海草抗氧化系統(tǒng)及其對逆境脅迫的響應特征

盧科宇, 裴炎沼, 何 晴, 周 斌

海草抗氧化系統(tǒng)及其對逆境脅迫的響應特征

盧科宇1, 2, 裴炎沼1, 2, 何 晴1, 2, 周 斌1, 2

(1. 中國海洋大學海洋生命學院, 山東 青島 266003; 2. 青島海洋科學與技術(shù)試點國家實驗室, 山東 青島 266237)

海草具有獨特的進化地位和重要的生態(tài)價值, 廣泛分布于潮間帶和潮下帶淺海海域, 易遭受多種環(huán)境因子變化的威脅, 抗氧化系統(tǒng)在海草抵御逆境脅迫的過程中具有非常重要的作用。本文綜述了海草抗氧化系統(tǒng)的組成、特征及其對主要逆境脅迫的響應特征研究進展, 闡述了海草主要酶促抗氧化機制, 并將北半球代表性海草物種——鰻草抗氧化系統(tǒng)相關(guān)酶基因歸類分析。目前對于海草逆境脅迫的研究主要集中于單一脅迫下主要抗氧化酶(如SOD、CAT、GST)及其轉(zhuǎn)錄組的變化, 對于多脅迫因子協(xié)同作用和非酶抗氧化物及其他抗氧化酶的響應特征研究較少; 另外對于逆境脅迫下不同海草種類間抗氧化系統(tǒng)和關(guān)鍵基因的響應差異也有待深入研究。

海草; 逆境脅迫; 抗氧化酶; 抗氧化系統(tǒng)

海草是地球上唯一能夠完全在海水中生活的被子植物, 起源于陸生高等植物, 經(jīng)演化而適應海洋環(huán)境[1-3], 具有重要的進化地位[4]。海草經(jīng)歷了海洋-陸地-海洋的演化進程, 具有區(qū)別于陸生高等植物和大型海洋藻類的獨特生理特征。鰻草()和牟氏鰻草()全基因組測序結(jié)果揭示了海草基因組具有區(qū)別于陸生高等植物的諸多特性, 如細胞壁組分基因變化、氣孔基因缺失、萜類物質(zhì)合成與乙烯信號轉(zhuǎn)導基因缺失、紫外線防護與遠紅外線感受基因缺失、抗氧化系統(tǒng)基因收縮等, 提示海草具有不同于陸生高等植物的特殊逆境響應機制[5-6]。

植物抗氧化系統(tǒng)兼具響應逆境脅迫和調(diào)控活性氧信號傳導的雙重功能, 對其進行研究有助于全面理解植物的逆境響應過程和機制。綠色植物進行有氧代謝時產(chǎn)生的副產(chǎn)物活性氧(reactive oxygen species, ROS), 包括超氧陰離子(O2·–)、過氧化氫(H2O2)、單態(tài)氧(1O2)和羥基自由基(·OH), 能夠?qū)χ参矬w產(chǎn)生氧化損傷[7-9], 而植物也進化出了精細的抗氧化機制清除活性氧[9-10]。正常情況下, 植物體內(nèi)的抗氧化系統(tǒng)能夠及時清除過量ROS[9-11]; 在逆境脅迫下, 植物體內(nèi)ROS生成速度加快, 抗氧化系統(tǒng)清除能力相對不足, ROS的動態(tài)平衡被打破而導致累積。累積的ROS在造成氧化應激和產(chǎn)生氧化傷害的同時, 也參與多種信息傳遞過程, 激活植物抗逆響應機制[9-17]。對于不同的逆境脅迫, 植物生成ROS的位點與種類存在差異, 而抗氧化系統(tǒng)本身的響應方式、程度和組分也有所區(qū)別[7, 9, 15]。

近年來, 對陸生高等植物和藻類抗氧化系統(tǒng)及其逆境響應已經(jīng)開展了較多研究[8, 18-19], 并取得了系統(tǒng)性的成果[20-24]。與陸生高等植物相比, 海草由陸地到海洋的進化歷程使其在適應過程中產(chǎn)生了獨特的生理特點和抗逆機制。同時, 海草能夠提供眾多生態(tài)系統(tǒng)服務, 具有重要的生態(tài)價值[25-29]。其獨特性與重要性促使人們對海草這一重要植物類群的抗氧化系統(tǒng)及其逆境響應機制進行深入研究。

