摘要: 鹽堿地改良和利用對拓展我國后備耕地資源及保障國家糧食安全具有重要意義。根際微生物作為植物的“第二基因組”,在提高作物抗鹽堿脅迫能力,促進作物“以種適地”方面具有巨大潛力。其中,叢枝菌根真菌(arbuscular mycorrhizal fungi,AMF) 和植物根際促生細菌(plant growth promoting rhizobacteria,PGPR) 均為重要的根際有益微生物,能顯著提高植物耐鹽性。本文分別總結(jié)了AMF 和PGPR 提高植物抗鹽能力的相關(guān)研究進展,并進一步梳理了兩者協(xié)同提高植物耐鹽的機制,包括提高養(yǎng)分效率、調(diào)節(jié)激素內(nèi)穩(wěn)態(tài)、提高植物誘導(dǎo)抗性以及調(diào)控轉(zhuǎn)錄因子表達等,最后提出了基于兩者跨界組合的改良方向。旨在充分理解鹽脅迫條件下微生物耐鹽促生的作用機制,為充分挖掘微生物資源潛力和發(fā)展生物技術(shù)治理鹽堿地提供重要的科學(xué)支撐。
關(guān)鍵詞: 鹽化土壤; 叢枝菌根真菌; 植物根際促生細菌; 植物耐鹽性
土壤鹽堿化是指土壤中鹽分離子在隨水分蒸發(fā)過程中,于土壤表層形成積鹽層,影響植物正常生長的現(xiàn)象[1]。鹽堿地遍布全球100 多個國家,面積高達1×109 hm2[2]。在我國,鹽堿地總面積高達3690 萬hm2,其中鹽堿耕地760 萬hm2。據(jù)估計,土壤的鹽堿脅迫將威脅到全球50% 以上的耕地,嚴重影響糧食安全和生態(tài)系統(tǒng)健康,因此鹽堿地改良成為當(dāng)前耕地產(chǎn)能提升的一個重要內(nèi)容[3]。長期以來,鹽堿地改良主要依賴水利工程、化學(xué)改良劑和農(nóng)藝措施等傳統(tǒng)方法,但在實際應(yīng)用中常常受到水資源匱乏、高投入成本等因素的制約。根際微生物組作為植物的“第二基因組”,在促進植物營養(yǎng)生長,維持植物健康方面發(fā)揮重要作用[4?5]。在加強田間管理和改良土壤的基礎(chǔ)上,結(jié)合現(xiàn)代生物學(xué)技術(shù),強化“植物?土壤?微生物”互作,是新時期“以種適地”和“以地適種”相結(jié)合的一個重要突破方向。
關(guān)于微生物響應(yīng)、適應(yīng)鹽堿生境及其對植物的促生機制的研究,正成為近年來微生物領(lǐng)域的研究熱點。作為兩類重要的根際有益微生物,叢枝菌根真菌(arbuscular mycorrhizal fungi, AMF) 和植物根際促生細菌(plant growth-promoting rhizobacteria, PGPR)在協(xié)同提高植物抗鹽性上存在巨大的潛力。AMF 可以與陸地上約70%~80% 高等植物建立互惠共生關(guān)系,對植物的養(yǎng)分吸收和抗逆性具有顯著的提升作用[6]。PGPR 是指分布于植物根際微域內(nèi)可以促進植物生長的有益微生物類群,參與根際多種關(guān)鍵物質(zhì)的循環(huán)轉(zhuǎn)化過程[7]。植物主要通過根系分泌物來招募和塑造特異性根際微生物組[8]。與植物類似,AMF可以分泌有機酸、氨基酸和糖類等物質(zhì),在菌絲際招募和塑造核心微生物組,兩者互作可顯著提高其對養(yǎng)分的利用效率并增強植物的抗逆性[ 9 ]。然而,目前對AMF、PGPR 及兩者互作提高植物耐鹽能力的機制仍不清楚。隨著基因組學(xué)、基因編輯等技術(shù)的快速發(fā)展,為深入研究植物?真菌?細菌三者跨界合作提供了可能性。本文總結(jié)了國內(nèi)外關(guān)于AMF 和PGPR 單獨及其互作提高植物耐鹽能力的研究進展,以期為理解植物?AMF?PGPR 聯(lián)合抗鹽的機制,充分挖掘微生物資源,為綠色、高效地改良鹽堿土壤提供重要理論支撐。
1 植物對鹽脅迫的響應(yīng)
