王曉娟，王文斌，楊 龍，金 樑，宋 瑜，姜少俊，秦蘭蘭

1 上?？萍拣^，上海自然博物館自然史研究中心，上海 200127 2 蘭州大學(xué)，草地農(nóng)業(yè)科技學(xué)院，蘭州 730020 3 蘭州大學(xué)，資源環(huán)境學(xué)院，蘭州 730000

重金屬鎘(Cd)在植物體內(nèi)的轉(zhuǎn)運途徑及其調(diào)控機(jī)制

王曉娟1,*，王文斌2，楊 龍2，金 樑1，宋 瑜2，姜少俊2，秦蘭蘭3

1 上?？萍拣^，上海自然博物館自然史研究中心，上海 200127 2 蘭州大學(xué)，草地農(nóng)業(yè)科技學(xué)院，蘭州 730020 3 蘭州大學(xué)，資源環(huán)境學(xué)院，蘭州 730000

重金屬鎘(Cd)的毒害效應(yīng)與其由土壤向植物地上部分運輸有關(guān)，揭示Cd2+轉(zhuǎn)運途徑及其調(diào)控機(jī)制可為提高植物抗鎘性以及鎘污染的植物修復(fù)提供依據(jù)。對Cd2+在植物體內(nèi)的轉(zhuǎn)運途徑，特別是限制Cd2+移動的細(xì)胞結(jié)構(gòu)和分子調(diào)控機(jī)制研究進(jìn)展進(jìn)行了回顧。Cd2+通過共質(zhì)體和質(zhì)外體途徑穿過根部皮層進(jìn)入木質(zhì)部的過程中，大部分在皮層細(xì)胞間沉積，少部分抵達(dá)中柱后轉(zhuǎn)移到地上部分。為了免受Cd2+的危害，植物體產(chǎn)生了多種限制Cd2+吸收和轉(zhuǎn)移的生理生化機(jī)制：1)環(huán)繞在內(nèi)皮層徑向壁和橫向壁上的凱氏帶阻止Cd2+以質(zhì)外體途徑進(jìn)入木質(zhì)部；2)螯合劑與進(jìn)入根的Cd2+螯合形成穩(wěn)定化合物并區(qū)隔在液泡中；3)通過H+/Cd2+離子通道等將Cd2+逆向轉(zhuǎn)運出根部。植物共質(zhì)體和質(zhì)外體途徑轉(zhuǎn)運重金屬鎘的能力以及兩條途徑的串?dāng)_尚待進(jìn)一步明晰和闡明。

重金屬；鎘；共質(zhì)體途徑；質(zhì)外體途徑；調(diào)控機(jī)制

鎘(Cd)是一種毒性很強(qiáng)的重金屬，對植物生長和發(fā)育而言屬于非必需元素，輕度脅迫導(dǎo)致植物葉片干枯萎黃，根莖縮短，側(cè)根數(shù)量減少，降低營養(yǎng)元素吸收，而重度脅迫則會減少葉綠素含量，擾亂水分平衡，抑制抗氧化酶活性，引起活性氧(reactive oxygen species，ROS)合成積累，降低細(xì)胞膜通透性，導(dǎo)致細(xì)胞損傷，進(jìn)而顯著抑制植物生長[1-2]。當(dāng)前，自然環(huán)境中植物地上部分的鎘含量呈現(xiàn)增加趨勢，既是環(huán)境中鎘污染程度增加所致，也是植物自身在鎘脅迫下進(jìn)化的結(jié)果[3]。隨著對鎘污染環(huán)境下土著植物的不斷篩選，一些鎘的超累積植物不斷被發(fā)現(xiàn)[4]。

