蘇達(dá) 吳良泉 S?ren K Rasmussen 周廬建 程方民,*

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低植酸水稻種質(zhì)資源篩選、遺傳生理調(diào)控與環(huán)境生態(tài)適應(yīng)性研究進(jìn)展

蘇達(dá)1,2吳良泉2S?ren K Rasmussen3周廬建4程方民4,*

(1福建農(nóng)林大學(xué) 作物科學(xué)學(xué)院 作物遺傳育種與綜合利用教育部重點(diǎn)實(shí)驗(yàn)室, 福州 350002;2福建農(nóng)林大學(xué) 國(guó)際鎂營(yíng)養(yǎng)研究所, 福州 350002;3哥本哈根大學(xué) 植物與環(huán)境科學(xué)系, 丹麥 哥本哈根;4浙江大學(xué) 農(nóng)業(yè)與生物技術(shù)學(xué)院, 杭州 310058;*通訊聯(lián)系人, E-mail: chengfm@zju.edu.cn)

提高或維持水稻產(chǎn)量的同時(shí)，提高稻米品質(zhì)已成為目前水稻育種的首要目標(biāo)之一。其中，通過降低籽粒中植酸等抗?fàn)I養(yǎng)因子，增加鋅、鐵生物有效性以提升水稻營(yíng)養(yǎng)品質(zhì)，是目前水稻品質(zhì)改良的一個(gè)重要方向。本文主要綜述了水稻籽粒中植酸合成的代謝路徑、低植酸水稻的篩選及相關(guān)功能基因的遺傳特點(diǎn)、植酸生理代謝的調(diào)控網(wǎng)絡(luò)、低植酸水稻農(nóng)藝性狀劣變和生態(tài)適應(yīng)性降低的生理原因、籽粒植酸合成的環(huán)境調(diào)控效應(yīng)等相關(guān)研究進(jìn)展?？蔀榈椭菜崴酒焚|(zhì)改良以及栽培調(diào)優(yōu)提供借鑒。

植酸；水稻；籽粒品質(zhì)；遺傳調(diào)控；生態(tài)效應(yīng)

植酸(C6H18O24P6，IP6，六磷酸肌醇)是作物籽粒中磷的最主要貯存形式，約占籽?？偭缀康?0%~90%[1]。植酸與籽粒其他內(nèi)含物(如淀粉、蛋白質(zhì)和脂類等)一起隨灌漿充實(shí)逐漸積累。在此過程中，植酸可與籽粒中的礦質(zhì)離子(Zn2+、Fe2+、Ca2+、Mg2+等)以及活性蛋白絡(luò)合，形成難溶性的植酸鹽(phytin)，并以圓球狀復(fù)合晶體(globoid crystal)的形式貯存于籽粒中[2]。由于人和單胃動(dòng)物消化系統(tǒng)中缺乏分解植酸的內(nèi)源植酸酶，因此，作物籽粒中植酸(鹽)的存在大幅降低了有關(guān)礦質(zhì)營(yíng)養(yǎng)元素(尤其是鋅和鐵)的生物有效性以及人體對(duì)活性蛋白和氨基酸的有效吸收[3, 4]。因此，大量攝取植酸含量相對(duì)較高的谷物和豆類，被認(rèn)為是發(fā)展中國(guó)家人群鐵、鋅缺乏的主要原因。同時(shí)，在畜牧業(yè)生產(chǎn)中，不能被單胃動(dòng)物有效吸收的植酸磷，70%以上以糞便形式排泄到環(huán)境，也加劇了土壤污染和水體富營(yíng)養(yǎng)化[1, 5]。

水稻是我國(guó)重要的糧食作物，在傳統(tǒng)高產(chǎn)育種和優(yōu)化栽培體系的基礎(chǔ)上，進(jìn)一步挖掘其品質(zhì)潛力，是水稻品質(zhì)育種的主要目標(biāo)之一。其中，培育和篩選低植酸種質(zhì)資源是目前解決水稻籽粒植酸等抗?fàn)I養(yǎng)問題以及谷物營(yíng)養(yǎng)品質(zhì)改良的重要手段，也是保障鋅、鐵生物強(qiáng)化，磷資源可持續(xù)利用以及農(nóng)業(yè)生態(tài)保護(hù)的有效措施[1, 6, 7]。為此，本文主要綜述了植酸的合成代謝及分子遺傳特點(diǎn)、植酸合成的生理調(diào)控網(wǎng)絡(luò)、低植酸水稻的農(nóng)藝性狀和環(huán)境生態(tài)適應(yīng)性表現(xiàn)等內(nèi)容，相關(guān)研究進(jìn)展可為低植酸水稻品質(zhì)改良以及相應(yīng)的栽培調(diào)控提供借鑒。

