植物ABI5轉錄因子的研究進展

李魯華,王忠妮,王文新,王懷玉,任明見,徐如宏*

植物ABI5轉錄因子的研究進展

李魯華1,2,王忠妮3,王文新1,王懷玉1,任明見1,2,徐如宏1,2*

1. 貴州大學農學院, 貴州 貴陽 550025 2. 貴州大學國家小麥改良中心貴州分中心, 貴州 貴陽 550025 3. 貴州省農業科學院水稻研究所, 貴州 貴陽 550006

植物脫落酸不敏感蛋白5（Abscisic acid-insensitive 5, ABI5）為堿性亮氨酸拉鏈類型（Basic leucine zipper, bZIP）的轉錄因子，在ABA信號途徑中起著重要作用。目前已經報道的ABI5功能研究主要是以模式植物擬南芥（）為研究對象，而在小麥（）、水稻（）等農作物中的研究報道極少。本文概述了植物中ABI5在種子發育、植物生長（特別是蔗糖信號途徑和蛋白質泛素化過程）、花青素積累等生物學過程以及植物對干旱、低氮等逆境脅迫響應方面的最新研究進展，闡明在農作物中開展ABI5分子網絡調控解析工作的重要性，以期為選育具有種子萌發可控（如抗穗發芽）、抗逆性強（如低氮）以及高產量等優良性狀的作物品種提供理論基礎，對作物分子育種具有指導意義。

ABI5轉錄因子; 研究進展

植物激素脫落酸（Abscisic acid, ABA）廣泛參與植物生長發育的調控以及植物對逆境脅迫的響應過程。Finkelstein借助遺傳學和分子生物學手段鑒定得到了一系列與ABA響應有關的組分如ABI1（Abscisic acid-insensitive 1）、ABI2、ABI3、ABI4、ABI5。其中，ABI5為堿性亮氨酸拉鏈（Basic leucine zipper, bZIP）類型的轉錄因子[1]。ABI5含有C1-C4 4個保守的結構域和1個bZIP結構域[2]（圖1），其中bZIP結構域中含有分別與泛素化和蘇素化有關的賴氨酸殘基（分別為K344和K391），而C1-C4結構域中含有與磷酸化有關的絲氨酸殘基（S42、S145和S439）和蘇氨酸殘基（T201）[3]。

圖 1 ABI5保守結構域示意圖

近年來研究發現，ABI5在種子休眠和萌發[4]、植物的生長發育[5]、植物對逆境脅迫的響應[6]、花青素的積累[7]以及26S蛋白酶體降解途徑[8]等生物學過程起著重要的作用。本文對近年來ABI5的研究進展進行了梳理和歸納，以期為培育具有優良性狀的農作物提供一定的思路。

1 ABI5與種子發育

ABA參與種子發育過程的調控，而在該生物學過程中ABI5起著重要的作用[1]。

1.1 通過對ABI5的轉錄調控

擬南芥中的研究發現，轉錄中介體（Mediator 25, MED25）和MADS-box轉錄因子AGL21（MADS-box transcription factor AGL21）能夠結合到ABI5的啟動子區域，分別負調控/正調控ABI5的表達進而參與種子發育過程的調控[9,10]；ABI5也能夠直接結合到過氧化氫酶1（Catalase 1, CAT1）基因和SHB1（Short hypocotyl under blue 1, SHB1）基因的啟動子區域，分別通過激活CAT1進而影響活性氧簇的動態平衡[11]和負調控SHB1[12]的表達進而調控ABI5下游信號組分的表達參與種子發育過程的調控。ABI5還能夠直接靶向編碼晚期胚胎豐富蛋白（Late embryogenesis abundant, LEA）的Em1和Em6基因并負調控兩者的表達[13]；擬南芥核因子Y家族蛋白（Nuclear factor Y family protein, NF-YC9）能夠直接與ABI5結合，進而使其結合并激活靶基因EM6的表達，實現對ABI5轉錄活性的正調控作用，從而參與種子萌發對ABA的響應[14]。小麥中的研究發現，ABI5基因的表達量隨著小麥種子成熟度的增加而增加，表明其參與種子發育過程的調控[15]。

此外，擬南芥SAG（A protein containing the midasin homologue 1 domain）[16]、萌發延遲基因1（Delay of Germination 1, DOG1）[17]、DELLA蛋白RGL2[4]以及轉錄因子AtMyb7[18]和RAV1[19]等都通過調控ABI5，進而參與ABA對種子萌發的調控。