重金屬鎘發(fā)揮毒性的關(guān)鍵步驟是其被吸收進(jìn)入根內(nèi)并向植物地上部分運輸，該過程受到植物外部和內(nèi)部條件的影響。其中，影響根吸收鎘的外部條件如土壤鎘濃度、有機(jī)物含量、pH值、氧化還原電位、溫度和其它元素濃度等研究進(jìn)展已有評述[5]。由于植物地上部分對鎘毒害作用更加敏感，為了減少Cd2+向地上部分的轉(zhuǎn)移，植物體內(nèi)部阻礙Cd2+進(jìn)入木質(zhì)部和韌皮部并限制其向上轉(zhuǎn)運的機(jī)制已經(jīng)引起了人們的關(guān)注。研究發(fā)現(xiàn)，細(xì)胞內(nèi)部的螯合劑可以將鎘螯合后隔離在液泡中，阻止Cd2+以共質(zhì)體途徑(symplastic pathway)橫向運輸，減少抵達(dá)木質(zhì)部和韌皮部Cd2+的數(shù)量，進(jìn)而減弱重金屬鎘向地上部分運輸。在細(xì)胞外部，鎘沿著細(xì)胞壁中的空隙從表皮、皮層到內(nèi)皮層，經(jīng)質(zhì)外體途徑(apoplastic pathway)進(jìn)入木質(zhì)部向上轉(zhuǎn)運至植物的枝葉中[6]。由于植物內(nèi)皮層細(xì)胞壁中不透水的凱氏帶會阻止Cd2+的運輸，因此Cd2+進(jìn)入內(nèi)皮層后又轉(zhuǎn)為共質(zhì)體途徑[7]。本文著重回顧了鎘轉(zhuǎn)運途徑及其調(diào)控機(jī)制研究進(jìn)展，為植物抗鎘性機(jī)制和鎘污染防治提供理論依據(jù)。

1 植物體內(nèi)Cd的吸收和轉(zhuǎn)運途徑

土壤中的鎘在轉(zhuǎn)移進(jìn)入植物體內(nèi)各組織器官后才會產(chǎn)生毒害效果，共質(zhì)體途徑是指Cd2+從植物根毛細(xì)胞膜上的通道進(jìn)入，再利用細(xì)胞與細(xì)胞間的胞間連絲，經(jīng)由皮層、內(nèi)皮層及周鞘進(jìn)入根內(nèi)導(dǎo)管細(xì)胞，而質(zhì)外體途徑則是土壤中的Cd2+經(jīng)由根吸收之后不進(jìn)入細(xì)胞內(nèi)，而是沿著細(xì)胞壁中的空隙從表皮、皮層到內(nèi)皮層，進(jìn)入木質(zhì)部和韌皮部[6]。鎘通過以上兩種運輸途徑抵達(dá)維管束并向枝葉轉(zhuǎn)運，隨著植物的生長和新陳代謝，逐漸被稀釋或排出體外，進(jìn)而減少對植物的危害。雖然植物地上部分的鎘含量會隨著土壤中鎘濃度的增加而增加，但達(dá)到一定程度后就不再升高,如龍葵(Solanumnigrum)和中國石竹(Dianthuschinensis)地上部分的鎘含量最高分別為1110mg/kg和414mg/kg[8]。由于鎘毒害嚴(yán)重威脅植物生長，導(dǎo)致其光合作用和蒸騰作用幾乎停止，進(jìn)而影響離子轉(zhuǎn)運，因此，植物產(chǎn)生了多種限制Cd2+吸收和轉(zhuǎn)移的生理生化機(jī)制以利于植物生長發(fā)育。研究發(fā)現(xiàn)，植物根系中的Cd2+含量通常高于地上部分，且地上部分對鎘更加敏感，表明植物可能通過限制鎘進(jìn)入根中具有傳導(dǎo)水分和養(yǎng)分功能的木質(zhì)部和韌皮部，以減少Cd2+向地上部分的轉(zhuǎn)運[9]。

1.1 影響鎘在植物體內(nèi)轉(zhuǎn)運的外部因素

重金屬鎘并非地殼中含量豐富的元素，通常在巖石中伴隨大量的鋅而形成，土壤中鎘含量的范圍在0.1—2 μg/g，大多低于1 μg/g[10]。Cd在土壤溶液中溶解后，以水合離子、復(fù)雜的有機(jī)或無機(jī)化合物的形態(tài)存在，鎘向植物體內(nèi)遷移受到諸多因素的影響，如土壤pH值、有機(jī)質(zhì)含量、土壤中存在的其它元素和氧化還原電位等，其中土壤pH值和有機(jī)質(zhì)含量是影響Cd2+在土壤中被植物吸收的主要因素。一方面，酸堿環(huán)境能夠決定土壤顆粒表面電荷的正負(fù)性質(zhì)和Cd2+的存在形態(tài)，對于pH值較高的堿性土壤，Cd2+的遷移能力較低[11]。另一方面，Cd2+容易和土壤有機(jī)質(zhì)的功能團(tuán)如羧基、烯醇羥基、醇羥基等形成有機(jī)鎘絡(luò)合物而降低活性[12]。研究發(fā)現(xiàn)，植物根系分泌的可溶性有機(jī)物質(zhì)導(dǎo)致根際土壤中Cd2+進(jìn)入根部表皮受阻[13]。與此同時，鐵、錳等金屬元素的氧化物對土壤中Cd2+也具有吸附作用，可使其失去遷移能力[14]。