1 植酸合成的代謝路徑及其關(guān)鍵酶

植酸的合成和代謝路徑主要分為三個(gè)階段：肌醇合成、多磷酸肌醇合成以及植酸合成后從胞質(zhì)向液泡的轉(zhuǎn)運(yùn)(圖1)。1)肌醇(-inositol)合成階段。肌醇-3-磷酸合成酶(-inositol-3-phosphate synthase，MIPS)以NADH(Nicotinamide adenine dinu cleotide)為輔酶，將光合產(chǎn)物葡萄糖-6-磷酸(Glucose-6-phosphate，G-6-P)轉(zhuǎn)化為肌醇-3-單磷酸[Ins(3)P1][8,9]。肌醇-3-磷酸隨后在肌醇-3-磷酸水解酶[Ins(3)P1- monophosphatase，IMP]的催化下水解肌醇環(huán)上的磷酸基團(tuán)，生成肌醇。2)多磷酸肌醇的合成階段?？煞譃橹?dú)立途徑(磷酸肌醇順序磷酸化路徑，lipid-independent)和脂依賴途徑(磷脂酰肌醇代謝路徑，lipid-dependent)。兩條代謝途徑的最初產(chǎn)物均為肌醇，終產(chǎn)物均為1,3,4,5,6-五磷酸肌醇[Ins(1,3,4,5,6)P5]，區(qū)別在于代謝分支路徑中是否會(huì)有磷酸酯的出現(xiàn)。其中，脂獨(dú)立途徑中，肌醇在肌醇激酶(-inositol kinase，MIK)的催化下重新生成肌醇-3-磷酸，之后在磷酸甘油酸激酶(phosphoglycerate kinase，PGK)和多磷酸肌醇激酶(如多磷酸-肌醇5,6-激酶，Inositol 1,3,4-trisphosphate 5/6-kinase)催化下，逐漸順序磷酸化并最終合成植酸。前人研究多表明，此途徑是作物籽粒植酸合成的主要路徑。而在脂依賴途徑中，肌醇首先在磷脂酰肌醇合酶(phosphatidylinositol synthase)的催化下合成磷脂酰肌醇(phosphatidylinositol)，隨后由磷酸磷脂酶C(Phospholipase C, PLC)水解生成肌醇-1,4,5-三磷酸[-inositol-1,4,5-trisphosphate，Ins(1,4,5)P3]，繼而進(jìn)一步磷酸化生成植酸。因此，脂依賴途徑中有磷脂酰肌醇以及肌醇-1,4,5-三磷酸等第二信使參與。與脂獨(dú)立途徑相比，脂依賴途徑對(duì)植物籽粒器官植酸含量的影響較小。上述兩條代謝路徑的共同終產(chǎn)物均為1,3,4,5,6-五磷酸肌醇，其后在1,3,4,5,6-5-肌醇-2-磷酸激酶[Ins(1,3,4,5,6) P5-2-kinase]的催化下生成植酸[10]；3)植酸合成后的轉(zhuǎn)運(yùn)路徑。胞質(zhì)中合成的植酸，需經(jīng)ABC跨膜轉(zhuǎn)運(yùn)蛋白(ATP-binding cassette)家族中MRP蛋白(multidrug resistance-associated protein，MRP)的運(yùn)輸作用，將胞質(zhì)中合成的植酸最終轉(zhuǎn)移至液泡中。此外，真核細(xì)胞植酸還可在六磷酸肌醇激酶的催化下繼續(xù)磷酸化生成高階磷酸肌醇鹽(如7、8-磷酸肌醇)，參與磷和ATP能量代謝等生理過程的調(diào)節(jié)[8]。同時(shí)，在籽粒萌發(fā)過程中，植酸可在內(nèi)源植酸酶或磷酸化酶的催化下去磷酸化，重新降解為不同價(jià)位的磷酸肌醇(如IP1、IP2、IP3、IP4、IP5)和肌醇?？傊?，植酸合成主要有以下三個(gè)特點(diǎn)：1)代謝途徑同時(shí)進(jìn)行，不同代謝途徑之間相互協(xié)調(diào)和補(bǔ)充[11-14]；2)反應(yīng)過程非線性，磷酸化和去磷酸化同時(shí)進(jìn)行；3)植酸合成過程中的關(guān)鍵酶大多數(shù)具有多功能性特性，不局限于作用某一特定底物[15]。

G-6-P，葡萄糖-6-磷酸； Inositol，肌醇；Ins(3)P1, 肌醇-3-單磷酸；Ins(3,4)P2，肌醇-3,4-二磷酸；Ins(3,4,6)P3，肌醇-3,4-6-三磷酸；Ins(3,4,5,6)P4，肌醇-3,4-5-6-四磷酸；Ptd Ins，磷酯酰肌醇；PtdIns(4)P，磷脂酰肌醇-4-單磷酸；PtdIns(4,5)P2，磷脂酰肌醇4,5二磷酸；Ins(1,4,5)P3，肌醇1,4,5三磷酸；Ins(1,4,5,6)P4，肌醇1,4,5,6四磷酸；Ins(1,3,4,5,6)P5，肌醇1-3,4-5-6-五磷酸；Ins(1,2,3,4,5,6)P6，肌醇1-2-3-4-5-6-六磷酸(植酸)。

[1]－肌醇-3-磷酸合成酶；[2]－肌醇-3-磷酸水解酶；[3]－肌醇激酶；[4]－磷酸甘油酸激酶；[5]－多磷酸-肌醇5,6-激酶；[6]－1,3,4,5,6-5-肌醇-2-磷酸激酶；[7]－磷脂酰肌醇合成酶；[8]－磷酸磷脂酶C；[9]－肌醇1,4,5-三磷酸激酶；[10]－ABC轉(zhuǎn)運(yùn)蛋白；MRP轉(zhuǎn)運(yùn)蛋白；[11]－六磷酸肌醇激酶；[12]－植酸酶或磷酸酶。

[1], MIPS，-inositol-3-phosphate synthase; [2], Ins(3)P1-monophosphatase IMP，-inositol-phosphate monophosphatase; [3], MIK，-inositol- kinase; [4], PGK，phosphoglycerate kinase; [5], ITP5/6K，inositol 1,3,4-triphosphate 5/6-kinase; [6], IPK1，inositol 1,3,4,5,6-pentakisphosphate 2-kinase; [7], PtdIns Synthase，phosphatidy linositol synthase; [8], Phospholipase C; [9], Inositol 1,4,5-tris-phosphate kinase; [10], ABC transporter; MRP transporter; [11], InsP6 Kinase; [12], Phytases or phosphatase.

圖1 植酸的生物合成

Fig. 1. Biosynthetic pathways of phytic acid.