鹽脅迫對植物的影響具有多重性,Munns 等[10]提出,鹽脅迫下植物存在兩個過程,即起始階段的快速滲透脅迫和后期葉片中鹽離子過量積累引起的離子脅迫階段。鹽脅迫還會導(dǎo)致一系列的次生效應(yīng),其中主要是氧化脅迫。鹽脅迫引起植物細胞內(nèi)生物分子的物理或化學(xué)變化,從而引發(fā)細胞應(yīng)激反應(yīng)并啟動一系列響應(yīng)機制,主要包括滲透調(diào)節(jié)機制、離子平衡機制和抗氧化機制3 個方面[11?12]。在許多情況下,這些調(diào)控途徑之間相互關(guān)聯(lián)形成了復(fù)雜的調(diào)控網(wǎng)絡(luò)。
1.1 滲透調(diào)節(jié)
隨著土壤鹽分的升高,土壤溶液的水勢低于植物根細胞的水勢,植物體內(nèi)的水分子排到細胞外進而導(dǎo)致植物生理性干旱,形成滲透脅迫[13?14]。植物細胞會通過多種方式降低細胞滲透勢,從而保持胞內(nèi)外滲透壓平衡,維持植物正常生理代謝[15]。植物體內(nèi)的滲透調(diào)節(jié)過程主要分為兩類:一類是在土壤中吸收并積累無機鹽離子(如K+和Ca2+),從而快速恢復(fù)細胞膨壓;另一類是植物通過自身合成有機物質(zhì)調(diào)節(jié)滲透壓,如合成多元醇類、甜菜堿、可溶性糖和脯氨酸等,降低細胞滲透勢[16]。
1.2 離子平衡
在鹽堿脅迫下,土壤中高濃度的Na+進入植物細胞并在細胞中積累,細胞中Na+的外排和木質(zhì)部中Na+的回收是植物耐鹽的重要途徑,該過程通過高親和力鉀轉(zhuǎn)運蛋白和Na+/H+逆向轉(zhuǎn)運蛋白的調(diào)控來實現(xiàn)。植物激活高親和力的K+轉(zhuǎn)運體(HKT) 調(diào)控木質(zhì)部對Na+再吸收,降低Na+的毒害。例如由擬南芥中AtHKT1、水稻中OsHKT1;4 和OsHKT1;5 以及高粱中GmHKT1;1 編碼的HKT1 蛋白,增強根部木質(zhì)部對Na+的回收[17]。當(dāng)植物遭受鹽堿脅迫后Ca2+會通過質(zhì)膜和細胞器膜流入細胞質(zhì), 導(dǎo)致細胞質(zhì)中的Ca2 +峰,Ca2 +信號依賴的鹽超敏感途徑(salt overlysensitive, SOS) 則在鹽離子轉(zhuǎn)運過程中發(fā)揮關(guān)鍵作用,磷脂酸與SOS2 的Lys57 殘基結(jié)合后,促進其質(zhì)膜定位,進而激活Na+/H+反轉(zhuǎn)運蛋白SOS1,植物將過量的Na+排除到細胞外和將其區(qū)域化隔離在液泡中兩種方式來緩解植物鹽脅迫[18]。此外,植物還能通過促進K+吸收和液泡對Cl?的區(qū)隔化來維持細胞內(nèi)離子穩(wěn)態(tài)[19?23]。
1.3 抗氧化機制
植物體內(nèi)活性氧(reactive oxygen species,ROS)的積累會導(dǎo)致細胞膜脂質(zhì)過氧化、DNA 損傷、蛋白質(zhì)變形、碳水化合物氧化、色素分解和酶活性受損等,并產(chǎn)生有毒物質(zhì)丙二醛(MDA),導(dǎo)致植物死亡[24]。植物體內(nèi)存在兩類抗氧化防御系統(tǒng):一是酶促抗氧化系統(tǒng),主要包括在ROS 加工中起作用的酶,如超氧化物歧化酶(SOD)、過氧化物酶(POD)、過氧化氫酶(CAT) 等。研究表明,當(dāng)脅迫強度超過植物的承受能力時,這些酶的活性下降,呈先升后降的趨勢[25]。二是非酶促抗氧化系統(tǒng),主要包括還原型谷胱甘肽(GsH)、抗壞血酸(ASA)、氧化型谷胱甘肽(GSSG) 和類胡蘿卜素等,兩個系統(tǒng)通過清除細胞中過度積累的ROS 來維持ROS 穩(wěn)態(tài)[26?27]。此外,植物還可以通過抑制miRNA ZmmiR169q 的積累,增強抗氧化酶基因ZmPER1 的表達,從而消除過量ROS[28]。