1.2 通過對ABI5的穩定性調控

研究發現擬南芥BIN2（Brassinazole insensitive 2）[20]、SOS2類似的蛋白激酶5（SOS2-like protein kinase 5, PSK5）[21]和蛋白磷酸化2A相關蛋白（Protein phosphatase2A (PP2A)-associated protein, TAP46）[22]通過磷酸化作用穩定ABI5，維持種子中較高含量的ABI5水平從而抑制種子萌發。相反地，擬南芥光敏色素相關的絲氨酸/蘇氨酸蛋白磷酸酶（Phytochrome associated serine/threonine protein phosphatase, FyPP）與SnRK2激酶拮抗作用調控ABI5的磷酸化，通過去磷酸化降低ABI5蛋白的穩定性，降低種子中ABI5的含量進而促進種子的萌發[23]。

擬南芥小泛素相關修飾物（Small ubiquitin-related modifier, SUMO）E3連接酶SIZ1通過SUMO修飾保護ABI5免受蛋白酶的降解，進而負調控ABA對種子萌發的調控[24]。相反，研究發現擬南芥CRWN（Crowded nuclei）蛋白家族通過調控ABI5的降解參與ABA控制的種子萌發，CRWN突變體對ABA超敏感并積累更高水平的ABI5蛋白[25]。此外，NO對ABI5蛋白153位半胱氨酸的S-亞硝基化有利于ABI5通過KEG（Keep on going）E3連接酶的生物學降解，從而促進種子的萌發。而ABI5蛋白153位半胱氨酸的突變能夠引起NO失去對ABI5蛋白穩定性的調控，進而表現出抑制種子萌發的表型[26]。

綜上可知，ABI5在種子發育過程中有著極其重要的作用，ABI5基因的表達水平及ABI5蛋白的穩定性直接影響種子的萌發和/或休眠等生物學過程。因此，在小麥、大豆、高粱等作物開展ABI5基因的研究是必要的，進而為種業技術的發展提供一定的理論基礎。

2 ABI5與植物的生長

蔗糖具有類生長素信號分子的功能，在擬南芥生長發育過程中起著重要的調控作用[27]。研究發現，ABI5能夠通過抑制生長素運輸載體PIN1的積累引起根中生長素水平的降低，通過調節根中生長素的水平進而參與蔗糖介導的對根分生組織區的抑制過程[28]。過表達ABI5能夠造成開花轉變過程的延遲，染色質免疫沉淀發現ABI5通過結合到花發育基因FLC（Flowering locus c）的啟動子元件參與植物的花發育過程，進一步研究發現蔗糖非發酵1型相關蛋白激酶2（Sucrose nonfermenting 1-related protein kinase2, SnRK2）介導的ABI5的磷酸化在該過程中起著重要的作用，SnRK2通過調控ABI5的磷酸化能夠促進FLC基因的表達，進而實現擬南芥中ABA對開花轉變過程的抑制作用[29]。

此外，ABI5還參與植物子葉變綠[30]以及黑暗誘導葉片衰老[31]的生物學過程，但具體的作用機理仍有待于進一步的研究。

3 ABI5與逆境脅迫的響應

ABA與植物對逆境脅迫的響應過程緊密相關。對擬南芥突變體hls1（HOOKLESS1）進行研究，發現突變體中ABA介導的抗真菌侵染的現象消失，進一步研究發現HSL1介導的ABI5表達水平降低與該過程具有相關性[6]。低氮脅迫處理發現，擬南芥ABI5的表達水平受到顯著誘導，上調表達超過100倍[32]。在棉花中共表達擬南芥ABI3/Viviparous1（命名為AtRAV2）和ABI5能夠通過調節活性氧簇清除以及滲透調節marker基因的表達提高棉花對干旱脅迫的抗性[33]。此外，水稻[34]、水曲柳[35]等中的研究也發現ABI5與植物對逆境脅迫的響應有關。此外，ABI5還參與逆境脅迫下三酰基甘油（triacylglycerol, TAG）的積累。研究發現逆境脅迫條件下，擬南芥abi5中TAG合成途徑的限速酶二酰基甘油轉移酶（diacylglycerol acyltransferase 1, DGAT1）基因的表達水平以及TAG的積累量均降低；相反地，ABI5過表達轉基因株系中DGAT1的表達水平以及TAG的積累量均升高[36]。ABI5參與植物對逆境脅迫響應的生物學過程，并在植物對逆境脅迫的響應過程中起著重要作用。

4 ABI5與花青素的積累

對擬南芥abi5-4進行研究，發現3%蔗糖處理使得突變體中花青素的積累量高于野生型[7]。對擬南芥絲氨酸/精氨酸豐富（serine/arginine-rich, SR）蛋白SR45的研究發現，3%蔗糖處理能夠顯著誘導sr45中花青素合成基因APL3（ADP-Glc pyrophosphorylase, large subunit）、CHS以及ABI5的表達[37]。此外，擬南芥中的研究發現外源蔗糖和葡萄糖處理能夠誘導調控花青素合成途徑的花色苷色素產生基因（Production of anthocyanin pigment, PAP）PAP1和PAP2的表達，而突變體abi5-1能夠減弱該處理對PAP1和PAP2表達的誘導作用[38]，進而調節花青素的積累。可知，ABI5能夠通過影響花青素合成相關基因的表達實現對花青素的生物學調控，但其作用機制仍需要進一步闡明。