2 植酸合成過程的重要功能基因以及低植酸水稻種質(zhì)篩選

培育低植酸新品種是提高作物籽粒微營(yíng)養(yǎng)及生物有效性的一條有效途徑。近年來隨著分子生物技術(shù)的發(fā)展，在傳統(tǒng)理化誘變的基礎(chǔ)上，利用基因操作已成為作物低植酸突變材料創(chuàng)建的常用手段之一。目前，國(guó)內(nèi)外已獲得大麥、玉米、水稻和大豆等作物的低植酸突變材料，其籽粒植酸下降30%~95%[1, 4, 9, 14, 16-18]。人體和動(dòng)物試驗(yàn)結(jié)果均表明，低植酸作物鐵、鈣的利用率增加了30%~50%，鋅的有效性增幅高達(dá)76%[1, 19-22]。其中，通過基因工程技術(shù)對(duì)植酸合成的限速酶基因進(jìn)行干擾或敲除，籽粒植酸含量降低了14.9%~75%[23-26]。Ali等[27]和Kuwano等[23]在前人利用組成型表達(dá)啟動(dòng)子進(jìn)行低植酸作物選育的基礎(chǔ)上，進(jìn)一步利用籽粒特異性啟動(dòng)子對(duì)()進(jìn)行RNA干擾，轉(zhuǎn)基因后代中籽粒的表達(dá)量為原來的21.8%，籽粒植酸含量降低了68%~75%，特異性啟動(dòng)子的選擇有效避免了相關(guān)基因突變對(duì)營(yíng)養(yǎng)器官中磷代謝的干擾。除外，近年來對(duì)參與植酸代謝過程其他基因進(jìn)行突變，也正不斷豐富低植酸的突變類型。據(jù)Kim等[28]報(bào)道，通過EMS篩選出的(肌醇激酶基因)型低植酸突變水稻，籽粒植酸含量降低了34%~75%，這與通過基因工程篩選出的型低植酸突變體的植酸降幅相近(37.0%~50.7%)[25, 28]。對(duì)以及()等基因進(jìn)行突變或干擾，也篩選出相應(yīng)的低植酸突變材料[29]，籽粒植酸降幅分別可達(dá)39%~71%和46%~68%[29-31]。雖然目前對(duì)PGK的功能研究相對(duì)較少，但研究者在細(xì)菌中觀察到PGK在ATP參與下會(huì)催化生成2,3-二磷酸甘油酸鹽，此物質(zhì)會(huì)抑制肌醇多磷酸的產(chǎn)生，這可能是基因缺失引起植物細(xì)胞中肌醇單磷酸鹽增加以及植酸含量降低的一個(gè)原因。IPK1催化植酸合成的最后一步。Ali等[32]利用特異性啟動(dòng)子()對(duì)基因進(jìn)行RNA干擾，轉(zhuǎn)基因后代中籽粒基因表達(dá)量降至原來的26.0%，植酸含量也同步降低了69%?；虻耐蛔円话悴⒉粫?huì)影響低價(jià)肌醇磷酸的合成，考慮到肌醇、磷脂酰肌醇以及低價(jià)磷酸肌醇(IP1-3)在作物生長(zhǎng)發(fā)育過程中參與了多個(gè)重要生理代謝過程，而高價(jià)磷酸肌醇，如4-磷酸肌醇或5-磷酸肌醇具有與植酸相似的生理功能，因此對(duì)該基因進(jìn)行突變后產(chǎn)生的低植酸突變材料，其農(nóng)藝性狀的劣變趨勢(shì)比突變小。同時(shí)，雖然IMP在植酸合成中的功能現(xiàn)已基本明確，主要參與催化肌醇的合成，但迄今尚未獲得此基因突變的低植酸水稻材料。

此外，對(duì)參與植酸合成后從胞質(zhì)向液泡轉(zhuǎn)運(yùn)/分儲(chǔ)的轉(zhuǎn)運(yùn)蛋白(如MRP)的相關(guān)基因進(jìn)行突變，也已在不同主栽糧食作物中篩選出相應(yīng)的低植酸突變材料，如擬南芥[33]、水稻[34, 35]、豆類[36, 37]、玉米[38-40]和小麥[41, 42]等。由于植酸合成過程中存在肌醇回補(bǔ)路徑[43, 44]，因此，MRP蛋白可能充當(dāng)了植酸在不同器官和組織中相互交流的媒介。Mitsuhashi等[43]在擬南芥中發(fā)現(xiàn)，IMP催化產(chǎn)生的肌醇在肌醇轉(zhuǎn)運(yùn)蛋白的作用下從胚乳轉(zhuǎn)移到胚中參與肌醇順序磷酸化，這一過程也受到MRP蛋白的調(diào)控。利用T-DNA插入技術(shù)對(duì)水稻MRP基因進(jìn)行突變，籽粒植酸降幅可達(dá)90%[45]。由于MRP同時(shí)還參與作物穎花發(fā)育、激素調(diào)控、信號(hào)代謝、氧化脅迫響應(yīng)等多個(gè)生理過程[46, 47]，該基因的突變還會(huì)影響除籽粒植酸合成外的其他生理過程，如突變后擬南芥對(duì)激素調(diào)控的響應(yīng)變得更為敏感[33]。MRP蛋白還可以通過與谷胱甘肽偶聯(lián)參與重金屬的抗性表達(dá)，對(duì)該基因進(jìn)行突變后，小麥根系對(duì)鎘脅迫的敏感度增加，同時(shí)還改變了籽粒中礦質(zhì)元素的空間分布以及鐵離子的再運(yùn)轉(zhuǎn)[41]。基于MRP酶的多功能性，此基因的突變可能是導(dǎo)致水稻、小麥等作物的生長(zhǎng)發(fā)育(如籽粒灌漿、營(yíng)養(yǎng)器官的形態(tài)建成)及農(nóng)藝性狀劣變(粒重降低，萌發(fā)延遲，胚芽鞘生長(zhǎng)緩慢，感病性增強(qiáng))的原因之一。同時(shí)，由于不同作物中MRP同源基因組成/功能有所差異，型低植酸突變體的農(nóng)藝性狀在不同作物中的表現(xiàn)有所不同。水稻中多表現(xiàn)為農(nóng)藝性狀劣變的趨勢(shì)，而豆科作物由于不同基因的同工型之間表現(xiàn)為一定程度的功能互補(bǔ)(如同工型在一定程度上可補(bǔ)償缺失對(duì)營(yíng)養(yǎng)器官中磷代謝造成的影響，而不參與籽粒中植酸合成)，因此在一定程度上補(bǔ)償了突變后造成的農(nóng)藝性狀劣變的多效性[21, 37, 48]。前人研究表明型低植酸大豆籽粒中肌醇含量表現(xiàn)為增加[49]，而()三基因突變后，肌醇含量變化不顯著，推測(cè)可能與位點(diǎn)突變補(bǔ)償了突變后肌醇的降低有關(guān)，對(duì)型低植酸大豆的轉(zhuǎn)錄譜分析也表明，基因突變促進(jìn)植酸水解生成肌醇/磷酸肌醇[50]，說明型低植酸突變體中肌醇和磷酸肌醇中間產(chǎn)物含量的增加和植酸的降解有關(guān)。但也有觀點(diǎn)認(rèn)為，沉默后肌醇含量增加，可能與植酸(或肌醇、磷酸肌醇)減少后通過反饋調(diào)節(jié)激活肌醇轉(zhuǎn)運(yùn)子相關(guān)基因的表達(dá)有關(guān)[51]。因此，關(guān)于基因突變后降低植酸積累的生理原因還有待進(jìn)一步研究。除MRP轉(zhuǎn)運(yùn)蛋白外，研究者通過對(duì)已篩選出的低植酸突變體-Z9B-1和-MH86-1的突變位點(diǎn)進(jìn)行定位分析，結(jié)果表明硫酸鹽轉(zhuǎn)運(yùn)蛋白(sulfate transporter，sultr3;3)酶基因突變也會(huì)顯著影響水稻籽粒植酸的合成。研究表明在大麥、水稻和擬南芥中對(duì);基因進(jìn)行突變，籽粒植酸降幅可達(dá)45%，同時(shí)籽?？偭椎暮恳矔?huì)顯著降低[52-56]，這種植酸磷和總磷同步降低的低植酸類型是未來低植酸作物選育的一個(gè)新目標(biāo)。此外，Zhang等[57]對(duì)玉米轉(zhuǎn)錄譜分析表明，除上述功能基因直接參與植酸合成外，還有三個(gè)候選基因可能也參與了植酸的合成，包括(編碼磷酸肌醇合成相關(guān)酶基因)、(參與磷酸肌醇在不同器官中的轉(zhuǎn)運(yùn))和(參與磷酸肌醇向液泡轉(zhuǎn)運(yùn))，但其同源基因在水稻中尚未見相關(guān)報(bào)道，這可能是未來水稻植酸合成相關(guān)基因克隆以及相應(yīng)低植酸突變篩選的一個(gè)新方向。