2 AMF 或PGPR 單獨作用緩解鹽脅迫的機制
基于可視化應(yīng)用軟件CiteSpace,選取Web ofScience (WOS) 核心合集數(shù)據(jù)庫,AMF 抗鹽脅迫檢索條件為主題=“AMF” and “saline” or “AMF” and“salt”,PGPR 抗鹽脅迫檢索條件為主題=“PGPR”and “saline” or “PGPR” and “salt”,文獻類型均選擇“Article”和“Review”,分別檢索出該研究領(lǐng)域發(fā)表的文獻635 篇和872 篇,進行了關(guān)鍵詞突現(xiàn)性分析(圖1 和圖2)??梢钥闯觯珹MF 和PGPR 均影響植物耐鹽性。AMF 主要通過調(diào)節(jié)植物激素,改善植物對水分和養(yǎng)分的利用效率,以及植物光合速率,調(diào)節(jié)離子平衡和產(chǎn)生抗氧化劑等發(fā)揮作用。PGPR 的相關(guān)研究則較多集中在ACC 脫氨酶、定殖過程和對植物的促生作用等方面[29]。
2.1 提高養(yǎng)分和水分利用效率
作為有益微生物,AMF 和PGPR 在促進土壤養(yǎng)分循環(huán)和植物養(yǎng)分吸收方面均發(fā)揮著重要作用。在鹽脅迫下,兩者在促進養(yǎng)分吸收的機制上有所區(qū)別。PGPR 主要通過直接作用,例如通過分泌有機酸、胞外磷酸酶、產(chǎn)生鐵載體等促進土壤養(yǎng)分活化。例如,溶磷菌Y2R2 可以直接分泌酸性磷酸酶將難溶性無機磷化物活化為有效磷[30]。Bacillus aquimaris 和Azospirillum brasilense 能夠增加生物固氮酶活性,加速氮素活化過程[31?32]。AMF 則通過菌絲的直接作用以及菌絲?菌絲際微生物的互作兩種方式促進植物對養(yǎng)分的利用。一方面,AMF 龐大的菌絲網(wǎng)絡(luò)擴大了根系與土壤的接觸面積,提高了對磷(氮) 的吸收效率[33];另一方面,AMF 根外菌絲分泌物釋放到菌絲際微域土壤,在菌絲際招募解磷菌以及氮循環(huán)相關(guān)細菌,促進了植物對養(yǎng)分的吸收[34?35]。
在水分吸收利用方面,AMF 和PGPR 均可通過調(diào)節(jié)植物的水通道蛋白基因的表達來促進植物吸收水分[36?37]。如賀忠群等[38]通過熒光定量研究表明,鹽脅迫下接種AMF 的番茄(Solanum lycopersicum) 根系中水通道蛋白LeAQP2 基因表達顯著上調(diào)。Azospirillumbrasilens、Bacillus megaterium 等PGPR 能夠激活水通道蛋白相關(guān)基因(如PIP2、ZmPIP1-1 和HvPIP2-1) 表達來幫助植物在鹽脅迫下吸收水分等[39]。Kakouridis等[40]通過18O 同位素標(biāo)記試驗發(fā)現(xiàn),AMF 可以作為根系沿著土壤?植物?空氣連續(xù)體水分運動的延伸,植物蒸騰作用驅(qū)動水分通過菌絲的胞質(zhì)外途徑輸送到植物體內(nèi)。
2.2 調(diào)節(jié)植物內(nèi)源激素的水平
植物激素的變化是植物適應(yīng)逆境脅迫的主要反應(yīng)。植物受到鹽脅迫時,PGPR 和AMF 顯著影響植物激素水平。AMF 和PGPR 在機制上存在一些異同點,兩者均可以誘導(dǎo)植物體內(nèi)脫落酸(ABA)、赤霉素(GA)、吲哚乙酸(IAA) 和細胞分裂素(CTK) 等植物內(nèi)源激素的合成[41?43]。但AMF 與PGPR 誘導(dǎo)植物激素水平上升并促進植物生長的途徑存在一定差異。例如,有研究發(fā)現(xiàn),AMF 可以調(diào)控水楊酸、茉莉酸、游離多胺類物質(zhì)(如腐胺、亞精胺和精胺) 和獨腳金內(nèi)酯的含量,進而維持植物細胞膜穩(wěn)定性[ 4 4 ? 