丁澤琴王志敏牛 義湯青林田時(shí)炳王永清楊 洋宋 明
(1西南大學(xué)園藝園林學(xué)院,南方山地園藝學(xué)教育部重點(diǎn)實(shí)驗(yàn)室,重慶市蔬菜學(xué)重點(diǎn)實(shí)驗(yàn)室,重慶 400715;2重慶市農(nóng)業(yè)科學(xué)院蔬菜花卉研究所,重慶 400055)
花是植物的主要生殖器官。在植物生殖發(fā)育階段,雄性生殖發(fā)育由于涉及復(fù)雜且精細(xì)的細(xì)胞發(fā)育分子機(jī)制而成為植物發(fā)育生物學(xué)重點(diǎn)研究的對(duì)象?;ㄋ幾鳛樾廴镒钪匾慕M成部分,含有與花粉粒形成及釋放有關(guān)的生殖和營(yíng)養(yǎng)組織?;ㄋ庍m時(shí)開(kāi)裂,花粉才能在成熟后適時(shí)釋放進(jìn)行授粉,從而保證傳粉與受精過(guò)程的順利進(jìn)行,所以花藥開(kāi)裂是花藥發(fā)育后期的一個(gè)重要特征。當(dāng)作物花藥開(kāi)裂不完善或完全不開(kāi)裂時(shí),就會(huì)影響傳粉作用的完成而導(dǎo)致作物減產(chǎn)。因此對(duì)花藥開(kāi)裂機(jī)理的研究不僅有利于揭示植物的雄性生殖發(fā)育過(guò)程,而且有利于通過(guò)人工控制花藥開(kāi)裂,從而在生產(chǎn)上節(jié)約育種成本,提高育種質(zhì)量?;ㄋ庨_(kāi)裂涉及花藥壁各層細(xì)胞的變化以及細(xì)胞內(nèi)的許多生理生化反應(yīng),研究者已在擬南芥(Sanders et al.,1999)、百合(Varnier et al.,2005)、水稻(Zhu et al.,2004)、玉米(Vernoud et al.,2009)和茄科(Bonner & Dickinson,1989;Beals & Goldberg,1997)的一些植物上開(kāi)展了花藥開(kāi)裂的相關(guān)研究。除了解剖學(xué)觀察外,近年來(lái)在分子生物學(xué)研究中又發(fā)現(xiàn)了受體蛋白激酶(Mizuno et al.,2007)和多聚半乳糖醛酸酶(Gorguet et al.,2009)等幾個(gè)與調(diào)節(jié)花藥開(kāi)裂相關(guān)的功能基因。本文對(duì)近年來(lái)在植物花藥發(fā)育過(guò)程及花藥開(kāi)裂機(jī)制上的相關(guān)研究進(jìn)行總結(jié),從而為植物生殖發(fā)育的系統(tǒng)研究提供參考。
花藥由最初的雄蕊原基形成。雄蕊原基角隅處的孢原細(xì)胞經(jīng)過(guò)平周分裂形成初生壁細(xì)胞和造孢細(xì)胞,造孢細(xì)胞經(jīng)過(guò)一系列的有絲分裂和減數(shù)分裂形成小孢子,即后來(lái)的花粉。而初生壁細(xì)胞則經(jīng)過(guò)平周分裂形成了藥室內(nèi)壁(纖維層)、中間層和絨氈層。當(dāng)花藥成熟時(shí),藥室內(nèi)壁細(xì)胞壁的內(nèi)切向壁和橫向壁發(fā)生帶狀加厚,從而有助于花藥的開(kāi)裂和花粉的散放。Sanders等(1999)通過(guò)對(duì)擬南芥花藥的細(xì)胞切片觀察,將從雄蕊原基發(fā)生到花藥開(kāi)裂脫落的整個(gè)過(guò)程分為15個(gè)時(shí)期,并做了詳細(xì)的描述。在1~4期中,從花藥原基開(kāi)始細(xì)胞分裂形成各層組織,逐漸形成成熟花藥的腔、壁、連接細(xì)胞和導(dǎo)管區(qū)域。第5期,具有四角結(jié)構(gòu)的花藥原基孢子體細(xì)胞逐漸分化成了內(nèi)壁層、中間層、絨氈層和小孢子母細(xì)胞。小孢子母細(xì)胞經(jīng)5~7期的減數(shù)分裂形成含有單倍體小孢子的四分體,并包裹有一層較厚的胼胝質(zhì)層。在第8期,胼胝質(zhì)酶復(fù)合體降解胼胝質(zhì)層,使得小孢子從四分體釋放出來(lái)。在9~12期,小孢子經(jīng)過(guò)2次有絲分裂形成具有三核的花粉粒。隨著花粉發(fā)育,花藥不斷增大,部分花藥特異組織降解,在12~13期裂口細(xì)胞破裂后形成兩室花藥。13期后花粉釋放,花藥逐漸衰老并脫落。