根據(jù)水稻籽粒中植酸磷、無機(jī)磷和總磷含量的變化特點(diǎn)，可將不同低植酸突變體分為4種類型(表1)：1)類低植酸突變。該類低植酸突變體的籽粒植酸磷變化特點(diǎn)表現(xiàn)為籽粒植酸含量明顯降低的同時(shí)，無機(jī)磷含量等摩爾增加，而不同價(jià)位的磷酸肌醇和籽?？偭缀勘３植蛔儭ｎ惖椭菜嵬蛔凅w通常是由于植酸合成代謝路徑中作用于肌醇供應(yīng)階段(從葡萄糖-6-磷酸到三磷酸肌醇合成)的相關(guān)功能基因發(fā)生突變所引起的(如和等)；2)類低植酸突變。該類低植酸突變體籽粒植酸含量明顯下降，而籽粒中無機(jī)磷和其他形態(tài)的磷酸肌醇(如IP1~IP5)均有所增加，總磷含量保持不變。類低植酸突變體多是由多磷酸肌醇磷酸化環(huán)節(jié)的重要功能基因發(fā)生突變所引起的(如、、和等)，籽粒植酸含量的降幅一般小于類低植酸突變；3)類低植酸突變。主要是肌醇激酶(MIK)基因發(fā)生突變所引起，其籽粒植酸磷和肌醇含量的變化特點(diǎn)表現(xiàn)為籽粒植酸含量明顯下降，肌醇含量也顯著降低。與和型低植酸突變有所不同，類低植酸突變體的籽?？偭缀恳矔?huì)隨籽粒植酸含量的降低而發(fā)生明顯改變；4)其他類低植酸突變。此類突變相關(guān)基因通常與植酸在植物不同組織中的轉(zhuǎn)運(yùn)分配有關(guān)。如ABC轉(zhuǎn)運(yùn)子家族中MRP蛋白以及硫酸鹽轉(zhuǎn)運(yùn)蛋白(sultr3;3)基因(表1)。其中，基因突變所導(dǎo)致的籽粒植酸磷、總磷含量變化特征與類低植酸突變體相似，因而也有研究將其劃歸為類低植酸突變體。而sultr3;3酶基因突變可引起籽粒植酸磷和總磷含量的同步下降，其籽粒磷組分變化與突變類型不一致。

3 植酸生理代謝的調(diào)控網(wǎng)絡(luò)