1 海草抗氧化系統(tǒng)的組成及其基因分類

海草抗氧化系統(tǒng)包括酶促和非酶促清除系統(tǒng)[30], 主要包括抗氧化酶類、非酶抗氧化物以及由二者共同組成的一些循環(huán)系統(tǒng)如: 抗壞血酸-谷胱甘肽循環(huán)、谷胱甘肽過氧化物酶循環(huán)、過氧化氫酶循環(huán)以及硫氧還蛋白還原酶系統(tǒng)和甲硫氨酸亞砜還原酶系統(tǒng)等[10, 30](圖1)。主要抗氧化酶有超氧化物歧化酶(superoxide dismutase, SOD)、過氧化氫酶(catalase, CAT)、抗壞血酸過氧化物酶(ascorbate peroxidase, APX)和谷胱甘肽過氧化物酶(glutathione peroxidase, GPX)等。非酶抗氧化物包括-胡蘿卜素、維生素A、-生育酚(維生素E)、抗壞血酸(維生素C)、谷胱甘肽、黃酮類化合物, 以及某些滲透調(diào)節(jié)物質(zhì), 如脯氨酸、甘露醇等[31-32]。目前對于海草抗氧化系統(tǒng)的研究多集中于主要酶促反應, 對于上游轉(zhuǎn)錄因子和基因的研究較少。

圖1 海草抗氧化系統(tǒng)組成圖[7-10, 13, 24, 30, 33-37]

注: O2·–: 超氧陰離子; H2O2: 過氧化氫;1O2: 單態(tài)氧; ·OH: 羥基自由基; SOD: 超氧化物歧化酶; APX: 抗壞血酸過氧化物酶; GPX: 谷胱甘肽過氧化物酶; CAT: 過氧化氫酶; AsA, s: 抗壞血酸; GSH: 谷胱甘肽; (M)DHA: (單)脫氫抗壞血酸; GSSG: 氧化型谷胱甘肽; (M)DHAR: (單)脫氫抗壞血酸還原酶; GR: 谷胱甘肽還原酶; PUFA: 多不飽和脂肪酸; Met: 甲硫氨酸; MetO: 甲硫氨酸亞砜; MSR: 甲硫氨酸亞砜還原酶; 2-Cys Prx: 2-半胱氨酸過氧化物還蛋白; NTRC: NADPH依賴的硫氧還蛋白還原酶C; FTR: 鐵氧還蛋白依賴的硫氧還蛋白還原酶。亮色箭頭代表氧化過程, 相應暗色箭頭代表還原過程, 紅色方框表示ROS, 紅色箭頭表示ROS的清除路徑; AsA和GSH是參與酶促抗氧化系統(tǒng)的兩種重要非酶抗氧化物。

SOD、CAT、APX和GPX等關(guān)鍵酶是海草體內(nèi)應對逆境脅迫的重要防衛(wèi)系統(tǒng)(圖1)。SOD作為抗氧化系統(tǒng)中的第一道防線, 將難以直接清除的O2·–歧化為H2O2, CAT、APX和GPX則將H2O2轉(zhuǎn)化為H2O(圖1)[7, 9, 33-37]。其中APX通過催化抗壞血酸(ascorbic acid, AsA)與H2O2的反應參與到抗壞血酸-谷胱甘肽循環(huán)中, 其氧化產(chǎn)物單脫氫抗壞血酸(monodehydroascorbic acid, MDHA)與脫氫抗壞血酸(dehydroascorbic acid, DHA)能夠通過相應的單脫氫抗壞血酸還原酶(MDHAR)和脫氫抗壞血酸還原酶(DHAR)還原為AsA, 從而維持這一重要非酶抗氧化物的濃度[7, 9, 13, 35]。而另一種重要的非酶抗氧化物谷胱甘肽(GSH)作為還原劑參加了DHA的還原反應。類似地, GPX催化GSH清除H2O2產(chǎn)生的氧化態(tài)谷胱甘肽(GSSG)也能通過谷胱甘肽還原酶(GR)再生GSH。這兩種主要非酶抗氧化物除了通過酶促反應清除H2O2外, 還能夠參與清除其他三種ROS[7-10, 24, 38-39]。重要的抗氧化酶還包括谷胱甘肽S-轉(zhuǎn)移酶(glutathione S-transferase, GST)和過氧化物酶(peroxidase, POD)等[34, 37, 40-42]。