花藥開(kāi)裂發(fā)生在四分體之后,主要涉及3個(gè)特殊的組織:藥室內(nèi)壁、藥隔和裂口。
Scott等(2004)發(fā)現(xiàn)藥室內(nèi)壁在花藥發(fā)育的第5階段開(kāi)始形成,第6階段開(kāi)始擴(kuò)張,第11階段由于木質(zhì)素和纖維素的沉積而加厚。在帶有肉桂酰輔酶A還原酶1(CCR1)、肉桂酰脫氫酶a(CAD)和d突變的擬南芥三突變體中,不能合成木質(zhì)素單體,其藥室內(nèi)壁未能加厚從而導(dǎo)致花藥不開(kāi)裂(Thevenin et al.,2011)。Mizuno等(2007)發(fā)現(xiàn)擬南芥受體蛋白激酶rpk2突變體表現(xiàn)為雄性不育,是由于藥室內(nèi)壁未加厚而導(dǎo)致花藥未開(kāi)裂。向珣等(2007)對(duì)經(jīng)普通白菜品種矮腳黃(父本)與甘藍(lán)型油菜OguCMS(母本)雜交后,并與父本連續(xù)回交6代獲得的普通白菜矮腳黃OguCMS的花藥進(jìn)行切片觀察,發(fā)現(xiàn)其藥室內(nèi)壁細(xì)胞完整,卻沒(méi)有次生加厚,因而花藥未開(kāi)裂。
裂口組織是一層特殊細(xì)胞構(gòu)成的單細(xì)胞層,是花藥最終開(kāi)裂的位置(Keijzer,1987)。裂口由藥室內(nèi)壁的表皮細(xì)胞分化而成,并形成了單個(gè)的細(xì)胞區(qū)域,裂口位置決定了花藥開(kāi)裂的位置。藥室內(nèi)壁次生加厚,而花粉囊之間的裂口和藥隔不加厚的方式對(duì)花藥的開(kāi)裂至關(guān)重要。Beals和Goldberg(1997)在番茄細(xì)胞消融的研究中分離了由TA35啟動(dòng)子控制的細(xì)胞毒素barnase基因,barnase基因在環(huán)形細(xì)胞簇、裂口組織和連接組織中表達(dá),使得這些組織加速消融,從而引起花藥不開(kāi)裂。而抗細(xì)胞毒素barstar基因表達(dá)時(shí)則會(huì)保護(hù)藥室周圍的組織,僅裂口消融,花藥表現(xiàn)不開(kāi)裂,從而揭示了裂口在花藥開(kāi)裂中的重要作用。
關(guān)于花藥開(kāi)裂是由細(xì)胞程序性死亡引起的已有報(bào)道。擬南芥突變體non-dehiscence1的花藥經(jīng)歷了不正常的細(xì)胞程序性死亡,導(dǎo)致藥室內(nèi)壁脫水并間接引起裂口細(xì)胞不降解,因而花藥不開(kāi)裂,但花粉有活力(Sanders et al.,1999)。一些植物雄性不育突變體的絨氈層無(wú)法正常降解,表明絨氈層細(xì)胞的程序性死亡對(duì)花粉形成至關(guān)重要(Parish & Li,2010)。Varnier等(2005)發(fā)現(xiàn)百合花的細(xì)胞程序性死亡開(kāi)始發(fā)生在絨氈層,并向外延伸到中間細(xì)胞層、裂口組織等花藥的各個(gè)組織,最后是藥室內(nèi)壁和藥隔的降解。Senatore等(2009)在細(xì)胞程序性死亡導(dǎo)致番茄花藥開(kāi)裂的研究中,通過(guò)觀察番茄的藥隔、中間層及裂口周圍表皮組織的超微結(jié)構(gòu),發(fā)現(xiàn)這些組織都發(fā)生了正常的細(xì)胞程序性死亡,并在藥隔和裂口的表皮中發(fā)現(xiàn)了半胱氨酸蛋白酶(SlCysEP),花藥裂開(kāi)時(shí)藥室周圍的小孢子體中也有SlcysEP的積累,認(rèn)為SlcysEP與花藥開(kāi)裂相關(guān)。
