礦質(zhì)養(yǎng)分和激素對根毛生長發(fā)育的影響及作用機制

張德健, 夏仁學(xué), 曹 秀

(華中農(nóng)業(yè)大學(xué)教育部園藝植物生物學(xué)重點實驗室, 湖北武漢 430070)

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礦質(zhì)養(yǎng)分和激素對根毛生長發(fā)育的影響及作用機制

張德健, 夏仁學(xué)*, 曹 秀

(華中農(nóng)業(yè)大學(xué)教育部園藝植物生物學(xué)重點實驗室, 湖北武漢 430070)

【目的】植物礦質(zhì)養(yǎng)分和水分的吸收利用賴于根系，根系中根毛的生長發(fā)育不僅擴大了根系吸收表面積，促進了礦質(zhì)養(yǎng)分和水分的吸收還有助于植物根的固定以及與土壤微生物的互作。本文從礦質(zhì)養(yǎng)分角度(氮、 磷、 鉀、 鈣、 鐵)和激素角度(生長素、 乙烯、 茉莉酸、 獨腳金內(nèi)酯、 油菜素內(nèi)酯)探討影響根毛生長發(fā)育的因子及作用機理?！局饕M展】氮對根毛生長發(fā)育的影響與茉莉酸和乙烯有關(guān)， 磷與生長素、 乙烯、 獨腳金內(nèi)酯互作調(diào)控根毛生長發(fā)育；生長素和乙烯以交互作用調(diào)控根毛生長發(fā)育，茉莉酸、 獨角金內(nèi)酯和油菜素甾醇對根毛生長發(fā)育的作用是部分依賴生長素或乙烯途徑；植物體內(nèi)生長素和乙烯等激素的平衡對根毛的生長發(fā)育起著重要作用?！窘ㄗh和展望】基于以上分析，從蛋白激酶及其相關(guān)調(diào)控基因及轉(zhuǎn)錄因子等方面可深入探析礦質(zhì)養(yǎng)分、 植物激素等對根毛和叢枝菌根生長發(fā)育的影響。

根毛； 發(fā)育； 氮； 磷； 生長素； 乙烯； 叢枝菌根

根毛是植物根表皮細胞特化而形成向外凸起且頂端封閉的管狀器官，是根的一種特殊結(jié)構(gòu)[1]。根毛擴大了根系吸收礦質(zhì)養(yǎng)分和水分的表面積，有利于提高根系吸收土壤中礦質(zhì)養(yǎng)分和水分的效率，尤其是對土壤中短距離運輸?shù)牧住?鉀等礦質(zhì)養(yǎng)分的吸收特別重要[2-7]。根毛生長發(fā)育過程大致可分為4個階段： 根毛細胞命運決定、 根毛起始、 根毛頂端生長、 根毛成熟[8]。礦質(zhì)養(yǎng)分和激素影響著根毛生長發(fā)育各個階段，尤其對根毛頂端生長影響很大，相關(guān)轉(zhuǎn)錄因子及特異基因參與礦質(zhì)養(yǎng)分和激素對其生長發(fā)育的調(diào)控[5, 9-12]。本文擬從氮、 磷、 鉀、 鈣、 鐵等礦質(zhì)養(yǎng)分和生長素、 乙烯等激素對根毛生長發(fā)育的影響及其作用機制進行綜述，為深入研究植物根毛的生長發(fā)育和提高植物對礦質(zhì)養(yǎng)分及水分的吸收利用提供參考。

1　礦質(zhì)養(yǎng)分對根毛生長發(fā)育的影響

1.1氮

1.2磷

磷(P)是植物生長發(fā)育所必需的元素之一，然而土壤中可直接被吸收利用的有效磷濃度較低，這迫使植物進化出一套應(yīng)對磷缺乏響應(yīng)機制，其中較典型的是促進根毛生長發(fā)育[21]。