另外, 硫氧還蛋白/硫氧還蛋白還原酶系統(tǒng)和甲硫氨酸亞砜/甲硫氨酸亞砜還原酶系統(tǒng)也是植物體內(nèi)應對氧化脅迫的重要途徑(圖1)[43]。在逆境脅迫下產(chǎn)生的過量ROS除了對肽主鏈造成一般的氧化損傷外, 一些含硫氨基酸側(cè)鏈的功能和結(jié)構(gòu)也可能會在氧化應激過程中被特定的修飾所改變。如甲硫氨酸(Met)和半胱氨酸(Cys)其側(cè)鏈中含有一個硫原子, 是最容易被氧化的氨基酸[44]。Met和Cys的氧化損傷可分別由甲硫氨酸亞砜還原酶(methionine sulfoxide reductase, MSR)系統(tǒng)和硫氧還蛋白還原酶(thioredoxin reductase, TrxR)系統(tǒng)修復, ROS則在蛋白質(zhì)修復的過程中被清除。其中Met被過量ROS和H2O2氧化形成的甲硫氨酸亞砜(MetO)可以通過MSR還原為Met[45-47]。因此MetO的形成是氧化損傷的標志之一, 該損傷可由MSRs控制或逆轉(zhuǎn)[44]; 而TrxR系統(tǒng)是由一系列蛋白質(zhì)組成: 硫氧還蛋白(Trx)、過氧化物氧還蛋白(Prx)和硫氧還蛋白還原酶(TrxR)。這些蛋白與還原性煙酰胺腺嘌呤二核苷酸磷酸(NADPH)相互作用, 作為輔因子清除H2O2或其他ROS[48-49]。

單態(tài)氧和羥基自由基的清除主要依靠非酶抗氧化物(圖1)。除GSH和AsA外, 還包括類胡蘿卜素、生育酚、黃酮類化合物和脯氨酸等。這些非酶抗氧化物都能夠清除單態(tài)氧, 多酚類化合物和脯氨酸還能夠參與清除羥基自由基[7-10, 14, 50-51]。其中生育酚還能參與清除超氧化物, 并通過GSH和AsA的作用再生, 表明抗氧化物之間存在協(xié)作關(guān)系[7-8, 10, 13, 24, 52]。由于抗氧化系統(tǒng)的各個組分能夠清除的活性氧種類存在差異, 而且存在的位點也有區(qū)別(例如生育酚、類胡蘿卜素和CAT的主要存在位點分別在細胞膜、葉綠體和過氧化物酶體), 所以除了濃度以外, 各組分之間比例的變化也將影響活性氧的清除效果以及海草整體上對逆境脅迫的響應[9, 14, 53]?？傮w而言, 抗氧化物基因的過表達能夠提高抗逆性, 且多種抗氧化酶基因同時過表達能夠起到相互協(xié)同的作用[10]。

控制海草抗氧化系統(tǒng)關(guān)鍵酶和非酶抗氧化物基因的上游核轉(zhuǎn)錄因子調(diào)控過程也是氧化應激的重要控制環(huán)節(jié), 影響抗氧化系統(tǒng)相關(guān)基因的轉(zhuǎn)錄。在正常生理狀態(tài)下這些轉(zhuǎn)錄因子在胞質(zhì)內(nèi)與各自的抑制蛋白結(jié)合為復合體而呈現(xiàn)非活性狀態(tài)。當受到脅迫時, 復合體會被氧化劑等物質(zhì)激活而解離, 轉(zhuǎn)錄因子被轉(zhuǎn)移入核內(nèi)誘導含有特異啟動子(如抗氧化元件antioxidant response element, ARE)的基因轉(zhuǎn)錄, 調(diào)控氧化應激。這些信號通路主要有Nrf2-Keap1信號通路、NF-кB信號通路等, 但在海草等水生高等植物中研究較少[54-61]。

其中核因子E2相關(guān)因子2(Nrf2)是抗氧化系統(tǒng)的關(guān)鍵調(diào)節(jié)因子。Nrf2-Keap1信號通路主要由轉(zhuǎn)錄因子NF-E2相關(guān)因子2(Nrf2)及其伴侶分子Keap1組成。Nrf2/Keap1系統(tǒng)通過與抗氧化反應元件(ARE)相互作用, 調(diào)控一系列解毒酶和抗氧化酶基因的表達來維持機體氧化還原的平衡狀態(tài)[54]。例如Nrf2控制谷胱甘肽生物合成的限速步驟也控制谷胱甘肽過氧化物酶2和還原酶1的表達, 從而影響抗壞血酸-谷胱甘肽循環(huán)[62]; Nrf2也能影響控制胞質(zhì)硫氧還蛋白1(TXN1)[63]、硫氧還蛋白還原酶1(TXNRD1)[64-67]和硫氧還蛋白1(SRXN1)[68]的表達, 并且調(diào)控體內(nèi)產(chǎn)生NADPH的各個途徑, 從而對抗氧化系統(tǒng)進行調(diào)控[69-72]。在非應激條件下, Keap1是E3泛素連接酶復合物的氧化還原調(diào)控底物接頭, 其結(jié)合Nrf2形成Nrf2-Keap1復合物, 從而抑制Nrf2的活性并持續(xù)引導NRF2降解以確保Nrf2以較低濃度存在, 并將Nrf2限制在細胞質(zhì)中[55-57]。當細胞暴露于氧化脅迫時, Keap1上的高活性半胱氨酸基團被過量ROS氧化使其構(gòu)象產(chǎn)生變化, Keap1被親電子分子修飾后, 阻止其靶向蛋白Nrf2的降解, 復合體釋放Nrf2并將其轉(zhuǎn)運到細胞核中, 從而使Nrf2在氧化脅迫下在細胞核中快速積累, 誘導含有抗氧化元件ARE的基因轉(zhuǎn)錄[58-60]。擁有ARE的基因編碼形成合作酶網(wǎng)絡(luò), Nrf2可通過轉(zhuǎn)錄網(wǎng)絡(luò)調(diào)控抗氧化系統(tǒng)以應對外界脅迫[61]。