1.2 Cd2+進(jìn)入根表皮層的途徑

植物根部吸收土壤溶液中Cd2+和被土壤顆粒吸附的Cd2+的部位主要在根尖，其中，根毛區(qū)吸收離子最為活躍，占鎘吸收的絕大部分[18]。意大利五針?biāo)?Pinuspinea)和海岸松(Pinuspinaster)根際表面會附著大量Cd2+，但其根尖上發(fā)達(dá)的根冠具有篩選離子并阻止鎘侵入的功能[19]。對玉米(Zeamays)根中鎘的沉積格局觀察發(fā)現(xiàn)，在低鎘濃度下Cd2+主要累積在根的頂端或距根尖3 cm的區(qū)域[20]。

圖1 玉米根中Cd2+的質(zhì)外體(紅色)和共質(zhì)體(綠色)轉(zhuǎn)運途徑示意圖Fig.1 Diagram of apoplastic (red) and symplastic (green) pathways to transport Cd2+ in Zea maysA 長細(xì)胞；B 短細(xì)胞；C 中柱鞘；D 木質(zhì)部薄壁細(xì)胞；E 管胞。①根具有成熟的外皮層：Cd2+以質(zhì)外體途徑進(jìn)入表皮細(xì)胞壁后，受到正常發(fā)育的外皮層凱氏帶的阻礙；②根缺少成熟的外皮層，但具有成熟的內(nèi)皮層：Cd2+以質(zhì)外體途徑進(jìn)入表皮層，之后通過中央皮層細(xì)胞壁的傳遞到達(dá)內(nèi)皮層，正常發(fā)育的內(nèi)皮層凱氏帶會阻礙Cd2+在細(xì)胞壁中進(jìn)一步的傳輸，內(nèi)皮層不具有凱氏帶時Cd2+可以順利通過內(nèi)皮層，到達(dá)維管束；③中柱中的Cd2+以質(zhì)外體途徑向中央皮層回流時會受到內(nèi)皮層凱氏帶的阻礙；④根缺少成熟的外皮層：Cd2+通過共質(zhì)體途徑能夠輕松穿過沒有凱氏帶的外皮層而進(jìn)入維管束；⑤根具有成熟的外皮層，但外皮層細(xì)胞較短?。篊d2+首先進(jìn)入外皮層和中央皮層細(xì)胞壁，進(jìn)而穿過外皮層和中央皮層細(xì)胞膜進(jìn)入表皮細(xì)胞的細(xì)胞質(zhì)，細(xì)胞間通過胞間連絲傳遞Cd2+，使Cd2+進(jìn)入維管束；⑥根具有成熟的外皮層，且外皮層細(xì)胞較長：Cd2+通過共質(zhì)體途徑無法進(jìn)入成熟的外皮層長細(xì)胞，不能到達(dá)維管束

1.3 Cd2+進(jìn)入根木質(zhì)部的途徑

本課題組對鎘脅迫條件下玉米幼苗的鎘沉積顯微觀察發(fā)現(xiàn)，Cd2+通過共質(zhì)體和質(zhì)外體兩種途徑進(jìn)入根木質(zhì)部，圖1示意了Cd2+在玉米根中的運輸途徑。Cd2+通過共質(zhì)體途徑進(jìn)入玉米的中柱，最后Cd2+又經(jīng)過質(zhì)外體擴(kuò)散到導(dǎo)管或管胞。質(zhì)外體途徑中的Cd2+在向中央皮層擴(kuò)散時，受到外皮層凱氏帶的阻礙(圖1①)。當(dāng)Cd2+到達(dá)發(fā)育正常的內(nèi)皮層時，內(nèi)皮層細(xì)胞壁中的凱氏帶一方面會阻止Cd2+的繼續(xù)擴(kuò)散(圖1②)，另一方面，對于以共質(zhì)體途徑進(jìn)入木質(zhì)部的Cd2+，凱氏帶也會阻止其通過質(zhì)外體途徑返回中央皮層(圖1③)，使得木質(zhì)部保持了較高的Cd2+濃度。共質(zhì)體途徑中的Cd2+能夠順利穿過發(fā)育不正常缺失凱氏帶的外皮層細(xì)胞(圖1④)和具有正常凱氏帶外皮層的短細(xì)胞(圖1⑤)而進(jìn)入細(xì)胞質(zhì)，但不能穿過正常外皮層的長細(xì)胞(圖1⑥)。

