非生物脅迫下植物表觀遺傳變異的研究進(jìn)展

王淑妍,郭九峰*,劉曉婷,苑號(hào)坤,李亞嬌,那　日

(1 內(nèi)蒙古大學(xué) 物理科學(xué)與技術(shù)學(xué)院,呼和浩特 010021;2 電子科技大學(xué) 生命科學(xué)與技術(shù)學(xué)院,成都 610054)

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非生物脅迫下植物表觀遺傳變異的研究進(jìn)展

王淑妍1,郭九峰1*,劉曉婷1,苑號(hào)坤2,李亞嬌1,那日1

(1 內(nèi)蒙古大學(xué) 物理科學(xué)與技術(shù)學(xué)院,呼和浩特 010021;2 電子科技大學(xué) 生命科學(xué)與技術(shù)學(xué)院,成都 610054)

摘要:植物在整個(gè)生命過(guò)程中固著生長(zhǎng),不能主動(dòng)躲避外界不良環(huán)境的危害,需要通過(guò)自身的防御機(jī)制來(lái)抵御和適應(yīng)外界脅迫,而表觀遺傳修飾在調(diào)控植物應(yīng)對(duì)不良環(huán)境脅迫中起重要作用。該文從DNA甲基化、組蛋白修飾、染色質(zhì)重塑和非編碼RNA等方面進(jìn)行了綜述,主要闡述了近年來(lái)國(guó)內(nèi)外有關(guān)非生物脅迫下植物的表觀遺傳變化,以期為利用表觀遺傳變異提高植物的抗脅迫能力提供參考。

關(guān)鍵詞:非生物脅迫;DNA甲基化;組蛋白修飾;染色質(zhì)重塑;非編碼RNA

逆境脅迫是影響植物生長(zhǎng)發(fā)育的重要因素之一。植物常暴露于環(huán)境脅迫中,能夠建立自我保護(hù)和適應(yīng)不良環(huán)境的機(jī)制[1]。脅迫對(duì)植物的生長(zhǎng)發(fā)育及繁殖具有顯著的影響[2]。它既可決定物種的分布,又可促進(jìn)種群發(fā)生選擇性進(jìn)化[3]。一般來(lái)說(shuō),脅迫分為生物脅迫和非生物脅迫。植物中,引起生物脅迫的生物因素(感染與競(jìng)爭(zhēng))主要包括病害、蟲害和雜草;而導(dǎo)致非生物脅迫的非生物因素則比較廣泛,包括物理(如靜電、輻射等)、化學(xué)(如鹽堿土、除草劑等)、溫度(如低溫冷凍害和高溫?zé)岷?和水分(如干旱、洪澇)等。由于植物沒(méi)有像動(dòng)物那樣的運(yùn)動(dòng)機(jī)能,整個(gè)生命過(guò)程基本上是固著生長(zhǎng)的,因此不能像動(dòng)物一樣主動(dòng)躲避不良環(huán)境的影響,但植物可通過(guò)多種方式和一系列調(diào)控機(jī)制來(lái)適應(yīng)或抵御不良環(huán)境的變化[4]。因此,研究植物對(duì)逆境脅迫的應(yīng)答機(jī)制具有重要意義。植物應(yīng)答逆境脅迫的機(jī)制主要有兩種,一是改變植物的代謝途徑;二是改變植物抗逆基因的表達(dá)水平,其中表觀遺傳修飾起重要調(diào)控作用[5]。

表觀遺傳學(xué)(epigenetics)最早由著名生物學(xué)家Conrad H.Waddington于 1939年在《現(xiàn)代遺傳學(xué)導(dǎo)論》中提出[6]。1942年,他首次將表觀遺傳學(xué)定義為“基因與環(huán)境的互作導(dǎo)致表型的出現(xiàn)”[7]。隨著生命科學(xué)研究的發(fā)展,20世紀(jì)70年代中期,Holliday對(duì)“表觀遺傳學(xué)”進(jìn)行了系統(tǒng)表述,即在DNA序列不發(fā)生變化的情況下,發(fā)生的可遺傳的基因表達(dá)的變化[8]。而且,表觀遺傳變異可通過(guò)有絲分裂或減數(shù)分裂遺傳給下一代[9]。表觀遺傳修飾主要包括DNA甲基化、組蛋白修飾、染色質(zhì)重塑和小RNA等[10]。近年來(lái),已有研究表明不同的生物和非生物脅迫可導(dǎo)致植物DNA甲基化水平、甲基化模式以及組蛋白修飾等發(fā)生變化,從而影響基因表達(dá),使植物體能夠適應(yīng)或抵御不良環(huán)境,以求更好地生長(zhǎng)發(fā)育。本文從DNA甲基化、組蛋白修飾、染色質(zhì)重塑和非編碼RNA調(diào)控等方面簡(jiǎn)要闡述了非生物脅迫對(duì)植物表觀遺傳調(diào)控的影響及其研究進(jìn)展。

