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(1.中國科學(xué)院 東北地理與農(nóng)業(yè)生態(tài)研究所 黑土區(qū)農(nóng)業(yè)生態(tài)重點實驗室,吉林 長春130102;2.中國科學(xué)院大學(xué),北京 100049)
馴化是指經(jīng)過人為干預(yù)技術(shù)對野生物種某些可利用的性狀進行選擇、修飾和利用[1],動物、植物甚至微生物的馴化過程為人類的文明帶來了巨大的貢獻。在距今約7 000年前,人類就從野生植物中不斷地尋求利用和馴化可食用的經(jīng)濟作物,在漫長的作物馴化過程中,作物馴化不僅增加了籽/果實的產(chǎn)量,改變了作物的根系形態(tài)或次級代謝產(chǎn)物[2],還引起宿主作物根際微生物的協(xié)同響應(yīng)[3],但人為選擇技術(shù)使栽培種逐漸丟失其野生種的部分生理和遺傳特性,進而導(dǎo)致栽培作物的遺傳多樣性顯著降低[4]。
根際(Rhizosphere)是德國微生物學(xué)家在1904年第一次提出的,它是土壤與植物間物質(zhì)交換的活躍界面[5]。作物根際土壤的物理、化學(xué)和生物學(xué)特性與周圍土壤存在著顯著差異[6];同時,從遺傳學(xué)角度而言,植物根系表面棲息的微生物數(shù)量要遠高于植物體,根際微生物基因的數(shù)量也遠遠超過了植物基因的數(shù)量[7]。目前,根際微生物對作物的生長、發(fā)育和抗病的作用機制研究已有很多的報道,尤其在分子生態(tài)學(xué)方面都取得了重要的成果[8-10]。然而,人們過多的是關(guān)注馴化后作物地上部產(chǎn)量及表型的變化,卻忽略了馴化對地下微生物的影響以及根際微生物對作物的反饋作用。為此,本文綜述了近年來作物馴化所導(dǎo)致的作物表型和根際微生物的變化,并在此基礎(chǔ)上預(yù)測未來發(fā)展趨勢,以期從根際微生物角度加大對馴化的利用程度。
作物在馴化過程中會引起形態(tài)學(xué)和生理學(xué)等一系列變化,這種變化充分滿足了人們對栽培作物的喜愛和需求[11]。近年來,人們針對馴化引起的宿主作物表型變化的機制,運用生化、分子等手段對宿主作物的生長習(xí)性[12]、葉型[13]、花期[14-15]、豆莢開裂程度[16]及果實大小[17]等進行了大量深入的研究,加深了馴化對作物表型變化的內(nèi)在機理認識,特別是一些有關(guān)植株矮化、花期等同源基因的克隆鑒定和分析,在很大程度上揭示了馴化對作物的表型變化。此外,相比于人工馴化后的栽培作物,野生作物具有廣泛的生境范圍,可以適應(yīng)各種極端惡劣的生長環(huán)境。與此同時,在長期的自然選擇下,野生作物能夠進化出豐富的變異類型和良好的環(huán)境適應(yīng)性,也說明了野生作物對非生物和生物脅迫都具有較強的防御能力。具體而言,如在一定程度的鹽脅迫條件下,野生大豆能夠通過大豆異黃酮次生代謝途徑來削弱鹽脅迫的損傷,而栽培大豆卻喪失了這個能力[18];同樣地,野生大豆對病毒的抗性與其葉片蠟質(zhì)含量呈正相關(guān)關(guān)系[19]。
同樣地,馴化也會引起作物地下根系形態(tài)的變化。研究發(fā)現(xiàn),根構(gòu)型對作物的養(yǎng)分獲取具有決定性作用,在特定的環(huán)境下,根構(gòu)型的改變能夠影響作物的根系吸收更多的礦質(zhì)營養(yǎng)和水分及抵抗逆境脅迫能力[20]。在野生和栽培作物馴化過程中,根構(gòu)型變化能夠促進作物吸收更多的營養(yǎng),該變化得到了大量生理生化試驗結(jié)論的支持,如馴化導(dǎo)致栽培種的根系變淺,側(cè)根數(shù)增加[21],降低野生植物的根/莖比和細根數(shù)[22]。特別是近期QTL等位基因的研究也證實了野生和栽培大麥根構(gòu)型發(fā)生了變化[23]。除了促進作物根系吸收營養(yǎng)外,根構(gòu)型的改變能使野生種更好的適應(yīng)逆境條件,如野生種根冠比的降低[24],根長、根干重、根體積和分蘗數(shù)的增加等[23,25]。此外,馴化引起根構(gòu)型的變化還會受到作物自身遺傳特性和土壤微生物等生物因子的影響[26-27]。
