劉建菊 肖寧 吳云雨 蔡躍 潘存紅 時薇 陳梓春 朱書豪 李育紅 余玲 王志平 劉廣青 周長海 黃年生 張小祥 季紅娟 李愛宏



摘要:基因編輯是一種能對特定基因進行修飾的基因工程技術,能快速對靶點基因編輯,是高效捕獲目的基因、快速研究目標基因功能的重要手段,在基因功能研究和作物育種等方面有著重要意義和廣闊的應用前景。基因編輯利用特異的DNA結合元件和切割元件開展編輯工作,然而該技術最需注意的是特異性和脫靶率問題,不同時期的基因編輯技術也針對上述2個問題進行改良,目前應用最為廣泛的是CRISPR/Cas9,Cas12a 由于其特異性高且脫靶率大大降低也受到越來越多的關注。本文對基因編輯的技術發展及特點、CRISPR/Cas9和Cas12a的技術優勢進行介紹,并對這2種技術在水稻產量、抗性及品質中的研究進展進行綜述,同時對拓展CRISPR/Cas基因編輯技術在水稻中的應用提出展望,為基因功能鑒定及遺傳改良提供參考。
關鍵詞:基因編輯;Cas9;Cas12a;水稻;性狀改良
中圖分類號:S511.01文獻標志碼:A文章編號:1002-1302(2023)11-0001-09
基因編輯(gene editing)是一種能對特定基因進行修飾的基因工程技術[1-2],該技術利用工程核酸酶切割目標基因組產生DNA雙鏈斷裂(DSB),進而激活細胞內源性DNA修復機制從而產生包括插入、缺失及基因片段替換等新的基因突變類型[3-5]。
1996年出現的鋅指核酸酶(ZFN)為基因編輯技術的發展奠定了基礎[6-7],利用該技術首次于2002年果蠅染色體上實現基因定點突變[8]。隨后轉錄激活樣效應因子核酸酶(TALENs)[9]及由RNA介導的Cas9蛋白相關的成簇規則間隔短回文重復序列(CRISPR)相繼被發現[10-11],特別是CRISPR/Cas9于2013年開始應用于植物基因組編輯,被Science列入2013年十大科學進展[10]。此外,用于切割雙鏈DNA的CRISPR/Cas12a(Cpf1)[12-13]及在crRNA指導下切割ssRNA的CRISPR/Cas13(C2c2)[14]于2015年和2016年相繼被發現(圖1)。
基因編輯利用特異的DNA結合元件和切割元件開展編輯工作,然而該技術最需注意的是特異性和脫靶率問題,基因編輯技術的更迭對這2個方面的改善也各不相同(表1)。ZFNs是第一個應用于基因定點編輯的技術,然而其ZFN 剪切DNA 形成同源二聚體的同時,可能會產生異源二聚體引起脫靶且難以實現多靶點編輯等問題,嚴重阻礙了其應用[15-16];TALENs技術是1個TALE基序識別1個堿基對,因此多個串聯的TALE基序與其識別的堿基對呈一一對應關系,大大提高了編輯特異性并降低脫靶率,但其編輯效率較低,且難以進行多基因編輯[17-20];CRISPR/Cas9技術在sgRNA的指導下與靶點結合,并利用HNH和RuvC對外源DNA進行切割,其編輯效率大大提高,且可以對多基因同時編輯,然而其缺點是靶向目標 DNA 序列容易出現錯配,存在脫靶率高、編輯特異性低等缺陷[4,16,21-22];Cas12a可以在crRNA引導下識別PAM,識別到正確序列才會形成封閉的R環,因此編輯準確性相對Cas9有了較大提高,其脫靶率也有所降低[12-13,23]。
CRISPR/Cas9及Cas12a是目前基因編輯技術中應用最為廣泛的2種技術,在水稻產量、品質、生物脅迫及非生物脅迫性狀關鍵基因的分子遺傳功能解析和目標性狀的精準改良上已成熟應用(表2)。
2CRISPR/Cas在水稻中的研究進展
2.1產量性狀
水稻產量由單株穗數、每穗粒數、粒型及粒重等多個性狀綜合組成[112-113]。目前已有29個產量相關基因被編輯,其中4個基因對產量起正調控作用,其他25個基因均作為負調控因子發揮作用。Li等對每穗粒數Gn1a、粒型DEP1、粒重GS3及理想株型基因IPA1定點突變,gn1a、dep1和gs3的T2突變體出現穗粒數增加、粒型變大,成功提高了產量[37]。其他研究分別對Gn1a&DEP1、GS3&DEP1、GS3、GS2/GRF4及SPL16/qGW8等開展基因編輯,在穗粒數、粒型、粒重等性狀上調控產量,改善農藝性狀同時提高產量[39,42,44,47-48]。