1DAN甲基化

DNA甲基化指在共價(jià)催化酶(甲基轉(zhuǎn)移酶)的作用下,將S-腺苷甲硫氨酸上的一個(gè)甲基轉(zhuǎn)移到胞嘧啶的5位碳原子上,從而形成5-甲基胞嘧啶(5mC)的過(guò)程[11]。植物中存在較高水平的5mC,根據(jù)物種的不同,其范圍大約在6%～25%之間[12]。在植物中,DNA胞嘧啶甲基化通常發(fā)生在CpG、CpNpG(N表示任何堿基)和CpNpN位點(diǎn)(不對(duì)稱的,N代表A,C或T)[13]。DNA甲基化模式分為2種類型。一種是從頭甲基化(de novo methylation),即2條鏈均未甲基化的DNA被甲基化;另一種是維持甲基化(maintenance methlation),即雙鏈DNA中的一條鏈已發(fā)生甲基化,另一條未甲基化的鏈(半甲基化序列)被甲基化。由于位點(diǎn)對(duì)稱性,CpG和CpNpG位點(diǎn)的甲基化通過(guò)甲基化維持機(jī)制實(shí)現(xiàn),而非對(duì)稱的CpNpN位點(diǎn)必須經(jīng)DNA復(fù)制后進(jìn)行從頭甲基化[14]。不同位點(diǎn)的甲基化需要不同的甲基化轉(zhuǎn)移酶來(lái)維持。MET1維持CpG位點(diǎn)的甲基化,CpG位點(diǎn)的甲基化最穩(wěn)定;CMT3是植物特異性甲基化酶,通常維持CpNpG位點(diǎn)的甲基化;而CpNpN位點(diǎn)的甲基化則通過(guò)RNA指導(dǎo)的DNA甲基化(RNA-directed DNA methylation,RdDM)途徑來(lái)維持[15-17]。DNA甲基化在調(diào)控植物應(yīng)答環(huán)境脅迫中起重要作用。據(jù)報(bào)道,植物可通過(guò)甲基化和去甲基化作用調(diào)控不同發(fā)育階段和不同環(huán)境下相關(guān)基因的表達(dá),進(jìn)而調(diào)節(jié)植物的生命活動(dòng)[18]。高鹽、干旱、溫度、重金屬等非生物脅迫都能通過(guò)誘導(dǎo)DNA胞嘧啶甲基化的變化來(lái)調(diào)控脅迫應(yīng)答基因的表達(dá),從而提高植物對(duì)不良環(huán)境的抗性,以保證植物正常的生長(zhǎng)發(fā)育。