作物與土壤微生物之間的作用是相互的,作物將光合作用固定的碳以根系分泌物或脫落物的形式釋放到土壤中,為土壤微生物提供充足的能量和基質(zhì)[28],反過來,土壤微生物群落也對植物生長產(chǎn)生一定的反饋作用,進而影響作物的生長和適應(yīng)性[29]。栽培作物的生存環(huán)境,作物生理生化特征或者自身遺傳特征的改變都會使作物根際微生物發(fā)生強烈的變化[30-31],同時進一步影響栽培作物根際有益微生物的富集。農(nóng)業(yè)生態(tài)系統(tǒng)中,某些有益細菌或真菌(如根瘤菌和菌根真菌)的聚集不僅能增加作物的產(chǎn)量,促進植物對營養(yǎng)元素的吸收,還能增強對逆境環(huán)境和病原菌的入侵抵抗能力[8-9]。
土壤中的大量微生物以細菌為主,但土壤中的細菌和真菌群落組成會因作物的不同差異很大[32-33]。在長期的馴化篩選過程中,常常會導(dǎo)致栽培作物的根際細菌群落多樣性降低,同時,顯著地影響根際真菌群落結(jié)構(gòu)和多樣性[33]。例如,Germida和Siciliano的研究指出,野生小麥(Triticumspp.)根際微生物群落豐度要高于其栽培作物[34];通過T-RFLP和SSCP技術(shù)也進一步證實了野生玉米和甜菜(Betavulgarisspp.maritime)根際細菌群落多樣性顯著高于其栽培作物[21,35]。對于根際真菌群落多樣性而言,能夠明確指出作物的基因型對根際真菌群落產(chǎn)生影響的研究較少[36],但Cassman等人認為植物根際真菌群落結(jié)構(gòu)的影響比細菌表現(xiàn)得更加敏感[37]。隨后, Leff等人對野生和栽培向日葵根際細菌和真菌群落多樣性的分析,也進一步證實了該結(jié)論[33]。雖然大量研究表明,作物馴化會影響作物根際微生物的種類和組分,但缺少對微生物群落的功能分析,這將成為今后的重點。
其次,長期的作物馴化過程會使某些菌屬的作用和功能逐漸丟失,或是被其它的菌屬代替。例如,在東鄉(xiāng)野生水稻(OryzarufipogonGriff.)的根際中,泛菌屬是根際優(yōu)勢菌屬之一,而栽培水稻根際常見的是固氮螺菌屬(Azospirillum)和氣單胞菌屬(Aeromonas)[33-39]。隨后,Shenton等人通過測序技術(shù)也進一步確定,馴化能夠顯著地影響作物根際微生物群落組成[3]。此外,馴化通過降低作物根際病原菌或增加菌根真菌的共生關(guān)系,引起了作物的根際真菌群落結(jié)構(gòu)和多樣性的變化,但在確定是哪些病原菌的降低或有益微生物的增加方面還未得出最終結(jié)論[40-41]。
除此之外,作物基因型的不同也會進一步影響根際微生物群落結(jié)構(gòu)和多樣性[42-43];Weinert通過對比研究3個不同栽培種的馬鈴薯根際微生物也證實了這一點[44]。馴化導(dǎo)致野生和栽培作物遺傳距離的改變可能是影響作物根際微生物群落結(jié)構(gòu)的重要因素,但是,Bounffaud[45]和Shenton[3]得到的結(jié)論卻是相反的,這也需要在今后的研究中進一步驗證。
菌根真菌和根瘤菌是未來農(nóng)業(yè)發(fā)展中最重要的兩類微生物。菌根(Mycorrhizae)是由土壤真菌與植物根系形成一種互惠共生體(Mutualistic symbiont),叢枝菌根真菌(Arbuscular mycorrhizal fungi,AMF)是一類最典型的菌根真菌,它能與80%的陸生微管植物形成共生菌根[46-47]。根瘤菌能夠與豆科作物形成高度專化作用,是一類具有互惠共生關(guān)系的共生微生物。反過來,宿主作物的種類和基因型對菌根真菌和根瘤菌都同樣具有一定的選擇性,這也會進一步影響共生微生物對宿主作物的侵染能力和共生關(guān)系。
2.2.1 作物馴化與菌根真菌的關(guān)系。菌根在物質(zhì)循環(huán)和能量流動方面具有不可替代的作用[48]。菌根真菌與宿主作物具有營養(yǎng)交換的互惠共生關(guān)系。首先,菌根真菌自身不能進行光合作用,而是依賴宿主作物的光合產(chǎn)物為菌根真菌提供充足的碳源;同時,菌根真菌的外生菌絲將吸收的氮、磷等礦質(zhì)營養(yǎng)和水分輸送給宿主作物作為回報[46,49]。其次,這種營養(yǎng)交換的互惠共生關(guān)系,一方面促進了菌根真菌外生菌根的伸長,另一方面也促進了宿主作物根系對土壤中礦質(zhì)營養(yǎng)的吸收和利用[50]。通過對比野生和栽培玉米的研究發(fā)現(xiàn),未馴化的玉米菌根侵染程度高于栽培種,且根尖的含磷量高[51]。
