說明GhWRKY39-1通過調控活性氧清除能力來提高煙草對鹽及紋枯病、青枯病的抗性。另外,超表達GhWRKY25基因也提高了煙草植株的耐鹽性,但降低了對干旱和灰霉病的耐受性[49]。
2.2.2 園藝作物WRKY基因
醋栗番茄(Solanumpim pinellifolium L3708)SpWRKY1基因受鹽、干旱、ABA、SA(salicylic acid)、病原菌誘導表達,超表達該基因提高了煙草植株在鹽、干旱脅迫條件下的光合速率、氣孔導度、葉綠素含量、SOD活性、POD活性及抗逆相關基因 (NtSOD、NtPOD、NtP5CS、NtLEA5、NtNCED1) 的表達量,降低了MDA含量,進而提高了耐鹽性[50]。綜上,超表達SpWRKY1基因通過調控活性氧清除和脅迫相關基因的表達量來提高煙草的耐鹽性和抗旱性。類似的,番茄SlWRKY基因受鹽、干旱誘導表達,超表達該基因也提高了煙草植株的耐鹽性和抗旱性,這主要得益于超表達SlWRKY基因煙草植株SOD、POD活性及PR基因[PR1、PR2]表達量提高,MDA含量和電導率降低[51]。另外,超表達小白菜(Brassica campestris ssp. chinensis)BcWRKY46基因不僅提高了煙草植株的耐鹽性,而且提高了抗旱性和耐冷性[52]。此外,辣椒(Capsicum annuum)CaWRKY27基因負調控鹽脅迫響應,超表達該基因抑制煙草在鹽脅迫條件下的發(fā)芽和生長,促進其枯萎,且降低了活性氧清除基因(NtSOD、NtGST1、NtPOD1、NtPOD2)、多胺合成基因(NtADC1、NtSAMDC)、ABA合成基因(NtNCED1)及脅迫相關基因NtDREB3的表達量[53]。相反的,沉默該基因提高了辣椒的耐鹽性[53]。可見CaWRKY27負調控活性氧清除能力及脅迫相關基因的表達量,進而負調控耐鹽性,可以通過沉默該基因表達的方式提高植物的耐鹽性。
地被菊(Dendranthema grandiflorum)DgWRKY3基因受鹽、干旱誘導表達,在煙草中超表達該基因提高了轉基因植株在鹽脅迫條件下的脯氨酸含量、抗氧化酶(SOD、POD、CAT、APX)活性、抗氧化物質[AsA(ascorbate)、GSH(glutathione)]含量及脅迫相關基因[NtP5CS、NtLEA5、NtERD10D、NtSOD、NtPOD、NtCAT、NtAPX]的表達量,降低了MDA和H2O2含量,進而提高了耐鹽性[54]。說明DgWRKY3通過提高活性氧清除能力和脅迫相關基因的表達量來提高煙草的耐鹽性。另外,超表達葡萄VvWRKY2基因提高了煙草植株在鹽脅迫條件下的發(fā)芽率、生長勢和滲透調節(jié)物質(可溶性糖、脯氨酸)含量[55]。此外,超表達野生葡萄(Vitis pseudoreticulata)VpWRKY3基因也提高了煙草植株的耐鹽性,并且還提高了對青枯病的抗性[56]。
2.2.3 其他植物WRKY基因
麻風樹(Jatropha curcas)JcWRKY基因受SA誘導表達,超表達該基因提高了煙草的耐鹽性[57-58]。Agarwal等研究發(fā)現(xiàn),在鹽脅迫條件下,超表達JcWRKY基因煙草發(fā)芽率、葉綠素含量、可溶性糖含量、膜穩(wěn)定性、K+/Na+ 及SA合成基因ICS1、抗氧化酶編碼基因(CAT、SOD) 的表達量提高,電解質滲透率和活性氧(H2O2、O-2·) 含量降低[57]??梢奐cWRKY通過調控活性氧清除系統(tǒng)來提高煙草的耐鹽性。More等進一步對超表達JcWRKY基因煙草的光合能力和表皮蠟質成分進行研究,發(fā)現(xiàn)在鹽脅迫條件下,超表達JcWRKY基因煙草葉片的光合速率、氣孔導度、細胞間CO2濃度/環(huán)境CO2濃度、電子傳遞速率、光合效率(Fv/Fm)、光化學猝滅(qP)、非光化學猝滅(NPQ)和PSⅡ電子傳遞的量子產(chǎn)率(ΦPSⅡ)提高,且表皮蠟質的主要成分烷烴及其他蠟質成分脂肪醇、羧酸、脂肪酸的積累量提高[58]。說明JcWRKY還可以通過調控光合作用和蠟質代謝來提高煙草的耐鹽性。另外,超表達JcWRKY2基因也提高了煙草植株的耐鹽性[59]。這主要得益于超表達JcWRKY2基因提高了煙草葉片葉綠素含量、抗氧化酶(CAT、SOD)活性、SA含量及抗氧化酶編碼基因(NtCAT、NtSOD)、鈣結合基因(NtCal)、脫水素基因(NtERD10D)、磷脂酶C基因(NtPLC3)的表達量,降低了活性氧(H2O2、O-2·)含量。此外,超表達毛果楊(Populus trichocarpa)PtWRKY39基因提高了鹽和干旱脅迫條件下轉基因煙草的發(fā)芽率、鮮質量、葉綠素含量,降低了MDA和H2O2含量,進而提高了煙草的耐鹽性和抗旱性[60]。