1.1高鹽和干旱脅迫下的DNA甲基化

高鹽會(huì)對(duì)植物造成滲透脅迫和離子脅迫,使植物細(xì)胞的滲透壓降低,吸水困難,甚至迫使細(xì)胞脫水,從而影響植物的正常生理代謝和生長(zhǎng)發(fā)育,嚴(yán)重時(shí)會(huì)導(dǎo)致植物死亡。因此,鹽脅迫與干旱脅迫通常相伴發(fā)生[19]。Gianpiero等[20]對(duì)鹽脅迫影響油菜籽DNA甲基化水平的研究表明:耐鹽性油菜籽DNA甲基化水平降低,而鹽敏感性植株DNA甲基化水平增加。李慧等[21]研究了經(jīng)鹽脅迫處理后紅花幼苗DNA甲基化變化的情況,結(jié)果表明:與對(duì)照相比,紅花經(jīng)鹽脅迫處理4、8、12 h后,甲基化水平分別降低了4.7%、0.8%和0.5%。棉花中,DNA甲基化水平隨鹽脅迫強(qiáng)度的增加而降低[22]。在高鹽脅迫和冷脅迫下,煙草中NtGDPL(glycero-phosphodiesterase-like protein)編碼序列的DNA甲基化水平降低,說(shuō)明逆境應(yīng)答基因的轉(zhuǎn)錄受DNA甲基化的調(diào)控[23]。然而,有文獻(xiàn)報(bào)道,鹽脅迫可導(dǎo)致基因組甲基化水平上升和位點(diǎn)特異性的低甲基化[24]。潘雅嬌等[25]用甲基化敏感擴(kuò)增多態(tài)性(methylation sensitive amplification polymorphism,MSAP)和高效液相色譜(high performance liquid chromatography,HPLC)方法研究了水稻抗旱回交導(dǎo)入系DK106和干旱敏感輪回親本IR64在干旱脅迫前后DNA甲基化變化的情況。結(jié)果表明:水稻基因組中約有20%的CCGG位點(diǎn)發(fā)生了胞嘧啶甲基化,干旱脅迫導(dǎo)致DNA甲基化平均水平顯著增加,根部變化較為顯著,同時(shí)提出了DNA甲基化水平和狀態(tài)在品種間存在差異且具有時(shí)期和組織特異性。Wang等[26]也研究了干旱脅迫對(duì)水稻基因組DNA甲基化水平的變化,結(jié)果表明,經(jīng)干旱脅迫處理后,水稻葉和根中的DNA甲基化率分別下降了1.49%和0.41%,同時(shí)提出基因的去甲基化是水稻響應(yīng)鹽脅迫的一種重要的表觀遺傳機(jī)制。Tang等[27]在黑麥草的研究中發(fā)現(xiàn)干旱脅迫可導(dǎo)致基因組DNA甲基化水平下降,與對(duì)照相比,黑麥草的總DNA甲基化水平下降了10.28%。在豌豆中,干旱脅迫導(dǎo)致DNA甲基化水平增加[28]。Liang等[29]發(fā)現(xiàn)胡楊經(jīng)干旱脅迫處理后,上游2 kb,下游2 kb和重復(fù)序列中胞嘧啶甲基化水平增加,同時(shí)發(fā)現(xiàn)轉(zhuǎn)錄起始位點(diǎn)(transcriptional start site,TSS)上游10 bp處的甲基化抑制基因的表達(dá),而TSS上游100～2 000 bp以及基因內(nèi)部的甲基化則促進(jìn)基因的表達(dá),同時(shí)發(fā)現(xiàn)基因的剪切方式也影響到甲基化水平,所有順式剪接的基因都是非甲基化的,80%反式剪接的基因是甲基化的。此外,在干旱脅迫下,1 156轉(zhuǎn)錄因子與甲基化和基因表達(dá)水平的下降相關(guān),而690 TFs則與甲基化和基因表達(dá)水平的增加相關(guān),這說(shuō)明這些轉(zhuǎn)錄因子(TFs)在干旱脅迫誘導(dǎo)DNA甲基化變化中起重要作用。唐曉梅等[30]發(fā)現(xiàn)干旱脅迫處理后,高羊茅基因組總甲基化水平下降了1.05%,發(fā)生甲基化/去甲基化變化的位點(diǎn)達(dá)27.58%,同時(shí)提出轉(zhuǎn)座子可能在高羊茅應(yīng)答干旱脅迫中起重要作用。研究表明,當(dāng)植物暴露于高鹽和干旱脅迫環(huán)境時(shí),植物可通過(guò)DNA甲基化和去甲基化作用來(lái)調(diào)控相關(guān)基因的表達(dá)以抵御或適應(yīng)不良環(huán)境。同一植物在不同的發(fā)育階段或不同植物在不同強(qiáng)度的外界環(huán)境脅迫的刺激下,DNA甲基化水平發(fā)生了不同程度的下降或增加。DNA甲基化與去甲基化均可提高植物對(duì)高鹽和干旱環(huán)境的適應(yīng)性,使植物在維持正常的生長(zhǎng)發(fā)育的同時(shí)發(fā)生進(jìn)化,但這方面的研究還需要進(jìn)一步深入。

1.2重金屬脅迫與DNA甲基化

重金屬脅迫也能使植物的DNA甲基化發(fā)生變化,從而導(dǎo)致轉(zhuǎn)座子和基因的表達(dá)發(fā)生改變。重金屬脅迫對(duì)植物的影響因物種和劑量的不同而不同。Massimo等[31]在重鉻酸鉀對(duì)油菜基因組DNA甲基化變化的研究中發(fā)現(xiàn),鉻脅迫增加了油菜DNA胞嘧啶甲基化水平,全基因組中甲基化/去甲基化是隨機(jī)發(fā)生的,同時(shí)表明DNA甲基化的變化與劑量相關(guān)。此外,鉻脅迫可誘導(dǎo)DNA從頭甲基化[32]。楊金蘭等[33]用MSAP技術(shù)分析了重金屬鎘對(duì)蘿卜基因組DNA甲基化變化的影響,結(jié)果表明,在濃度為50、250和500 mg/L的鎘脅迫下,蘿卜基因組中甲基化敏感擴(kuò)增多態(tài)性和全甲基化的比率分別增加了3%、9%、17%和1%、3%、5%,且引起甲基化水平增加的主要是從頭甲基化。據(jù)報(bào)道,鎘脅迫也能提高油菜[34]和擬南芥[35]基因組DNA甲基化的水平。然而,并不是所有的重金屬脅迫都可以導(dǎo)致植物基因組DNA甲基化水平的增加。比如在鎳、鉻和鎘脅迫下,三葉草和大麻基因組DNA胞嘧啶甲基化水平則表現(xiàn)出下降的現(xiàn)象[36]。重金屬脅迫下水稻基因組DNA甲基化水平也發(fā)生了下降[37]。重金屬脅迫是一個(gè)相對(duì)復(fù)雜的過(guò)程,不同植物防御重金屬脅迫的確切表觀遺傳調(diào)控機(jī)制不同,但都是通過(guò)甲基化修飾改變DNA構(gòu)象,進(jìn)而導(dǎo)致染色體結(jié)構(gòu)的變化,使DNA與蛋白質(zhì)之間的相互作用受到一定的影響,從而調(diào)控基因表達(dá)以應(yīng)答重金屬脅迫。