磷對根毛生長發(fā)育的影響已在擬南芥(Arabidopsisthaliana)、 番茄、 薊草(Cirsiumjaponicum)、 油菜(Brassicacampestris)、 甜菜(Betavulgaris)、 大麥(Hordeumvulgare)、 菜豆(Phaseolusvulgaris)、 枳(Poncirustrifoliate)等植物中有較深入研究。缺磷能夠使枳主根根毛密度從150條/mm2提升到250條/mm2, 長度從45 μm增加到60 μm[5, 9]。缺磷條件下的擬南芥根毛密度和長度是不缺磷時的35倍，且生長速度也提高了1倍，使根的表面積從0.21 mm2/mm提升到1.44 mm2/mm[22-24]。磷脅迫能引起擬南芥根毛密度增加主要是由于促進生毛細胞最終凸起成為根毛的比例， 促進非生毛細胞凸起成為異位根毛， 降低生毛細胞直徑以提高單位面積生毛細胞數(shù)量[22, 25-26]。

不同磷效率植物根毛對磷脅迫的響應(yīng)不一樣，如菜豆磷吸收低效品種在低磷條件下有大量根毛形成，而磷吸收高效品種卻沒有[27]。磷對根毛的影響在不同基因型植株中也有區(qū)別，如擬南芥根毛缺失突變體，缺磷促進了其根毛長度的增加，卻對根毛密度沒有顯著影響[28]。

很多研究認為磷對根毛生長發(fā)育的調(diào)控伴隨著內(nèi)源激素信號途徑，且主要有以下3種狀況：

1) 磷與生長素互作于根毛生長發(fā)育擬南芥生長素信號傳導(dǎo)突變體axr1、axr2、aux1因缺乏生長素信號致使根毛生長發(fā)育受阻，而缺磷能回復(fù)根毛正常生長[23]。生長素運輸抑制劑2-(對-氯苯氧基)-2-甲基丙酸(CMPA)會顯著抑制低磷對根毛生長發(fā)育的促進作用[25]。

2) 磷與乙烯互作于根毛生長發(fā)育乙烯信號突變體ein2因缺乏乙烯信號導(dǎo)致根毛生長發(fā)育受阻，而缺磷能誘導(dǎo)其根毛正常生長[23, 29]。深入研究發(fā)現(xiàn)缺磷誘導(dǎo)根產(chǎn)生乙烯的量是不缺磷的2倍左右，因此認為缺磷刺激乙烯信號突變體ein2產(chǎn)生乙烯，進而促進根毛正常生長[23, 29]。采用乙烯生物合成抑制劑AVG處理發(fā)現(xiàn)其能夠抑制低磷對根毛生長的促進效應(yīng)[30]。

3) 磷與獨腳金內(nèi)酯互作于根毛生長發(fā)育磷脅迫對根毛生長促進效應(yīng)在擬南芥獨腳金內(nèi)酯突變體max2和max4中被降低，該突變體也降低了磷脅迫響應(yīng)基因PSI的表達量，而外源施加獨腳金內(nèi)酯(GR24)能夠提高PSI的表達量，并促進根毛生長[31]。

1.3鉀

近些年的研究發(fā)現(xiàn)鉀(K)能影響根毛的生長發(fā)育[32]。如低鉀(0.05 mmol/L)條件下，煙草的根毛密度、 長度顯著大于正常供鉀(5 mmol/L)的根毛密度和長度[33]。低鉀對根毛生長作用機理可能是低鉀能激活植物高親和性鉀離子轉(zhuǎn)運蛋白和打開鉀離子通道[34]。鉀離子轉(zhuǎn)運蛋白突變體akt1和鉀離子生化阻斷劑能夠證明鉀離子通道對根毛頂端生長十分重要[35-36]。在擬南芥trh1突變體中，發(fā)現(xiàn)生長素輸出載體PIN1異位表達，從而影響生長素體內(nèi)運輸和分布，最終阻礙根毛生長，其中TRH1是編碼鉀離子運輸載體，由此可推測鉀通過生長素正調(diào)控根毛生長發(fā)育[36-37]。然而在枳的研究中發(fā)現(xiàn)缺鉀處理時，主根和側(cè)根的根毛密度、 長度及直徑均顯著低于正常供鉀處理[3-5]。由此可見鉀對不同植物種類根毛生長發(fā)育的影響及作用機理或許存在差異。

1.4鈣

鈣(Ca)作為細胞信號傳導(dǎo)的第二信使，已被證實在植物生長、 發(fā)育以及抗逆性等方面具有重要作用。研究發(fā)現(xiàn)鈣也參與根毛的生長發(fā)育，外界鈣離子濃度對根毛發(fā)育具有顯著影響。如當外加鈣離子濃度為0.33 mmol/L時，擬南芥根毛長度達到最大值[38]。鈣離子螯合劑乙二醇-雙-(2-氨基乙基醚)四乙酸(EGTA) 和鈣調(diào)素拮抗劑三氟拉嗪(TFP)及氯丙嗪 (CPZ)均可顯著抑制根毛發(fā)生和生長，外源施加鈣調(diào)素可減弱此抑制作用[32, 39]。由此表明鈣離子在根毛形成和生長過程中具有十分重要的作用。

