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植物體細(xì)胞胚胎發(fā)生的分子機(jī)制

 昵稱37581541 2022-08-09 發(fā)布于江蘇

   孫思敏   孫偉男   楊細(xì)燕*   張獻(xiàn)龍

華中農(nóng)業(yè)大學(xué)作物遺傳改良國(guó)家重點(diǎn)實(shí)驗(yàn)室,湖北武漢 430070

摘要 Abstract


植物細(xì)胞的全能性是指每個(gè)細(xì)胞均具有該植物的全部遺傳信息,其離體組織或細(xì)胞在適當(dāng)培養(yǎng)條件下具有發(fā)育成完整植株的潛能。植物體細(xì)胞胚胎發(fā)生是最能體現(xiàn)植物細(xì)胞全能性的一種方式,其在人工種子、單倍體育種、無性繁殖和種質(zhì)保存等領(lǐng)域具有廣闊的應(yīng)用前景,其發(fā)生的機(jī)理也是基礎(chǔ)研究領(lǐng)域的熱點(diǎn)。近年來,隨著技術(shù)的進(jìn)步及研究的深入,植物體細(xì)胞胚胎發(fā)生的分子調(diào)控機(jī)制取得了重要進(jìn)展。植物體細(xì)胞胚胎發(fā)生是一系列基因在時(shí)空順序上表達(dá)調(diào)控的結(jié)果。本文系統(tǒng)綜述了體細(xì)胞胚胎發(fā)生過程中激素及逆境脅迫信號(hào)轉(zhuǎn)導(dǎo)、胚胎發(fā)育相關(guān)轉(zhuǎn)錄因子、胞外蛋白和表觀遺傳調(diào)控的作用,并對(duì)本領(lǐng)域未來的研究重點(diǎn)及方向進(jìn)行了展望。


植物體細(xì)胞胚胎發(fā)生是指由體細(xì)胞經(jīng)過離體培養(yǎng)產(chǎn)生胚狀體的過程,該過程可直接從外植體表皮、亞表皮、懸浮細(xì)胞、原生質(zhì)體發(fā)生,也可以從脫分化的外植體形成愈傷組織的外部或內(nèi)部產(chǎn)生。從體細(xì)胞向胚性細(xì)胞轉(zhuǎn)變是體細(xì)胞胚胎發(fā)生的前提,該過程中離體的植物細(xì)胞經(jīng)歷脫分化形成愈傷組織,然后脫分化狀態(tài)的愈傷組織和細(xì)胞經(jīng)歷再分化過程,再度分化成不同類型的細(xì)胞、組織和器官,甚至最終再生成完整的植株。這一過程涉及細(xì)胞的重編程、激活細(xì)胞周期、細(xì)胞分化及器官發(fā)育過程,受到眾多轉(zhuǎn)錄因子、激素信號(hào)傳導(dǎo)途徑及表觀遺傳修飾等構(gòu)成的復(fù)雜網(wǎng)絡(luò)調(diào)控。本研究就體細(xì)胞胚胎發(fā)生過程中關(guān)鍵基因的功能及調(diào)控機(jī)制、表觀遺傳機(jī)制及關(guān)鍵基因的應(yīng)用等進(jìn)行綜述,為深入解析體細(xì)胞胚胎發(fā)生的分子機(jī)制及細(xì)胞全能性的研究提供理論依據(jù)。
1 激素信號(hào)轉(zhuǎn)導(dǎo)與植物體細(xì)胞胚胎發(fā)生

在體細(xì)胞胚胎發(fā)生過程中,通過不同激素的配合使用可以有效調(diào)控體細(xì)胞胚胎發(fā)生的各個(gè)發(fā)育階段。比如,通過調(diào)節(jié)生長(zhǎng)素(auxin)與細(xì)胞分裂素(cytokinin,CTK)比率促進(jìn)脫分化和愈傷增殖、通過添加低濃度的乙烯(ethylene)促進(jìn)體細(xì)胞胚胎發(fā)生、通過赤霉素(gibberellinsGAs)調(diào)控胚性培養(yǎng)物到球形胚的轉(zhuǎn)化、通過添加脫落酸(abscisic acidABA)來提高體細(xì)胞胚的質(zhì)量等。外源激素對(duì)體細(xì)胞胚胎的作用主要是通過胞外或胞內(nèi)的激素受體將外界刺激信號(hào)轉(zhuǎn)到核內(nèi),從而調(diào)控基因的表達(dá),啟動(dòng)發(fā)育程序。

1.1  生長(zhǎng)素信號(hào)傳導(dǎo)及體細(xì)胞胚胎發(fā)生

生長(zhǎng)素是誘導(dǎo)植物體細(xì)胞胚胎發(fā)生過程的重要植物生長(zhǎng)調(diào)節(jié)劑。其中2,4-D在植物體細(xì)胞胚胎發(fā)生中的應(yīng)用較為廣泛。一定濃度的生長(zhǎng)素能夠促進(jìn)愈傷組織的誘導(dǎo)和增殖。在棉花體細(xì)胞到胚性細(xì)胞的轉(zhuǎn)變中,伴隨著內(nèi)源生長(zhǎng)素水平的升高,表現(xiàn)為生長(zhǎng)素信號(hào)途徑的激活[1]。與生長(zhǎng)素合成、極性運(yùn)輸和響應(yīng)相關(guān)的基因被認(rèn)為是植物體細(xì)胞胚胎發(fā)生的關(guān)鍵基因。目前已從不同植物中已解析出多個(gè)受生長(zhǎng)素誘導(dǎo)表達(dá)的基因,這些基因包括GH3s (Gretchen Hagen 3)、PINs (Pin-formed)、生長(zhǎng)素/吲哚乙酸蛋白基因(auxin/indole-3-acetic acid, AUX/IAAs)、生長(zhǎng)素響應(yīng)因子基因(auxin response factors, ARFs)SAURs (small auxin-up RNAs)。生長(zhǎng)素信號(hào)通路依賴生長(zhǎng)素響應(yīng)因子,尤其是JMJ30 (JUMONJI C DOMAIN-CONTAINING PROTEIN 30)ARF7ARF19相互作用并直接與LBD (LATERAL ORGAN BOUNDARIES DOMAIN)家族基因(LBD16,LBD17)的瞬時(shí)元件結(jié)合激活它們的表達(dá),LBD家族基因誘導(dǎo)E2Fa的表達(dá),促進(jìn)愈傷組織的增殖(1)[2-4]。另外,在體細(xì)胞胚胎發(fā)生過程中,外源生長(zhǎng)素能激活胚胎發(fā)育相關(guān)轉(zhuǎn)錄因子調(diào)控網(wǎng)絡(luò),轉(zhuǎn)錄因子BABY BOOM (BBM)LEC1-ABI3-FUS3-LEC2 (LAFL)復(fù)合物是體細(xì)胞胚胎發(fā)生的主要調(diào)節(jié)因子。BBM編碼AIL (AINTEGUMENTA-LIKE)并直接調(diào)節(jié)所有LAFL基因[5]。同時(shí)LAFL基因也可以激活生長(zhǎng)素合成和運(yùn)輸進(jìn)而調(diào)控體細(xì)胞胚胎發(fā)生。在體細(xì)胞胚胎發(fā)生過程中,在SAM區(qū)域的較高濃度的生長(zhǎng)素會(huì)誘導(dǎo)WUS (WUSCHEL)基因的表達(dá),進(jìn)而激活PIN1蛋白的極性定位,促進(jìn)體細(xì)胞胚胎的生長(zhǎng)和發(fā)育[6]