鎘進(jìn)入根細(xì)胞后通過共質(zhì)體途徑到達(dá)中柱鞘，該轉(zhuǎn)運方式主要通過胞間連絲完成。有關(guān)Cd2+的共質(zhì)體轉(zhuǎn)運形式尚不清楚，可能的形式有Cd2+和鎘螯合物兩種。重金屬鎘通過共質(zhì)體途徑進(jìn)入木質(zhì)部主要利用重金屬酶P1B-ATP，如AtHMA2和AtHMA4編碼轉(zhuǎn)運蛋白，也有可能和YSL蛋白質(zhì)結(jié)合進(jìn)入木質(zhì)部[16]。此外，擬南芥(Arabidopsisthaliana)AtPDR8基因編碼的三磷酸腺苷(ATP，adenosine triphosphate)轉(zhuǎn)運體已經(jīng)被證明能夠?qū)d2+從根毛和表皮細(xì)胞的細(xì)胞膜上導(dǎo)出[21]。業(yè)已證明，外皮層作為環(huán)境變化的屏障控制著水和離子的吸收[22]。大多數(shù)被子植物的外皮層與內(nèi)皮層同時發(fā)育，外皮層是阻礙Cd進(jìn)入根內(nèi)的初級屏障，外皮層的存在可有效降低Cd2+通過質(zhì)外體途徑進(jìn)入根中[23]。外部環(huán)境因素能夠改變外皮層的發(fā)育速度，研究發(fā)現(xiàn)Cd2+會刺激外皮層使其發(fā)育加快，從而減少根系對鎘的吸收量[24]。

1.4 Cd2+在植物體內(nèi)的轉(zhuǎn)運與沉積

鎘脅迫下植物的耐受機(jī)理主要包括解毒和轉(zhuǎn)運體系，其中，對鎘的解毒涉及到抗氧化防衛(wèi)機(jī)制、鎘的螯合隔離以及外排機(jī)制等[25-26]；對鎘的轉(zhuǎn)運過程包括：根際活化吸附、經(jīng)質(zhì)外體途徑和共質(zhì)體途徑的短距離運輸、經(jīng)木質(zhì)部及韌皮部裝載的長距離運輸[24]。大多數(shù)植物根中Cd2+的濃度高于莖葉，受根際Cd2+濃度的影響，根中鎘含量可能達(dá)到地上部分的10倍[27]。研究發(fā)現(xiàn)，根內(nèi)鎘含量受土壤Cd2+濃度、鎘的有效性和鎘脅迫持續(xù)時間等因素的影響[28]。根際Cd2+濃度較低時，吸收的Cd2+主要向地上部分轉(zhuǎn)運，隨著鎘濃度進(jìn)一步升高，根內(nèi)Cd2+濃度迅速增加直至與外部達(dá)到平衡[29]。

根中的鎘濃度從外皮層薄壁組織到外皮層逐漸降低，中柱鞘中Cd2+的累積量很少，推測是Cd2+向不斷生長的側(cè)根轉(zhuǎn)移所致[30-31]。在維管束中鎘主要累積在傳導(dǎo)養(yǎng)分和水分的部位及其鄰近的薄壁細(xì)胞，表明Cd2+在長途運輸中不斷沉積[32]。值得注意的是在內(nèi)皮層與木質(zhì)部之間的薄壁細(xì)胞，其鎘含量高于鄰近維管束的薄壁細(xì)胞，這種結(jié)構(gòu)可能與內(nèi)皮層的細(xì)胞通道有關(guān)，相比于內(nèi)皮層的凱氏帶，鎘更容易透過內(nèi)皮層的細(xì)胞膜[29]。研究發(fā)現(xiàn)，高濃度鎘脅迫條件下Cd2+大量集中在根的中柱鞘和維管束組織，對主根的生長和側(cè)根的分生具有明顯的抑制作用[33]。