利用激光共聚焦技術(shù)發(fā)現(xiàn)鈣離子在根毛頂端生長過程中濃度梯度的變化。根毛進入頂端生長階段，其頂端形成一個鈣離子濃度梯度，當根毛進入成熟階段或因外界環(huán)境干擾使其停止生長時，鈣離子濃度梯度將消失[40]。人為讓鈣離子濃度梯度消失，根毛頂端生長直接被終止[41]。外源處理使頂端一側(cè)的鈣離子濃度升高時，根毛生長方向被改變[42]。這種鈣離子濃度梯度可能通過影響根毛中細胞骨架的形成以及肌動蛋白、 囊泡的移動、 微管排列等來影響其頂端生長，質(zhì)膜上PCaP2作為鈣離子結(jié)合蛋白以信號傳導(dǎo)模式參與此過程[43]。最近的研究發(fā)現(xiàn)鈣離子相關(guān)蛋白CAP1通過維持細胞質(zhì)中鈣離子濃度梯度調(diào)控根毛頂端生長[20]。根毛頂端生長與其頂端鈣離子濃度梯度密切相關(guān)。

1.5鐵

鐵(Fe)廣泛參與植物一系列重要的生理生化代謝，如光合作用、 葉綠素的合成、 植株體內(nèi)氧化還原反應(yīng)和電子傳遞等。當土壤中可直接被吸收利用的有效鐵較少時，植物將做出一系列反應(yīng)以應(yīng)對鐵脅迫，其中典型的是促進根毛生長發(fā)育，如鐵脅迫可促進枳和擬南芥根毛的生長發(fā)育，增加其根吸收表面積，進而提高了鐵元素的吸收[3-5, 26]。鐵脅迫可能通過鐵氧化還原蛋白PFLP (plant ferredoxin-like protein)在NADPH氧化酶(NOX)作用下影響體內(nèi)ROS含量[44-47]。ROS能夠激活A(yù)CC合成酶(ACO)促進乙烯合成，進而促進根毛生長發(fā)育[48]。因此鐵可能通過乙烯信號影響根毛生長發(fā)育。然而缺鐵和低鐵對擬南芥根毛作用機理有所區(qū)別， 缺鐵是通過促進根毛分叉以增加根毛數(shù)量，而低鐵促進根毛生長發(fā)育主要是通過誘發(fā)更多的表皮細胞凸起成為根毛并促進凸起后的頂端生長[26]。

2　激素對根毛生長發(fā)育的調(diào)控

2.1生長素

生長素(auxin)作為調(diào)控植物生長發(fā)育的主要激素，影響著植物根毛凸起和伸長生長。擬南芥生長素響應(yīng)突變體axr1的根毛長度顯著低于野生型，證明生長素對根毛頂端生長很重要，但其表皮細胞發(fā)育成為根毛的比例與野生型無顯著差異[49]，這與先前生長素對生毛細胞比例的作用無顯著影響的結(jié)論相似[50]。然而后期研究結(jié)果表明生長素也參與調(diào)控根毛起始。外源生長素類似物萘乙酸(NAA)提高了擬南芥根毛數(shù)量和長度，外源施加的生長素運輸抑制劑1-萘氧乙酸(1-NOA)和1-萘氨甲酰苯甲酸(NPA)則阻礙了根毛凸起和伸長生長[51]。這些不同的結(jié)果可能與其施用外源生長素的濃度等有關(guān)，而生長素在根毛細胞中的濃度對根毛的凸起和頂端生長十分重要[52-53]。根毛細胞中的生長素濃度與生長素運輸相關(guān)，然而生長素不是從根尖直接運輸?shù)缴毎?，而是以非生毛細胞作為中轉(zhuǎn)站進行極性運輸，從而影響生毛細胞中生長素的濃度進而影響根毛生長發(fā)育[54]。外源施加NAA可導(dǎo)致生長素響應(yīng)報告基因DR5:GUS在非生毛細胞中大量表達，非生毛細胞積累生長素，然后運輸?shù)缴毎倨浒l(fā)育成為根毛[55]。生長素在根中的極性運輸是需要載體的，擬南芥生長素流入載體基因PIN2突變后，將抑制生長素從根尖向根毛區(qū)運輸，同時抑制生長素從非生毛細胞運輸?shù)缴毎?，生毛細胞缺乏生長素信號，將阻礙根毛的凸起和頂端生長[56]。PIN7、PGP4和PDR8作為生長素流出載體基因，其表達量下降能降低擬南芥生毛細胞中的生長素流出速度，因而生毛細胞積累較多的生長素，在一定的濃度范圍內(nèi)促進根毛生長發(fā)育[52, 55, 57-58]。生長素作為信號物質(zhì)影響根毛發(fā)育，合適濃度的生長素對根毛生長發(fā)育有利。如NAA施加過量將使擬南芥根毛的長度顯著下降[59-60]，1 μmol/L的吲哚丁酸(IBA)促進枳根毛生長效果最佳[9-10]。