目前在基因水平對海草抗氧化系統(tǒng)的研究相對較少。由于僅有鰻草和牟氏鰻草完成了全基因組測序, 因此本文以北半球代表性海草鰻草為代表, 根據(jù)鰻草全基因組測序結(jié)果等研究成果, 對鰻草抗氧化酶基因進行了分類注釋。根據(jù)抗氧化酶參與ROS清除的不同作用, 將相關(guān)酶分為3類進行注釋, 分別為: 直接清除ROS的抗氧化酶(表1)、參與清除ROS的蛋白和酶(表2)、催化抗氧化劑再生的酶(表3)[6]。

表1 鰻草直接清除ROS的抗氧化酶基因[6]

表2 鰻草參與清除ROS的蛋白和酶基因[6]

表3 鰻草催化抗氧化劑再生酶基因[6]

2 海草抗氧化系統(tǒng)對逆境脅迫的響應特征

2.1 對高溫脅迫的響應

全球變暖導致的高溫脅迫是海草面臨的主要環(huán)境問題之一。高溫脅迫能夠影響海草的光合作用與呼吸進程, 影響其生長和代謝平衡, 并導致海草體內(nèi)ROS過量生成和累積而引起氧化損傷[73-78]。具體表現(xiàn)為在高溫脅迫下海草的光合酶系統(tǒng)受到損傷, 最終呼吸作用會大于光合作用, 導致消耗的O2增加以及ROS的生成增加, 并且海水溫度升高可以直接抑制海草體內(nèi)抗氧化酶(如GST)的活性, 從而削弱抗氧化系統(tǒng)清除ROS的能力。因此, 過量的ROS在植物體內(nèi)累積, 能夠影響正常的生理過程, 造成氧化損傷[79-80]。鰻草、大洋波喜蕩草()、小絲粉草()、泰來草()等均通過增加抗氧化酶的含量或提高抗氧化酶的活性來應對高溫脅迫, 但參與響應的抗氧化酶種類、響應程度和速度等受到海草種類、生境、脅迫強度與持續(xù)時間等諸多因素的影響。大多數(shù)情況下, SOD作為抗氧化系統(tǒng)的第一道防線在清除ROS過程中具有重要作用, 比如高溫脅迫能夠誘導大洋波喜蕩草、小絲粉草、泰來草基因表達量顯著上調(diào)以應對ROS的累積, 避免氧化損傷[34, 74, 80]; 鰻草的基因會在非極端高溫誘導時上調(diào)表達, 并在一段時間內(nèi)保持其過表達狀態(tài), 同時保持較高的光合能力[81-84]。一定范圍內(nèi)的高溫脅迫能夠通過激活SOD活性而不是提高基因表達程度來提高活性氧清除能力, 而在極端高溫下(≥25 ℃)SOD酶活性急劇下降, 使抗氧化系統(tǒng)的高溫脅迫響應能力急劇下降[77, 83]。相對于其他抗氧化酶組分活性受到抑制而逐漸降低、自由基清除能力變?nèi)? SOD的重要性還體現(xiàn)在能夠在連續(xù)多次的高溫脅迫過程中始終保持有效響應[34, 77]。其他種類的抗氧化酶也在海草響應高溫脅迫中發(fā)揮重要作用, 比如高溫能夠誘導泰來草和諾氏鰻草和基因表達量增高[80, 85], 鰻草中這兩種酶的活性也有顯著升高[78, 84]。海草抗氧化系統(tǒng)的其他組分對高溫脅迫的響應相對于上述幾種主要抗氧化酶具有較高的可變性[34], 這種可變性與物種和生境的相關(guān)性較高, 總體而言高緯度物種和深水生態(tài)型的適應程度低, 其抗氧化系統(tǒng)響應速度慢、強度低, 除主要抗氧化酶外需要更多的抗氧化系統(tǒng)組分參與響應, 且高溫脅迫后的恢復較慢[34, 81, 86-87]。可見, 包括溫度升高在內(nèi)的氣候變化能夠影響海草抗氧化能力等生理過程, 進而改變海草的分布與種群結(jié)構(gòu)[76]; 而基于氣候馴化的生態(tài)適應對于海草應對高溫脅迫具有重要意義。