明確植酸合成代謝的調(diào)控網(wǎng)絡(luò)，并建立起植酸代謝與其他生理過程之間的聯(lián)系，對(duì)于籽粒植酸積累的調(diào)控、低植酸“優(yōu)質(zhì)種性”的發(fā)揮具有重要的意義。研究表明植酸合成與糖代謝、信號(hào)轉(zhuǎn)導(dǎo)[Ca2+和Ins(1,4,5)-P3]、激素調(diào)控、肌醇/磷酸肌醇代謝、ROS響應(yīng)密切相關(guān)。例如，型低植酸作物降低籽粒植酸含量的同時(shí)，還影響了糖代謝[64]。Edwards等[65]通過QTL定位分析表明，水稻籽粒植酸和堊白形成相關(guān)基因高度連鎖，推測(cè)植酸含量降低是水稻堊白率增加的原因之一。Zhou等[66]分析低植酸水稻(9311-)及其野生對(duì)照(9311)的萌發(fā)生理差異發(fā)現(xiàn)，低植酸水稻籽粒中Ins(1,4,5)-P3升高抑制了ROS活性(SOD、CAT、POD、NOX和APX)。Redekar等[51]對(duì)低植酸大豆發(fā)育籽粒的轉(zhuǎn)錄譜分析表明，在發(fā)育籽粒的不同階段，(低植酸基因型，單基因突變)和1MWT(野生對(duì)照基因型)相比共有250個(gè)差異表達(dá)基因。而(，三基因突變)與3MWT(野生對(duì)照)相比，差異表達(dá)的基因增加到4000個(gè)。同時(shí)，低植酸突變體與其野生對(duì)照在肌醇代謝、磷酸肌醇代謝、激素信號(hào)代謝(如auxin-ABA信號(hào)，肌醇-生長(zhǎng)素信號(hào))均表現(xiàn)出顯著差異。對(duì)大豆和玉米的植酸代謝研究表明，植酸代謝相關(guān)基因的表達(dá)同時(shí)受到轉(zhuǎn)錄因子的調(diào)控，如(調(diào)控逆境脅迫和激素響應(yīng))、(參與ABA信號(hào)轉(zhuǎn)導(dǎo)、寡聚糖合成以及鈣調(diào)蛋白結(jié)合轉(zhuǎn)錄激活因子表達(dá))、、和等[50, 51, 57]。Zhang等[57]用系統(tǒng)生物學(xué)方法，利用轉(zhuǎn)錄組測(cè)序(RNA-Seq)以及小RNA測(cè)序(microRNA-Seq)對(duì)不同植酸遺傳背景玉米(Qi319，低植酸基因型；B73，高植酸基因型)進(jìn)行轉(zhuǎn)錄表達(dá)分析，發(fā)現(xiàn)低植酸突變體和野生對(duì)照基因型之間差異最大的基因?yàn)楹?，這與IPs含量的變化一致。對(duì)參與植酸合成的代謝網(wǎng)絡(luò)進(jìn)行分析表明，在植酸和激素關(guān)系上，磷酸肌醇合成路徑和赤霉素(GA)合成路徑之間通過泛素化途徑相關(guān)聯(lián)。而在植酸與信號(hào)代謝上，Ca2+信號(hào)路徑是植酸和其他代謝路徑之間的橋梁。在植酸和碳水化合物代謝上，植酸(磷酸肌醇)轉(zhuǎn)運(yùn)相關(guān)基因與碳水化合物轉(zhuǎn)運(yùn)代謝密切相關(guān)。此外，、乙烯響應(yīng)相關(guān)轉(zhuǎn)錄因子(參與初級(jí)和次級(jí)代謝調(diào)控)、以及(參與調(diào)控作物生長(zhǎng)發(fā)育，逆境響應(yīng)等)，是突變體及其野生對(duì)照比較中表達(dá)差異最顯著的轉(zhuǎn)錄因子。Redekar等[50]在低植酸大豆中的轉(zhuǎn)錄譜分析表明，和相比，肌醇轉(zhuǎn)運(yùn)子相關(guān)基因都表現(xiàn)為顯著上調(diào)表達(dá)，推測(cè)兩種突變類型均誘導(dǎo)了相同的信號(hào)代謝。明確植酸代謝和調(diào)控的網(wǎng)絡(luò)聯(lián)系，為今后利用栽培調(diào)優(yōu)或外源調(diào)控效應(yīng)降低籽粒植酸積累具有重要的借鑒意義，相關(guān)轉(zhuǎn)錄因子的發(fā)現(xiàn)也為未來低植酸品種選育提供了新的研究思路。

4 低植酸水稻的農(nóng)藝性狀和環(huán)境生態(tài)適應(yīng)性表現(xiàn)

低植酸作物在增加籽粒鋅、鐵生物有效性的同時(shí)，農(nóng)藝性狀卻表現(xiàn)出不同程度的劣變特征[60, 67, 68]，如籽?；钚?、發(fā)育、萌發(fā)率、出苗率或花粉育性降低，灌漿充實(shí)不暢，籽粒充實(shí)度下降，易早衰，營(yíng)養(yǎng)器官生長(zhǎng)緩慢，花期及成熟延遲等[43, 67, 69, 70]，并最終導(dǎo)致產(chǎn)量降低[1, 46, 71, 72]。且籽粒植酸的降幅與農(nóng)藝性狀劣變程度還表現(xiàn)出正相關(guān)趨勢(shì)，即籽粒植酸降幅越大，其灌漿充實(shí)度和產(chǎn)量水平的劣變趨勢(shì)就越明顯[9, 30, 67, 68, 73]。低植酸水稻籽粒植酸降幅從35%增加到63.6%時(shí)，對(duì)應(yīng)產(chǎn)量降幅從12.5%上升至25.6%。籽粒植酸降幅超過70%時(shí)，農(nóng)藝性狀劣變的多效性開始加劇。對(duì)植酸降幅90%的低植酸大麥(-M955)進(jìn)行轉(zhuǎn)錄分析表明，在發(fā)育籽粒的信號(hào)代謝、激素代謝(細(xì)胞分裂素、乙烯)、碳水化合物轉(zhuǎn)運(yùn)和合成代謝路徑中，相關(guān)酶基因的表達(dá)量均表現(xiàn)出顯著降低的趨勢(shì)[74]。當(dāng)籽粒植酸含量降幅達(dá)90%以上時(shí)，作物的正常生長(zhǎng)發(fā)育都會(huì)受到嚴(yán)重影響[1, 9, 73]；植酸降幅超過95%時(shí)甚至無法完成正常的發(fā)育進(jìn)程(表現(xiàn)為胚發(fā)育缺陷或致死)[1, 46, 75, 76]。因此，低植酸作物的優(yōu)質(zhì)種性，往往因其農(nóng)藝性狀劣變表現(xiàn)，難以進(jìn)一步在育種中得以推廣和應(yīng)用。探明低植酸突變作物籽粒灌漿特點(diǎn)，以及低植酸作物籽粒灌漿不良和產(chǎn)量下降的生理機(jī)制及其與植酸合成積累間的代謝生理聯(lián)系，對(duì)于通過育種、栽培等途徑協(xié)調(diào)作物籽粒植酸含量降低與產(chǎn)量性狀劣變之間的矛盾具有重要的理論和生產(chǎn)指導(dǎo)意義。