2.3 糧食及經(jīng)濟作物耐鹽基因工程
水稻、玉米均是重要的糧食作物,利用耐鹽WRKY基因通過基因工程技術提高水稻、玉米的耐鹽性,對于保障國家糧食安全具有重要的現(xiàn)實意義[61]。研究發(fā)現(xiàn),超表達水稻OsWRKY50基因提高了水稻的耐鹽性,這在一定程度上得益于OsLEA3(late embryogenesis abundant 3)、OsRAB21(responsive to ABA 21)、OsHKT1;5(high affinity K+transporter 1;5)、OsP5CS1(pyrroline-5-carboxylate synthase 1)基因表達量的提高[62]。另外,超表達小麥TaWRKY13基因也提高了水稻的耐鹽性,主要表現(xiàn)為超表達TaWRKY13基因水稻根系更發(fā)達(主根長增長、根系總表面積增加)、脯氨酸含量提高、MDA含量降低[63]。WRKY基因除了正調控植物的耐鹽性外,還有一些WRKY基因負調控植物的耐鹽性。如玉米(Zea mays L.)ZmWRKY11基因[64]。ZmWRKY11基因在鹽脅迫條件下下調表達,在ABA處理下上調表達。超表達該基因削弱了水稻對ABA的敏感性,降低了水稻在鹽脅迫條件下的株高、根長和存活率。這主要歸因于超表達ZmWRKY11基因降低了水稻脯氨酸含量,提高了MDA含量、電解質滲透率??梢奪mWRKY11通過ABA介導的信號通路負調控水稻的耐鹽性,可以通過沉默該基因表達的方式提高玉米的耐鹽性。類似的,ZmWRKY86基因也負調控耐鹽性,wrky86突變體在鹽脅迫條件下的存活率提高[65]。這主要歸因于其葉片CAT活性和K+含量增加,MDA含量、相對電解質滲透率、Na+含量降低。
大豆、棉花均是重要的經(jīng)濟作物,利用耐鹽WRKY基因提高大豆、棉花的耐鹽性對于農(nóng)業(yè)豐產(chǎn)、農(nóng)民增收具有重要意義。GmWRKY27基因受鹽、干旱脅迫誘導表達,超表達該基因提高了大豆在鹽脅迫條件下的毛狀根數(shù)量和長度,進而提高了耐鹽性[66]。這在一定程度上得益于超表達GmWRKY27基因降低了水稻活性氧含量,提高了脯氨酸含量;且GmWRKY27與GmMYB174互作,二者共同抑制GmNAC29的表達,而GmNAC29負調控脅迫耐受性。類似的,GmWRKY12基因受鹽、干旱脅迫誘導表達,在大豆中超表達該基因促進了在鹽、干旱脅迫條件下的毛狀根發(fā)育(根長和數(shù)量增加),提高了葉片脯氨酸含量,降低了MDA含量,進而提高了大豆的耐鹽性和抗旱性[67]。另外,紫花苜蓿(Medicago sativa)MsWRKY11基因受鹽、高堿、干旱、低溫、ABA誘導表達,超表達該基因不僅提高大豆在營養(yǎng)生長期的耐鹽性,還提高了生殖生長期鹽脅迫條件下的株高、單株莢果數(shù)、單株籽粒數(shù)和百粒質量[68]。這主要歸因于超表達MsWRKY1基因提高了大豆葉片葉綠素、脯氨酸、可溶性糖含量及CAT、SOD活性,降低了相對電導率及MDA、H2O2、O-2· 含量。綜上,MsWRKY11通過調控活性氧清除系統(tǒng)和滲透調節(jié)物質來提高大豆的耐鹽性。此外,海島棉(Gossypium barbadense)GbWRKY1基因受鹽、干旱、ABA誘導表達,負調控耐鹽性[69],可以通過沉默該基因表達的方式來提高海島棉的耐鹽性。
2.4 園藝作物耐鹽基因工程