開展多基因同時編輯也可快速調控產量,Xu等同時對負調控粒重、粒型基因GS3、GW2、GW5及TGW6進行編輯,快速改良突變體粒重及產量[41]。Zhou等同時編輯GS3、Gn1a及GW2,相關突變體出現籽粒變大、穗粒數增多從而提高水稻產量[38]。Zeng等同時編輯PIN5b、GS3和MYB30,突變體兼顧了高產和耐冷性[43]。非產量調控基因突變也會提高產量,Miao等獲得ABA受體突變體pyl1/4/6,通過增加31%籽粒數量從而提高產量[57],除此之外,對FWL4、SD1(OsGA20ox2)及PYL9進行定點突變也可不同程度提高產量[49,51-52,58]。然而產量正調控基因如RGA1、SWEET11被編輯后會分別引起植株極端矮化及灌漿功能受損,從而減產[42,50]。
CRISPR/Cas12a在水稻產量調控中應用也日漸增多,Malzahn等對粒長基因DEP1和葉片卷曲度基因ROC5進行敲除提高產量。對水稻PDS、DEP 和ROC5基因所有靶點進行突變,能同時改良農藝性狀及抗性[45,54],而將葉綠素a加氧酶基因CAO1靶向敲入水稻中,突變體的產量及品質降低[32,53],Zheng等同時利用Cas9和Cas12a對細胞分裂素家族基因OsCKX1-11進行編輯,獲得了農藝性狀及產量均有提升的單基因及多基因突變體,Cas9的編輯效率為26.9%~90.0%,有8個基因的編輯效率高于50.0%,而Cas12a的編輯效率為368%~100%且9個基因的編輯效率高于60%,Cas12a的多基因編輯效率高于Cas9(91.7%>545%)[40]。上述研究表明,對負調控基因進行定點突變后可快速獲得目標性狀改善的編輯系,然而有些基因突變后會對其他性狀產生不利影響,因此多重基因編輯技術的應用為多個性狀同時改良提供了方案和可行性,在開展基因編輯時Cas12a的編輯效率及穩定性均高于Cas9。
2.2品質性狀
稻米品質是水稻商業價值的核心賣點,受到多個基因綜合調控,已有大量基因被證實直接或間接調控稻米品質,可用于定向改良直鏈淀粉含量、蛋白、香味等性狀。目前有13個品質基因被編輯,其中4個基因(ISA、ITPK、GL3.2和BEL)正調控稻米品質,其他基因負調控稻米品質。Wx基因的基因編輯位置差異對稻米品質影響不同,對Wx基因功能位點進行突變,可以將直鏈淀粉含量降至與糯稻相似,在不影響產量前提下改良稻米品質[59-61];對 Wxb基因啟動子轉錄因子結合位點進行突變,獲得新的Wx等位基因并獲得直鏈淀粉含量不同程度降低的突變體,改良了稻米品質[62]。fad2突變體的油酸濃度提高,gs9突變體的粒型、堊白及外觀等品質顯著改善,or突變體籽粒β-胡蘿卜素含量顯著提高,isa突變體總淀粉含量下調,ZmPsy和SSU-crtI突變體水稻的籽粒類胡蘿卜素含量提高,badh2突變體籽粒產生香味,均可改良稻米品質[66-67,69-70,72,114]。多基因同時突變可綜合提升水稻性狀,如app6/10雙突變體的直鏈淀粉、蛋白及谷蛋白含量均下調[65];細胞色素P450家族基因(Os03g0603100、Os03g0568400和GL3.2)和香味基因BADH2同時突變后改良稻米香味并提高產量[71];PDS和BELs同時突變穩定提高水稻產量和品質[73]。對正調控基因進行突變,有助于理解基因在稻米品質改良中的作用,敲除Wxb第一內含子、SBEIIb進行精準敲除,突變體直鏈淀粉含量上調,且引起營養特性改變[63-64]。Jiang等突變ITPK1-6,降低籽粒植酸含量然而卻提高無機磷含量,不利于水稻生長繁殖,證實該基因對水稻正常生長發育的重要性[68]。對負調控稻米品質基因的敲除加速了優質水稻品種選育的進程,與其他產量性狀相關基因同時編輯,有望在保證產量的同時提高品質。
2.3生物脅迫