2.2 對重金屬脅迫的響應

重金屬是影響海草的主要脅迫因素之一[88-92]。重金屬的過量累積能夠?qū)２莓a(chǎn)生嚴重的生理傷害, 包括抑制生長、降低光合速率等, 同時也能誘導海草體內(nèi)ROS的過量生成而產(chǎn)生氧化損傷[93-100]?？寡趸到y(tǒng)對于重金屬的敏感性可能是由于重金屬離子能通過與抗氧化酶的巰基蛋白結(jié)合來破壞其結(jié)構(gòu)[94, 98-99], 或是在高濃度下造成嚴重的細胞損傷, 使與抗氧化系統(tǒng)相關(guān)的代謝循環(huán)受損而導致整個抗氧化系統(tǒng)的崩潰[94]。海草抗氧化系統(tǒng)對重金屬脅迫的響應特征與重金屬的種類和濃度有關(guān), 如抗氧化酶基因的表達程度與重金屬濃度在一定限度內(nèi)呈正相關(guān)[37, 40, 42]。海草對Cu2+脅迫非常敏感, 在短時間、低濃度的Cu2+脅迫下, 海草能夠上調(diào)抗氧化酶(GPX、CAT、SOD、GR、APX和GST)和抗氧化劑(GSH)的基因表達來清除額外生成的ROS; 而在長時間或高濃度的Cu2+脅迫下, 抗氧化系統(tǒng)組分的基因表達受到抑制, ROS得不到及時清除而導致氧化損傷[37, 41, 93-94, 100-101]。但是對牟氏鰻草的研究表明, 在低濃度Cu2+脅迫下并不響應的與基因隨著Cu2+濃度的升高而出現(xiàn)過表達[98]。

對于其他重金屬(Fe、Mn、Zn、Cd、Cr、Pb、Hg、Ni)而言, 即使在較高濃度下, 海草抗氧化系統(tǒng)的相關(guān)組分也能通過基因上調(diào)表達來提高ROS清除能力[41, 46]。其中, 海草對Cd2+的耐受能力最強, 例如大洋波喜蕩草只有受到高濃度Cd2+脅迫時才需要提高GST的活性[102]; 鰻草在Cu2+脅迫下會增加GSH的生成量來協(xié)助清除ROS, 而即使暴露于高濃度的Cd2+脅迫下,基因的表達也沒有顯著升高[93]。Greco等認為Cu2+與Cd2+在海草體內(nèi)的累積是競爭性的, 因此低濃度Cd2+的存在甚至可能會減輕Cu2+對海草的氧化損傷[93]。此外, 乙酰膽堿酯酶(AchE)對重金屬也具有高敏感性, 因而被認為通過基因上調(diào)表達參與了對重金屬脅迫的響應, 并與SOD共同組成第一道防線[42]。

對于納米金屬顆粒脅迫的響應研究主要集中于納米銀微粒, 實驗證明其在極低含量下也能導致海草體內(nèi)ROS過量生成, 而不同種類的海草對其響應程度存在差異。小絲粉草能夠通過提高SOD和APX等主要抗氧化酶的活性, 有效防止氧化損傷[103]; 而長萼喜鹽草()只能通過增加SOD的生成來應對氧化脅迫且APX的活性受到抑制, 因此并不能避免脂質(zhì)過氧化[104-105]。與之相似的, 氧化鋅微粒也能導致氧化應激, 引起海草抗氧化系統(tǒng)的響應變化, 但其作用機制尚不明確[106]。