圖片

生長(zhǎng)素信號(hào)傳導(dǎo)及體細(xì)胞胚胎發(fā)生

Fig. 1  Auxin signaling and somatic embryogenesis

1.2  細(xì)胞分裂素信號(hào)與愈傷組織的增殖

在植物體細(xì)胞胚胎發(fā)生過程中,大部分情況下生長(zhǎng)素不是獨(dú)立發(fā)揮作用,只有當(dāng)生長(zhǎng)素和細(xì)胞分裂素維持在合適的比例時(shí),才能促進(jìn)愈傷組織的產(chǎn)生[7]。細(xì)胞分裂素信號(hào)轉(zhuǎn)導(dǎo)是一個(gè)磷酸接力傳遞的過程。細(xì)胞分裂素首先結(jié)合受體組氨酸激酶(histidine kinases,HK)并使其磷酸化,將磷酸基團(tuán)轉(zhuǎn)移給胞質(zhì)中的磷酸轉(zhuǎn)運(yùn)蛋白(histidine-phosphotransfer protein,HP),磷酸化的HP進(jìn)入細(xì)胞核并將磷酸基團(tuán)轉(zhuǎn)移到A型和B型反應(yīng)調(diào)節(jié)因子(ARABIDOPSIS RESPONSE REGULATOR, ARR)上,進(jìn)而進(jìn)行信號(hào)的傳遞和激活。擬南芥中超表達(dá)ARR1促使細(xì)胞周期相關(guān)基因的表達(dá),從而促進(jìn)愈傷組織的分裂與增殖[8]。細(xì)胞分裂素信號(hào)的抑制基因ARR7ARR15的超表達(dá)會(huì)抑制擬南芥根系頂端分生組織的起始和擬南芥的體細(xì)胞胚胎發(fā)生,在擬南芥細(xì)胞分裂素受體基因ahk2ahk4ahk3ahk4雙突變體中,再生體細(xì)胞胚的根尖和莖尖發(fā)育畸形[9]

1.3  乙烯信號(hào)與體細(xì)胞胚胎發(fā)生

植株的再生能力與乙烯的敏感性有一定的關(guān)聯(lián),乙烯含量的升高及其信號(hào)途徑的激活有利于體細(xì)胞胚胎發(fā)生[10]。擬南芥中的研究表明,用乙烯合成抑制劑處理或超表達(dá)乙烯合成抑制基因ETO1 (ETHYLENE-OVERPRODUCER 1)會(huì)降低外植體的再生能力,乙烯不敏感突變體(etr1-1ein2-1)的再生能力下降,而乙烯響應(yīng)負(fù)相關(guān)因子突變體(ctr1-1ctr1-12)的再生能力增強(qiáng)[11]。在大豆體細(xì)胞胚胎發(fā)生過程中,添加一定濃度的乙烯合成前體1-氨基環(huán)丙烷-1-羧酸(1-aminocylopropane-1-carboxylic acid, ACC)顯著促進(jìn)大豆的體細(xì)胞胚胎發(fā)生效率,而利用乙烯抑制劑氨基乙氧基乙烯甘氨酸(aminoethoxyvinylglycine, AVG)等處理則顯著抑制大豆的體細(xì)胞胚胎發(fā)生效率[12];在棉花中,乙烯及其信號(hào)對(duì)愈傷組織的增殖起著正向調(diào)控作用。超表達(dá)SPL10 (SQUAMOSA-PROMOTER BINDING PROTEIN-LIKE 10)促使乙烯合成酶相關(guān)基因ACOs (1-aminocyclopropane carboxylic acid oxidases)的表達(dá),從而提高愈傷組織的增殖率,乙烯合成抑制劑AVG抑制處理降低乙烯的含量并顯著抑制愈傷組織的增殖;而乙烯合成前體ACC處理顯著則促進(jìn)外植體愈傷組織的增殖[13]
2  逆境脅迫信號(hào)與愈傷組織再生

理論上所有離體植物細(xì)胞在合適的外界環(huán)境下均可表現(xiàn)全能性。外界環(huán)境因素(特別是逆境因素)是體細(xì)胞胚胎發(fā)生的重要影響因素。逆境因子(包括機(jī)械損傷)在體細(xì)胞胚胎發(fā)生的幾個(gè)主要階段都起著重要的作用,很多研究者都將逆境因子的調(diào)控作為優(yōu)化體細(xì)胞胚胎發(fā)生體系的重要手段。在現(xiàn)有的誘導(dǎo)愈傷組織體系中,培養(yǎng)基多采用MS培養(yǎng)基,相對(duì)于維持植株正常生長(zhǎng)或萌發(fā)胚生根成苗的低糖、低鹽、低滲的SH培養(yǎng)基及1/2MS培養(yǎng)基來說,MS培養(yǎng)基無機(jī)鹽含量較高,微量元素種類較全,濃度也較高。另外,很多物種,包括胡蘿卜、苜蓿、煙草等,均有通過利用逆境處理來促進(jìn)體細(xì)胞胚胎形成及發(fā)育的研究,涉及到的逆境因素也多種多樣,主要有ABA處理、饑餓處理、滲透脅迫、高溫處理等[14-15]。