水稻生長過程對生物脅迫的抗性也可利用基因編輯方法改良,對抗性相關基因MPK1、MPK2、MPK5和MPK6的敲除能夠提高抗病性[85-86]。ERF922、SEC3A、ALB1、RSY1 和Pi21敲除后,突變體對稻瘟病的抗性提高,同時農藝性狀也得到改良[74-78]。SWEET13和SWEET14敲除后突變體對白葉枯病菌的抗性提高,且SWEET14突變體無產量損失[79,81]。對SWEET11/8N3/Xa13編碼區及啟動子區定點突變,也能提高水稻對白葉枯病的抗性[80,82]。Liang等對稻曲病相關基因USTA和UvSLT2進行編輯,顯著提高了水稻對稻曲病抗性[84]。利用Cas12a低水平同源性核酸酶MAD7對水稻基因EPSPS、NRAMP、PDS、Xa13及ALS等進行多重基因敲除,同步提升了突變體的品質、除草劑及白葉枯病抗性[83]。Wang等利用Cas12a對受體樣激酶(OsRLK)相關基因(OsRLK-798、OsRLK-799、OsRLK-802和OsRLK-803)及CYP81A家族基因(OsBEL-230、OsBEL-240、OsBEL-250和OsBEL-260)開展多重基因編輯,獲得了陽性植株,相關突變體調控了水稻的抗逆性[105]。
對水稻負調控抗性基因進行敲除或替換可快速改善目標性狀,提升水稻抗性,然而有些編輯以損失產量為代價[109],而有些編輯在不損害甚至優化農藝性狀前提下同步改善水稻品質[77-78,81,90,95],因此在進行水稻抗性改良時需要考慮基因對水稻的綜合影響,從而制定相應編輯策略。
2.4非生物脅迫
水稻生長發育過程中會受到多種非生物脅迫的影響,如干旱、低溫、鹽、除草劑等,相關基因的大量挖掘促進了基因編輯在水稻非生物脅迫中的應用,目前有24個相關基因被編輯,其中8個基因起正調控作用,即Ann3、OTS1、RAV2、SAPK2、BELs、MKK5、RLKs和SAP。在水稻抗旱性方面,PYL9、ERA1、PDS、半卷葉基因(SRL1和SRL2)和MIR535的基因突變會增強突變體的抗旱性[58,88-90,106]。而敲除SAPK2和SAP基因后,突變體對干旱脅迫和活性氧更敏感,農藝性狀顯著下降[87,111]。在水稻響應鹽脅迫方面,敲除水稻中的RR22、DST及PQT3基因,可顯著提高耐鹽性且不影響農藝性狀[92,94-95],但對OTS1編碼區及RAV2啟動子的GT-1元件突變后,其耐鹽性下降[91,93]。在水稻抗除草劑方面,通過將EPSPS、ALS突變基因敲入,或點突變野生型基因(ALS、FTIP1e)均能使水稻獲得除草劑抗性[96-103]。
除此之外,敲除Nramp5能降低Cd的積累且不影響產量[107-108];Ann3敲除后對低溫的耐受性降低[110];敲除MKK5后,突變體抗逆性降低[104];同時突變抽穗基因Hd2、Hd4和Hd5后突變體開花期及成熟期提前有助于逃避脅迫[109],然而農藝性狀受到較大影響,因此在應用時可進行單基因編輯,從而消除對產量的損害。
3CRISPR/Cas的技術展望
基因編輯技術為生命科學帶來重大進展,然而幾種技術的脫靶率及特異性問題仍需重點關注。研究人員優化了相關技術,開發了DB-PACE法從而降低基因編輯工具酶的脫靶效應,大大提高TALEN核酸酶的DNA結合能力和切割特異性[115];開發出提高Cas9基因編輯和堿基編輯特異性的選擇性核輸出抑制劑(SINE)[116];Sheng利用腙介導CRISPR/Cas12a系統,通過互補堿基配對引起的鄰近效應來加速整個激活鏈的形成,從而提高Cas12a 系統的特異性[117]。除此之外,CRISPR系統的sgRNA的優化、PAM修飾、crRNA優化及Cas蛋白突變體挖掘也會進一步提高編輯范圍及特異性并降低脫靶率[12,46,104,118-120]。此外Cas12a蛋白表現出對低溫敏感的特征,目前Cas12a突變體是解決該問題的主要方式,而引起低溫敏感的分子機制尚不明確。上述問題的解決,將大大提高基因編輯水平,對目標基因進行定向編輯,產生無外源DNA插入的新品種,從而加快育種速度、縮短育種年限。
水稻產量、抗性和品質相關基因的挖掘及分子機理解析,有助于更全面了解基因功能,目前基因編輯主要集中在編碼區,有少量研究是編輯啟動子的轉錄結合位點實現性狀調控的。已有研究表明,DNA結構本身,如拓撲異構結構等也會影響基因表達水平[121],因此,未來也可能作為基因編輯靶點,增加目標性狀精準改良的可能性。隨著人工智能的發展,Alphafold等技術對蛋白預測精準度提高,越來越多的蛋白結構被預測,對目標基因的模擬突變有助于挖掘關鍵堿基序列,可進行靶向預測,實現新的目標性狀的改良已經成為可能。相信隨著基因編輯技術的不斷完善、生物信息學和人工智能的不斷發展,水稻育種將會迅猛發展。
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