開(kāi)花期的最后一個(gè)階段涉及藥室內(nèi)壁和表皮細(xì)胞的脫水,引起藥室向外彎曲,繼而花藥開(kāi)裂。在花藥壁脫水期間,Bonner和Dickinson(1989)發(fā)現(xiàn)了連接組織中淀粉的流失,并且提出淀粉轉(zhuǎn)化為糖能增加花藥組織的滲透勢(shì)。Stadler等(1999)在擬南芥花藥藥隔周圍觀察到了H+-蔗糖轉(zhuǎn)運(yùn)體AtSUC1,由于AtSUC1增加了藥隔的滲透勢(shì)而導(dǎo)致花藥周圍組織脫水。對(duì)矮牽牛花藥開(kāi)裂過(guò)程的研究表明,花藥的脫水在一定程度上是水從花藥向蜜腺中轉(zhuǎn)移而引起的。Ge等(2000)研究證實(shí),矮牽牛NECTARY(NEC1)和NEC2基因作用于花絲和裂口細(xì)胞的上部分,因此打破了淀粉轉(zhuǎn)化為糖的平衡并調(diào)節(jié)裂口和蜜腺的水勢(shì),最終導(dǎo)致花藥脫水。Thompson等(2010)發(fā)現(xiàn)擬南芥中黃酮轉(zhuǎn)運(yùn)因子FFT的缺乏會(huì)影響黃酮類化合物的水平,并且FFT可通過(guò)調(diào)節(jié)花藥脫水來(lái)控制花藥開(kāi)裂,在fft-1突變體中花藥不開(kāi)裂。
煙草中2種水通道蛋白PIP1和PIP2在花藥和柱頭中特異表達(dá)(Bots et al.,2005a);花藥開(kāi)裂期間,PIP2蛋白不表達(dá);利用RNA干擾技術(shù)使PIP2表達(dá),發(fā)現(xiàn)花藥延遲開(kāi)裂,從而發(fā)現(xiàn)水通道蛋白參與花藥脫水的過(guò)程(Bots et al.,2005b)。
水解細(xì)胞壁的幾種酶及蛋白包括聚半乳糖醛酸酶(PGs)、β-1,4-糖苷酶和擴(kuò)展蛋白(Cho& Cosgrove,2000)等,這些酶在大量的特定細(xì)胞中表達(dá)。Torki等(2000)發(fā)現(xiàn)一組相關(guān)的PGs在花和花芽中表達(dá),其他的PGs在營(yíng)養(yǎng)組織中表達(dá)。Roberts等(2002)認(rèn)為花藥開(kāi)裂的過(guò)程與水解細(xì)胞間膠質(zhì)的細(xì)胞壁降解酶有關(guān)。Rhee等(2003)發(fā)現(xiàn)擬南芥中許多PGs蛋白與花粉壁的發(fā)育有關(guān),如QRT1、QRT2和QRT3是花粉母細(xì)胞壁退化所必需的。另外,擬南芥開(kāi)裂區(qū)域多聚半乳糖醛酸酶ADPG1、ADPG2和QRT2均與花藥的開(kāi)裂有關(guān)(Ogawa et al.,2009)。有研究表明這3種PGs均受到茉莉酸的調(diào)節(jié),其中ADPG2也受到乙烯的調(diào)節(jié)(Gonzalez-Carranza et al.,2007),QRT2則受到乙烯和脫落酸的共同調(diào)節(jié)(Ogawa et al.,2009)。Gorguet等(2009)在研究番茄花藥開(kāi)裂突變中發(fā)現(xiàn)了新的PG基因ps-2,其表達(dá)和花藥的開(kāi)裂同時(shí)發(fā)生。
對(duì)擬南芥花藥開(kāi)裂突變體的研究表明,茉莉酸有助于調(diào)控花藥的開(kāi)裂、花絲的伸長(zhǎng)和花粉活力(Scott et al.,2004),與茉莉酸代謝途徑相關(guān)的基因有DAD1、FAD、LOX、AOS、AOC、OPR。DAD1編碼參與茉莉酸合成途徑中的磷脂酶A1,dad1突變體的花藥不開(kāi)裂。AOS基因編碼一種叫丙二烯氧化合酶的羥脂通道酶,突變體aos的花粉發(fā)育正常,但是花藥不開(kāi)裂,從而導(dǎo)致雄性不育。OPR編碼12-氧植二烯還原酶,該酶催化茉莉酸反應(yīng)途徑中最關(guān)鍵的一步。
擬南芥突變體dde1(延遲開(kāi)裂)和opr3,其茉莉酸生物合成的途徑中缺乏12-氧植二烯還原酶,導(dǎo)致裂口不正常脫水和花藥延遲開(kāi)裂(Sanders et al.,2000)。對(duì)擬南芥突變體coil、opr3和dad1的花藥形態(tài)學(xué)分析發(fā)現(xiàn),在茉莉酸調(diào)控途徑各個(gè)階段中的缺陷都會(huì)導(dǎo)致花絲變短、花藥不開(kāi)裂(Ishiguro et al.,2001)。