番茄(Solanum lycopersicum L.)SlWRKY3基因受鹽、干旱、SA誘導表達,超表達該基因提高了番茄在鹽脅迫條件下的葉片相對含水量、葉綠素含量、氣孔導度、K+含量、Ca2+含量和抗氧化酶編碼基因[GST(glutathione-S-transferases)、POD]、離子及水轉運蛋白編碼基因(水通道蛋白基因)、防御蛋白編碼基因(PR6)表達量,降低了Na+含量、電解質滲透率、TTC(2,3,5-triphenyltetrazolium chloride)含量,進而提高耐鹽性[70]。可見SlWRKY3通過調控滲透勢、活性氧平衡來提高番茄的耐鹽性。類似的,超表達SlWRKY8基因也提高了番茄的耐鹽性,并提高了其對假單胞桿菌、干旱的抗性[71]。超表達SlWRKY8基因番茄耐鹽性和抗旱性的提高主要歸因于滲透調節(jié)物質(脯氨酸)含量、葉綠素含量、抗氧化酶(SOD、POD、CAT)活性及脅迫響應基因[SlAREB(ABA response element binding protein)、 SlDREB2A、SlRD29]表達量提高,H2O2、O-2·、MDA含量降低[71]。綜上,SlWRKY8通過調節(jié)滲透調節(jié)物質、活性氧清除來提高番茄的耐鹽性和抗旱性。另外,超表達受鹽脅迫誘導表達的茄子(Solanum melongna L.)SmWRKY40基因提高了茄子的耐鹽性[72]。在鹽脅迫條件下,超表達SmWRKY40基因茄子生長狀態(tài)更好,株高、葉面積和生長速率均高于野生型對照,活性氧含量降低,葉綠素含量更高[72]。
地被菊DgWRKY5基因受鹽、H2O2、ABA、GA誘導表達,超表達該基因提高了地被菊在鹽脅迫條件下的根長、氣體交換參數(shù)(凈光合速率、氣孔導度、蒸騰速率)、葉綠素含量、滲透調節(jié)物質(脯氨酸、可溶性蛋白、可溶性糖)含量和抗氧化酶(SOD、POD、CAT)活性及一些脅迫相關基因[DgAPX、DgCAT、DgNCED3A、DgNCED3B、DgCuZnSOD、DgP5CS]的表達量,降低了H2O2、O-2·、MDA含量,進而提高了耐鹽性[73]。類似的,DgWRKY4[74]、DgWRKY2[75]基因均受鹽誘導表達,超表達這2個基因也均提高了地被菊的耐鹽性。其中,超表達DgWRKY4基因地被菊耐鹽性的提高主要歸因于其光合能力(凈光合速率、氣孔導度、蒸騰速率)、脯氨酸含量、可溶性糖含量、抗氧化酶(SOD、POD、CAT)活性及一些脅迫相關基因[DgCuZnSOD、DgCAT、DgAPX、DgP5CS、DgDREB1A、DgDREB2A]的表達量提高,MDA、H2O2、O-2· 含量降低。可見DgWRKY4通過調控光合系統(tǒng)、活性氧清除系統(tǒng)、滲透調節(jié)物質及脅迫相關基因的表達量來提高地皮菊的耐鹽性。超表達DgWRKY2基因地皮菊耐鹽性的提高主要歸因于其葉片脯氨酸、可溶性糖、可溶性蛋白、葉綠素含量和抗氧化酶(SOD、POD、CAT)活性及一些脅迫相關基因(DgCAT、DgAPX、DgZnSOD、DgP5CS、DgDREB1A、DgDREB2A)表達量提高,H2O2、O-2·、MDA含量降低。說明DgWRKY2通過提高抗氧化和滲透調節(jié)能力來提高地皮菊的耐鹽性。另外,杭菊(Chrysanthemum morifolium)CmWRKY17是一個轉錄抑制子,超表達CmWRKY17基因降低了杭菊的耐鹽性[76]。這主要是因為超表達CmWRKY17降低了杭菊葉片脯氨酸含量SOD活性、POD活性及一些脅迫相關基因(AtRD29、AtDREB2B、AtSOS1、AtSOS2、AtSOS3、AtNHX1)的表達量,提高了電導率??梢奀mWRKY17負調控地皮菊的耐鹽性,可以通過沉默該基因表達的方式來提高杭菊的耐鹽性。
蘋果(Malus×domestica Borkh.)MdWRKY30基因受鹽、干旱誘導表達,超表達該基因提高了蘋果的耐鹽性,伴隨著提高了一些脅迫相關基因(MdABI1、MdABF3、MdRD22、MdRD29A、MdDREB1A、MdCAT1、MdSOD1)的表達量[77]。另外,超表達香蕉(Musa sapientum)MusaWRKY71基因提高了香蕉在鹽脅迫條件下的葉片光合效率(Fv/Fm),降低了MDA含量和膜損傷程度,進而提高了耐鹽性[78]。
