劉立軍 周沈琪 劉 昆 張偉楊 楊建昌

水稻大穗形成及其調(diào)控的研究進展

劉立軍*周沈琪 劉 昆 張偉楊 楊建昌

揚州大學江蘇省作物遺傳生理重點實驗室 / 江蘇省糧食作物現(xiàn)代產(chǎn)業(yè)技術協(xié)同創(chuàng)新中心 / 江蘇省作物基因組學和分子育種重點實驗室 / 農(nóng)業(yè)農(nóng)村部耕地質(zhì)量監(jiān)測與評價重點實驗室, 江蘇揚州 225009

水稻每穗粒數(shù)是構成產(chǎn)量的關鍵因素, 現(xiàn)代高產(chǎn)水稻品種多表現(xiàn)為穗大粒多。增加每穗粒數(shù)、促進大穗形成是提高水稻產(chǎn)量的重要途徑。本文概述了水稻每穗粒數(shù)形成與幼穗發(fā)育的關系, 并結合作者相關研究, 從水稻穗型的遺傳調(diào)控、營養(yǎng)狀況與氮肥管理、水分、溫光條件和內(nèi)源激素等方面分析了其對每穗粒數(shù)形成的影響。從根系形態(tài)生理與幼穗發(fā)育、水肥管理和溫光條件以及植物激素間的相互作用調(diào)節(jié)穎花分化與退化的生理和分子機制等方面提出了未來加強水稻大穗形成研究的重點, 以期為大穗高產(chǎn)水稻品種選育和栽培調(diào)控提供依據(jù)。

水稻; 大穗; 穎花分化與退化; 水肥管理; 激素

水稻是世界上最重要的糧食作物之一, 全球一半人口以稻米為主食[1-3]。目前水稻單產(chǎn)年增長率僅為1.70%, 顯著低于2050年將總產(chǎn)翻一番所需2.4%的年增長率[4-5]。水稻增產(chǎn)主要通過擴大種植面積和提高單位面積產(chǎn)量2條路徑, 但由于耕地面積逐漸減少、水資源匱乏的現(xiàn)狀, 靠擴大種植面積實現(xiàn)水稻增產(chǎn)的潛力非常有限[6]。因此, 提高單位面積產(chǎn)量是增加水稻總產(chǎn)量的有效途徑。水稻產(chǎn)量由單位面積穗數(shù)、每穗粒數(shù)、結實率和粒重構成。單位面積穗數(shù)受空間條件制約, 存在一定程度上的飽和; 粒重主要受遺傳因素控制, 表現(xiàn)相對穩(wěn)定; 結實率不僅取決于水分和養(yǎng)分的供應, 還取決于成熟期的光溫條件; 而每穗粒數(shù)的可變性與可調(diào)性較大, 高產(chǎn)水稻品種通常具有穗型大、每穗粒數(shù)多、單穗重量高(大穗)的特點[7-8]。因此, 增加每穗粒數(shù)、促進大穗形成是提高水稻產(chǎn)量的重要途徑[9-11]。本文綜述了水稻每穗粒數(shù)與幼穗發(fā)育的關系以及影響每穗粒數(shù)形成的主要因素, 提出了未來加強水稻每穗粒數(shù)研究的重點。旨在進一步揭示水稻大穗形成的機制, 為大穗高產(chǎn)水稻品種選育和栽培調(diào)控提供依據(jù)。