研究表明其他植物激素也參與了花藥開(kāi)裂。生長(zhǎng)素在花的發(fā)育、器官形成和花粉發(fā)育中起著至關(guān)重要的作用,并且在協(xié)調(diào)花粉成熟和花藥開(kāi)裂之間也起主要作用(Cheng et al.,2006)。根癌農(nóng)桿菌基因rolB的定向表達(dá)增加了生長(zhǎng)素的敏感性,轉(zhuǎn)基因煙草中生長(zhǎng)素水平的增加導(dǎo)致花藥延遲開(kāi)裂(Cecchetti et al.,2007)。通過(guò)對(duì)擬南芥生長(zhǎng)素受體突變體的分析,證實(shí)了生長(zhǎng)素對(duì)花藥發(fā)育的調(diào)節(jié)作用;在tir1、afb1、afb2、afb3生長(zhǎng)素受體多突變中,存在花藥提前開(kāi)裂和花粉提前成熟的現(xiàn)象,在絨氈層退化之前發(fā)生藥室內(nèi)壁木質(zhì)化,藥隔和裂口同時(shí)且不連續(xù)降解;生長(zhǎng)素誘導(dǎo)細(xì)胞分裂,導(dǎo)致花粉提前進(jìn)行有絲分裂,并且花絲延長(zhǎng)率降低(Cecchetti et al.,2008)。生長(zhǎng)素的累積導(dǎo)致了棉花花藥不開(kāi)裂,主要是由于細(xì)胞骨架的改變以及藥室內(nèi)壁增厚方向從縱向向橫向的轉(zhuǎn)變(Yasuor et al.,2006)。Nagpal等(2005)從T-DNA插入突變的擬南芥群體中,篩選到與花藥開(kāi)裂后期、花絲和花瓣均較野生型短的突變體,從突變體中分離到了一類轉(zhuǎn)錄因子,生長(zhǎng)素影響因子ARF,通過(guò)研究arf6-2和arf8-3單突變體及二者的雙突變體,發(fā)現(xiàn)其花藥延遲開(kāi)裂。
近期的研究表明,生長(zhǎng)素并不是獨(dú)立調(diào)節(jié)花藥開(kāi)裂的,而是通過(guò)影響其他植物激素如茉莉酸來(lái)調(diào)控花藥開(kāi)裂。擬南芥dad1突變體中,生長(zhǎng)素受體因子ARF6和ARF8的缺失影響了茉莉酸的形成,因此導(dǎo)致了花藥延遲開(kāi)裂、花絲變短和花瓣伸長(zhǎng)(Tabata et al.,2010),通過(guò)噴灑外源茉莉酸,這些特征得到了明顯的改善(Cecchetti et al.,2007)。Tabata等(2010)研究發(fā)現(xiàn),生長(zhǎng)素受體因子ARF6和ARF8有激活DAD1表達(dá)和調(diào)節(jié)茉莉酸合成的作用。
通過(guò)對(duì)5個(gè)擬南芥C19-GA2-氧合酶突變體的研究發(fā)現(xiàn),赤霉素對(duì)花器官發(fā)育的協(xié)調(diào)性和同步性是必需的(Rieu et al.,2008)。在絨氈層降解前和花藥開(kāi)裂初期檢測(cè)到GA3ox3和GA3ox4基因的最大表達(dá)量,表明絨氈層是產(chǎn)生赤霉素的主要位置(Hu et al.,2008)。HvGAMYB是一類轉(zhuǎn)錄因子,受到GA的上游調(diào)節(jié),Murray等(2003)研究發(fā)現(xiàn)轉(zhuǎn)基因擬南芥中HvGAMYB的過(guò)量表達(dá)可導(dǎo)致花藥不開(kāi)裂。
Rieu等(2003)在2個(gè)煙草乙烯不敏感突變體etr1轉(zhuǎn)基因煙草植株和使用乙烯受體抑制劑1-甲基-環(huán)丙烯(MCP)處理的野生型煙草的研究中,發(fā)現(xiàn)其花藥均延遲開(kāi)裂;用乙烯處理后花藥提前開(kāi)裂。矮牽?;ǖ囊蚁┦荏wPhETR2的反義抑制作用使得裂口在減數(shù)分裂前脫水,且花藥提前開(kāi)裂(Wang & Kumar,2007)。這些研究表明乙烯可能是作為一種信號(hào)分子調(diào)節(jié)煙草和矮牽牛的花藥開(kāi)裂。