籽粒植酸含量的降低也會(huì)伴隨作物生態(tài)適應(yīng)性的改變。據(jù)Meis等[69]報(bào)道，型大豆和玉米低植酸突變體在適宜的溫度區(qū)域種植，其子代的田間出苗率為63%，同一低植酸材料種植于高溫區(qū)域，其子代的出苗率僅為8%，而野生型對(duì)照品種在不同生態(tài)區(qū)域的出苗率差異卻不明顯。相似地，在非脅迫環(huán)境中具有優(yōu)良田間農(nóng)藝表現(xiàn)的低植酸大麥(胚乳特異性)，在逆境條件下產(chǎn)量也表現(xiàn)出顯著下降的趨勢(shì)[77]。Bregitzer等[68]研究表明，低植酸作物農(nóng)藝性狀的劣變程度在逆境條件下尤為明顯，并把這一現(xiàn)象稱為“種源效應(yīng)”。前人對(duì)“種源效應(yīng)”的分析認(rèn)為，低植酸作物早衰以及籽粒成熟過程中化學(xué)結(jié)構(gòu)/組分的變化可能是后代籽粒萌發(fā)率降低的主要原因[1, 46, 69]。然而，環(huán)境生態(tài)因子或外源物質(zhì)調(diào)控對(duì)作物籽粒植酸積累的影響是否還與品種本身的植酸遺傳特性有關(guān)？即關(guān)于低植酸作物的農(nóng)藝表現(xiàn)的爭(zhēng)議是否忽略了其環(huán)境生態(tài)穩(wěn)定性變化？Naidoo等[78]研究表明，相比野生對(duì)照，型低植酸玉米()對(duì)干旱脅迫表現(xiàn)更敏感。Su等[71]利用多對(duì)低植酸水稻進(jìn)行灌漿期高溫脅迫處理，結(jié)果證實(shí)水稻籽粒植酸的種性與其在逆境條件下的生態(tài)穩(wěn)定性之間也存在密切聯(lián)系。與野生型對(duì)照相比，低植酸水稻對(duì)逆境表現(xiàn)更“敏感”，其籽粒植酸含量和結(jié)實(shí)特性(結(jié)實(shí)率、花粉育性和千粒重)的生態(tài)穩(wěn)定性也明顯變差。

雖然近年來利用基因工程選用器官特異性啟動(dòng)子(如胚、糊粉層等特異性啟動(dòng)子和)對(duì)植酸代謝相關(guān)基因(如、、和等)進(jìn)行沉默，篩選出的低植酸突變體(玉米、大麥、水稻)表現(xiàn)出了與野生型對(duì)照相似的農(nóng)藝性狀特征[1, 23, 26, 39, 46, 79]。如Kuwano等[23]以糊粉層特異表達(dá)的啟動(dòng)子對(duì)水稻()基因進(jìn)行沉默，籽粒植酸含量顯著降低的同時(shí)，產(chǎn)量性狀并未受到顯著影響。對(duì)植酸代謝路徑中下游基因的沉默和敲除(如等)，由于未影響到低價(jià)磷酸肌醇及1,4,5-3-磷酸肌醇等信號(hào)物質(zhì)的合成，低植酸突變體的產(chǎn)量?jī)?yōu)勢(shì)也得以保留[32]。然而上述低植酸突變體及其野生型的產(chǎn)量對(duì)照多在初代遺傳材料之間或單一生態(tài)環(huán)境條件下進(jìn)行，低植酸性狀的同源性還有待進(jìn)一步提高。因此，低植酸突變體是否能表現(xiàn)出與野生型對(duì)照相似的農(nóng)藝表現(xiàn)還有待進(jìn)一步觀察。事實(shí)上，Raboy等[77]經(jīng)多代篩選，并進(jìn)行了多年、多點(diǎn)的大田試驗(yàn)，發(fā)現(xiàn)初代具有產(chǎn)量?jī)?yōu)勢(shì)的低植酸突變體，純合后的農(nóng)藝性狀再次表現(xiàn)出一定的劣變特征，如花期延遲、萌發(fā)率降低等。因此，低植酸作物品質(zhì)和產(chǎn)量的同步提升在未來品質(zhì)育種與改良中依然是個(gè)挑戰(zhàn)。

5 低植酸水稻的農(nóng)藝性狀劣變和生態(tài)適應(yīng)性降低的生理原因

明確低植酸作物產(chǎn)量降低的生理機(jī)制及其與植酸合成積累的生理聯(lián)系，是協(xié)調(diào)與優(yōu)化低植酸作物品質(zhì)與產(chǎn)量潛力的基礎(chǔ)。低植酸作物農(nóng)藝性狀變劣、結(jié)實(shí)/灌漿障礙、產(chǎn)量水平下降可能與以下四方面有關(guān)：