5 ABI5與26S蛋白酶體降解途徑

蛋白的泛素化過程在植物生長過程中具有廣泛而重要的生物學功能，該過程一般由泛素激活酶（Ubiquitin-activating enzymes, E1）、泛素結合酶（Ubiquitin-conjugating enzymes, E2）和E3催化的級聯反應完成[39]。26S蛋白酶體能夠識別泛素化的蛋白，經過去泛素化酶（Deubiquitinating enzymes, DUBs）去泛素化后被26S蛋白酶體中的蛋白酶降解[40]。擬南芥中研究發現定位于細胞質和內膜系統的泛素化E3連接酶KOG能夠通過泛素化作用抑制ABI5的積累水平[8]，進一步研究發現泛素化的ABI5通過26S蛋白酶體進行降解，從而實現KOG對ABI5蛋白表達水平的調控[3]。此外，Cullin4 E3復合體底物受體DWA1/DWA2（DWD hypersensitive to ABA1/2）和ABD1（ABA-hypersensitive DCAF1）也能夠通過26S蛋白酶體途徑調控細胞核中ABI5的水平[41]。總之，植物可以通過26S蛋白酶體降解途徑實現對細胞核和細胞質中ABI5表達水平的調節，進而對ABA信號通路進行調控。

目前，對農作物中ABI5的功能報道較少，然而由于該蛋白是ABA信號途徑中重要的轉錄因子，闡明農作物中ABI5的調控網絡為最終應用于分子育種具有指導性的意義。

6 總結與展望

植物ABI5是ABA信號通路中重要的轉錄因子，參與種子發育、植物生長、花青素積累等生物學過程，并在植物對逆境脅迫如干旱、低氮等的響應方面起著重要的作用。目前擬南芥中ABI5的功能研究已有較多的報道，而對小麥、水稻等農作物中的研究報道極少。綜上可知，ABI5在種子發育、植物生長（特別是蔗糖信號途徑和蛋白質泛素化過程）、植物對逆境脅迫的響應以及花青素積累等生物學過程中起重要作用，在農作物中可以重點開展該蛋白在上述方面的研究。然而，農作物中ABI5具有的生物學功能是否與擬南芥中的功能相同？哪些蛋白（靶向或者非靶向）參與調控ABI5的生物學功能？ABI5又能夠通過調控哪些蛋白實現上游信號的傳遞？在某一特定生物學過程中ABI5分子調控網絡是如何協同的？等問題仍需要進行系統深入的研究和探討。對小麥、水稻、玉米等重要農作物中ABI5進行深入的研究，有助于人們深入了解ABI5的分子調控網絡，為培育具有抗穗發芽、抗逆性強、延緩衰老等優良性狀的品種提供一定的理論基礎。綜上所述，在農作物中開展闡明ABI5分子調控網絡的相關研究是必要可行的。

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Research Advances of Plant ABI5 Transcription Factors

LI Lu-hua1,2, WANG Zhong-ni3, WANG Wen-xin1, WANG Huai-yu1, REN Ming-jian1,2, XU Ru-hong1,2*

1.550025,2.550025,3.550006,

Plant abscisic acid insensitive 5 (ABI5) protein is transcription factors of basic leucine zipper (bZIP) type and plays an important role in ABA signaling pathway. Currently, the functional research of ABI5 in plant mainly focused on, while few studies have been reported on crops such as wheat () and rice (). In this paper, we summarized the advanced researches of ABI5 in biological process such as seed development, plant growth (especially the sucrose signaling pathway and protein ubiquitination process), anthocyanin accumulation and in stress responses such as drought and low nitrogen, and elucidated the importance of carrying out the molecular regulation research of ABI5 in crops, which would provide theoretical basis for crop varieties with excellent traits, such as controllable seed germination (e.g. preharvest sprouting resistance), strong stress resistance (e.g. low nitrogen) and high yield, and has instructive significance of molecular breeding in crops.

ABI5 transcription factors; research advance

Q7

A

1000-2324(2019)04-0537-05

2018-08-22

2018-10-10

貴州省科技計劃項目(黔科合基礎[2019]1073號);國家自然科學基金項目(31660390); 貴州省農業成果轉化計劃項目(黔科合成果(2016)4022號);貴州大學引進人才科研項目(貴大人基合字2017(49號))

李魯華(1985-),男,博士,講師,主要從事作物遺傳育種工作. E-mail:luhua_li@163.com

Author for correspondence. E-mail:xrhgz@163.com