4 6 ]。PGPR 可以通過釋放揮發(fā)性有機化合物(VOCs) 介導(dǎo)并維持鹽脅迫條件下植物生長的穩(wěn)態(tài)[47?48]。鹽脅迫致使植物產(chǎn)生大量的乙烯,對植物生長有抑制作用,一些耐鹽的PGPR 含有acdS 基因,能夠編碼1-氨基環(huán)丙烷-1-羧酸脫氫酶(aminocyclopropane-1-carboxylate deaminase,ACCD),ACCD 將乙烯的前體ACC 水解成堿和α-酮丁酸鹽作為氮源,從而降低乙烯的過量積累[49?51]。近80% 的固氮菌(Azotobacter)和熒光假單胞菌(Pseudomonas fluorescens) 及近20% 的芽孢桿菌屬(Bacillus) 能分泌IAA,在鹽脅迫下接種這些促生菌不僅能夠直接促進生長,也能激發(fā)ACCD 活性,誘導(dǎo)植物抵抗鹽脅迫損傷[52]。
2.3 調(diào)控植物體內(nèi)離子平衡
AMF 和PGPR 可通過截留Na+以緩解離子毒害。AMF 可以將Na+截留在根外菌絲中,抑制其從根向地上部的轉(zhuǎn)運[53]。PGPR 可在根際分泌胞外多糖(exopolysaccharides, EPS),其能夠通過羥基、羧基和磷?;雀黝惞倌軋F形成龐大的生物膜,在根際微域阻隔Na+進入植物體,進而降低離子毒害[29]。此外,AMF 和PGPR 還通過調(diào)控K+轉(zhuǎn)運蛋白(HKT)和Na+/H+逆向轉(zhuǎn)運蛋白(NHX) 的特異性表達,并調(diào)節(jié)SOS 途徑基因的表達,協(xié)同緩解離子毒害。例如,解淀粉芽孢桿菌SQR9 可以調(diào)控玉米體內(nèi)與Na+外排相關(guān)基因(HKT1、NHX1、NHX2 和NHX3) 表達,將Na+排到細胞外[54?55]。AMF 可以通過增加OsSOS1OsHKT2;1 兩類基因的表達促進Na+的排出,并且上調(diào)水稻中的OsNHX3 將Na+區(qū)域化隔離到根系細胞的液泡中,減少向地上部的轉(zhuǎn)運[56?59]。
2.4 緩解滲透脅迫
AMF 和PGPR 可能通過促進植物體內(nèi)的碳水化合物、胺、氨基酸及其衍生物等滲透調(diào)節(jié)物質(zhì)的合成來緩解滲透脅迫,如脯氨酸、甜菜堿、海藻糖、有機酸和可溶性糖等[29, 47, 60?61]。其中,脯氨酸是植物體內(nèi)一種重要的滲透保護物質(zhì),可通過穩(wěn)定膜和蛋白亞結(jié)構(gòu)、增加胞內(nèi)溶質(zhì)濃度來增加植物對鹽的耐受性。Bharti 等[62]發(fā)現(xiàn)接種PGPR (Bacillus pumilus、Halomonas desiderata 等) 處理,野薄荷葉片中的脯氨酸含量顯著提升。同時,在擬南芥上接種綠膿假單胞菌腸桿菌(Enterobacter sp. EJ01) 使植株葉片中脯氨酸合成相關(guān)基因P5CS1 和P5CS2 均表達上調(diào)[63]。關(guān)于植物遭受鹽脅迫時AMF 通過分泌滲透調(diào)節(jié)物質(zhì)幫助植物抵抗?jié)B透脅迫的分子機制還缺乏證據(jù)。例如,在低鹽條件下,萵苣接種菌根真菌Rhizophagusintraradices 后,其脯氨酸合成酶的調(diào)控基因P5CS 表達上調(diào),但在高鹽條件下P5CS 基因的表達量不受AMF 影響[ 6 4 ]。然而,目前仍缺乏關(guān)于PGPR 或AMF 直接促進植物合成滲透調(diào)節(jié)物質(zhì)的直接證據(jù)。
2.5 調(diào)節(jié)植物抗氧化防御機制