1 幼穗發(fā)育與每穗粒數(shù)形成

稻穗為圓錐花序, 由穗軸、一次枝梗、二次枝梗、小穗梗和小穗(小花)組成。每穗粒數(shù)在穗分化期形成并確定, 是小花分化、發(fā)育和退化等一系列復雜生理過程的最終體現(xiàn)[12-13]。國內(nèi)外學者從形態(tài)和解剖等方面對稻穗的發(fā)育過程進行了詳細研究[14]。水稻幼穗發(fā)育大致可分為2個階段, 第一階段自穗軸分化期(苞分化期)至穎花原基分化期結束, 此階段主要分化一、二次枝梗和穎花; 第二階段自雌雄蕊分化期至抽穗, 主要完成枝梗伸長、穎花發(fā)育、花器官形成、減數(shù)分裂和花粉粒充實完成等過程[15]。幼穗分化第一階段主要影響穎花分化數(shù)的形成, 以二次枝梗及穎花原基分化期對穎花分化的促進作用最為顯著, 且穎花分化數(shù)與二次枝梗分化數(shù)呈極顯著正相關[16]。第二階段主要影響穎花退化數(shù), 以花粉母細胞減數(shù)分裂期對穎花退化的影響最為明顯?；ǚ勰讣毎麥p數(shù)分裂期是水稻每穗穎花數(shù)從分化增加到退化減少的消長轉(zhuǎn)折時期, 這一時期的幼穗正在快速伸長且接近全長, 絕大多數(shù)穎花已經(jīng)分化完成, 劣勢穎花正在急劇退化[17]。

穎花退化是糧食減產(chǎn)的一個重要原因。闡明穎花分化與退化的機制是當今水稻栽培學家和育種學家關注的熱點問題之一。前人對穎花分化與退化的認識歸納起來主要有以下4種理論。第一種是“資源限制”假設。在非脅迫條件下, 水稻節(jié)間伸長與幼穗原基分化發(fā)育之間存在“器官同伸”關系, 此時幼穗與莖鞘會發(fā)生養(yǎng)分競爭, 從而影響幼穗分化發(fā)育。在脅迫條件下, 處于下部的穎花養(yǎng)分供應變得不足[18-19], 缺少非結構性碳水化合物供應會導致其在強烈的養(yǎng)分競爭下營養(yǎng)不良, 最終劣勢穎花退化[20]。但亦有研究表明, 即使在良好的營養(yǎng)供應條件下, 穎花仍會嚴重退化。因此, “資源限制”可能只是穎花退化的原因之一, 而非主要原因。第二種是“自組織過程”假設。該假設認為幼穗發(fā)育之初同一稻穗內(nèi)部不同穎花相互競爭, 營養(yǎng)物質(zhì)分配不均勻, 這種初始的資源不平衡在穎花分化前期表現(xiàn)為優(yōu)勢穎花分化能力提升, 在后期擴大并形成優(yōu)勢, 通過自催化過程獲得更多的資源, 不能獲得足夠資源的劣勢穎花則發(fā)生退化[21]。然而大多數(shù)谷物中的穎花退化并不是隨機的, 如水稻小穗的穎花退化通常發(fā)生在穗基部和頂部。因此, 上述2種假說尚未找到更加有力的證據(jù)來支持。第三種是“植物激素平衡”理論。植物激素是影響穎花分化和退化的關鍵調(diào)節(jié)因子。一般認為, 生長素(indole-3-acetic, IAA)、細胞分裂素(cytokinins, CTK)和赤霉素(gibberellins, GAs)可促進穎花分化, 而脫落酸(abscisic acid, ABA)和乙烯(ethylene, ET)則會引起穎花退化[16]。第四種是“碳氮代謝平衡”理論。穎花分化和退化與植株器官的碳氮代謝平衡存在密切關系。有研究表明, 在穎花原基分化期, 水稻地上部的含氮量與穎花分化數(shù)呈顯著正相關, 植株含氮量越高, 穎花分化數(shù)越多, 但穎花退化數(shù)也會增加[22]。在花粉母細胞減數(shù)分裂期, 每穗穎花數(shù)與穗分化期整株碳積累量呈直線相關, 植株體內(nèi)積累的非結構性碳水化合物越多, 穎花退化率越低[22]。幼穗發(fā)育過程中的碳氮比例達到一個相互促進的平衡狀態(tài)才能有效促進穎花分化, 減少退化, 增加穎花現(xiàn)存數(shù)[23]。