1)肌醇及磷酸肌醇是植酸代謝的主要產(chǎn)物，在胞間信號(hào)轉(zhuǎn)導(dǎo)以及磷酸肌醇信號(hào)轉(zhuǎn)導(dǎo)[如1,4,5-3-磷酸肌醇作為胞內(nèi)第二信使通過誘導(dǎo)Ca2+離子釋放激活信號(hào)級(jí)聯(lián)、肌醇1-3,4-5-6-5磷酸(IP5)是COI1-JAZ信號(hào)代謝路徑的配體][80]、激素調(diào)控/代謝與平衡(肌醇與生長(zhǎng)素受體結(jié)合參與逆境響應(yīng)、葡萄糖醛酸代謝)[50]、鈣和糖信號(hào)生理[81, 82]、糖轉(zhuǎn)運(yùn)、碳水化合物代謝(寡聚糖、蔗糖和淀粉合成)、生長(zhǎng)發(fā)育調(diào)節(jié)、生物和非生物脅迫[83, 84]、滲透調(diào)節(jié)和保護(hù)、磷貯存和平衡[85, 86]、萌發(fā)、光合形態(tài)建成[87]、質(zhì)膜和細(xì)胞壁合成、膜/囊泡轉(zhuǎn)運(yùn)、能量調(diào)控、基因調(diào)控、染色體修飾和重組[88]、DNA 修復(fù)、mRNA輸送/轉(zhuǎn)運(yùn)、細(xì)胞程序性死亡、環(huán)醇合成、多元化合物貯存以及病原體防御中均發(fā)揮著重要的生理調(diào)控作用[50, 74, 89-92]。以肌醇為例，肌醇會(huì)通過抗壞血酸、磷脂酰肌醇等途徑參與植物抗逆生理。研究表明肌醇甲基化后形成的甲酯、D-芒柄醇或D-松醇等代謝物質(zhì)，可通過保護(hù)細(xì)胞結(jié)構(gòu)和維持膨壓提高植物對(duì)干旱脅迫的耐受能力[93, 94]。過表達(dá)基因會(huì)引起肌醇含量同步增加，作物耐鹽性也同步增強(qiáng)[94, 95]。這些代謝過程涉及籽粒從萌發(fā)、出苗、營(yíng)養(yǎng)生長(zhǎng)以及灌漿等發(fā)育過程的各個(gè)方面，植酸代謝路徑中相關(guān)基因的突變，不僅會(huì)直接降低籽粒植酸含量，還會(huì)影響作物生長(zhǎng)發(fā)育的其他生理過程，降低作物的逆境適應(yīng)性，并最終體現(xiàn)為農(nóng)藝性狀的變化[50]。除植酸代謝的中間產(chǎn)物外，調(diào)控植酸代謝相關(guān)的酶基因(如等)也參與了多個(gè)生理代謝過程，如除是植酸合成的限速酶外，對(duì)逆境響應(yīng)也較為敏感，高溫、冷害、干旱、脫水和強(qiáng)光等逆境脅迫均會(huì)誘導(dǎo)基因的顯著上調(diào)表達(dá)[8, 96-100]。Das-Chatterjee等[101]將基因轉(zhuǎn)入煙草，轉(zhuǎn)基因后代的耐鹽性得以顯著提升。此外，植酸合成代謝途徑的中間產(chǎn)物還可能通過調(diào)控磷酸肌醇信號(hào)傳導(dǎo)和ATP能量轉(zhuǎn)換等生理代謝過程對(duì)低植酸作物的生長(zhǎng)發(fā)育和結(jié)實(shí)產(chǎn)生影響。

2)低植酸作物在籽粒植酸含量降低的同時(shí)，無機(jī)磷含量顯著增加。由于常規(guī)水稻品種的籽粒植酸主要隔離于液泡中，其相對(duì)獨(dú)立的細(xì)胞區(qū)位保證了胞內(nèi)的離子平衡，可能并不會(huì)影響籽粒中包括淀粉、蛋白以及脂類在內(nèi)的其他內(nèi)含物的合成和積累。但當(dāng)植酸含量顯著降低后，游離出的過量無機(jī)磷不僅會(huì)打破細(xì)胞內(nèi)的離子平衡，同時(shí)還會(huì)通過抑制ADPG焦磷酸化酶和淀粉磷酸化酶的活性影響淀粉的合成及積累，從而影響發(fā)育籽粒的正常灌漿和充實(shí)[102]。也有研究認(rèn)為過量無機(jī)磷的胞外泄漏可能是低植酸作物易感病和籽?；盍档偷脑蛑籟1]；

3)植酸代謝路徑較為復(fù)雜，涉及多條代謝路徑、多種酶/基因、多底物、多器官共同協(xié)調(diào)參與調(diào)控。除籽粒高表達(dá)的植酸代謝相關(guān)基因(如、、)外，營(yíng)養(yǎng)器官(、、、、)及花器()中植酸(或磷)代謝受阻也會(huì)影響光合同化物的合成、運(yùn)輸與分配[10]。

6 籽粒植酸合成的環(huán)境調(diào)控效應(yīng)

作物籽粒植酸積累除與作物類型和品種基因型的高遺傳力有關(guān)外，還顯著受生態(tài)因素和栽培環(huán)境的影響。土壤類型/結(jié)構(gòu)、種植區(qū)域、肥水管理(磷肥、氮肥和鋅肥運(yùn)籌)、氣象因素、種植年份或播期、溫室效應(yīng)等，均會(huì)顯著影響作物籽粒的植酸積累[103-115]。Liu等[116]通過多點(diǎn)生態(tài)區(qū)域種植，對(duì)24個(gè)常規(guī)粳稻品種的籽粒植酸進(jìn)行分析，結(jié)果表明品種基因型、環(huán)境條件以及基因型和環(huán)境的互作效應(yīng)(G×E)均會(huì)顯著影響水稻籽粒的植酸含量，其中以環(huán)境(種植地點(diǎn))效應(yīng)最為顯著。Magallane-Lopez等[117]對(duì)小麥的研究結(jié)果表明，環(huán)境效應(yīng)對(duì)籽粒中植酸和鐵的生物有效性的影響最為顯著，且對(duì)鐵的生物有效性的影響(57.8%)高于對(duì)植酸(46%)的影響。Su等[71]和Hummel等[118]在水稻和大豆的研究結(jié)果表明，灌漿期高溫或全球氣候變化所導(dǎo)致的干旱等逆境脅迫會(huì)顯著增加作物籽粒中的植酸含量。由于植酸是磷的最主要貯存形式，多數(shù)研究結(jié)果均表明土壤磷水平會(huì)對(duì)作物籽粒中的植酸積累產(chǎn)生顯著正調(diào)控的影響。這可能是不同種植區(qū)域、土壤類型條件下作物籽粒植酸含量顯著變化的原因之一。Gibson等[119]和Thavarajah等[110]以豆科作物為研究對(duì)象，發(fā)現(xiàn)高溫脅迫下籽粒植酸含量和總磷積累量均呈顯著增加的趨勢(shì)。同一大田中同一作物品種在不同播期和年際效應(yīng)所引起的籽粒植酸含量變異，可能就與作物灌漿結(jié)實(shí)期間的溫度變化有關(guān)。此外，肥水管理也會(huì)顯著影響作物籽粒的植酸積累，合理氮肥運(yùn)籌可同步實(shí)現(xiàn)小麥籽粒植酸降低以及蛋白質(zhì)含量增加[120]，且不同氮源形態(tài)(氯化銨、硫酸銨和尿素)對(duì)籽粒植酸含量的影響一致[121]。灌漿初期葉片噴施鋅肥可抑制植酸積累、提高籽粒鋅的生物有效性[121, 122]。適宜的水肥調(diào)控也會(huì)使水稻籽粒中植酸含量有所降低[123]。相反，增施磷肥可引起谷物籽粒中植酸含量顯著上升[124-128]。最近的研究進(jìn)一步表明，外源無機(jī)磷供應(yīng)雖然能在一定程度上提高籽粒植酸含量和積累量，但過量磷水平下，水稻籽粒的植酸積累量卻表現(xiàn)出顯著降低的趨勢(shì)[124]。明確低植酸突變水稻的環(huán)境調(diào)控效應(yīng)，對(duì)進(jìn)一步提高低植酸作物產(chǎn)量的同時(shí)，保持低植酸的優(yōu)質(zhì)種性發(fā)揮具有重要的參考價(jià)值。