2.2乙烯

乙烯(ethylene)以雙重作用影響根毛發(fā)育，即提高凸起率和促進頂端生長。擬南芥乙烯不敏感突變體etr1的根毛數(shù)量和長度顯著低于野生型，而乙烯過量突變體eto1由于產(chǎn)生了大量乙烯，提高了根毛的數(shù)量和長度[61]。經(jīng)乙烯合成前體1-氨基環(huán)丙烷-1-羧酸(ACC)誘導(dǎo)的野生型擬南芥，它們根毛長度和密度均得到了提高，通過切片等試驗證明其密度的增加是由于提高了生毛細胞最終凸起成為根毛的比例和更多異位根毛的產(chǎn)生[62]。乙烯生物合成抑制劑氨基乙氧基乙烯甘氨酸(AVG)則能阻礙生毛細胞凸起以及凸起后的頂端生長[50]。在生毛細胞凸起啟動之后，采用乙烯生物合成抑制劑1-甲基環(huán)丙烯(1-MCP)處理，發(fā)現(xiàn)根毛的密度無顯著變化，但頂端生長受抑制[63]。這說明乙烯作為信號物質(zhì)在根毛凸起之前調(diào)控表皮細胞凸起發(fā)育成根毛，在根毛凸起之后調(diào)控其頂端生長。

值得注意的是生長素和乙烯可能以交互作用調(diào)控根毛生長發(fā)育[64]。擬南芥生長素響應(yīng)突變體axr1和乙烯響應(yīng)突變體etr1的根毛生長受到阻礙，外源施加乙烯前體ACC均能回復(fù)其根毛正常生長，甚至其長度和密度超過野生型植株，切片分析其密度的提高是由于產(chǎn)生了異位根毛[49]。擬南芥乙烯信號傳導(dǎo)突變體ein2-1的根毛頂端生長受到顯著抑制，外源施加NAA能夠使其回復(fù)至野生型水平[65]。研究認為表皮細胞中生長素水平對根毛的凸起和頂端生長至關(guān)重要，乙烯突變體根毛生長回復(fù)到正常水平需要NAA的量是生長素突變體的1倍[65]。通過轉(zhuǎn)錄組數(shù)據(jù)發(fā)現(xiàn)生長素和乙烯均能夠使90%與根毛發(fā)育相關(guān)的基因上調(diào)表達，這充分證明生長素和乙烯部分通過共同的路徑參與調(diào)控根毛的發(fā)育[66]。然而它們在調(diào)控根毛發(fā)育過程中的作用和關(guān)聯(lián)有兩種相反看法。

1)生長素在乙烯上游調(diào)控根毛發(fā)育。生長素能夠促進乙烯的生物合成，因此生長素促進根毛的生長被認為可能是通過內(nèi)源乙烯途徑[49-50]。外源生長素能夠回復(fù)擬南芥短根毛突變體rhd6的根毛長度至野生型水平，而乙烯生物合成抑制劑1-MCP能夠阻礙該效應(yīng)[63]。擬南芥生長素信號缺失突變體arf7和arf9的根毛凸起和頂端生長受阻，外源ACC處理能夠回復(fù)根毛的正常生長[67]。據(jù)報道，根毛的頂端生長需要生長素誘導(dǎo)的微管不規(guī)則分布，而乙烯能夠啟動微管不規(guī)則分布[68]。由此認為生長素位于乙烯的上游調(diào)控根毛生長發(fā)育。