1.3溫度脅迫與DNA甲基化

溫度是影響植物分布和生長(zhǎng)發(fā)育的重要因素之一。植物生長(zhǎng)對(duì)溫度的反應(yīng)有3個(gè)基點(diǎn),即最低溫度、最適溫度和最高溫度。低于最低溫度植物就會(huì)受到冷害,而當(dāng)超過(guò)最高溫度時(shí),植物便會(huì)遭受熱害。植物可通過(guò)表觀遺傳修飾應(yīng)答極端溫度。比如冷害會(huì)導(dǎo)致玉米根部核小體核心區(qū)DNA甲基化水平下降[38]。轉(zhuǎn)座子是植物基因組的重要組成部分,并在DNA甲基化的作用下保持沉默。研究表明,溫度脅迫可誘導(dǎo)甲基化狀態(tài)發(fā)生變化并激活轉(zhuǎn)座子,從而影響基因組的穩(wěn)定性,比如冷脅迫導(dǎo)致金魚草DNA甲基化水平下降,使CpNpN位點(diǎn)的Tam-3轉(zhuǎn)座子甲基化狀態(tài)發(fā)生改變,引起Tam-3轉(zhuǎn)座[39]。此外,冷脅迫誘導(dǎo)可導(dǎo)致玉米MET1表達(dá)下調(diào)及Ac/Ds轉(zhuǎn)座子甲基化水平下降[40]。據(jù)報(bào)道,除冷脅迫外,高溫脅迫也可引起植物基因組DNA甲基化的變化,比如,高溫脅迫導(dǎo)致辣椒基因組中64.80%～75.89%的CCGG位點(diǎn)發(fā)生了胞嘧啶甲基化,且甲基化水平及其狀態(tài)的變化存在品種差異[41]。研究表明,在熱脅迫下,擬南芥DRM2,NUCLEAR RNA POLYMERASE D1(NRPD1)和NRPE1上調(diào),基因組甲基化水平升高[42-43]。除此之外,在油菜[44]和栓皮櫟[45]中也發(fā)現(xiàn)了類似的現(xiàn)象。然而,熱脅迫似乎對(duì)不同物種的DNA甲基化水平的變化沒(méi)有持續(xù)性影響,它能夠改變特定位點(diǎn)的甲基化狀態(tài)[46]。

綜上所述,冷害和熱害均可導(dǎo)致植物DNA甲基化水平發(fā)生變化,但均無(wú)持續(xù)性影響。這說(shuō)明在不同的溫度脅迫下,不同物種間DNA甲基化水平存在一定的差異。而且冷脅和熱脅均可影響特定位點(diǎn)的甲基化狀態(tài),它們通過(guò)甲基化變化來(lái)調(diào)控脅迫應(yīng)答基因的表達(dá)以抵御和適應(yīng)不良環(huán)境的變化。

1.4電磁輻射等物理因素與DNA甲基化

地球本身就是一個(gè)巨大的磁場(chǎng)。因此,植物在生長(zhǎng)過(guò)程不僅會(huì)受干旱、高鹽、溫度和重金屬的脅迫,在一定程度上也受高壓靜電、電磁和離子輻射等物理因子的影響。據(jù)報(bào)道,自20世紀(jì)60年代起,陸續(xù)有科研工作者開始研究電場(chǎng)、激光、磁場(chǎng)等對(duì)植物生長(zhǎng)發(fā)育的影響,直到90年代,才開始進(jìn)行大量的研究并取得顯著的成果,且已應(yīng)用到實(shí)踐中[47]。