機(jī)械損傷是愈傷組織誘導(dǎo)原初誘導(dǎo)觸發(fā)因子[16]。再生的發(fā)生一般都是在創(chuàng)傷部位開始的。創(chuàng)傷可以誘導(dǎo)許多細(xì)胞反應(yīng),包括激素的積累(茉莉酸)Ca2+快速流入、活性氧(reactive oxygen species, ROS)爆發(fā)、細(xì)胞間遠(yuǎn)程通訊中斷等。擬南芥WOUND INDUCED DEDIFFERENTIATION 1(WIND1)及其同源物WIND2~WIND4是這一過程的中心調(diào)節(jié)因子。WIND1~WIND4AP2/ERF轉(zhuǎn)錄因子家族成員,在愈傷組織誘導(dǎo)過程中會(huì)在受傷處誘導(dǎo)迅速表達(dá),并促進(jìn)細(xì)胞的脫分化及愈傷組織增殖。異位表達(dá)WIND1可使成熟的體細(xì)胞擺脫正常的分化程序而啟動(dòng)誘導(dǎo)在組織和結(jié)構(gòu)上無定形的細(xì)胞增殖,以及保持其脫分化狀態(tài)[17]。轉(zhuǎn)基因植株(35S:WIND1)細(xì)胞狀態(tài)與2,4-D所誘導(dǎo)的愈傷組織細(xì)胞類似。WIND1蛋白是細(xì)胞分裂素應(yīng)答的正調(diào)節(jié)因子,主要通過對(duì)BARR基因促進(jìn)細(xì)胞分裂素信號(hào)傳導(dǎo)并直接激活ESR1 (ENHANCER OF SHOOT REGENERATION1)從而促進(jìn)愈傷組織的形成(2)[17]。在損傷時(shí)也會(huì)誘導(dǎo)IPT3 (SOPENTENYLTRANSFERASE 3)、LOG1 (LONELY GUY 1)、LOG4 (LONELY GUY 4)LOG5(LONELY GUY5)并提高細(xì)胞分裂素水平,進(jìn)而通過CYCD3;1 (D-type cyclin 3;1)重新激活細(xì)胞周期[18]。其他AP2/ERF (APETALA2/ethylene response factor)基因,如ERF115PLT3(PLETHORA3)-5、-7也在傷口誘導(dǎo)的愈傷組織形成過程中上調(diào)表達(dá),并且還發(fā)現(xiàn)AP2/ERFCTK介導(dǎo)的途徑調(diào)控網(wǎng)絡(luò)廣泛串聯(lián)[19]。

圖片

機(jī)械損傷通過調(diào)節(jié)因子WINs調(diào)控愈傷組織形成

Fig. 2  Mechanical damage regulates callus formation through the regulatory factor WINs


3  轉(zhuǎn)錄因子與體細(xì)胞胚胎發(fā)生

雖然體細(xì)胞胚胎發(fā)生過程受諸多外界環(huán)境因素的影響,但歸根結(jié)底是在各種因素的作用下,體細(xì)胞中某些特異的基因啟動(dòng)表達(dá),從而使體細(xì)胞脫分化并再分化轉(zhuǎn)變?yōu)榕咝约?xì)胞。許多研究人員致力于解析體細(xì)胞胚胎發(fā)生過程的相關(guān)基因,目前已鑒定和克隆了大量與體細(xì)胞胚胎發(fā)生相關(guān)的轉(zhuǎn)錄因子基因,其中許多參與調(diào)節(jié)合子胚發(fā)生、分生組織分化和維持的轉(zhuǎn)錄因子都在體細(xì)胞胚胎發(fā)生過程中起著重要用。

3.1  核因子Y (nuclear factor Y, NF-Y)

核因子Y (nuclear factor Y, NF-Y),又稱CCAAT盒結(jié)合因子(CCAAT-binding factor, CBF)或亞鐵血紅素激活蛋白(heme activator protein, HAP),是一類普遍存在于酵母、動(dòng)物、植物等真核生物中的轉(zhuǎn)錄因子,通常由3種不同亞基組成,即NF-YA (CBF-BHAP2)、NF-YB (CBF-AHAP3)NF-YC (CBF-CHAP5)3個(gè)家族[20]。植物NF-YB對(duì)胚胎發(fā)生具有重要作用[21]。擬南芥中NF-YB家族共有13個(gè)成員,分為LEAFY COTYLEDON 1 (LEC1)類和非LEC1兩類蛋白,其中LEC1類包括LEC1LEC1-LIKE (L1L),是植物胚胎形成的中樞調(diào)控因子[22]。LEC1在胚性細(xì)胞、體細(xì)胞胚和未成熟種子中高表達(dá),并可賦予體細(xì)胞向胚性細(xì)胞發(fā)育的能力,在體細(xì)胞胚胎發(fā)育的早期能維持胚性細(xì)胞的命運(yùn),目前在多個(gè)物種中將其作為體細(xì)胞胚胎發(fā)生的標(biāo)記基因[23-24]。L1LLEC1類似,在胚性愈傷組織、體細(xì)胞胚和未成熟種子中高表達(dá),而在非胚性愈傷組織、營(yíng)養(yǎng)組織中表達(dá)量較低,L1L的異位表達(dá)可以代替LEC1發(fā)揮作用,在柑橘營(yíng)養(yǎng)組織中異位表達(dá)L1L會(huì)誘導(dǎo)出胚[25]。

3.2  B3-結(jié)構(gòu)域轉(zhuǎn)錄因子

B3結(jié)構(gòu)域轉(zhuǎn)錄因子家族屬于植物特異性轉(zhuǎn)錄因子家族。其中LEC2、FUS3 (FUSCA3)ABI3是含有B3結(jié)構(gòu)域的與胚胎發(fā)育相關(guān)的調(diào)控因子,是一類胚胎發(fā)育相關(guān)的標(biāo)志基因,與LEC1存在協(xié)同調(diào)控作用[26-27]。過量表達(dá)(或異位表達(dá)) LEC2基因調(diào)控LEC1、L1L、FUS3、ABI3 (ABSCISIC ACID-INSENSITIVE3)WRI1 (WRINKELED1)等轉(zhuǎn)錄因子基因的表達(dá),并促使體細(xì)胞向胚性細(xì)胞的轉(zhuǎn)變,從而使轉(zhuǎn)基因植物具有了胚胎的特性[27]。LEC1促進(jìn)胚性細(xì)胞的形成不同,LEC2可直接誘導(dǎo)形成體細(xì)胞胚胎,兩者可能激活不同的調(diào)控路徑[28]。同樣,FUS3基因在頂端分生組織特異表達(dá),可以促使轉(zhuǎn)基因植物在頂端分生組織產(chǎn)生體細(xì)胞胚[27]。LEC2、FUS3ABI3的下調(diào)則顯著抑制直接和間接體細(xì)胞胚胎發(fā)生[5]。LEC2、FUS3ABI3同時(shí)受生長(zhǎng)素的誘導(dǎo)表達(dá),而且LEC2也可以對(duì)生長(zhǎng)素合成基因YUC4 (YUCCA4)直接調(diào)控,同時(shí)激活生長(zhǎng)素響應(yīng)基因的表達(dá),進(jìn)而激活生長(zhǎng)素信號(hào)傳導(dǎo)途徑[29]