對于大多數(shù)植物，根中質(zhì)外體通道含鎘最多，主要位于表皮、細(xì)胞壁和皮層細(xì)胞上，而細(xì)胞內(nèi)的Cd含量很少，主要集中在液泡和細(xì)胞核內(nèi)，有時也出現(xiàn)在細(xì)胞質(zhì)和葉綠體中[34-35]。鎘敏感的植物較耐鎘植物其細(xì)胞壁中的鎘含量低而液泡中鎘含量高[36]。研究發(fā)現(xiàn)，大麥(Hordeumvulgrar)根中36%的Cd2+存在于細(xì)胞壁中，51%的Cd2+存在于細(xì)胞質(zhì)中，后者34%—50%的Cd2+以螯合肽復(fù)合物的形式存在[37]，然而，在重金屬超累積植物東南景天(Sedumalfredii)和遏藍(lán)菜(Thlaspicaerulescens)中，70%—90%的Cd2+集中在細(xì)胞壁/質(zhì)外體通道而不是細(xì)胞質(zhì)/液泡中[38]。

研究發(fā)現(xiàn)，植物受鎘脅迫后細(xì)胞壁的陽離子交換能力加強(qiáng)[39-40]，透射電子顯微觀察顯示棉花(Gossypiumhirsutum)和遏藍(lán)菜中鎘顆粒出現(xiàn)在外層根組織細(xì)胞壁和細(xì)胞膜之間，維管束中少有分布[41-42]。也有研究發(fā)現(xiàn)，鎘顆粒主要出現(xiàn)在內(nèi)皮層與木質(zhì)部之間的質(zhì)外體通道[29]以及內(nèi)皮層與中柱鞘細(xì)胞的中間殼層[43]。通常，液泡中鎘顆粒聚集并形成大的沉淀物，其數(shù)量和大小隨鎘濃度的增加而增加[33]，成熟的根內(nèi)鎘沉積在分生組織或外皮層薄壁細(xì)胞液泡中[29]。內(nèi)皮層細(xì)胞中鎘則被隔離在液泡中或其大顆粒沉淀分布在靠近細(xì)胞壁的細(xì)胞質(zhì)中，而維管束的鎘沉積發(fā)生在內(nèi)皮層和木質(zhì)部之間的薄壁細(xì)胞[42]，沉積在篩管和韌皮部細(xì)胞中的鎘進(jìn)一步證明了植物對鎘向地上部分轉(zhuǎn)運的限制[29]。

2 植物抗鎘性的解剖結(jié)構(gòu)基礎(chǔ)

根部外皮層由分裂成多層細(xì)胞的表皮構(gòu)成，剩余外圍組織稱為皮質(zhì)，內(nèi)皮層將皮質(zhì)與中柱鞘隔開，中柱鞘決定性的控制著溶質(zhì)向地上部分的轉(zhuǎn)運，而形成凱氏帶的木栓質(zhì)是影響中柱鞘細(xì)胞壁滲透性的主要物質(zhì)[29, 44]。木栓質(zhì)是構(gòu)成凱氏帶的基本材料，其在成熟的內(nèi)皮層細(xì)胞壁上呈線狀橫向分布，這些物質(zhì)占滿內(nèi)皮層細(xì)胞間的空隙，在細(xì)胞壁之間緊密連接，和質(zhì)膜共同構(gòu)成了根的質(zhì)外體屏障，阻止溶質(zhì)通過質(zhì)外體途徑進(jìn)入木質(zhì)部與韌皮部[45]。

2.1 Cd2+在植物根系內(nèi)部轉(zhuǎn)運的屏障

凱氏帶的形成代表根內(nèi)皮層細(xì)胞發(fā)育的最初階段，是非常關(guān)鍵的時期[46]。凱氏帶的存在使水分和離子不能以質(zhì)外體途徑穿過內(nèi)皮層，只能通過內(nèi)皮層細(xì)胞具有選擇性的質(zhì)膜以共質(zhì)體途徑向木質(zhì)部轉(zhuǎn)運。根部內(nèi)皮層細(xì)胞壁表面片狀木栓質(zhì)的沉積過程，是內(nèi)皮層發(fā)育的第二階段[45]。該階段內(nèi)皮層表現(xiàn)出對質(zhì)外體轉(zhuǎn)運的屏障作用，根尖以后的成熟區(qū)域嚴(yán)格控制水和溶質(zhì)以質(zhì)外體通道向木質(zhì)部的流動[47]。栓質(zhì)化能夠增強(qiáng)細(xì)胞對鎘脅迫的耐性，木栓質(zhì)是栓質(zhì)化過程的產(chǎn)物，其在根細(xì)胞壁中的含量影響著水和離子的流動。研究發(fā)現(xiàn)，擬南芥突變體由于內(nèi)皮層木栓質(zhì)的數(shù)量增加，顯著降低了水分進(jìn)入木質(zhì)部的流通性和地上部分Ca2+、Mn2+、Zn2+的富集量[48]。但是，木栓質(zhì)在根中的數(shù)量差異可能不是唯一影響水分和溶質(zhì)通過質(zhì)外體途徑向木質(zhì)部轉(zhuǎn)移的因素，其化學(xué)性質(zhì)和沉積的微觀位置尚需深入研究[49]。此外，植物的周皮組織能夠抑制水、離子、氣體和病原菌的活動，對Cd2+的轉(zhuǎn)運也具有限制作用[50]。