在一定的重金屬濃度范圍內(nèi), 抗氧化系統(tǒng)的活性隨著重金屬濃度升高而提升, 但這并不一定代表其能夠有效地清除額外生成的ROS, 保護植物組織免受氧化損傷。例如, 大洋波喜蕩草隨著Hg2+脅迫強度的提高不斷增加GST和CAT的生成量, 而代表脂質(zhì)過氧化程度的丙二醛(MDA)含量也在增加, 其在高濃度Hg2+脅迫下通過植物螯合物才能有效降低氧化損傷的程度[44]。不同抗氧化酶對不同重金屬的響應程度也不同, 例如日本鰻草SOD的含量及活性在Cd2+脅迫下不發(fā)生明顯變化, CAT則對Pb2+脅迫無顯著響應[37]; 泰來草在受到Zn2+、Cu2+、Cd2+脅迫時,基因的表達受到顯著抑制, 而POD則一直保持過表達狀態(tài)[100]。迄今為止, 重金屬對海草抗氧化系統(tǒng)的影響研究仍限于單一重金屬對單一海草物種的脅迫和響應, 對多種重金屬協(xié)同效應的研究較少, 且普遍缺少脅迫的分子機制研究。

2.3 對光脅迫的響應

光是海草生產(chǎn)力、分布和豐度的重要決定因素。海草的光合速率低于陸生植物, 具有較低的光補償點與光飽和點, 可以保證在水下弱光環(huán)境中正常生長, 是海草對弱光環(huán)境的適應[107-109]。但在近岸海域中, 由于懸浮顆粒增加、附生植物或浮游藻類過度生長而導致的光散射和/或光衰減增加, 可能會進一步削弱海草的可利用光照, 導致海草光合產(chǎn)能不足, 從而影響海草生長繁育[110]。弱光脅迫對海草的影響可能涉及更多的光適應基因, 對抗氧化系統(tǒng)影響較小。長期弱光脅迫使大洋波喜蕩草光合作用碳反應關(guān)鍵調(diào)節(jié)酶Rubisco的基因表達下調(diào), 參與碳水化合物裂解的酶和蛋白水解酶表達上調(diào), 而抗氧化系統(tǒng)CAT、SOD和APX三種抗氧化酶的合成減少, 但未呈現(xiàn)受脅迫狀態(tài)[110-113]。

相比于光照不足, 海草對強光脅迫更加敏感, 而抗氧化系統(tǒng)作為光保護機制的重要部分, 對海草響應強光脅迫至關(guān)重要。生長在潮間帶和淺水環(huán)境中的海草經(jīng)常在一天中的部分時間暴露在過飽和光強下, 甚至直接暴露于陽光直曬之下, 這可能會導致強光脅迫。由于海草的光飽和點較低不能充分利用光能, 過剩的光能會導致ROS產(chǎn)生, 破壞海草體內(nèi)的代謝平衡, 產(chǎn)生光抑制[114]。在強光脅迫下, 海草的光適應相關(guān)基因(Rubisco酶、鐵氧還蛋白、葉綠素結(jié)合蛋白)和光保護相關(guān)基因(抗氧化酶、葉黃素循環(huán)相關(guān)基因、生育酚的生物合成)上調(diào), 表明了抗氧化系統(tǒng)作為防御機制被激活[115]。持續(xù)強光脅迫能夠激活牟氏鰻草和大洋波喜蕩草AsA-GSH循環(huán), 表現(xiàn)為APX、GPX、MDHAR等抗氧化酶和相關(guān)基因上調(diào)表達, 并顯著提高AsA和GSH的合成相關(guān)的酶與蛋白的表達量; 同時CAT、POX、GST、生育酚和類黃酮化合物的合成量增加[110, 115]。大洋波喜蕩草在強光脅迫下并沒有產(chǎn)生光損傷或氧化損傷, 說明抗氧化系統(tǒng)能夠起到有效的保護作用[110]。與之不同的是, 卵葉喜鹽草()與泰來草作為典型的熱帶海草, 在強光脅迫下一般不會顯著增加ROS生成量或抗氧化系統(tǒng)的活躍程度, 體現(xiàn)了其對熱帶強光環(huán)境的適應性[116]。深水生態(tài)型與淺水生態(tài)型大洋波喜蕩草之間存在的抗逆基因差異, 也說明其對于光脅迫具備足夠的適應能力[110]。在不同緯度及深度環(huán)境下, 不同生態(tài)型海草的抗氧化系統(tǒng)均對光脅迫表現(xiàn)出適應特征, 表明了海草抗氧化系統(tǒng)能夠應對光強變化產(chǎn)生的脅迫。