3.3  AP2/ERF結(jié)構(gòu)域蛋白

AP2/ERF結(jié)構(gòu)域蛋白是植物特異性轉(zhuǎn)錄因子家族,它們參與許多發(fā)育過程的調(diào)節(jié)。其中有幾個(gè)AP2/ERF家族成員參與調(diào)控體細(xì)胞胚胎發(fā)生,其中研究得最多的是BBMBBM最初是從甘藍(lán)型油菜花粉體細(xì)胞胚中分離得到的,并在花粉體細(xì)胞胚和合子胚中表達(dá)。在擬南芥、辣椒、毛白楊等物種中的研究表明BBM基因是植物細(xì)胞全能性的關(guān)鍵調(diào)控因子,在體細(xì)胞胚胎發(fā)生過程中該基因能促進(jìn)細(xì)胞分裂和形態(tài)發(fā)生,異位表達(dá)BBM基因或者擬南芥AtBBM基因能在沒有外源植物生長(zhǎng)調(diào)節(jié)劑或脅迫的情況下誘導(dǎo)體細(xì)胞胚胎的發(fā)生[30]。BBM能結(jié)合LAFL基因的啟動(dòng)子,從而調(diào)控這些基因的表達(dá),進(jìn)而調(diào)控WOX2 (WUSCHEL-RELATED HOMEOBOX 2)WOX3等早期胚胎發(fā)生基因;反過來,BBM的表達(dá)也受到LAFL蛋白質(zhì)的調(diào)節(jié),它們形成交互網(wǎng)絡(luò)共同促進(jìn)體細(xì)胞胚胎發(fā)生[31-32]。另一個(gè)成員是擬南芥EMK (EMBRYO MAKER)在胚發(fā)育的早期和成熟胚中表達(dá),異位表達(dá)EMK促進(jìn)擬南芥子葉體細(xì)胞胚胎發(fā)生的啟動(dòng)[34]。在機(jī)械損傷部分提到過的WIND1~WIND4也是AP2/ERF轉(zhuǎn)錄因子家族中調(diào)控體細(xì)胞胚胎發(fā)生的成員,WIND1可以與LEC途徑相互作用,與單獨(dú)激活WIND1LEC2相比,在外植體中按順序依次激活WIND1LEC2能誘導(dǎo)出更多的胚性愈傷[19]。

3.4  同源異形域轉(zhuǎn)錄因子

同源異形域(homeodomainHD)是指真核生物中一個(gè)由約60個(gè)氨基酸組成的高度保守的具有轉(zhuǎn)錄因子特征的DNA結(jié)合域,其最早是在果蠅體節(jié)發(fā)育中發(fā)現(xiàn)并命名的,之后高等植物中發(fā)現(xiàn)包含該類結(jié)構(gòu)域的基因有重要作用。其中WOX轉(zhuǎn)錄因子家族在胚的發(fā)育、頂端分生組織建成和器官形成中發(fā)揮重要作用。WUSWOX家族中最早發(fā)現(xiàn)的成員,在分生組織階段該基因的表達(dá)受生長(zhǎng)素和細(xì)胞分裂素的誘導(dǎo),調(diào)控分生組織維持的特性;反之,WUS基因調(diào)控體細(xì)胞胚胎發(fā)生過程中生長(zhǎng)素依賴的營(yíng)養(yǎng)組織向胚性組織轉(zhuǎn)變[35-36]。超表達(dá)WUS會(huì)誘導(dǎo)體細(xì)胞胚胎發(fā)生及莖尖和根尖器官發(fā)生,異位表達(dá)WUS基因可使那些難以誘導(dǎo)體細(xì)胞胚胎發(fā)生的頑拗型材料去分化產(chǎn)生不定芽和體細(xì)胞胚[36-37]。WUS基因是細(xì)胞分裂素應(yīng)答的負(fù)調(diào)節(jié)因子,通過抑制AARR基因的表達(dá),與細(xì)胞分裂素信號(hào)傳導(dǎo)途徑協(xié)同作用,進(jìn)而決定體細(xì)胞胚胎過程中某些細(xì)胞的命運(yùn)[37]。另外由于WUS可以穿過頂細(xì)胞層并激活其負(fù)調(diào)控因子CLV3 (CLAVATA3)的轉(zhuǎn)錄,這種WUS-CLV的反饋回路可以維持干細(xì)胞庫的穩(wěn)定和頂端分生組織的發(fā)育[34,36]。WOX家族其他成員也被發(fā)現(xiàn)具有調(diào)控體細(xì)胞胚胎發(fā)生的功能。在擬南芥中,AtWOX2是胚胎發(fā)育中細(xì)胞命運(yùn)決定和頂端分化的調(diào)控因子,包括頂端分生組織干細(xì)胞的啟動(dòng),發(fā)育后期WOX2還參與側(cè)生器官的形成和分離[38]。在松樹中WOX2被作為體細(xì)胞胚胎發(fā)生的標(biāo)志基因[39]。STM (SHOOT MERISTEMLESS)是同源異性域轉(zhuǎn)錄因子KNOXI (KNOTTED1-like homeobox I)家族的成員,與WUSCLV3共同作用保持頂端分生組織的干細(xì)胞微環(huán)境,并調(diào)節(jié)細(xì)胞增殖和分化的平衡[40]。甘藍(lán)型油菜中STM的異位表達(dá)可以調(diào)節(jié)對(duì)外源生長(zhǎng)素的敏感性并促進(jìn)擬南芥體細(xì)胞胚胎發(fā)生[41]
4  體細(xì)胞胚胎發(fā)生過程中的信號(hào)轉(zhuǎn)導(dǎo)