與許多植物鹽脅迫加速根內(nèi)皮層和外皮層的發(fā)育相似[51]，在含重金屬的廢棄礦渣上培育的玉米受到脅迫誘導(dǎo)內(nèi)皮層細(xì)胞壁大范圍增厚[52]。因此，根外皮層和內(nèi)皮層扮演著屏障的角色，限制Cd2+以質(zhì)外體途徑進(jìn)入根部[6]。當(dāng)植物受到重金屬鎘脅迫時，質(zhì)外體屏障受到感應(yīng)，會在距離根尖很近的部位形成凱氏帶。Vaculík 等[53]觀察在不含鎘和含鎘(5 μmol/L)的營養(yǎng)液中培養(yǎng)10d的玉米幼苗，發(fā)現(xiàn)對照組發(fā)育形成的木栓質(zhì)遠(yuǎn)離根尖，占總根長的10%，而鎘處理組木栓質(zhì)靠近根尖，占總根長的45%，表明鎘的脅迫刺激增大了內(nèi)皮層木栓質(zhì)區(qū)域，促使木栓質(zhì)化提前和內(nèi)皮層成熟加快。進(jìn)一步研究發(fā)現(xiàn)，鎘脅迫不僅會導(dǎo)致玉米根內(nèi)皮層木栓質(zhì)和木質(zhì)素含量增加，還會改變其化學(xué)構(gòu)成[54]。以上改變可能是植物為減少鎘通過質(zhì)外體途徑進(jìn)入木質(zhì)部而做出的適應(yīng)性反應(yīng)。

2.2 Cd2+脅迫下植物根形態(tài)結(jié)構(gòu)的抗逆變化

3 Cd2+在植物體內(nèi)的轉(zhuǎn)運調(diào)控

研究發(fā)現(xiàn)，許多植物的耐受性與液泡富集鎘的能力相關(guān)，液泡能夠大量富集鎘，則植物對鎘的耐受性強(qiáng)，反之則弱[6]。目前，已鑒定克隆了H+/ Cd2+逆向轉(zhuǎn)運通道同源基因AtCAX2和AtCAX4[65]、重金屬轉(zhuǎn)運酶P1B同源基因AtHMA3和ABC轉(zhuǎn)運同源基因AtMRP3[18]。與此同時，研究還發(fā)現(xiàn)巨噬細(xì)胞蛋白(NRAMP，natural resistance-associated macrophage protein)可以從液泡向細(xì)胞質(zhì)中轉(zhuǎn)移鎘，相同功能的轉(zhuǎn)運蛋白還包括同源基因AtNRAMP3和AtNRAMP4的蛋白質(zhì)[66]。

許多植物感應(yīng)到鎘的侵入后，會產(chǎn)生螯合肽合成酶以促進(jìn)螯合肽的形成，這些螯合肽能夠捕獲Cd2+形成無毒螯合物，并將其隔離在液泡中[5]。目前，植物螯合肽對鎘的解毒作用已經(jīng)得到證實，大多突變會增強(qiáng)植物螯合肽合成酶基因的表達(dá)，對鎘的耐受性強(qiáng)于野生型，但也有突變會導(dǎo)致植物螯合肽合成酶產(chǎn)生缺陷，解毒性能降低，弱于野生型[67]。然而，自然生態(tài)系統(tǒng)中許多植物對鎘的耐受性和植物螯合肽合成酶的含量并不成正比，表明植物的抗鎘性還存在其他機(jī)理[68]。研究發(fā)現(xiàn)，鎘脅迫誘導(dǎo)了小麥和水稻金屬硫蛋白基因的表達(dá)，增加了植物中金屬硫蛋白的含量，說明其對提高植物抗鎘性和緩解鎘毒害具有積極作用[65,69]。