2 水稻穗型的遺傳調(diào)控

轉(zhuǎn)錄組研究顯示, 在22,000個基因中, 有357個基因在穗部發(fā)育中發(fā)揮了不同的作用, 表明仍有許多基因尚未被識別。在目前已被識別的基因中,()基因是第一個被克隆的控制水稻穗粒數(shù)的主效數(shù)量性狀基因座, 它是與細胞分裂素氧化酶/脫氫酶高度同源的()基因。當表達水平降低時, CTK會在花序分生組織上大量積累, 增加枝梗數(shù)和籽粒數(shù), 而活性CTK合成缺陷突變體的穗部枝梗以及籽粒數(shù)目顯著下降[24]。()基因可直接通過調(diào)控的表達來影響水稻穗整體枝梗與籽粒的數(shù)目[25]。另外, 過表達以及干擾CTK應答的GATA轉(zhuǎn)錄因子都會降低水稻枝梗數(shù)以及穗粒數(shù)[26]。()基因編碼一個赤霉素20-氧化酶, 參與GAs信號調(diào)控。該基因在水稻穗分生組織中通過調(diào)控()基因的轉(zhuǎn)錄反饋環(huán), 增強CTK活性, 進而促進編碼赤霉素20-氧化酶的的轉(zhuǎn)錄, 抑制活性GA1和GA3的積累, 從而增加穗粒數(shù)[27]。這些結果也從另一方面說明了CTK相關基因的表達對每穗粒數(shù)的形成起到了至關重要的調(diào)控作用。()、()和()是與水稻抽穗期感光性相關的3個主效數(shù)量性狀基因座, 它能同時控制水稻株高、每穗粒數(shù)和抽穗期3個性狀。單(敲除和)促進穗粒數(shù)形成, 而單(敲除和)抑制穗粒數(shù)形成。日照長短對上述3個基因的表達水平也有重要影響作用, 長日照下,促進表達, 并被和招募形成不同的抑制復合體, 產(chǎn)生不同程度的延遲抽穗或不抽穗。而短日照下,表達量很低,與抑制復合體間表現(xiàn)出競爭關系,表達量的增加可以不同程度地促進抽穗和穗粒數(shù)形成[28-29]。主要在幼嫩組織中表達, 它的高表達量能夠顯著增加一次枝梗數(shù)和二次枝梗數(shù), 尤其是二次枝梗的數(shù)量。也有研究觀察到,()能促進細胞分裂, 增加枝梗數(shù)和每穗籽粒數(shù), 進而促進水稻增產(chǎn)[30]。除此之外, 每穗粒數(shù)還受到()、()、()和()等基因調(diào)控[28,31]。

3 影響水稻每穗粒數(shù)形成的主要因素

3.1 營養(yǎng)狀況與氮肥管理

水稻穎花現(xiàn)存數(shù)與氮素積累量和穗發(fā)育階段的營養(yǎng)狀況、生物積累量密切相關。水稻大穗形成的前提是充足的營養(yǎng)生長量和碳氮代謝水平的協(xié)調(diào)穩(wěn)定, 這也是源強的直接表現(xiàn)[13]。當?shù)鼗蛏锓e累量減少時, 穎花分化數(shù)會隨之減少。水稻幼穗發(fā)育過程中吸收的氮、磷、鉀等礦物質(zhì)營養(yǎng)元素主要用于構成碳水化合物、蛋白質(zhì)和核酸。碳水化合物主要用來構建穗、枝梗和小花, 蛋白質(zhì)和核酸則主要用于減少枝梗和穎花的退化、充實生殖器官[32]。在水稻由營養(yǎng)生長向生殖生長轉(zhuǎn)化的階段, 莖稈和幼穗處于共同旺盛生長時期, 互相競爭同化物[33-34]。葉片含糖量的增加是保證幼穗分化和水稻植株迅速生長的基礎, 幼穗分化始期營養(yǎng)不足, 穎花分化數(shù)將明顯減少, 影響結實粒數(shù)[8,35]。穎花干物質(zhì)量越低, 退化就越多; 反之, 穎花干物質(zhì)量越高, 枝梗和穎花退化越少, 每穗粒數(shù)則越多[11,36]。幼穗與莖稈競爭同化物的能力直接影響到穗粒數(shù)的形成。相對于莖稈來說, 幼穗是一個較弱的庫。當水稻進入花粉母細胞減數(shù)分裂期后, 幼穗對營養(yǎng)物質(zhì)的競爭能力大于莖稈[37]。此期過多無效分蘗和穗數(shù)將再次引起空間和養(yǎng)分的競爭, 最終導致稈細穗弱, 穎花發(fā)育不良。