4.1  類受體激酶與體細(xì)胞胚胎發(fā)生

SERK (SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE)是體細(xì)胞胚胎發(fā)生過程中促進(jìn)體細(xì)胞向胚性細(xì)胞轉(zhuǎn)變的關(guān)鍵激酶。它首先是在研究胡蘿卜體細(xì)胞向胚性細(xì)胞轉(zhuǎn)變的過程中發(fā)現(xiàn)的,其僅在胚性細(xì)胞的球形期表達(dá),而在非胚性細(xì)胞或其他時(shí)期的體細(xì)胞胚胎中均不表達(dá)[42]SERK屬于富含亮氨酸重復(fù)序列受體類似激酶(leucine-rich repeat receptor-like kinase, LRR-RLK)亞家族的成員,通過識(shí)別分子信號(hào)進(jìn)而介導(dǎo)其蛋白的LRR結(jié)構(gòu)域與胞外蛋白結(jié)合,誘導(dǎo)細(xì)胞內(nèi)的信號(hào)級(jí)聯(lián)放大。這些信號(hào)可識(shí)別不同的靶標(biāo),并利用染色質(zhì)重塑提高體細(xì)胞胚胎發(fā)生早期響應(yīng)基因的表達(dá)(LECBBM),從而誘導(dǎo)組織或體細(xì)胞向胚性細(xì)胞發(fā)生轉(zhuǎn)變[43]。異位表達(dá)擬南芥SERK1基因可促進(jìn)其體細(xì)胞胚胎發(fā)生,該基因可作為體細(xì)胞胚胎發(fā)生過程中標(biāo)記基因[44]。植物激素對(duì)SERK基因的表達(dá)和信號(hào)轉(zhuǎn)導(dǎo)起重要作用。SERK1的表達(dá)受到生長(zhǎng)素和細(xì)胞分裂素的協(xié)同調(diào)控,SERK2SERK3則參與生長(zhǎng)素特異性反應(yīng),并且SERK1SERK5與油菜素內(nèi)酯信號(hào)傳導(dǎo)有關(guān)[45]。CLV1也是屬于LRR-RLK亞家族的成員,雖與SERK同屬于一個(gè)家族,但它們?cè)隗w細(xì)胞胚胎發(fā)生中的功能卻是相反的,在蕓薹屬植物中,CLV1抑制體細(xì)胞胚胎發(fā)生轉(zhuǎn)錄因子(WUS)的表達(dá)進(jìn)而抑制體細(xì)胞胚胎發(fā)生[38]。

4.2  鈣信號(hào)與體細(xì)胞胚胎發(fā)生

Ca2+作為第二信使,在植物生長(zhǎng)發(fā)育及環(huán)境響應(yīng)中起著重要的作用。有研究認(rèn)為液泡Ca2+濃度增加是識(shí)別胚胎發(fā)生細(xì)胞的原初信號(hào),胚性細(xì)胞啟動(dòng)胚胎發(fā)育時(shí)伴隨著較大的Ca2+濃度的波動(dòng),Ca2+的梯度調(diào)控胚胎發(fā)育階段極性分化及器官建成[46]。較高濃度的Ca2+提高胚性愈傷組織的形成,并胚胎發(fā)生的頻率,缺乏Ca2+會(huì)阻止體細(xì)胞胚的形成。
細(xì)胞Ca2+信號(hào)由Ca2+感受器進(jìn)行感受并傳遞。植物中存在3種主要的Ca2+感受器。最著名的是鈣調(diào)素(calmodulin, CaM)和類鈣調(diào)素蛋白(calmodulin-like proteins, CMLs)[47]CaM受生長(zhǎng)素誘導(dǎo)表達(dá),并在誘導(dǎo)體細(xì)胞胚胎發(fā)生后期(胚性愈傷到胚發(fā)育階段)特異性表達(dá),一般定位于發(fā)育中胚胎的分生組織區(qū)域。CaM具有促進(jìn)細(xì)胞增殖的作用;采用鈣調(diào)素抑制劑W7 [N-(6-氨基己基)-5--1-萘磺胺鹽酸鹽]處理則會(huì)抑制體細(xì)胞胚胎發(fā)生。鈣依賴蛋白激酶(calcium-dependent protein kinase, CDPK)是第2Ca2+感受器,它包含一個(gè)C-末端CaM樣結(jié)構(gòu)域,可以直接結(jié)合Ca2+并進(jìn)行下游信號(hào)的傳遞,阻斷CDPK介導(dǎo)的信號(hào)通路抑制胚胎發(fā)生[48]。第3Ca2+感受器是鈣調(diào)磷酸酶B亞基類似蛋白(calcineurin B-like protein, CBL)家族,與CaM不同,CBLs只在高等植物中存在,并特異與蛋白質(zhì)激酶CIPKs (CBL-interacting protein kinases)家族相互作用進(jìn)行鈣信號(hào)轉(zhuǎn)導(dǎo)[49]。在棉花體細(xì)胞胚胎發(fā)生過程中發(fā)現(xiàn)CBLCIPK家族基因差異表達(dá),超表達(dá)CIPK6基因影響棉花體細(xì)胞的脫分化過程,并可能與生長(zhǎng)素信號(hào)通路存在交叉互作[1]。
5  胞外蛋白與體細(xì)胞胚胎發(fā)生
為了在植物中誘導(dǎo)體細(xì)胞胚胎發(fā)生,已經(jīng)開發(fā)了多種誘導(dǎo)系統(tǒng)。這些系統(tǒng)中的分子有助于激發(fā)植物細(xì)胞的胚性潛能,促進(jìn)體細(xì)胞胚胎的發(fā)生。在此過程中也篩選到一些胚胎特異性基因作為區(qū)分胚性和非胚性細(xì)胞培養(yǎng)物的標(biāo)記,并發(fā)現(xiàn)了許多胞外蛋白也可作為胚胎發(fā)生潛力的標(biāo)記物或調(diào)控體細(xì)胞胚胎發(fā)生的信號(hào)分子[47]。

5.1  阿拉伯半乳糖蛋白

阿拉伯半乳糖蛋白(arabinogalactan-proteins,AGPs)是一類富含羥脯氨酸或脯氨酸的糖蛋白,廣泛存在于植物的細(xì)胞壁、細(xì)胞膜和組織的細(xì)胞間隙,并可以分泌到培養(yǎng)細(xì)胞的培養(yǎng)基中[50]。AGPs能促進(jìn)多個(gè)物種的體細(xì)胞胚胎發(fā)生,有些可作為細(xì)胞能否進(jìn)行體細(xì)胞胚胎發(fā)生的早期標(biāo)志。添加外源AGPs提高胡蘿卜、仙客來、云杉、香蕉和小麥非胚性細(xì)胞系的胚性能力,增加體細(xì)胞胚胎的數(shù)量[51-52]。去除細(xì)胞壁上的AGPs會(huì)使原生質(zhì)體形成體細(xì)胞胚胎的能力下降,而加入胞外AGPs則會(huì)逆轉(zhuǎn)去除細(xì)胞壁的部分效果。添加胡蘿卜種子AGP可使失去體細(xì)胞胚胎發(fā)育能力的細(xì)胞系重新啟動(dòng)胚形成,添加與AGP特異結(jié)合的抑制劑Yariv (1,3,5-[4-β-D葡吡喃糖基-氧化苯基-偶氮基]-2,4,6-三羥基苯)導(dǎo)致細(xì)胞增殖受阻,抑制體細(xì)胞胚胎發(fā)生[53-54]。AGPs調(diào)節(jié)體細(xì)胞胚胎發(fā)生的作用機(jī)制可能體現(xiàn)在兩方面:(1) AGPs一般存在于胞外或膜的界面,與細(xì)胞壁非共價(jià)結(jié)合,能快速移動(dòng)、降解,可作為信號(hào)分子影響細(xì)胞識(shí)別及信號(hào)傳遞,進(jìn)而影響細(xì)胞增殖、分化,導(dǎo)致發(fā)育軌跡的改變;(2) AGP可作為多種酶的底物,裂解后產(chǎn)生不同種類的寡糖分子,寡糖分子作為信號(hào)分子參與發(fā)育調(diào)控。有研究表明同時(shí)添加幾丁質(zhì)酶和AGP能進(jìn)一步加快體細(xì)胞胚胎發(fā)生的頻率,幾丁質(zhì)酶能裂解AGP的寡糖基鏈,產(chǎn)生信號(hào)分子,促進(jìn)體細(xì)胞胚胎的發(fā)生[55]。