2)生長素在乙烯下游調(diào)控根毛發(fā)育。乙烯能夠刺激生長素的生物合成和信號傳導(dǎo)，并能促進生長素運輸?shù)礁扉L區(qū)的表皮細胞[69-70]。外源生長素能夠回復(fù)擬南芥乙烯不敏感突變體ein2-1短根毛表型[65]。擬南芥乙烯超量突變體eto1根毛較長，生長素突變體aux1的根毛發(fā)育受阻，而其雙突變體eto1aux1的根毛凸起和伸長生長受阻[71]。所以認為生長素在乙烯下游調(diào)控根毛生長發(fā)育。

但也有研究認為生長素提高根毛密度的機理可能獨立于乙烯。乙烯主要是通過異位根毛影響根毛密度，外源施加IAA和生長素類似物2, 4-二氯苯氧乙酸(2, 4-D)不能誘導(dǎo)擬南芥產(chǎn)生異位根毛。生長素增加根毛數(shù)量的機理是：增加皮層細胞數(shù)量、 減小表皮細胞的長度，從而增加生毛細胞的數(shù)量；促進更多的生毛細胞凸起成為根毛[11, 50-51, 62]。

2.3茉莉酸和茉莉酸甲酯

除了生長素和乙烯對根毛生長發(fā)育有著顯著促進作用外，茉莉酸(jasmonic acid, JAs)和茉莉酸甲酯(methyl jasmonate, MeJAs)也具有類似作用。適宜濃度的茉莉酸和茉莉酸甲酯能夠提高擬南芥根毛長度和密度，其密度的提高部分是由于產(chǎn)生了分叉根毛[72]。茉莉酸可通過上調(diào)生長素合成基因YUCCA8 和YUCCA9 的表達，促進擬南芥內(nèi)源生長素的合成[73]。擬南芥生長素信號傳導(dǎo)突變體axr1的主根生長受抑制，外源施加茉莉酸能夠回復(fù)主根正常生長，推測茉莉酸可能通過生長素路徑調(diào)控根系及根毛生長發(fā)育[74]。另有研究表明茉莉酸和茉莉酸甲酯促進擬南芥根毛生長發(fā)育的效應(yīng)可被乙烯效應(yīng)抑制劑銀離子(Ag+)和乙烯合成抑制劑AVG消除，茉莉酸合成抑制劑布洛芬和水楊羥肟酸能夠抑制ACC促進根毛生長，它們也能夠減少擬南芥乙烯過量突變體eto2根毛的數(shù)量和長度[16, 72]，由此說明茉莉酸和茉莉酸甲酯對根毛生長發(fā)育的作用是部分依賴根中生長素或乙烯途徑。

2.4獨角金內(nèi)酯

新型激素獨角金內(nèi)酯(strigolactone, SL)也能促進根毛的生長發(fā)育，并且與乙烯、 生長素在調(diào)控根毛發(fā)育過程中具有協(xié)同效應(yīng)[67, 75]。雖有研究認為獨腳金內(nèi)酯可通過乙烯途徑調(diào)控根毛生長發(fā)育，然而其更有可能是依托生長素來調(diào)控根毛生長發(fā)育，因為獨角金內(nèi)酯能夠調(diào)控PINs和TIR1的表達，以此影響生長素的運輸和信號傳導(dǎo)[76-77]。擬南芥生長素運輸突變體arf7、arf9添加外源獨角金內(nèi)酯，突變體根毛恢復(fù)正常生長[75]，這說明獨角金內(nèi)酯可能獨立或位于生長素下游調(diào)控根毛生長。然而，外源生長素也可恢復(fù)獨角金內(nèi)酯信號傳導(dǎo)突變體max2、max4根毛至野生型水平[77]，說明獨角金內(nèi)酯也可能位于生長素的上游調(diào)控根毛發(fā)育。