李娜等[48]發(fā)現(xiàn)高壓靜電場(chǎng)可導(dǎo)致羽衣甘藍(lán)基因組DNA總甲基化水平下降,其中有19.64%甲基化位點(diǎn)發(fā)生了甲基化狀態(tài)的變化,且以去甲基化為主。熊建平等[49]研究表明,強(qiáng)電場(chǎng)輻射導(dǎo)致國(guó)稻6號(hào)甲基化水平下降,引起表觀遺傳變異。此外,高壓可導(dǎo)致水稻轉(zhuǎn)座子的激活及DNA甲基化模式的變化[50],且甲基化變化存在基因型差異[51]。Kovalchuk等[52]研究了輻射對(duì)松樹基因組DNA甲基化水平變化的情況,發(fā)現(xiàn)輻射可增加DNA甲基化水平,且隨輻射劑量的增加而升高。史金銘等[53]的研究結(jié)果顯示,經(jīng)空間飛行和重離子輻射處理后,植株當(dāng)代表現(xiàn)出的表型變化、基因組序列多態(tài)性的增加以及抗逆基因表達(dá)的變化均與DNA 甲基化變化相關(guān),這表明表觀遺傳調(diào)控機(jī)制可能參與調(diào)控相關(guān)基因的表達(dá)以應(yīng)答輻射脅迫。因此,空間環(huán)境可導(dǎo)致水稻發(fā)生DNA甲基化和去甲基化,且個(gè)體無(wú)統(tǒng)一變化的趨勢(shì)。同時(shí),基因組CCGG位點(diǎn)的胞嘧啶的甲基化和去甲基化是隨機(jī)發(fā)生的,且存在個(gè)體差異性[54]。激光輻射誘導(dǎo)水稻吉粳88號(hào)M1代基因組DNA發(fā)生去甲基化,且去甲基化比率達(dá)2.25%[55]。李思圓等[56]也證明了激光輻射可導(dǎo)致水稻發(fā)生去甲基化并且可遺傳給后代。

2組蛋白修飾

在真核細(xì)胞中,組蛋白和基因組DNA結(jié)合形成染色質(zhì)。核小體是染色質(zhì)的基本單位,它是由大約146 bp的DNA和由H2A、H2B、H3、H4各2個(gè)分子生成的八聚體組成的[57]。因此,細(xì)胞可通過(guò)組蛋白修飾來(lái)調(diào)控基因轉(zhuǎn)錄的激活或抑制,同時(shí)可通過(guò)組蛋白修飾來(lái)影響DNA甲基化的變化以調(diào)控基因表達(dá)。組蛋白N端尾部可以進(jìn)行多種翻譯后修飾,主要包括乙?；?甲基化,泛素化,磷酸化和類泛素化。每一個(gè)組蛋白都有不同基因編碼的變體。在細(xì)胞中,組蛋白變體和翻譯后修飾的組合以“組蛋白密碼”的形式儲(chǔ)存表觀遺傳記憶。這在染色質(zhì)結(jié)構(gòu)中具有關(guān)鍵作用,它決定了基因的轉(zhuǎn)錄狀態(tài)和表達(dá)水平[58]。據(jù)報(bào)道,組蛋白修飾在調(diào)控植物應(yīng)答非生物脅迫中具有重要作用[59-60]。不同的組蛋白修飾可促進(jìn)或抑制基因的表達(dá)。比如組蛋白修飾中的乙酰化、某些磷酸化和泛素化可以促進(jìn)基因的表達(dá)[61-62],而生物素化和類泛素化則抑制基因的表達(dá)[63-64]。

2.1組蛋白乙?；?/p>

組蛋白乙酰化是組蛋白修飾中重要的修飾方法之一,組蛋白賴氨酸殘基N末端的乙?；谡{(diào)控真核基因活性中起重要作用。核心組蛋白的乙?；瘯?huì)導(dǎo)致染色質(zhì)松散促進(jìn)基因表達(dá),而組蛋白去乙?；瘎t導(dǎo)致染色質(zhì)結(jié)構(gòu)“閉合”抑制基因表達(dá)[65]。組蛋白乙?；腿ヒ阴；謩e由組蛋白乙酰轉(zhuǎn)移酶(histone acetyltransferase,HATs)和組蛋白去乙?；D(zhuǎn)移酶(histone deacetylation transferase,HDACs)催化。

在植物中,HATs包含4個(gè)家族分別是GNAT(GCN5-相關(guān)N末端乙酰轉(zhuǎn)移酶)家族,MYST(MOZ,Ybf2/Sas3,Sas2和Tip60)家族,CBP(CREB-結(jié)合蛋白)家族和TAFII250家族[66]。其中,GNAT家族中的GCN5是多個(gè)乙酰轉(zhuǎn)移酶復(fù)合體的催化亞基,而ADA2接頭蛋白是GCN5復(fù)合物不可或缺的部分。擬南芥中有2個(gè)與ADA2相關(guān)的因子,即ADA2a和ADA2b[67]。據(jù)報(bào)道,GCN5和ADA2在植物生長(zhǎng)發(fā)育中起重要作用[68-69]。比如ADA2b對(duì)鹽脅迫和ABA高度敏感[70]。鹽脅迫下,擬南芥GCN5和ADA2b突變株H3和H4乙酰化水平下降,同時(shí)COR6.6,RAB18和RD29b基因的轉(zhuǎn)錄活性下降[71]。冷脅迫促進(jìn)CBF1轉(zhuǎn)錄因子與GCN5和ADA2相互作用[72],誘導(dǎo)GCN5和ADA2調(diào)控?cái)M南芥冷脅迫誘導(dǎo)基因的表達(dá)[73]。水稻[74]中也存在類似的現(xiàn)象,比如ABA導(dǎo)致OsHAC701、OsHAC703、OsHAG702、OsHAG703和OsHAM701的轉(zhuǎn)錄水平明顯提高;鹽脅迫促進(jìn)OsHAC701、OsHAC703、OsHAC704和OsHAG703的表達(dá),而冷脅迫則抑制它們的表達(dá)。因此,HATs可能在植物應(yīng)答非生物脅迫中起重要作用。