5.2  脂質(zhì)轉(zhuǎn)運(yùn)蛋白

脂質(zhì)轉(zhuǎn)運(yùn)蛋白(lipid transfer protein,LTP)是一類分子量小于10 kD的小分子可溶性堿性蛋白[56]。在動(dòng)物中,有些LTP蛋白參與癌細(xì)胞增殖的調(diào)控。植物中LTP含量高,占可溶性蛋白的4%左右。有些非特異性LTP基因在幼嫩組織、營(yíng)養(yǎng)生長(zhǎng)時(shí)期的分生組織、幼胚中優(yōu)勢(shì)表達(dá),并在體細(xì)胞胚胎發(fā)生過程進(jìn)行富集,被認(rèn)為是體細(xì)胞胚胎發(fā)生誘導(dǎo)的早期標(biāo)記[58]。EP2 (extracellular protein 2)是胡蘿卜胚性培養(yǎng)中第一個(gè)被分離和鑒定的編碼LTP的基因,其分泌到胞外,在原胚性細(xì)胞中高表達(dá),而在非胚性細(xì)胞中不表達(dá)[57-58]。棉花LTP基因在下胚軸、非胚性細(xì)胞組織和植株中不表達(dá),但在胚性細(xì)胞、球形胚高量表達(dá),而在球后期體細(xì)胞胚胎中表達(dá)量又下降[59]。LTP1是葡萄體細(xì)胞胚胎發(fā)育過程中表皮原形成的標(biāo)志,異位表達(dá)35S:VvLTP1導(dǎo)致胚胎嚴(yán)重畸形,導(dǎo)致表皮層出現(xiàn)異常[60]
6  體細(xì)胞胚胎發(fā)生的表觀遺傳調(diào)控
一般認(rèn)為,植物體細(xì)胞比動(dòng)物體細(xì)胞具有更強(qiáng)的可塑性,已分化的離體植物細(xì)胞在一定培養(yǎng)條件下可脫分化并獲得全能性或亞全能性,在這個(gè)過程中,植物細(xì)胞必須獲得改變細(xì)胞發(fā)育命運(yùn)的潛能,這一過程也伴隨著染色質(zhì)水平和基因表達(dá)水平的重編程,DNA甲基化、組蛋白去乙?;?/span>/甲基化、miRNA等重要的表觀調(diào)控因子也是影響體細(xì)胞胚胎發(fā)生的重要因素[61]。

6.1  DNA甲基化

植物體細(xì)胞胚胎發(fā)生過程受DNA甲基化的調(diào)控。一般來說,非胚性愈傷組織基因組甲基化水平高,而胚性愈傷組織的基因組甲基化水平較低,這一現(xiàn)象在刺五加、甜菜、棉花、黑松等中都能觀察到[62]。體細(xì)胞胚的發(fā)育過程也伴隨著DNA甲基化水平的變化,體細(xì)胞胚胎發(fā)育早期階段較低,隨著體細(xì)胞胚胎的發(fā)育逐漸增加,到子葉胚階段達(dá)到最高值[63]。在擬南芥等中發(fā)現(xiàn)體細(xì)胞胚胎發(fā)生過程的調(diào)節(jié)需要一定水平的DNA甲基化維持,甲基化抑制劑5-氮雜胞苷(5-azacitidine,5-AzaC)處理誘導(dǎo)DNA胞嘧啶甲基化水平降低,抑制非胚性愈傷組織的增殖或胚性愈傷組織的體細(xì)胞胚胎發(fā)生能力[64]。DNA甲基化可通過引起胚胎發(fā)生特定基因的啟動(dòng)進(jìn)而影響體細(xì)胞胚胎發(fā)生。LEC1基因啟動(dòng)子在體細(xì)胞中被甲基化,隨著外源培養(yǎng)條件的施加,其甲基化狀態(tài)被消除,促進(jìn)體細(xì)胞向胚性細(xì)胞的轉(zhuǎn)變及體細(xì)胞胚的產(chǎn)生,隨后在胚胎發(fā)育成熟后再次被甲基化[65];同樣,在擬南芥中經(jīng)過5-AzaC處理不能誘導(dǎo)體細(xì)胞胚胎發(fā)生的培養(yǎng)物中發(fā)現(xiàn)LEC2、LEC1BBM基因的顯著被抑制,但在DNA甲基化酶基因(drm1 drm2drm1 drm2 cmt3)突變體中這些基因明顯上調(diào)并改善體細(xì)胞胚胎發(fā)生[64]DNA甲基化受2,4-D的調(diào)控,在誘導(dǎo)階段,高濃度的2,4-D通過DNA甲基化關(guān)閉原有分化細(xì)胞內(nèi)基因的表達(dá);去除2,4-D后使DNA去甲基化。

6.2  組蛋白甲基化

在體細(xì)胞胚胎發(fā)生過程中,發(fā)現(xiàn)DNA甲基化作用的同時(shí)也發(fā)現(xiàn)組蛋白甲基化對(duì)體細(xì)胞胚胎形成的影響。其中組蛋白H3的第27位賴氨酸的三甲基化(H3K27me3)、組蛋白H3的第4位賴氨酸的三甲基化(H3K4me3)2個(gè)研究得較多的組蛋白甲基化修飾,H3K27me3是動(dòng)、植物發(fā)育重要基因的沉默基因轉(zhuǎn)錄標(biāo)記物,而H3K4me3是轉(zhuǎn)錄激活標(biāo)記物。