每穗穎花數(shù)由穎花分化數(shù)和退化數(shù)決定。研究表明, 適宜的氮素水平可以顯著提高水稻的穎花分化數(shù), 從而增加每穗粒數(shù)[38-39]。氮肥的合理施用, 尤其是穗肥的合理用量與每穗粒數(shù)形成密切相關。作者研究觀察到, 在0～216 kg hm–2氮素穗肥范圍內(nèi), 增加施氮量可不同程度提高不同穗型超級稻品種一次枝梗和二次枝梗穎花分化數(shù)。穎花退化數(shù)和一次枝梗穎花退化率隨穗肥施氮量的增加而增加, 而二次枝梗穎花退化率隨穗肥施氮量增加呈先降后升的變化趨勢。不同穗型水稻品種對穗肥的響應存在明顯差異, 小穗型水稻品種每穗穎花數(shù)對穗肥的響應更大。如與不施穗肥相比, 施用穗肥后的小穗型品種南粳9108、中穗型品種揚兩優(yōu)6號和大穗型品種甬優(yōu)1540每穗二次枝梗穎花現(xiàn)存數(shù)分別增加了25.0%～57.6%、10.5%～40.8%和7.1%～25.3% (圖1)。

碳氮代謝平衡是水稻幼穗正常發(fā)育的基礎, 一般認為, 氮代謝旺盛有利于營養(yǎng)生長, 而碳代謝旺盛有利于生殖生長[41-42]。在幼穗分化前期, 施用一定量的氮素穗肥對葉片的當時效應是“得氮耗糖”, 但由于葉片等營養(yǎng)器官的相對生長量變化, 在花粉母細胞減數(shù)分裂期后會出現(xiàn)“低氮高糖”現(xiàn)象, 這種現(xiàn)象有利于增加每穗穎花數(shù)。氮素營養(yǎng)可通過調(diào)節(jié)葉片光合作用和碳氮物質(zhì)分配比例, 使一部分碳氮物質(zhì)繼續(xù)供應植株生長, 另一部分貯藏于幼穗、葉片等器官中, 并調(diào)節(jié)其相對生長[43-44]。李剛?cè)A等[45]觀察到從穗分化開始到抽穗前20 d, 植株含氮量越高, 穎花分化數(shù)越多; 從抽穗前16 d開始, 植株體內(nèi)積累的非結構性碳水化合物越多, 穎花退化率越低。在花粉母細胞減數(shù)分裂期前后, 幼穗急劇生長, 穎花急劇增大, 水稻從營養(yǎng)生長過渡到生殖生長, 此過程以碳積累為主, 需要大量碳水化合物, 此期施用適當比例氮肥可以防止穗下部的枝梗和穎花退化。水稻葉片需要保持一定的氮含量, 才有利于光合作用的進行。施氮肥的植株比不施或少施氮肥的植株得到更多的氮素供應, 可使一些弱勢穎花得以生存而減少退化[46]。氮肥一方面通過影響穗分化期植株的碳氮代謝平衡影響穎花量的多少, 另一方面還可通過調(diào)節(jié)幼穗中IAA和CTK的時空分布來調(diào)控穎花發(fā)育[47], 這也表明植物激素在養(yǎng)分管理調(diào)控水稻穎花發(fā)育的過程中發(fā)揮著重要作用。