環(huán)境和胞內(nèi)信使可通過(guò)組蛋白去乙?；种颇康幕虻谋磉_(dá),真核生物的組蛋白去乙酰化轉(zhuǎn)移酶主要分為3個(gè)家族,即RPD3/HDA1超家族,SIR2家族和HD2家族。RPD3家族的HDA6和HDA19組蛋白去乙酰化酶可調(diào)控?cái)M南芥對(duì)生物和非生物脅迫的應(yīng)答,HDA6與基因沉默和RNA指導(dǎo)的DNA甲基化相關(guān)[75-76]。損傷、病菌感染和植物激素(JA和乙烯)都會(huì)導(dǎo)致HDA19/HD1/AtRPD3A基因的表達(dá)。轉(zhuǎn)基因植物中HDA19的過(guò)量表達(dá)會(huì)導(dǎo)致組蛋白乙?；较陆?促進(jìn)ETHYLENE RESPONSE FACTOR-1(ERF1)和PATHOGENESIS-RELATED(PR)基因的表達(dá)。脅迫和內(nèi)源信使可促進(jìn)HDA16和HDA19表達(dá),影響多個(gè)位點(diǎn)的染色質(zhì)修飾。對(duì)水稻分別進(jìn)行冷、高鹽及ABA和JA處理,發(fā)現(xiàn)不同脅迫處理可導(dǎo)致HDAC家族中不同成員的表達(dá)水平不同[77]。HOS15(high expression of osmotically responsive gene)能編碼一種類似于TBL1(Transducin Beta-Like protein-1)的蛋白,與H4相互作用導(dǎo)致H4去乙?；?抑制抗逆基因的表達(dá)[78]。

2.2組蛋白甲基化

組蛋白甲基化也是組蛋白修飾的重要方式之一,主要發(fā)生于賴氨酸和精氨酸殘基上,分別由組蛋白賴氨酸甲基轉(zhuǎn)移酶(histone lysine methyltransferases,HKMTs)和組蛋白精氨酸甲基轉(zhuǎn)移酶(protein arginine methyltransferases,PRMTs)催化。組蛋白甲基化可通過(guò)組蛋白去甲基化酶消除[79]。一般來(lái)說(shuō),組蛋白H3K9和H3K27與轉(zhuǎn)基因沉默相關(guān),H3K4和H3K36與基因活性相關(guān)。

擬南芥中組蛋白賴氨酸甲基化主要發(fā)生在H3的Lys 4、Lys 9、Lys 27和Lys 36上,組蛋白賴氨酸甲基轉(zhuǎn)移酶具有一個(gè)SET域(SET domain)。在植物中,SET 域分為4種類型:SU(VAR)3-9,E(Z)( enhancer of zeste),TRX(trithorax)和ASH1(absent,small,or homeotic discs 1)[80-81]。其中,TRX中的ATX1參與干旱脅迫應(yīng)答。比如干旱脅迫導(dǎo)致atx-1突變株的抗旱性提高[82],擬南芥H3K4三甲基化(H3K4me3)的水平下降[83]。據(jù)報(bào)道,H3K27me3可標(biāo)記多種脅迫基因[84]。植物中,K3K27me3標(biāo)記的基因表達(dá)水平較低,同時(shí)參與植物對(duì)環(huán)境脅迫應(yīng)答的調(diào)控[85-86]。水稻和擬南芥中大約有20%的基因由H3K27me3標(biāo)記[87-88]。植物中由H3K4me3標(biāo)記的基因大約有40%,主要集中于基因的5′端[89],具有較高的轉(zhuǎn)錄活性。H3K4me3在基因表達(dá)過(guò)程中具有重要作用,參與植物生長(zhǎng)及對(duì)脅迫的適應(yīng)性調(diào)控[90-91]。據(jù)報(bào)道,精氨酸甲基化主要發(fā)生在H3的Arg2、Arg8、Arg17、Arg26和組蛋白H4的Arg3上,由PRMTs催化,且SKB1(PRMT5)參與鹽脅迫調(diào)控。SKB1與染色質(zhì)相互作用,導(dǎo)致H4R3sme2水平增加,抑制脅迫基因的轉(zhuǎn)錄與表達(dá)。鹽脅迫下,H3R3sme2與染色質(zhì)分離,H3R3sme2水平下降,促進(jìn)脅迫基因表達(dá)[92]。因此,SKB1可通過(guò)改變H3R3sme2脅迫基因的甲基化狀態(tài)應(yīng)答鹽脅迫。