2.5油菜素甾醇

油菜素甾醇(brassinosteroids, BR)是近幾十年來確認的新型植物激素，被稱為繼生長素、 細胞分裂素、 赤霉素、 脫落酸、 乙烯之后的第六大激素。雖然它在植物莖、 葉、 根的生長以及維管組織的分化、 育性、 種子萌發(fā)、 頂端優(yōu)勢的維持、 植物光形態(tài)建成等方面與生長素具有協(xié)同作用，然而在根毛生長發(fā)育方面卻與生長素差異較大，即對根毛生長發(fā)育既有促進又有抑制作用。在雙子葉模式植物擬南芥中的研究表明油菜素甾醇抑制根毛凸起和頂端伸長生長，而在單子葉模式植物水稻(Oryzasativa)中，油菜素甾醇對根毛的發(fā)育存在劑量效應(yīng)，在一定范圍內(nèi)促進根毛發(fā)育[78-80]。對擬南芥的研究還發(fā)現(xiàn)，油菜素甾醇促進AXR3/IAA17(生長素信號抑制基因)上調(diào)表達[78]，因此可推測油菜素甾醇有可能通過AXR3/IAA17抑制生長素信號傳輸，從而抑制根毛生長。

3　結(jié)語

在植物根系中，礦質(zhì)養(yǎng)分脅迫通過誘導(dǎo)根系細胞內(nèi)源激素信號變化以調(diào)控根毛發(fā)育，從而增加植物對有限養(yǎng)分的吸收能力以適應(yīng)脅迫環(huán)境。如磷、 鐵、 鉀等脅迫能夠增加根系內(nèi)源生長素和乙烯含量，促進根毛生長發(fā)育[29-30]。APSR1是磷脅迫響應(yīng)因子，其突變后，能使PIN7(生長素流出載體基因)表達下調(diào)，促進根毛生長發(fā)育[58, 81]。乙烯生物合成抑制劑AVG處理能夠抑制低磷對根毛生長的促進效應(yīng)[30]。乙烯過量突變體eto3的根毛密度和長度得到顯著提高，然而缺鐵能促進其密度和長度進一步增加[23]。鉀脅迫能夠促進煙草根毛發(fā)育，推測生長素輸出載體PIN1可能參與鉀對根毛發(fā)育的影響[33, 37, 82]。然而本課題組研究發(fā)現(xiàn)鉀脅迫能夠降低枳根毛密度和長度[3-5]，這或許與鉀脅迫程度或不同植物應(yīng)對鉀脅迫機制不同有關(guān)?？傊?，礦質(zhì)養(yǎng)分對植物根毛生長發(fā)育的影響不僅與激素信號相關(guān)，而且與根毛發(fā)育的遺傳相關(guān)，相關(guān)基因的表達等影響著根毛的發(fā)生和生長。

已有研究表明缺磷不僅能夠促進根毛發(fā)育，也能促進叢枝菌根發(fā)育，且根毛和叢枝菌根不僅能夠提高植物對磷的吸收，也能增強植物抗旱能力[3, 9, 83-84]?？梢姼途际侵参镞m應(yīng)環(huán)境脅迫的一種途徑，然而它們抵抗脅迫環(huán)境的能力差別較大。大麥根毛相對于叢枝菌根在磷脅迫環(huán)境條件下更具優(yōu)勢，而叢枝菌根相對于根毛能夠更有效的幫助植物適應(yīng)干旱環(huán)境[85-86]。近年來的研究從轉(zhuǎn)錄組數(shù)據(jù)中得到缺磷條件下有3000個差異表達基因，通過全基因組共表達分析可知缺磷主要是通過蛋白激酶(protein kinases)促進根毛發(fā)育[87]。同時，蛋白激酶對菌根的形成也是不可或缺的[88]。從蛋白激酶及其相關(guān)調(diào)控基因或轉(zhuǎn)錄因子等方面進行研究，或許可深入探析礦質(zhì)養(yǎng)分(如磷、 鐵等)對根毛和叢枝菌根的影響及其相互關(guān)系，進一步挖掘植物對脅迫環(huán)境的適應(yīng)機制。

影響植物根毛生長發(fā)育的激素較多，但其中起調(diào)控作用的主要為生長素和乙烯，茉莉酸、 獨腳金內(nèi)酯、 油菜素甾醇等激素對根毛生長發(fā)育的作用主要是通過影響生長素和乙烯而起作用。雖然有研究認為，生長素提高根毛密度的機理可能獨立于乙烯，但多數(shù)研究認為二者相關(guān)聯(lián)，即它們在調(diào)控根毛發(fā)育過程中的作用或是生長素在乙烯上游調(diào)控根毛生長，亦或是生長素在乙烯下游調(diào)控根毛發(fā)育[70-71]。不同植物根毛生長發(fā)育過程中生長素和乙烯的作用位置不同或許與試材不同等有關(guān)，因為不同植物根毛發(fā)育方式有差別，且調(diào)控根毛發(fā)育的基因及其表達亦有差異。