圖1 穗肥施氮量對不同穗型超級稻品種一、二次枝梗穎花分化和退化的影響

Nanjing 9108、Yangliangyou 6和Yongyou 1540分別代表小穗型(每穗粒數(shù)130粒左右)、中穗型(每穗粒數(shù)220粒左右)和大穗型(每穗粒數(shù)300粒左右)品種南粳9108、揚兩優(yōu)6號和甬優(yōu)1540。0N、54N、108N、162N和216N分別代表穗肥施氮量為0、54、108、162和216 kg hm–2。DiN、SN、DeN和DR分別代表分化數(shù)、退化數(shù)、現(xiàn)存數(shù)和退化率。該圖根據(jù)參考文獻[40]改制。

Nanjing 9108, Yangliangyou 6, and Yongyou 1540 respectively represent small panicle size (about 130 spikelets per panicle), medium panicle size (about 220 spikekets per panicle) and large panicle size (about 300 spikelets per panicle) varieties of rice. 0N, 54N, 108N, 162N, and 216N represent the panicle nitrogen fertilizer rate of 0, 54, 108, 162, and 216 kg hm–2, respectively. DiN, SN, DeN, and DR represent differentiated number, degenerated number, surviving number, and degeneration rate, respectively. The figure is adapted from the reference of [40].

3.2 水分

幼穗分化期是水稻一生中生理需水量最多的時期?；ǚ勰讣毎麥p數(shù)分裂期對水分最為敏感, 此期遭受水分脅迫可能造成穎花退化數(shù)增加或花粉不育而影響總穎花量和結實粒數(shù), 不利于形成大穗[40,48]。有研究表明, 水分脅迫可顯著提高水稻葉鞘中氮的比例, 水分的缺失使得葉片光合作用的同化產(chǎn)物優(yōu)先運往新生葉片和正在伸長的莖稈, 而后運往穗部, 此時營養(yǎng)物質(zhì)供應不足就會導致穎花分化數(shù)減少、退化數(shù)增加[49]。因此, 自減數(shù)分裂開始至抽穗開花結束, 生產(chǎn)上一般采用水層灌溉的方法。但也有研究表明, 長期淹水條件下根層土壤環(huán)境惡化會影響水稻根系和地上部的生長發(fā)育, 不利于穎花發(fā)育和最終穗粒數(shù)形成[13,50]。

我們最近的研究表明, 與傳統(tǒng)淹水灌溉相比, 輕干濕交替灌溉(土壤落干期土水勢不低于-15 kPa或中午葉片水勢不低于-1.2 MPa)有利于促進穎花分化、減少穎花退化, 最終有利于促進水稻大穗形成[40]。其主要原因是土壤落干后復水使根系產(chǎn)生一種積極的自我調(diào)節(jié)能力(補償效應), 即水稻受到輕度水分脅迫時, 某些生理功能略有降低或生長受阻, 但在復水后卻能超過正常供水狀態(tài)[48,51-53]。具體表現(xiàn)為增加了根系縱向伸長和橫向擴展, 提高了根系生物量和根毛數(shù)量以及根系吸收表面積, 擴大了根系對水分和養(yǎng)分的吸收范圍, 增加了穗分化前期單莖干物質(zhì)和非結構性碳水化合物的積累量, 進而有利于增加水稻二次枝梗穎花分化數(shù)、降低二次枝梗穎花退化數(shù)與退化率, 提高水稻穎花現(xiàn)存數(shù), 最終獲得高產(chǎn)(表1)。

表1 穗分化期水分處理對不同穗型水稻品種穎花分化和退化的影響

CF: 常規(guī)灌溉; AWMD: 干濕交替灌溉; DiN: 分化數(shù); DeN: 退化數(shù); SN: 現(xiàn)存數(shù); DR: 退化率。標以不同字母表示在= 0.05水平上差異顯著, 同欄內(nèi)同一品種間比較。該表根據(jù)參考文獻[54]改制。

CF: conventional irrigation; AWMD: alternate wetting and moderate soil drying; DiN: differentiated number; DeN: degenerated number; SN: surviving number; DR: degeneration rate. Different lowercase letters within the same column indicate significant differences at the= 0.05 level within the same variety. The table is adapted from the reference of [54].