7 研究展望

關(guān)于植物籽粒植酸合成的大致路徑現(xiàn)已基本清楚，相關(guān)關(guān)鍵酶及其編碼基因的功能、克隆和定位也已在多種作物中得到驗(yàn)證，如、、、、、、、和等。然而在脂獨(dú)立途徑中，低價(jià)磷酸肌醇之間(如IP1和IP2等)、低價(jià)磷酸肌醇向高價(jià)磷酸肌醇轉(zhuǎn)化的過程，以及脂依賴途徑中磷脂酰肌醇向1,3,4,5,6-5磷酸肌醇轉(zhuǎn)化的過程，究竟還受到哪些其他關(guān)鍵調(diào)控位點(diǎn)或酶(基因)催化的影響，目前的研究還較少。未來利用數(shù)量性狀基因座(quantitative trait locus, QTL)、精細(xì)定位、基因組測(cè)序、關(guān)聯(lián)分析、關(guān)鍵調(diào)控基因克隆和定位以及CRISPR-Cas9基因編輯等方法，并結(jié)合ICS-HPLC對(duì)不同價(jià)位磷酸肌醇衍生物及其同分異構(gòu)體(InsP1-InsP5)變化進(jìn)行分析，明確低價(jià)肌醇磷酸的磷酸化過程，有利于進(jìn)一步明確植酸的生物合成過程、遺傳機(jī)理以及兩條代謝路徑的功能特點(diǎn)。同時(shí)，明確相關(guān)轉(zhuǎn)錄因子在植酸合成以及調(diào)控中的作用，可繼續(xù)豐富低植酸的突變類型，為篩選具有產(chǎn)量?jī)?yōu)勢(shì)的低植酸基因型提供依據(jù)。此外，篩選低植酸和低總磷雙性狀水稻基因型，在改善作物品質(zhì)的同時(shí)，還能進(jìn)一步節(jié)約磷肥資源。考慮到水稻籽粒植酸積累的環(huán)境調(diào)控效應(yīng)以及低植酸水稻突變體在逆境生態(tài)系統(tǒng)中的不穩(wěn)定性，如何利用栽培調(diào)優(yōu)(如肥料運(yùn)籌、環(huán)境調(diào)控、株型改造)或外源調(diào)控等措施，確保低植酸的優(yōu)質(zhì)種性發(fā)揮的同時(shí)，進(jìn)一步提升其產(chǎn)量潛力，也是栽培學(xué)研究的重要方向。這些研究將會(huì)對(duì)以鋅、鐵生物強(qiáng)化為目標(biāo)的水稻品質(zhì)育種提供借鑒和補(bǔ)充。

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Research Advances on the Low Phytic Acid Rice Breeding and Their Genetic Physiological Regulation and Environmental Adaptability

SU Da1,2, WU Liangquan2, S?ren K Rasmussen3, ZHOU Lujian4, CHENG Fangmin4,*

(,,,,,;International Magnesium Institute,,;Department of Plant and Environmental Sciences,,,,;,,,;,:)

Breeding variety with improved quality while maintaining or improving yields is one of the primary objectives in rice breeding. Among which, reducing the anti-nutritional factors, such as grain phytic acid content, is an effective strategy to cope with hidden hunger and increase grain bioavailabilities of zinc and iron. In this paper, we reviewed the biosynthesis of phytic acid and the genetic characteristics of related functional genes, the co-regulatory networks of phytic acid synthesis and other physiological metabolism, breeding of low phytic acid () germplasm resource and their genetic characteristics, agronomic performance and environmental ecological adaptability ofmutants, the possible reasons for their agronomic deterioration and ecological adaptation change, and the environmental regulation of grain phytic acid accumulation. Those contents could provide reference for production ofrice with suitable agronomic cultivation practices.

phytic acid; rice (L.); grain nutrition; genetic regulation; ecological effect

10.16819/j.1001-7216.2019.8083

S482.8; S511.02

A

1001-7216(2019)02-0095-13

2018-07-16;

2018-12-31。

國(guó)家自然科學(xué)基金資助項(xiàng)目(31571602和31271655); 福建省中青年教師教育科研項(xiàng)目（JAT170156）; 國(guó)家留學(xué)基金委資助項(xiàng)目; 國(guó)家重點(diǎn)研發(fā)計(jì)劃資助項(xiàng)目(2017YFD0200200)。