發(fā)生在根部外皮層的一些突變也與鎘的轉(zhuǎn)運調(diào)控有關(guān)，如圖1中④、⑤所示，正常的外皮層細(xì)胞應(yīng)由成熟的長細(xì)胞(圖1中途徑⑥中所示)構(gòu)成，但發(fā)生突變后長細(xì)胞缺失凱氏帶或變短小均會喪失對Cd2+的抵抗能力[59]。在根部缺少凱氏帶的區(qū)域，Cd2+和鎘的螯合物可能會唯一取道細(xì)胞外基質(zhì)，以質(zhì)外體途徑進(jìn)入木質(zhì)部(如圖1中途徑②所示)[70]。研究發(fā)現(xiàn)，陽離子通過質(zhì)外體途徑進(jìn)入木質(zhì)部的過程一般會受到根尖末端橫向傳輸?shù)南拗芠71]。雖然共質(zhì)體和質(zhì)外體途徑對傳遞Cd進(jìn)入木質(zhì)部的相對能力大小尚不清楚，但質(zhì)外體途徑隨著根際溶液中Cd2+濃度的升高而增加Cd的吸收量，與Zn和Na的吸收機(jī)制一致[72]。支持質(zhì)外體途徑參與鎘轉(zhuǎn)運的觀點認(rèn)為根尖是根系中鎘流通最活躍的區(qū)域，且鎘的累積量與其根尖數(shù)成正比[73]。相反的觀點認(rèn)為質(zhì)外體途徑對鎘的運輸影響不大，以遏藍(lán)菜為材料的研究發(fā)現(xiàn)，24 h內(nèi)從根部吸收轉(zhuǎn)運至地上部分的Cd2+與質(zhì)外體途徑中的水流量呈負(fù)相關(guān)關(guān)系[74]，這與其他植物中蒸騰作用與鎘吸收富集呈正相關(guān)的研究結(jié)論不一致[75]。第三種觀點認(rèn)為以共質(zhì)體途徑傳遞鎘進(jìn)入木質(zhì)部之前，Cd2+和其它陽離子的吸收存在競爭作用，遏藍(lán)菜對Cd和Zn在地上部分的富積能力相差很大，表明植物對金屬離子的轉(zhuǎn)運具有選擇性和專一性，表明共質(zhì)體途徑轉(zhuǎn)運Cd2+與蛋白質(zhì)誘導(dǎo)有關(guān)[74,76]。

4 展望

由于鎘的共質(zhì)體與質(zhì)外體途徑之間呈現(xiàn)緊密的聯(lián)系和錯綜復(fù)雜的關(guān)系，迄今為止，關(guān)于鎘誘導(dǎo)根中央部位組織和細(xì)胞發(fā)生改變的研究較少，今后應(yīng)進(jìn)一步關(guān)注Cd2+以共質(zhì)體和質(zhì)外體途徑進(jìn)入木質(zhì)部的轉(zhuǎn)運過程，探析細(xì)胞間隙在鎘轉(zhuǎn)運過程中的作用。與此同時，木質(zhì)部擔(dān)負(fù)著調(diào)節(jié)Cd2+向莖葉轉(zhuǎn)運的重要作用，木質(zhì)部在受到鎘脅迫時會產(chǎn)生一定的應(yīng)激反應(yīng)，目前有關(guān)鎘離子對木質(zhì)部功能與形態(tài)的影響尚不清楚，通過研究這種應(yīng)激反應(yīng)的機(jī)制并促進(jìn)反應(yīng)發(fā)生，有可能為減少根中Cd2+向地上部分轉(zhuǎn)移提供依據(jù)[77-78]。

進(jìn)一步豐富植物耐鎘突變體庫，嘗試定向誘導(dǎo)植物產(chǎn)生較強(qiáng)的外皮層和缺失功能的內(nèi)皮層，并開展相應(yīng)的細(xì)胞學(xué)、生理生化和分子調(diào)控機(jī)制研究，在抵御外部重金屬鎘侵襲的同時使根中Cd2+容易進(jìn)入木質(zhì)部而向地上部分轉(zhuǎn)運并收獲移除，將為改造植物重金屬超累積性能和實施重金屬植物修復(fù)提供新思路。