2.4 對其他環(huán)境因子脅迫的響應

除上述的主要逆境脅迫外, 海草抗氧化系統(tǒng)還會響應其他環(huán)境脅迫, 包括附生生物、高鹽度、海洋酸化、缺氧及硫化物等。

1)對附生生物脅迫的響應。有關(guān)海草抗氧化系統(tǒng)對附生生物響應的研究主要局限于大洋波喜蕩草。附生生物一般附著在海草葉片表面上, 能夠?qū)е鹿庹账p, 降低光合作用, 但是不會導致氧化脅迫。附生生物一般通過直接創(chuàng)傷導致海草體內(nèi)產(chǎn)生過量ROS, 引起海草體內(nèi)APX、GPX、CAT、SOD、DHAR和GSH等多種抗氧化組分活性或含量的增加, 抗氧化系統(tǒng)總體清除能力顯著提升; 但現(xiàn)有研究認為這種清除能力的提升不足以應對ROS的快速產(chǎn)生和過量累積, 導致氧化脅迫產(chǎn)生, 表現(xiàn)為MDA含量顯著增加[115, 117]。不同海草物種對附生生物的響應可能存在差異, 因此需要更多相關(guān)研究來完善認識。

2) 對鹽脅迫的響應。海草在高或低鹽度脅迫下均能激活其抗氧化系統(tǒng)的大部分組分, 包括非酶抗氧化物和與AsA-GSH循環(huán)相關(guān)的抗氧化酶, 然而由于SOD和CAT這兩種主要抗氧化酶的活性受到抑制, 海草還是會受到一定程度的氧化損傷[118-119]。在陸生植物中, 高鹽脅迫誘導的氧化應激會導致植物卡爾文循環(huán)消耗的NADPH和光合作用固定的CO2減少而影響ASA-GSH循環(huán), 電子可能從PSI轉(zhuǎn)移至O2而形成O2–, 并引發(fā)鏈式反應[120-122]。對海草而言, 在高鹽脅迫下其線粒體和葉綠體的相對面積隨鹽度的增加而增加, 總光合受到抑制, 凈光合作用減少[123-124], 表明高鹽度脅迫導致了海草光合器官損傷。在光、鹽度與營養(yǎng)鹽聯(lián)合脅迫下, 光脅迫成為主導因素。強光脅迫能夠促進SOD和POD等抗氧化酶活性提高以應對氧化脅迫[124]。我們推測海草與陸生植物的抗氧化系統(tǒng)具有相同的鹽脅迫響應機制, 如黃秋葵()、水稻()、大豆植株在鹽脅迫下能夠誘導CAT、POD等主要抗氧化酶活性增強[121, 125-126]。此外, 有研究表明植物可以通過一種非典型的雙特異性蛋白酪氨酸磷酸酶ATPFA- DSP3(DSP3)調(diào)節(jié)蛋白質(zhì)磷酸化介導植物抗氧化系統(tǒng)對鹽脅迫的響應[127]。

3) 對缺氧及硫化物脅迫的響應。全球變暖及富營養(yǎng)化引起的缺氧現(xiàn)象正成為海洋生態(tài)系統(tǒng)的重要威脅[128-130]。缺氧脅迫會顯著降低海草的光合作用, 并影響海草的碳/氮代謝, 而海草抗氧化系統(tǒng)在缺氧脅迫下也會被激活以應對ROS產(chǎn)生的危害, 如鰻草在缺氧脅迫下顯著上調(diào)編碼CAT、SOD、POD、GST、MDHAR等抗氧化系統(tǒng)相關(guān)酶的基因[128]。缺氧不僅對海草有直接影響, 還會引起硫化物對海草的入侵。海草一般生長在高度還原性的沉積物中, 沉積物中的有機質(zhì)經(jīng)過厭氧代謝產(chǎn)生的硫化物在低氧環(huán)境下會侵入海草內(nèi)部, 進而破壞海草的分生組織并抑制海草的光合作用[131]。而目前海草對硫化物脅迫響應的分子機制研究較少, 關(guān)于海草抗氧化系統(tǒng)如何應對硫化物脅迫尚不清晰。

4) 對海洋酸化的響應。對海草抗氧化系統(tǒng)而言, 海水中CO2含量(pCO2)升高導致的海洋酸化并非逆境脅迫, 反而能夠在一定程度上緩解海草的氧化壓力。高pCO2馴化(一般需要持續(xù)數(shù)月)能夠使海草穩(wěn)態(tài)下抗氧化酶和抗氧化物(SOD、CAT、APX、GR和GSH)活性降低, 表明抗氧化系統(tǒng)在相對不活躍的情況下也能有效地阻止氧化損傷, 表明了海草的高pCO2偏好; 而未馴化的海草在低pCO2下抗氧化酶和抗氧化物表達量短期內(nèi)提高, 在高pCO2下表達下降, 但下降趨勢不明顯[132]。相關(guān)研究一般利用海洋火山口這一自然實驗室, 但是其導致的溫度變化和排放的其他氣體帶來了大量的干擾, 例如Lauritano等用同種同源的海草在不同火山口進行培養(yǎng)實驗卻得出了截然不同的結(jié)果[133]; 而Ravaglioli等在證明火山口高不確定性的同時, 還表明營養(yǎng)鹽的濃度會顯著影響海草抗氧化系統(tǒng)對pCO2變化的響應[134]。由于pH的改變還會影響海草附生生物群落[135], 因此在氣候變化及海洋酸化的背景下, 揭示海草抗氧化系統(tǒng)的響應情況需要更多深入全面的研究。