菌根真菌對宿主作物仍然具有一定的偏好性,菌根真菌的分布受宿主作物的種類和基因型的影響[52-53]。研究表明,在長期作物馴化過程中,過多的人為干預(yù)會使栽培作物與菌根真菌的共生能力低于與野生作物的[30],這種差異可能是由宿主作物自身基因型決定的,如不同基因型的野生和栽培小麥造成了菌根侵染程度的差異, Hetrick[54]等人也進一步證實六倍體野生小麥的菌根真菌侵染程度要高于栽培種。
目前,菌根真菌與野生和栽培作物間的侵染程度也是學(xué)者關(guān)注的主要問題之一,但卻沒有得到一致的結(jié)論。Koide等人及Bryla和Koide通過研究發(fā)現(xiàn),野生燕麥和番茄的菌根真菌侵染程度要低于其栽培種[55-56],同樣地,與栽培種面包樹(Artocarpusaltilis)相比,野生種面包樹的根際菌根真菌侵染率明顯低。然而,近些年的研究結(jié)論卻與之前的相反,Hetrick等人通過對比野生和栽培小麥發(fā)現(xiàn),野生小麥的菌根真菌侵染要高于栽培種[54]。隨著對菌根真菌與作物間共生關(guān)系的深入分析,Lehmann等人發(fā)現(xiàn)即使菌根真菌侵染野生宿主作物的強度高于栽培種,但栽培種也擁有更強烈的菌根響應(yīng)能力(Mycorrhiza-responsive)[57]。此外,Sangabriel-Conde等人通過對比當(dāng)?shù)匚瘩Z化玉米和雜交種,也進一步證實了該結(jié)論[58]。雖然有關(guān)宿主作物與菌根真菌的侵染程度的研究結(jié)果存在分歧,但宿主作物與菌根真菌仍然有著密切的聯(lián)系。
2.2.2 作物馴化與根瘤菌的關(guān)系。根瘤菌(Rhizobium)是一類與豆科作物形成高度專化的共生細菌,豆科作物把光合作用固定的碳水化合物傳遞給根瘤菌,為根瘤菌提供充足的能量;根瘤菌能夠把空氣中分子態(tài)氮轉(zhuǎn)化成為能夠被豆科作物吸收利用形態(tài)的氮,為豆科作物提供大量的礦質(zhì)營養(yǎng)[59]。從根瘤菌識別豆科作物分泌信號開始,就已強烈地受到豆科作物的控制[60]。長期的作物馴化同樣會影響根瘤菌與豆科作物的共生關(guān)系,與栽培種相比,野生豆科作物與根瘤菌的共生關(guān)系的改變主要體現(xiàn)在增加根瘤菌群落多樣性或者是增加根瘤菌的數(shù)量,如Mutch和Young[61]在研究豌豆(Pisumsativum)、蠶豆(Viciafaba)等野生豆科作物與根瘤菌共生關(guān)系時,發(fā)現(xiàn)這些野生豆科作物能夠擁有更豐富的根瘤菌群落多樣性;Kim等人[62]通過對比野生和栽培鷹嘴豆的研究也得到了相同的結(jié)論。
長期的馴化導(dǎo)致豆科作物與根瘤菌共生關(guān)系發(fā)生變化,栽培種的根際根瘤菌多樣性的降低主要有兩方面的原因,一是通過減少一些無效的共生關(guān)系,投入更多的資源到其他共生關(guān)系上[63];二是由于長期的氮肥施用導(dǎo)致了作物與根瘤菌共生關(guān)系的降低[64]。野生和栽培豆科作物與根瘤菌的互惠共生關(guān)系,在長期的馴化過程中如何提高宿主作物的固氮效率需要我們進一步研究。
作物馴化導(dǎo)致了根際微生物的變化,并引起根際一部分有益微生物逐漸丟失,這主要是由于生態(tài)環(huán)境因子、作物本身的生理生化和遺傳信息的發(fā)生變化所導(dǎo)致。
為探索如何把馴化帶來的影響應(yīng)用到生產(chǎn)實踐,并發(fā)揮根際微生物組的促生效益,未來應(yīng)加強以下3方面的研究力度。
(1)微生物多樣性、群落結(jié)構(gòu)及功能的變化;
(2)微生物與植物營養(yǎng)傳遞和伴生效益改變的關(guān)系;
(3)微生物和植物之間選擇和被選擇趨勢加強的關(guān)系。
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