綜上所述，今后應(yīng)進(jìn)一步明晰植物共質(zhì)體和質(zhì)外體途徑轉(zhuǎn)運重金屬鎘的能力，深入探究鎘脅迫下植物形態(tài)結(jié)構(gòu)響應(yīng)及其與抗鎘性的關(guān)系，闡明共質(zhì)體和質(zhì)外體途徑的分子調(diào)控機(jī)制，以增強(qiáng)根部耐受重金屬脅迫并促進(jìn)Cd2+向植物地上部分轉(zhuǎn)移為目的，培育適合我國生態(tài)環(huán)境治理的超富集植物，最終實現(xiàn)植物修復(fù)技術(shù)在鎘污染土壤上的應(yīng)用。

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Transport pathways of cadmium (Cd) and its regulatory mechanisms in plant

WANG Xiaojuan1,*, WANG Wenbin2, YANG Long2, JIN Liang1, SONG Yu2, JIANG Shaojun2, QIN Lanlan3

1NaturalHistoryResearchCenter,ShanghaiNaturalHistoryMuseum,ShanghaiScience&TechnologyMuseum,Shanghai200127,China2SchoolofPastoralAgricultureScienceandTechnology,LanzhouUniversity,Lanzhou730020,China3SchoolofEarthandEnvironmentalScience,LanzhouUniversity,Lanzhou730000,China

Heavy metal (HM) toxicity is a worldwide concern because it damages plants by altering their major physiological and metabolic processes. The heavy metal cadmium (Cd) is a nonessential element, and is a valid inhibitor of plant growth. The toxic effect of cadmium is closely related to its transfer from the soil to the plant above ground parts. Understanding the transport pathway and regulatory mechanism of cadmium in plants may improve plant resistance to this heavy metal, in addition to providing a theoretical basis for the phytoremediation soils contaminated by cadmium. In this paper, we reviewed the transport pathways of Cd2+in plants and what limits its mobility based on the cytological structural and molecular regulation mechanism of plants. As the main organ for transporting water and nutrients to the plant body, the plant root is also the main organ that absorbs toxic metals, such as cadmium. During the process of Cd2+transfer from the root cortex to the xylem, most Cd2+is deposited between the cells of the root cortex, with some reaching stele, before being transferred to the plant organs, such as the leaves in the above ground part of the plant. The transport pathway of Cd2+through the root cortex is mainly apoplastic, with the cytoplasmic accumulation of Cd2+possibly causing apoplastic transport towards the vascular cylinder to decline. The transport pathway of Cd2+in the vascular cylinder is also mostly apoplastic, with cytoplasmic accumulation reducing Cd2+transfer to the xylem. Since the aboveground parts of plants are more susceptible to Cd2+poisoning, two cellular strategies to restrict the absorption and transfer of cadmium have evolved. First, the Casparian strip surrounding radial wall and the endodermis wall prevents Cd2+from entering the root xylem via the apoplastic pathway. In addition, the Casparian strip promotes Cd2+transport via the endodermis, leading to vacuolar isolation and cytoplasmic precipitation. Second, heavy metal detoxification occurs by chelating Cd2+to form stable compounds, which are then deposited inside the vacuole. Third, excess cadmium also activates oxidative stress defense mechanisms and the synthesis of heavy metal stress related proteins to minimize metal toxicity, which includes the use of metallothiones and ion channels, such as H+/Cd2+binding or sequestrating Cd2+into vacuoles. For systematic improvements in the phytoremediation of heavy metal pollution, a more comprehensive understanding of cellular mechanisms involved in Cd avoidance, uptake, transport, and accumulation is required. Furthermore, the excluder strategy by extensive sequestration and retranslocation of cadmium through symplastic and apoplastic pathways should be confirmed and explored in future studies.

heavy metal; cadmium; symplastic pathway; apoplastic pathway; regulatory mechanism

國家自然科學(xué)基金(31270558)

2014- 04- 17; < class="emphasis_bold">網(wǎng)絡(luò)出版日期:

日期:2015- 05- 19

10.5846/stxb201404170754

*通訊作者Corresponding author.E-mail:wangxj@sstm.org.cn

王曉娟，王文斌，楊龍，金樑，宋瑜，姜少俊，秦蘭蘭.重金屬鎘(Cd)在植物體內(nèi)的轉(zhuǎn)運途徑及其調(diào)控機(jī)制.生態(tài)學(xué)報,2015,35(23):7921- 7929.

Wang X J, Wang W B, Yang L, Jin L, Song Y, Jiang S J, Qin L L.Transport pathways of cadmium (Cd) and its regulatory mechanisms in plant.Acta Ecologica Sinica,2015,35(23):7921- 7929.