AMF 和PGPR 均可以調(diào)控植物體內(nèi)的抗氧化酶與非酶抗氧化物的活性。在抗氧化酶方面,AMF 和PGPR 能分泌清除ROS 的SOD、CAT、POD 和APX等抗氧化酶,減輕氧化損傷[65]。在抗氧化劑方面,二者能夠提高如AsA、GSH 等的含量,從而阻止自由基和膜質(zhì)脂肪酸相互作用,增加細胞膜穩(wěn)定性,增強植物細胞的抗氧化能力。例如,Kim 等[63]發(fā)現(xiàn)鹽脅迫下接種綠膿假單胞菌腸桿菌(Enterobacter sp. EJ01)的擬南芥中APX 含量比對照高11 倍。AMF 還能誘導(dǎo)上調(diào)抗壞血酸-谷胱甘肽(AsA-GSH) 循環(huán)相關(guān)酶的表達,使植株體內(nèi)保持較高的抗氧化劑含量,阻止自由基破壞膜質(zhì)脂肪酸[63?69]。
2.6 增強植物的光合作用
鹽脅迫下AMF 和PGPR 均能促進植物體內(nèi)光合作用相關(guān)基因的表達來提高植物的光合作用。例如,在鹽脅迫下,AMF 提高了光反應(yīng)核心亞基蛋白合成基因RppsbA 和RppsbD 的表達量,提高了植株光反應(yīng)速率,且當(dāng)PSⅡ反應(yīng)中心的D1 和D2 蛋白降解時,AMF 對這2 個基因的上調(diào),可以使接種AMF 的刺槐中PSⅡ有更好的修復(fù)能力[70]。同時,接種AMF 后,植物體中尿卟啉原脫羧酶(UROD)、葉綠素還原酶(NYC) 和葉綠素合成酶(CHLG) 等一些調(diào)控葉綠素合成的蛋白的基因顯著上調(diào)[71]。在小麥、番茄、玉米、擬南芥和苜蓿等植物上接種PGPR 發(fā)現(xiàn)有同樣的效果[54, 72]。接種解淀粉芽孢桿菌FZB42后,擬南芥中與光合作用相關(guān)的基因表達上調(diào)[41]。
3 AMF-PGPR 互作協(xié)同提高耐鹽機制
目前對于微生物增強植物耐鹽的認識大多局限于單菌的耐鹽促生功能,但是在土壤中,微生物之間的協(xié)同作用更利于植物的生長。通過Web of science核心合集數(shù)據(jù)庫搜索,以“AMF” and “saline” and“PGPR” or “AMF” and “salt” and “PGPR”為檢索詞,文獻類型選擇“Article”和“Review”,共檢索出已發(fā)表的文獻僅58 篇,表明鹽土環(huán)境下AMF+PGPR聯(lián)合效應(yīng)的研究仍然相對較少,且多集中在優(yōu)化植物生長、改善土壤微生物群落結(jié)構(gòu)、促進菌根侵染等方面。
3.1 AMF 和PGPR 對鹽脅迫下植物生長的影響
目前AMF 和PGPR 聯(lián)合接種對于植物生長和耐鹽性的影響多集中于農(nóng)作物。研究發(fā)現(xiàn),AMF 和PGPR 互作提高了水稻(Oryza sativa L)、高粱(Sorghumbicolor)、玉米(Zea mays L.)、豆類、油菜(Brassicacampestris L.) 和番茄等作物的生長和抗鹽性。在大田研究中,二者聯(lián)合接種可使作物產(chǎn)量增加約30%~40%[ 7 3 ? 7 8 ]。例如,在水稻田鹽土上雙接種AMF 和PGPR,通過改善根際環(huán)境,促進植物吸收水分來緩解生長抑制,雙接種水稻的分蘗數(shù)、穗數(shù)和每穗粒數(shù)及產(chǎn)量提升最高,在正常土壤和鹽土上產(chǎn)量分別提高了23%~44.5% 和32.5%~56%[79]。然而,當(dāng)在不同試驗田進行試驗時,AMF 和PGPR 共接種時,發(fā)現(xiàn)在Kolli Hills 試驗田中小米和木豆的作物產(chǎn)量提升了128%,但在Bangalore 試驗田增產(chǎn)效果不顯著,這可能與作物種植方式、環(huán)境條件、接菌菌種以及植物種類有關(guān)[80]。