組蛋白去甲基化酶對(duì)調(diào)控生物體內(nèi)甲基化平衡具有重要作用。據(jù)報(bào)道,有兩種組蛋白去甲基化酶,一種是JMJs(jumonji proteins)蛋白家族,首次在人類和酵母細(xì)胞中發(fā)現(xiàn)[93],其中許多成員為組蛋白賴氨酸去甲基化酶(histone lysine demethylase,KDM)。另一種組蛋白去甲基化酶為賴氨酸特異性去甲基化酶1(lysine-specific demethylase1,KDM1/LSD1)[94]。目前,有關(guān)植物HDMs參與非生物脅迫應(yīng)答的調(diào)控尚未報(bào)道。近期研究表明,組蛋白去甲基化可能參與植物脅迫應(yīng)答,比如脫水處理會(huì)降低H3K4me和H3K4me3水平,H3K4me2表現(xiàn)出微量下降,這表明HDMs可能調(diào)控脫水脅迫基因的表達(dá)[95]。也有研究表明干旱脅迫可導(dǎo)致H3K4me3發(fā)生動(dòng)態(tài)變化。干旱脅迫下,H3K4me3修飾水平的增加伴隨著基因的低表達(dá),而H3K4me3下降則與基因高表達(dá)相關(guān),這說(shuō)明水稻HDMs可能參與干旱脅迫的調(diào)控[96]。

3染色質(zhì)重塑

染色質(zhì)重塑是基因表達(dá)調(diào)控過(guò)程中出現(xiàn)的一系列染色質(zhì)結(jié)構(gòu)變化的總稱。目前研究比較多的是ATP依賴的染色質(zhì)重塑。在真核基因表達(dá)調(diào)控中,三磷酸腺苷依賴的染色質(zhì)重塑復(fù)合物通過(guò)ATP水解釋放的能量來(lái)改變?nèi)旧|(zhì)結(jié)構(gòu)是一種重要的表觀遺傳調(diào)控機(jī)制[97]。據(jù)報(bào)道,ATP依賴的染色質(zhì)重塑因子在植物非生物脅迫應(yīng)答中起重要作用。ATP依賴的染色質(zhì)重塑復(fù)合物主要分為3種類型:第1種是SWI/SNF;第2種是ISF;第3種是CHD。植物中研究較為廣泛的是SWI/SNF染色質(zhì)重塑復(fù)合物。比如干旱和熱脅迫條件下,擬南芥中CHR12(一種SNF/BRM型染色質(zhì)重塑因子)是一種負(fù)調(diào)控因子,將CHR12過(guò)表達(dá)突變體暴露于脅迫環(huán)境下會(huì)導(dǎo)致初生芽和初生莖生長(zhǎng)停止[98]。高溫脅迫下,AtCHR12基因敲除突變型表現(xiàn)出的生長(zhǎng)停滯比野生型低;而無(wú)脅迫處理時(shí),很難區(qū)分野生型和突變型,且生長(zhǎng)停滯應(yīng)答依賴于脅迫的嚴(yán)重程度。綜上所述,CHR12參與植物對(duì)不良環(huán)境的脅迫應(yīng)答。AtSWI3B是擬南芥SWI/SNF復(fù)合物的核心組成部分,ABA脅迫下swi3ba突變株的RD29B和RAB18脅迫應(yīng)答基因表達(dá)水平下降,其對(duì)生長(zhǎng)的抑制作用也出現(xiàn)了下降的現(xiàn)象[99]。豌豆中,ABA和干旱脅迫可促進(jìn)SWI/SNF復(fù)合物組分PsSNF5基因表達(dá)[100]。研究表明,擬南芥SWI2/SNF2染色質(zhì)重塑因子ATPase BRM(BRAHMA)在脅迫應(yīng)答中起重要作用,比如干旱處理提高了brm突變體的抗旱性[101]。