組蛋白甲基化由組蛋白甲基轉(zhuǎn)移酶完成的。擬南芥中,組蛋白甲基轉(zhuǎn)移酶PRC2通過H3K27me3來抑制相關(guān)基因的表達(dá),從而促進(jìn)細(xì)胞分化,反之則引起細(xì)胞脫分化,誘導(dǎo)體細(xì)胞胚胎發(fā)生[66]。在擬南芥中PRC2 (polycomb repressive complex 2)基因(CURLY LEAF,CLFSWINGER,SWN)(VERNALIZATION 2,VRN2EMBRYONIC FLOWER 2,EMF2)雙突變體在莖尖脫分化形成愈傷組織,間接導(dǎo)致體細(xì)胞胚胎發(fā)生并形成異位根[67],并且與野生型相比,PRC2的突變體在營(yíng)養(yǎng)組織中顯示出更高的體細(xì)胞胚胎誘導(dǎo)能力[68]。大部分胚性相關(guān)基因LEC1LEC2、AGL15BBM以及分生組織調(diào)節(jié)因子STM、WUSWOX5等基因的染色質(zhì)區(qū)域都含有H3K27me3等甲基化位點(diǎn)[69]。而PRC1PRC2與胚胎發(fā)生轉(zhuǎn)錄抑制因子VAL1VAL2等互作,并通過表觀修飾抑制胚胎發(fā)生相關(guān)靶標(biāo)基因的表達(dá)從而抑制愈傷組織的形成和體細(xì)胞胚胎發(fā)生。除了H3K27me3在體細(xì)胞胚胎發(fā)生發(fā)揮功能外,在擬南芥還發(fā)現(xiàn)賴氨酸特異性去甲基酶LDL3可以在愈傷組織形成過程中特異性消除H3K4me2,進(jìn)一步使愈傷組織具有芽分化的能力[70]。

6.3  組蛋白去乙?;?/span>

組蛋白去乙酰化也是一種與植物體細(xì)胞胚胎發(fā)生密切關(guān)聯(lián)的組蛋白修飾。在激素誘導(dǎo)下組蛋白乙?;胶徒M蛋白去乙?;?/span>(histone deacetylaseHDAC)的活性在體細(xì)胞胚胎發(fā)生過程中會(huì)發(fā)生明顯變化。而且組蛋白H3H4的乙?;瘯?huì)促進(jìn)體細(xì)胞胚胎發(fā)生。組蛋白乙?;乃胶臀恢檬艿浇M蛋白乙酰轉(zhuǎn)移酶(histone acetyltransferase,HAT)和組蛋白去乙?;傅膰?yán)格控制,并影響植物的許多發(fā)育過程[71]。HDAC抑制劑曲古抑菌素A (trichostatin A,TSA)處理小麥培養(yǎng)物可增加胚性愈傷誘導(dǎo)率和芽分化率[72]。HDAC雙突變體hda6/hda19TSA處理誘導(dǎo)胚性標(biāo)記基因LEC1、FUS3ABI3的上調(diào)表達(dá),促進(jìn)體細(xì)胞胚胎發(fā)生[73]。在擬南芥利用成熟的合子胚進(jìn)行體外培養(yǎng)時(shí),經(jīng)過TSA處理無需額外施加生長(zhǎng)素便能夠誘導(dǎo)體細(xì)胞胚形成,這可能是由于TSA處理后生長(zhǎng)素合成通路基因(YUC1、YUC10)、胚胎發(fā)育相關(guān)轉(zhuǎn)錄因子(LEC1、LEC2BBM)及脅迫響應(yīng)因子PGA37/MYB118等顯著上調(diào)表達(dá)引起的[74-75]。HDA6HDA19分別特異性結(jié)合VAL1 (VP1/ABSCISIC ACID INSENSITIVE 3-LIKE1)VAL2 (VP1/ABSCISIC ACID INSENSITIVE 3-LIKE2),并通過表觀修飾抑制胚胎發(fā)生相關(guān)靶標(biāo)基因的表達(dá)從而抑制愈傷組織形成和體細(xì)胞胚胎發(fā)生[76]。PKL (PICKLE)是擬南芥中主要的染色質(zhì)重塑因子,在抑制細(xì)胞過度增殖中起核心作用。其突變體pkl在種子萌發(fā)后形成愈傷組織,在pkl突變體中LEC1LEC2H3K27me3水平下降導(dǎo)致甲基化抑制解除;其另一突變體cytokinin hypersensitive 2易誘導(dǎo)形成愈傷組織,而該表型可被外施TSA模擬[77]