在堿蓬( S u a e d a g l a u c a )、高羊茅( F e s t u c aarundinacea) 和白刺(Nitraria tangutorum Bobrov) 等鹽生植物上,AMF 和PGPR 的促生作用顯著[81]。在鹽堿地環(huán)境下,在堿蓬上單獨接種AMF 或PGPR 的效果低于聯(lián)合接種,聯(lián)合接種顯著促進了菌根定殖,提高了光合性能、氣孔調(diào)節(jié)能力,降低了鹽分對植物幼苗的不利影響[82?83]。
3.2 AMF 和PGPR 協(xié)同調(diào)控植物抗鹽機制
關(guān)于AMF 和PGPR 聯(lián)合接種促進植物生長及幫助植物抗脅迫機制方面的研究,主要集中在土壤養(yǎng)分活化、植物生理生化變化、調(diào)節(jié)植物耐鹽基因表達和二者直接互作的界面機制等方面(圖3)。
1) 在養(yǎng)分活化方面,PGPR 直接提高土壤養(yǎng)分的有效性,而AMF 一方面可將PGPR 通過菌絲網(wǎng)絡(luò)運輸?shù)金B(yǎng)分(如有機磷) 富集斑塊活化養(yǎng)分,同時也可直接吸收養(yǎng)分[84]。例如,Chen 等[74]發(fā)現(xiàn)單獨接種Rhizophagus irregularis 對玉米生長無顯著促進,但由于PGPR 提高了AMF 的侵染率,擴大養(yǎng)分吸收范圍,可以有效提高玉米的生物量。同時,AMF 提高PGPR 分泌胞外酶的活性,而PGPR 也可增強AMF在植物根系表面的定殖。在油菜籽上單獨和聯(lián)合接種枯草芽孢桿菌(Bacillus subtilis) 和Rhizophagusintraradice,聯(lián)合接種相比于單獨接種顯著提高了堿性磷酸酶、β-葡萄糖苷酶和脫氫酶等土壤胞外酶活性,改善土壤養(yǎng)分循環(huán)功能[85]。
2) 在植物生理生化方面,AMF 和PGPR 的協(xié)同作用在維持K+和Na+離子穩(wěn)態(tài)、調(diào)節(jié)細胞內(nèi)外滲透平衡和清除ROS 方面有顯著效果[86]。例如,在鹽脅迫環(huán)境中,聯(lián)合接種Bacillus subtilis 和Funneliformismosseae 增加了植物體內(nèi)原花青素、類黃酮、抗壞血酸、SOD、APX 的含量,同時增加總可溶性糖和蛋白質(zhì)[87],提升了番茄的總抗氧化能力。在研究單獨和雙接種AMF (Funneliformis mosseae) 和固氮菌(Sinorhizobium meliloti) 對苜蓿的影響時,發(fā)現(xiàn)雙接種處理中葉片蛋白質(zhì)和脯氨酸含量顯著最高,根系和葉片中鈉離子含量最低,且結(jié)瘤量(每株結(jié)瘤數(shù)和根瘤菌重量) 最高[ 7 5 ]。在種植水稻的鹽土上雙接種Pseudomonas putida strain S34、Pseudomonasfluorescens strain R167 和Rhizophagus irregularis,可通過降低水稻的CAT 活性,提高脯氨酸含量和清除活性氧來降低其對作物生長限制[79]。
3) 在植物耐鹽基因方面,目前已挖掘出大量耐鹽關(guān)鍵基因及調(diào)控網(wǎng)絡(luò),如鈉離子通道、ABA 信號和SOS 通路的上調(diào)提高了植物對高鹽堿脅迫的抗性。聯(lián)合接種AMF 和PGPR 可誘導(dǎo)相關(guān)耐鹽基因的表達,例如,研究發(fā)現(xiàn)水稻的多種過氧化物酶基因受OsQHB 調(diào)控,導(dǎo)致水稻中ROS 清除酶活性更高,MDA 積累更低[ 8 8 ]。鹽脅迫下AMF (Funneliformismossea) 和兩種PGPR (Piriformospora indica 和Agrobacterium rhizogenes) 聯(lián)合接種處理,水稻的OsQHB、OsHBP1b、OsNCX2 等與抗氧化酶合成、光合作用和調(diào)控離子穩(wěn)態(tài)的相關(guān)基因表達量上調(diào)[89]。