3 展望

1) 海草抗氧化系統(tǒng)組分在逆境脅迫下的協(xié)同變化和多層次關(guān)聯(lián)分析: 目前對海草抗氧化系統(tǒng)響應逆境脅迫的研究關(guān)注于主要抗氧化酶的變化, 往往忽視非酶抗氧化物和參與AsA-GSH循環(huán)的其他抗氧化酶。抗氧化系統(tǒng)是復雜的多酶多劑系統(tǒng), 不同組分之間存在響應差異, 某一種或某幾種組分的變化不能全面反映抗氧化系統(tǒng)的總體狀態(tài)。應當結(jié)合基因組、轉(zhuǎn)錄組、蛋白質(zhì)組等多個研究層次的成果, 將抗氧化酶活性變化與基因表達變化關(guān)聯(lián)分析, 綜合分析逆境脅迫下抗氧化系統(tǒng)的響應特征及機制, 這是全面開展海草抗逆生理機制研究的重要基礎(chǔ)。

2) 不同海草物種抗氧化系統(tǒng)對逆境脅迫的響應差異: 海草抗氧化系統(tǒng)對逆境脅迫的響應存在顯著的種間差異, 需要將海草種間差異響應特征與關(guān)鍵差異基因相關(guān)聯(lián), 進而基于差異基因的功能闡明抗氧化系統(tǒng)響應存在種間差異的原因, 這是揭示海草逆境脅迫耐受性種間差異機制的重要前提。

3) 多個脅迫因子聯(lián)合作用下海草抗氧化系統(tǒng)的響應特征與機制: 海草抗氧化系統(tǒng)的逆境脅迫響應研究仍局限于少數(shù)脅迫因子, 與海草面對的復雜實際環(huán)境相距甚遠, 需要開展多個脅迫因子聯(lián)合作用下海草抗氧化系統(tǒng)的響應特征與機制研究, 這是全面分析復雜環(huán)境條件下海草逆境響應機制的基礎(chǔ)。

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Research progress on the antioxidant system of seagrass and its response to stress

LU Ke-yu1, 2, PEI Yan-zhao1, 2, HE Qing1, 2, ZHOU Bin1, 2

(1. College of Marine Life Science, Ocean University of China, Qingdao 266003, China; 2. Pilot National Laboratory for Marine Science and Technology (Qingdao), Qingdao 266237, China)

Seagrass has a unique evolutionary status and important ecological value. It is widely distributed in the intertidal and infralittoral zones of littoral waters. Seagrass is often threatened by a variety of environmental stressors. The seagrass antioxidant system plays a very important role in the tolerance to abiotic stress. This study reviews the composition and characteristics of the antioxidant system of seagrass and its response to stress. Moreover, the major enzymatic antioxidation mechanisms of, arepresentative seaweed species in the Northern Hemisphere, were elaborated and genes of antioxidation enzymes were classified and analyzed. Current research on stress in seagrass has concentrated on changes in major antioxidases (e.g., superoxide dismutase, catalase, and glutathione S-transferase) and the relevant transcriptomes under a single stressor. However, few studies have considered the response to multiple stressors and non-enzymatic antioxidants as well as the responses of other antioxidant enzymes. Besides, there is a great research gap in antioxidant systems and key genes among different seagrass species under adversity stress.

seagrass; stress tolerance; antioxidant enzymes; antioxidant system

Dec. 21, 2021

Q945.78

A

1000-3096(2022)08-0171-15

10.11759/hykx20211221002

2021-12-21;

2022-05-19

國家自然科學基金面上項目(42076147); 中央高?；究蒲袠I(yè)務費專項項目(202066001)

[National Nature Science Foundation of China, No. 42076147; The Fundamental Research Funds for the Central Universities, No. 202066001]

盧科宇(1996—), 男, 碩士, 主要從事海洋生態(tài)學研究, E-mail: 1056016762@qq.com; 周斌(1981—),通信作者, 從事海草生理生態(tài)學研究, E-mail: zhoubin@ouc.edu.cn

(本文編輯: 叢培秀)