4非編碼RNA調(diào)控

非編碼RNA(non-coding RNA)泛指那些不能編碼蛋白質(zhì),但能夠調(diào)控基因和蛋白表達(dá)的RNA。根據(jù)功能不同可將其分為調(diào)控非編碼RNA(regulatory non-coding RNA)和看家非編碼RNA(housekeeping non-coding RNA);調(diào)控非編碼RNA根據(jù)長(zhǎng)短不同可分為長(zhǎng)鏈非編碼RNA(long non-coding RNA,lncRNA)和短鏈非編碼RNA(small non-coding RNA,sncRNA)。sncRNA主要包括siRNA、miRNA和piRNA。通常,sncRNA在轉(zhuǎn)錄水平(轉(zhuǎn)錄基因沉默,TGS)和轉(zhuǎn)錄后水平(轉(zhuǎn)錄后基因沉默,PTGS)上對(duì)基因表達(dá)表進(jìn)行調(diào)控使基因沉默。在表觀遺傳學(xué)中,非編碼RNA(non-coding RNAs)是一種重要的表觀遺傳調(diào)控方式。目前,研究比較多的是siRNA(small interfering RNA)和miRNA(microRNA)。

據(jù)報(bào)道,參與調(diào)控轉(zhuǎn)錄和轉(zhuǎn)錄后水平基因表達(dá)的內(nèi)源siRNA分子有nat-siRNAs、ta-siRNAs和hc-siRNAs等。siRNA由長(zhǎng)雙鏈RNA通過(guò)不同的生物過(guò)程而來(lái)[102],dsRNA經(jīng)DCLs蛋白剪切為21～24nt,即siRNA。siRNA通過(guò)RNA指導(dǎo)的DNA甲基化途徑來(lái)調(diào)控基因的轉(zhuǎn)錄水平或轉(zhuǎn)錄后水平[103]。miRNA與mRNA高度互補(bǔ),與mRNA的3′-UTR區(qū)結(jié)合,從而導(dǎo)致轉(zhuǎn)錄后基因沉默[9]。已有研究發(fā)現(xiàn)siRNA和miRNA在調(diào)控植物應(yīng)答生物和非生物脅迫的過(guò)程中起重要作用。比如SRO5-P5CDH nat-siRNAs與P5CDH和SRO5蛋白是鹽脅迫應(yīng)答過(guò)程中的重要組成部分,P5CDH下調(diào)導(dǎo)致脯氨酸的積累,這在植物應(yīng)對(duì)高鹽脅迫中具有重要作用[104]。冷害、熱害、鹽脅迫及干旱脅迫導(dǎo)致小麥siRNA的表達(dá)水平發(fā)生不同程度的上調(diào)或下調(diào)[105]。miRNA在擬南芥應(yīng)對(duì)高鹽、干旱和冷脅迫應(yīng)答過(guò)程中具有重要的調(diào)控作用[106]。Zhao等[107]發(fā)現(xiàn)干旱脅迫導(dǎo)致miR169大幅度上調(diào),miR393則發(fā)生瞬時(shí)上調(diào)。熱脅迫下,小麥miRNAs的表達(dá)水平發(fā)生了不同程度變化,如miR172表現(xiàn)明顯的下降,而miRNAs(miR156、 miR159、 miR160、miR166、miR168、miR169、miR393和miR827)的表達(dá)則上調(diào)[108]。據(jù)報(bào)道,miRNAs除參與調(diào)控冷、熱和干旱脅迫外,在作物應(yīng)對(duì)鹽、紫外輻射、低氧和氧化應(yīng)激脅迫以及生物脅迫中也具有重要的調(diào)控作用。

5展望

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(編輯:宋亞珍)

Research Progress of Abiotic Stress Induced Epigenetic Variation in Plants

WANG Shuyan1,GUO Jiufeng1*,LIU Xiaoting1,YUAN Haokun2,LI Yajiao1,NA Ri1

(1 College of Physical Science and Technology,Inner Mongolia University,Hohhot 010021,China;2 School of Life Science and Technology,University of Electronic Science and Technology of China,Chengdu 610054,China)

Abstract:Because of sedentary life style,plants can not avoid bad environmental stimulus,hence need to defense and avoid environmental stress through their own defense mechanism.Epigenetic plays an important role in regulating plans response to environmental stress.This paper summarizes the current research status of epigenetic variations of plants induced by abiotic stress,including DNA methylation,histone modification,chromatin remodeling and non-coding RNA.We are expecting to exploit epigenetic changes improve the stress resistance of plants.

Key words:abiotic stress;DNA methylation;histone modification;chromatin remodeling;non-coding RNA

中圖分類號(hào):Q789

文獻(xiàn)標(biāo)志碼:A

作者簡(jiǎn)介:王淑妍(1990-),女,在讀碩士研究生,主要從事環(huán)境生物物理研究。E-mail:wsykk0914@126.com*通信作者:郭九峰,博士,教授,主要從事生物物理與生物技術(shù)研究。E-mail:guojf101@sina.com

基金項(xiàng)目:國(guó)家自然科學(xué)基金(51467014)

收稿日期:2015-12-07;修改稿收到日期:2016-01-18

文章編號(hào):1000-4025(2016)03-0631-10

doi:10.7606/j.issn.1000-4025.2016.03.0631