聯(lián)合接種AMF 和孢子表面細菌能顯著提高ZmAKT2、ZmSOS1 和ZmSKOP 等離子穩(wěn)態(tài)相關(guān)基因的表達,提高水稻對鹽堿脅迫的適應(yīng)性[90]。另外,接種AMF處理促進了小麥根系分泌物中苯并惡嗪類代謝物的釋放,誘導(dǎo)增強PGPR 的趨化作用[91]。雖然AMF 和PGPR 在調(diào)控植物基因表達方面已取得了一定的研究進展,但AMF 及PGPR 對植物體內(nèi)眾多基因表達過程的調(diào)控機制尚不明確。
4) 在AMF 和PGPR 直接互作的界面機制方面,AMF 和PGPR 在植物根際微域內(nèi)發(fā)生互作[92]。在與植物漫長的協(xié)同進化過程中,AMF 失去了腐生功能,無法通過分泌胞外酶來直接活化土壤中復(fù)雜的有機化合物,難以利用土壤有機態(tài)的養(yǎng)分[93?94]。然而AMF 的菌絲可以作為土壤細菌的棲息場所,通過釋放菌絲分泌物吸引一些假單胞菌和芽孢桿菌在菌絲際定殖,這些細菌可產(chǎn)生有機酸和分泌磷酸酶等活化土壤中的磷,這些微生物同時可以通過釋放揮發(fā)性物質(zhì)來調(diào)控AMF 孢子的萌發(fā),促進菌絲生長和菌根定殖,增強養(yǎng)分活化,達到抗鹽促生的目的[95]。
鹽脅迫下二者互作過程中,AMF 菌絲可能發(fā)揮了“高速公路”的作用,AMF 的根外菌絲可以將溶磷菌運輸至富含有機磷的斑塊[83]。同樣的,AMF 可使PGPR 附著于菌絲表面并沿其游動至鹽分較低/生態(tài)位適宜的區(qū)域;或者將植物地上部光合作用獲取的碳源,通過海藻糖、果糖等菌絲分泌物的方式釋放到土壤中,驅(qū)動PGPR 沿著菌絲趨化運動,最終定殖于菌絲表面。這個過程可能是復(fù)雜的化學(xué)和分子水平的互作。例如,菌絲可以將PGPR 分解的養(yǎng)分(如氮、磷) 運輸至植物根細胞,并且菌絲介導(dǎo)的細菌運動可以促進細菌細胞間的水平基因轉(zhuǎn)移,從而推動PGPR 的快速進化,以適應(yīng)脅迫環(huán)境并發(fā)揮功能,從而間接協(xié)助植物耐鹽[96?98]。反過來,細菌對AMF 的作用機制還不清楚,例如PGPR 能否通過代謝物刺激菌絲細胞壁代謝或細胞膜形成,促使菌絲固定土壤溶液中的鹽基離子仍未可知[99?100]。
這些研究結(jié)果(表1) 表明,PGPR 和AMF 雙接種能夠顯著降低鹽分對幼苗的不利影響,為提升植物抗鹽脅迫能力奠定基礎(chǔ),然而,植物生長受到土壤條件、真菌和細菌群落的多重調(diào)節(jié),其影響因素還取決于植物與微生物之間的功能相容性以及AMF和PGPR 的特定組合效果。
4 結(jié)論與展望
植物?微生物以及微生物?微生物的互作關(guān)系一直是農(nóng)業(yè)生命科學(xué)領(lǐng)域的研究熱點。綜上所述,AMF與PGPR 聯(lián)合提高植物耐鹽堿能力的研究還剛起步,未來還需加強的研究方向包括:
1) 鹽堿地中AMF 與PGPR 在多重脅迫效應(yīng)下的互作機制研究。鹽堿地存在高鹽和高堿等類型,有機質(zhì)含量低,土壤結(jié)構(gòu)差,同時存在多種脅迫因素。目前有關(guān)在鹽堿地土壤AMF 和PGPR 互作的研究主要集中在鹽脅迫方面,對于其他脅迫,如Na2SO4鹽脅迫、NaHCO3 和Na2CO3 堿脅迫以及鹽堿混合脅迫的報道相對較少。因此,未來研究需要綜合考慮多種脅迫條件下微生物互作的效果和機制。