各種轉(zhuǎn)錄因子通過(guò)調(diào)控藥室內(nèi)壁次生加厚來(lái)控制花藥開(kāi)裂。在擬南芥MYB26突變體中,花藥的發(fā)育最初是正常的,減數(shù)分裂之后絨氈層和中間層細(xì)胞開(kāi)始降解;然而在野生型花藥中存在藥室內(nèi)壁的擴(kuò)張和次生加厚,突變體中卻未見(jiàn)藥室內(nèi)壁加厚(Steiner-Lange et al.,2003),從而導(dǎo)致花藥不能正常開(kāi)裂引起雄性不育。Yang等(2007)研究發(fā)現(xiàn)轉(zhuǎn)基因擬南芥中因AtMYB24過(guò)量表達(dá)導(dǎo)致藥室內(nèi)壁無(wú)纖維狀加厚,因而花藥未開(kāi)裂。水稻中AID1(花藥開(kāi)裂)基因編碼1個(gè)與花藥發(fā)育相關(guān)的MYB轉(zhuǎn)錄因子,主要調(diào)節(jié)花粉成熟階段的細(xì)胞程序性死亡和次生代謝物的沉積,從而影響花粉成熟和花藥開(kāi)裂,該基因的突變體由于花藥開(kāi)裂推遲而導(dǎo)致部分不育(Zhu et al.,2004)。
Mitsuda等(2005)研究發(fā)現(xiàn)NAC轉(zhuǎn)錄因子(次生壁加厚促進(jìn)因子)中的NST1和NST2的抑制表達(dá)可引起藥室內(nèi)壁不加厚,進(jìn)而導(dǎo)致花藥不開(kāi)裂。在擬南芥ucl1突變體中,MYB26、NST1和NST2通過(guò)下游調(diào)控UCL1(HD-ZIP亮氨酸拉鏈蛋白的同源結(jié)構(gòu)域轉(zhuǎn)錄因子)的表達(dá)可導(dǎo)致花藥不開(kāi)裂(Li et al.,2007)。玉米的插入型突變體中HD-ZIP IV轉(zhuǎn)錄因子OCL4基因的表達(dá)抑制藥室內(nèi)壁增厚,進(jìn)而導(dǎo)致花藥不開(kāi)裂 (Vernoud et al.,2009)。對(duì)擬南芥轉(zhuǎn)基因植物SAF1-OX的研究表明,在營(yíng)養(yǎng)生長(zhǎng)階段,SAF1-OX與野生型相比,其形態(tài)學(xué)特征表現(xiàn)正常,在生殖生長(zhǎng)階段,80%的植株表現(xiàn)為部分不育或者完全不育,其不育程度與SAF1基因的表達(dá)量有關(guān);在光學(xué)顯微鏡和電子顯微鏡下觀察,SAF1-OX的花藥和野生型花藥的形態(tài)結(jié)構(gòu),發(fā)現(xiàn)SAF1-OX的花藥不開(kāi)裂,且藥室內(nèi)壁未加厚,這也證實(shí)了F-box蛋白有阻礙藥室內(nèi)壁加厚,從而導(dǎo)致花藥不開(kāi)裂的作用(Yun et al.,2012)。
隨著對(duì)顯花植物花藥開(kāi)裂機(jī)理研究的深入,從細(xì)胞分化、細(xì)胞分裂、降解和程序性死亡等組織細(xì)胞的觀察,到生理生化變化以及分子水平的基因調(diào)控研究,都為不斷揭示植物生殖發(fā)育中前期花的生長(zhǎng)發(fā)育奠定了一定的基礎(chǔ)。然而,關(guān)于花藥開(kāi)裂過(guò)程的研究還有很多值得探索的內(nèi)容,比如關(guān)于花藥的藥室內(nèi)壁增厚的精確定位、裂口的調(diào)節(jié)、隔膜的形成以及花藥中水運(yùn)動(dòng)的調(diào)節(jié)過(guò)程等。相信通過(guò)對(duì)花藥開(kāi)裂過(guò)程調(diào)控網(wǎng)絡(luò)的深入研究,在生產(chǎn)上將有助于加快雄性不育材料的創(chuàng)造及雜種優(yōu)勢(shì)育種的進(jìn)程。
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