3.3 溫光條件

溫度和光照是影響水稻每穗粒數(shù)的重要環(huán)境因子。幼穗分化期最適溫度為26～30℃, 溫度過高或過低均不利于每穗粒數(shù)形成。幼穗分化期遇高溫脅迫會減少水稻穎花分化數(shù)、增加退化數(shù)[33,55]?；ǚ勰讣毎麥p數(shù)分裂期對環(huán)境條件最為敏感, 此期受到高溫脅迫會導致IAA、CTK含量下降, ET釋放速率增加, 花粉代謝紊亂[56-57]。水稻在幼穗分化期遇高溫將積累大量氧化物, 氧離子、過氧化氫(H2O2)、丙二醛(MDA)等氧化物含量上升。隨著高溫脅迫時間的增加, 活性氧的生成速度已經(jīng)遠遠大于清除速度, 過氧化物酶(POD)、過氧化氫酶(CAT)和超氧化物歧化酶(SOD)等抗氧化酶活性受到抑制, 細胞膜受到傷害而影響水稻器官正常建成, 導致穎花退化數(shù)增加[58]。幼穗發(fā)育過程中, 花粉母細胞減數(shù)分裂期后2～3 d是水稻對低溫最為敏感的時期, 這一時期如遇17℃以下的低溫環(huán)境, 會降低穎花分化數(shù)和現(xiàn)存數(shù)、增加穎花退化數(shù)[59]。穗分化期低溫還會影響花粉粒的正常發(fā)育, 導致籽粒充實不良[48]?？傊? 極端高溫或極端低溫均不利于穎花分化, 且低溫天氣對穎花退化的影響明顯大于高溫天氣[60]。日照時數(shù)和光照強度對一、二次枝梗數(shù)和穎花數(shù)的形成有較大影響作用, 水稻幼穗分化期如沒有足夠的光強, 將影響營養(yǎng)物質(zhì)的積累, 導致枝梗分化速度減緩、穎花退化數(shù)增加[3,6]。

3.4 內(nèi)源激素

目前公認的植物激素有6類: IAA、GAs、CTK、ABA、ET和油菜素甾醇(brassinosteriods, BRs)。此外, 多胺(polyamines, PAs)、水楊酸(salicylic acid, SA)和茉莉酸(jasmonic acid, JA)也具有植物激素的特征[61]。

CTK主要是一種由根系合成、通過輸導組織運輸?shù)降厣喜糠謱χ参锷L發(fā)育起調(diào)控作用的植物激素[40,47,62]。CTK除具誘導細胞分裂的作用外, 還參與延緩葉片衰老[63-64]、頂端優(yōu)勢[65]、根系增殖[24]和生殖生長[66]等各種生理代謝過程。有研究表明, 水稻幼穗發(fā)育與CTK含量有明顯的同步性, 穎花分化期高濃度的CTK及其與IAA的比值有利于延長分化時間, 促進穎花分化, 抑制穎花退化[67]。CTK促進穎花發(fā)育的同時, 還能抑制IAA從分化較早的穎花中向外輸出, 削減幼穗生長的頂端優(yōu)勢, 使小穗生長具有一致性, 從而穩(wěn)定穎花分化數(shù)、減少退化數(shù)[63,65]。除了CTK和IAA, GAs也是一種促進植物生長發(fā)育的激素。有學者觀察到, 當水稻遭受鹽脅迫時, 噴施GAs可以有效抑制穎花退化[68]。但亦有研究發(fā)現(xiàn), 水稻在花粉母細胞減數(shù)分裂期遭受干旱時, 穎花退化數(shù)顯著增加, 但此時幼穗中的GAs含量并沒有下降[69]。因此, 水稻逆境脅迫下GAs水平對穎花分化與退化調(diào)控還有待進一步研究。