2) 探索植物?微生物互作過程中的關(guān)鍵信號通路和挖掘植物主效耐鹽堿基因。其中,組學(xué)技術(shù)如轉(zhuǎn)錄組學(xué)、宏蛋白質(zhì)組學(xué)、代謝組學(xué)有利于深入挖掘微生物間的物質(zhì)傳遞和信號交流;利用CRISPR/Cas9 基因編輯技術(shù)和基因沉默技術(shù)等,研究AMF和PGPR 的關(guān)鍵耐鹽基因的具體功效。理想的耐鹽作物需要在保持抗逆能力的同時穩(wěn)定產(chǎn)量,但目前僅依靠分子技術(shù)難以優(yōu)化作物的生長?抗逆權(quán)衡,可以通過分子設(shè)計育種增強作物抗性,然后利用AMF與PGPR 改善作物生長環(huán)境,可能是一個重要的突破口。
3) 鹽堿脅迫條件下AMF 與菌絲際細菌互作在作物生長中的應(yīng)用潛力。AMF 龐大的根外菌絲網(wǎng)絡(luò)可以與土壤中功能細菌形成互惠關(guān)系,菌絲特異性招募并構(gòu)建菌絲際核心微生物組,彌補AMF 缺失的關(guān)鍵功能基因。然而,目前關(guān)于AMF 菌絲際的研究大多集中在養(yǎng)分吸收方面,對AMF?菌絲際細菌互作幫助植物抵御非生物脅迫的研究偏少。未來應(yīng)利用現(xiàn)代分子生態(tài)學(xué)的研究手段,監(jiān)測鹽堿脅迫下根際微生物組及菌絲際細菌的動態(tài)變化,并結(jié)合合成生物學(xué)的方法構(gòu)建耐鹽堿合成菌群,建立菌絲際核心微生物組、AMF 和植物表型之間的互惠關(guān)系,以改良作物根際微環(huán)境,促進作物在鹽堿土壤環(huán)境下的養(yǎng)分獲取、水分吸收,最終提高作物產(chǎn)量。
4) 構(gòu)建功能互補型的合成菌群并研發(fā)相關(guān)微生物菌劑。目前,鹽堿地物理和化學(xué)改良技術(shù)已有較多的研究,但鹽堿地理化修復(fù)技術(shù)耦合微生物產(chǎn)品,系統(tǒng)提升鹽堿地產(chǎn)能的相關(guān)研究較少,且大多是在受控的生長室或溫室條件下進行的。當(dāng)前合成菌群已被廣泛應(yīng)用于環(huán)境治理、人體健康和工業(yè)生產(chǎn)領(lǐng)域,未來需要構(gòu)建并利用AMF 和PGPR 的合成菌群研發(fā)抗鹽促生的微生物菌劑,結(jié)合種子包衣/顆粒技術(shù),配以合理的農(nóng)學(xué)管理措施,系統(tǒng)解決耐鹽堿合成微生物定殖及功能高效穩(wěn)定問題。
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作者簡介:
秦敬澤,中國農(nóng)業(yè)大學(xué)資源與環(huán)境學(xué)院博士研究生,主要研究領(lǐng)域為鹽堿地土壤改良與菌根生理生態(tài)。
張俊伶,中國農(nóng)業(yè)大學(xué)資源與環(huán)境學(xué)院教授,博士生導(dǎo)師。主要從事土壤健康和根際微生物研究方向的工作。在土壤健康和資源可持續(xù)利用、微生物多樣性和生態(tài)功能、土壤生物肥力以及生物肥料等方面也開展了大量的研究工作。主持國家自然科學(xué)基金、國家重點研發(fā)項目課題,參加國家自然科學(xué)基金委創(chuàng)新群體和重點項目、科技部973 項目、農(nóng)業(yè)部公益性行業(yè)項目、參加科技部國家重點研發(fā)計劃中澳可持續(xù)農(nóng)業(yè)管理項目、中荷農(nóng)業(yè)綠色發(fā)展項目等。相關(guān)研究成果發(fā)表在Nature Communications、Microbiome、Global Change Biology、EcologyLetters、Environmental Microbiology、New Phytologist 等期刊上。任中國土壤學(xué)會土壤健康工作組組長、中國菌物學(xué)會內(nèi)生菌和菌根真菌專業(yè)委員會主任、中國植物營養(yǎng)與肥料學(xué)會第十屆教育工作委員會副主任等。
基金項目:國家自然科學(xué)基金聯(lián)合基金項目(U23A201464)。