值得注意的是，在植物根毛發(fā)育研究中曾有人提出“乙烯中心”假說，即土壤環(huán)境、 生長調(diào)節(jié)劑、 礦質(zhì)養(yǎng)分等能影響根系生毛細胞中乙烯的濃度，進而調(diào)控根毛的生長發(fā)育[89]。如生長素、 茉莉酸、 獨腳金內(nèi)酯以及磷脅迫等因子對根毛生長發(fā)育的影響是通過乙烯途徑作用的，同時從細胞學(xué)角度證實了乙烯提高根毛密度的機理是提高生毛細胞最終凸起成根毛的比例和更多異位根毛的產(chǎn)生[29-30, 62, 72, 75-76]。這些相互矛盾的結(jié)果不僅說明乙烯和生長素等激素對植物根毛生長發(fā)育的調(diào)控較為復(fù)雜，而且還受其它因素的影響(圖1)。當遇到逆境時(如磷脅迫、 干旱等)，植物為了規(guī)避風(fēng)險，通過改變生長素合成運輸和乙烯合成來打破其體內(nèi)生長素和乙烯的平衡，從而觸發(fā)根作出反應(yīng)以保證根生長的可塑性。筆者認為，生長素和乙烯在植物體內(nèi)的平衡狀態(tài)可能對其根毛的生長發(fā)育起著重要作用。此外，不同種類植物可能對不同激素種類反應(yīng)不同，因而所得到的根毛生長發(fā)育結(jié)果不同。即使是同一種植物，應(yīng)用同一種激素，不同濃度其結(jié)果也不相同，如1 μmol/L的IBA促進枳根毛生長發(fā)育效果最佳，較高或較低濃度的IBA的效果都不顯著甚至相反[10]。這從一個側(cè)面再次說明植物體內(nèi)生長素和乙烯等激素的平衡對其根毛的生長發(fā)育起著重要作用。

圖1　礦質(zhì)養(yǎng)分和植物激素調(diào)控根毛生長發(fā)育模式圖Fig.1　The model of mineral nutrients and phytohormones regulating root hairs development

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Effects and mechanism of mineral nutrient and phytohormone on root hair development

ZHANG De-jian, XIA Ren-xue*, CAO Xiu

(MinistryofEducationKeyLaboratoryofHorticulturalPlantBiology,HuazhongAgriculturalUniversity,Wuhan430070,China)

【Objectives】 Root hairs are tubular projections from root epidermal cells. Their formation make a significant increase in root surface area, which play important roles in nutrients and water uptake, anchorage, and interaction with soil microorganisms. In this review, we discussed the effects of mineral nutrients (nitrogen, phosphorus, potssium, clacium, iron) and phytohormones (auxin, ethylene, jasmonic acid, strigolactone, brassinosteroids) on root hairs development and their relevant mechanisms. 【Major advances】The effect of nitrogen on root hairs development is related to jasmonic acid and ethylene. Interaction existes between phosphorus and phytohormones (auxin, ethylene, strigolactone) on root hairs growth. The interaction between auxin and ethylene regulates root hairs development. The effects of jasmonic acid, strigolactone and brassinosteroids partly depend on the way of auxin and ethylene. The balance of auxin/ethylene maybe important on root hairs growth and development.【Suggestions and expectations】 On the basis of the above review, it is important to study the regulatory genes and transcription factors, which will be helpful to deeper understand the effects of mineral nutrient and phytohormone on arbuscular mycorrhizas and root hairs development.

root hairs; development; nitrogen; phosphorus; auxin; ethylene; arbuscular mycorrhiza

2014-09-29接受日期: 2015-04-08

華中農(nóng)業(yè)大學(xué)自選項目(52207-05019)資助。

張德健(1988—)， 男， 安徽蕪湖人， 博士研究生， 主要從事根系栽培生理學(xué)研究。

Tel: 027-87284181, E-mail： zhangdejian0551@126.com。 *通信作者 Tel: 027-87286913, E-mail: renxuexia@mail.hzau.edu.cn

Q945.12

A

1008-505X(2016)03-0802-09