6.4  miRNA的調(diào)控作用

植物的miRNA通過轉(zhuǎn)錄和轉(zhuǎn)錄后水平的基因沉默對(duì)植物細(xì)胞發(fā)育命運(yùn)進(jìn)行調(diào)控[78]。水稻中發(fā)現(xiàn)miR156在分化的胚性愈傷比未分化的胚性愈傷中有更高的表達(dá)量;而miR397表達(dá)則相反[79]。柑橘和棉花等物種中也發(fā)現(xiàn)類似結(jié)果,同時(shí)也發(fā)現(xiàn)miR168、miR171、miR159、miR164、miR390等也在體細(xì)胞胚胎發(fā)生過程中差異表達(dá)[78]。miR156通過調(diào)控其靶基因SPLs的表達(dá)參與調(diào)控體細(xì)胞胚胎發(fā)生。在柑橘中超表達(dá)miR156a或敲除其兩個(gè)靶基因CsSPL3CsSPL14株系的體細(xì)胞胚的數(shù)量顯著高于對(duì)照[80],而且在體細(xì)胞胚胎發(fā)生的標(biāo)志基因CsFUS3的超表達(dá)材料中可以發(fā)現(xiàn)miR156a在整個(gè)體細(xì)胞胚胎過程均顯著上調(diào)[81]。miR157amiR156的另一個(gè)成員。棉花中對(duì)GhmiR157a及其靶標(biāo)SPL10研究發(fā)現(xiàn)其通過調(diào)控乙烯信號(hào)和類黃酮生物合成調(diào)控逆境下棉花體細(xì)胞胚胎發(fā)育[13]。miRNA的調(diào)控體細(xì)胞胚胎發(fā)生可能通過2個(gè)方面:(1) miRNA參與植物激素信號(hào)的轉(zhuǎn)導(dǎo)。植物體細(xì)胞胚胎發(fā)生過程中多個(gè)激素信號(hào)途徑相關(guān)基因被發(fā)現(xiàn)受miRNA調(diào)控;(2)體細(xì)胞胚胎發(fā)生過程中一些重要轉(zhuǎn)錄因子可作為miRNA的直接靶標(biāo)。miRNA可通過降解互補(bǔ)的mRNA或影響mRNA翻譯降低蛋白水平。
7  關(guān)鍵基因在體細(xì)胞胚胎發(fā)生和遺傳轉(zhuǎn)化中的應(yīng)用
隨著研究的不斷深入和新技術(shù)的涌現(xiàn),體細(xì)胞胚胎發(fā)生所涉及的激素信號(hào)轉(zhuǎn)導(dǎo),基因轉(zhuǎn)錄調(diào)控、表觀遺傳修飾與代謝物動(dòng)態(tài)變化等復(fù)雜分子生物學(xué)過程都在逐步深入的闡釋。然而,目前雖然通過對(duì)組織培養(yǎng)、農(nóng)桿菌侵染等方法的優(yōu)化,擬南芥、楊樹、水稻、玉米、棉花等植物的遺傳轉(zhuǎn)化體系得以成功建立,但植物的高效遺傳轉(zhuǎn)化仍存在許多問題,如基因型依賴性嚴(yán)重、轉(zhuǎn)化效率不高。隨著對(duì)體細(xì)胞胚胎發(fā)生機(jī)理深入研究,一些調(diào)控體細(xì)胞胚胎發(fā)生的關(guān)鍵基因被發(fā)現(xiàn)并逐漸用于改進(jìn)植物遺傳轉(zhuǎn)化體系和再生植株的效率,從而實(shí)現(xiàn)植物的高效遺傳轉(zhuǎn)化[82]。
BBMWUS2個(gè)提高植物遺傳轉(zhuǎn)化效率的關(guān)鍵基因。在玉米中用ZmBBMZmWUS2兩個(gè)基因共轉(zhuǎn)化未成熟胚時(shí),愈傷組織的比例顯著提升,且在多個(gè)不易轉(zhuǎn)化的玉米自交系中取得較高的轉(zhuǎn)化效率[83]。此外,在水稻、高粱中利用ZmBBMZmWUS2的共轉(zhuǎn)化也實(shí)現(xiàn)轉(zhuǎn)化效率提高[83]。在雙子葉植物甜椒中,Heidmann等發(fā)現(xiàn)瞬時(shí)表達(dá)BnBBM基因可高效誘導(dǎo)體細(xì)胞再生,并分化出大量可發(fā)育成完整植株的體細(xì)胞胚胎[84];將WUS2IPTWUS2STM共轉(zhuǎn)化煙草、番茄、葡萄等植物中可實(shí)現(xiàn)了芽的原位誘導(dǎo)[85]。因此解析體細(xì)胞胚胎發(fā)生過程中關(guān)鍵基因的功能及調(diào)控機(jī)制,開發(fā)更高效的遺傳轉(zhuǎn)化方法,有望實(shí)現(xiàn)更多植物的遺傳高效再生體系建立。
8  總結(jié)與展望
體細(xì)胞胚胎發(fā)生是體細(xì)胞在沒有受精的情況下再生為胚狀體的過程,是展現(xiàn)植物細(xì)胞具有全能性的一個(gè)重要途徑。自20世紀(jì)50年代以來,體細(xì)胞胚胎發(fā)生的研究經(jīng)歷了從經(jīng)驗(yàn)方法到系統(tǒng)性理論研究的發(fā)展,并在多個(gè)物種成功建立起再生體系[86]。對(duì)于體細(xì)胞胚胎發(fā)生的研究,從最開始通過對(duì)外植體的選擇、培養(yǎng)基類型、激素含量的配比及組合等進(jìn)行優(yōu)化開展研究;發(fā)展到對(duì)植物體細(xì)胞胚胎發(fā)生調(diào)控基因的研究,并利用調(diào)控體細(xì)胞胚胎發(fā)生的關(guān)鍵基因去提高植物的轉(zhuǎn)化及再生效率;再到對(duì)植物體細(xì)胞胚胎發(fā)生過程基因的調(diào)控網(wǎng)絡(luò)、代謝物質(zhì)的變化、表觀遺傳調(diào)控機(jī)制的研究,說明對(duì)體細(xì)胞胚胎發(fā)生的分子機(jī)制不斷深入及完善。
隨著分子生物學(xué)的進(jìn)步和測(cè)序技術(shù)的發(fā)展。多組學(xué)技術(shù)的應(yīng)用加速了對(duì)體培發(fā)生分子機(jī)制的解析。利用RNA-seq揭示體細(xì)胞胚胎發(fā)生過程中基因的動(dòng)態(tài)變化,借助代謝組技術(shù)解析體胚發(fā)育各個(gè)階段的內(nèi)源物質(zhì)變化,并通過對(duì)表觀組學(xué)的研究揭示體細(xì)胞胚胎發(fā)生過程中表觀遺傳機(jī)制(DNA甲基化、組蛋白甲基化、組蛋白去乙?;?/span>miRNA)對(duì)基因表達(dá)調(diào)節(jié)。并且隨著將來單細(xì)胞測(cè)序技術(shù)、空間轉(zhuǎn)錄組等新技術(shù)在植物體細(xì)胞胚胎發(fā)生上的應(yīng)用,體細(xì)胞胚胎發(fā)生起源單細(xì)胞還是多細(xì)胞的問題有可能得到解決。此外,隨著轉(zhuǎn)基因技術(shù)及基因編輯技術(shù)利用,對(duì)植物體細(xì)胞胚胎過程中基因型依賴嚴(yán)重、轉(zhuǎn)化效率低等問題也在逐步得到改善。
盡管當(dāng)前體細(xì)胞胚胎發(fā)生的研究進(jìn)展迅速,但仍然存在許多謎團(tuán)。2005年,Science創(chuàng)刊125周年之際公布了125個(gè)最具挑戰(zhàn)性的科學(xué)問題,其中植物細(xì)胞全能性被列入最重要的25個(gè)科學(xué)問題。20219月,Plant Cell公布了植物細(xì)胞生物學(xué)15個(gè)最重要的問題,其中有多個(gè)問題都涉及到植物細(xì)胞全能性和多能性的表達(dá)。理論上,只要條件合適,所有的植物細(xì)胞都可以表現(xiàn)全能性,得到再生植株。然而,植物細(xì)胞再生實(shí)際上只能在有限的植物中實(shí)現(xiàn);而且離體細(xì)胞胚胎發(fā)生和器官再生的細(xì)胞學(xué)和分子機(jī)理的研究都不夠完善。體細(xì)胞胚胎發(fā)生的起源是單細(xì)胞還是多細(xì)胞?在不同的發(fā)生體系中究竟是哪一類或哪幾類細(xì)胞如何被選中進(jìn)行重編程,從而再生?雖然已有研究表明這些再生大都來源于植物的干細(xì)胞,但植物的干細(xì)胞究竟如何界定?表觀遺傳修飾如何參與并協(xié)調(diào)再生過程?這些問題都是未來研究中的重點(diǎn)。
________________

本研究由國(guó)家重點(diǎn)研發(fā)計(jì)劃(2018YFD1000907)項(xiàng)目資助。

*通信作者: 楊細(xì)燕, E-mail: yxy@mail.hzau.edu.cn

第一作者聯(lián)系方式: E-mail: bhan_z@163.com


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