2.5 其他植物耐鹽基因工程
蒙古柳(Salix linearistipularis)WRKY28基因受堿性鹽脅迫誘導表達,超表達該基因提高了柳樹對堿性鹽脅迫的耐受性[79]。這主要得益于超表達WRKY28基因降低了柳樹葉片H2O2含量,提高了葉綠素含量、光合能力(Fv/Fm)、APX活性及一些脅迫相關基因[APX、SOD、SPDS(Spermidine synthase)]的表達量。說明SlWRKY28通過調控抗氧化酶編碼基因的表達來提高抗氧化酶活性,進而正調控柳樹對堿性鹽脅迫的耐受性。相反的,新疆楊(Populus alba var. pyramidalis)PalWRKY77基因負調控耐鹽性,超表達該基因降低了新疆楊的耐鹽性[80]。采用CRISPR/Cas9對該基因進行編輯,Palwrky77突變體葉片脯氨酸、光合色素含量提高,MDA含量和電解質滲透率降低,且PalWRKY77調控PalNAC002、PalRD26基因的表達,進而提高了耐鹽性[80]。綜上,可以通過沉默、突變PalWRKY77基因的方式來提高新疆楊的耐鹽性。類似的,沉默楊樹(Populus simonii×Populus nigra)PsnWRKY70基因表達也提高了楊樹的耐鹽性[81]。
3 展望
植物的耐鹽性是一個復雜的數(shù)量性狀,受基因型及環(huán)境等多種因素影響。因此,利用傳統(tǒng)育種方法培育耐鹽品種周期較長、效果較差。隨著分子生物技術的迅速發(fā)展,挖掘優(yōu)異耐鹽基因,通過基因工程技術提高植物的耐鹽性是一條行之有效的途徑。WRKY是植物特有的在植物生命活動中具有重要作用的轉錄因子,調控植物生長發(fā)育和對生物、非生物脅迫的響應。目前,利用基因工程技術已經(jīng)獲得了一批耐鹽性提高的轉WRKY基因材料,且有些材料提高了籽粒產(chǎn)量,為植物耐鹽性改良和育種奠定了堅實的基礎。但是,該領域仍存在一些亟待解決的問題。(1)由于耐鹽性是復雜的數(shù)量性狀,所以轉單個WRKY基因對植物耐鹽性的提高程度有限,大多只能提高營養(yǎng)生長期的耐鹽性,可以提高生殖生長期耐鹽性并提高產(chǎn)量的很少。因此,以后應將WRKY基因與其他耐鹽基因尤其是調節(jié)基因共轉化植物,以更好地提高植物的耐鹽性乃至產(chǎn)量。(2)對于那些提高植物營養(yǎng)生長期耐鹽性的WRKY基因,應該進一步進行生殖生長期耐鹽性鑒定及對產(chǎn)量的影響情況分析,驗證其提高植物耐鹽性程度,以便于后期植物耐鹽性遺傳改良及育種的選擇使用。(3)大部分WRKY基因都是由組成型啟動子驅動的,組成型超表達WRKY基因會引起轉基因植株生長緩慢、花期推遲甚至產(chǎn)量降低等不良后果。因此,今后應該選擇使用誘導型啟動子(如鹽脅迫誘導啟動子)或者組織特性啟動子(如根特異啟動子)來驅動WRKY基因,在提高植物耐鹽性的同時不會影響植物的生長發(fā)育。(4)WRKY基因家族成員眾多,不同成員功能不盡相同,即使都能提高植物耐鹽性,其提高耐鹽程度也不盡相同,大部分WRKY調控植物耐鹽性的機制研究尚不是很清楚。因此,今后應加強WRKY基因功能的進一步深入研究,剖析WRKY調控植物耐鹽性的分子機制及調控網(wǎng)絡。
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收稿日期:2022-05-25
基金項目:河南省農(nóng)業(yè)科學院高層次人才科研啟動經(jīng)費(編號:豫財教[2013]232號2060503)。
作者簡介:段俊枝(1981—),女,河北滄州人,博士,助理研究員,主要從事作物遺傳育種研究。E-mail:junzhi2004@163.com。
通信作者:卓文飛,博士,副研究員,主要從事農(nóng)業(yè)信息及期刊編輯方面的工作,E-mail:kjcankao@126.com;齊學禮,博士,副研究員,主要從事小麥遺傳育種研究,E-mail:xueliqi888@163.com。