ET和ABA常被認為是抑制型植物激素[70-71]。前人觀察到, 高濃度的ET會誘發(fā)玉米和水稻等禾谷類作物的穎花(小花)退化或種子敗育。內(nèi)源ABA和ET還可能相互拮抗調(diào)控水稻穎花發(fā)育, 較高的ABA與ET比值能減少穎花退化[50,72]。此外, PAs、SA、JA對水稻穎花發(fā)育也有重要調(diào)控作用[63,66,73-74]。PAs廣泛存在于細胞各處并參與植物細胞代謝的整個過程, 如細胞分裂、DNA復制、轉(zhuǎn)錄和翻譯、細胞的生長衰老以及對環(huán)境脅迫的響應等。PAs通過與ET之間的拮抗作用響應適度的土壤干旱, 從而調(diào)控水稻穎花退化[35]。在高溫脅迫下, 噴施SA能顯著提高水稻穗部可溶性糖、脯氨酸、GAs、BRs、IAA等激素的含量和抗氧化酶的活性, 減少小穗退化[75-76]。JA是植物體內(nèi)一種重要的脂類激素。研究表明, JA可通過激活()、()和()等已克隆的茉莉素合成及轉(zhuǎn)導基因的表達來調(diào)控穎花的分化與退化[77]。

BRs主要由油菜素內(nèi)酯(brassinolid, BL)和油菜素甾酮(castasterone, CS)及其衍生物組成, 對花粉育性、穎花分化與退化有明顯調(diào)控作用[78]。其中, 24-表油菜甾酮(24-epocastasterone, 24-epiCS)和28-高油菜素內(nèi)酯(28-homobrassinolide, 28-homoBL)被認為是水稻體內(nèi)重要的BRs[79-80]。作者觀察到, 穎花原基分化期幼穗中較高含量的BRs (24-epiCS和28-homoBL)能促進穎花分化, 提高花粉母細胞減數(shù)分裂期幼穗中BRs含量有利于減少穎花退化(圖2)。BRs主要通過以下三方面調(diào)控穎花分化和退化。(1)增加BRs含量可促進()和()基因表達, 從而提高花序分生組織的活性, 延遲小穗分生組織分化, 延長枝梗發(fā)育時間, 有利于促進穎花分化, 最終增加每穗穎花數(shù)。(2) BRs生物合成量的增加提高了水稻具有MYB結構域蛋白的()基因表達, 并直接觸發(fā)幼穗中糖利用相關基因的表達, 可使幼穗從營養(yǎng)組織中獲得更多的碳同化物, 從而增加穎花分化數(shù)并降低穎花退化率。(3) BRs含量的提高發(fā)育幼穗中抗氧化系統(tǒng)的活性, 減少活性氧對幼穗細胞的傷害, 最終促進穎花分化, 并減少穎花退化。BRs調(diào)控幼穗發(fā)育的機制分析見圖3[81]。

4 研究與展望

增加每穗粒數(shù)、促進水稻大穗形成、增加庫容一直是水稻高產(chǎn)栽培和育種的重要目標。目前, 此方面已有大量研究, 并取得明顯進展。近年來, 以甬優(yōu)系列為代表的大穗型水稻品種在生產(chǎn)上應用廣泛,多地多年均表現(xiàn)高產(chǎn)穩(wěn)產(chǎn), 代表性品種甬優(yōu)1540、甬優(yōu)2640和甬優(yōu)12的每穗粒數(shù)超過300粒[83-84], 遠高于大面積生產(chǎn)中常用高產(chǎn)水稻品種的每穗粒數(shù), 但這些品種大穗形成的機制仍然不是十分明確?；谝延醒芯? 筆者認為將來對水稻大穗形成機制的研究有以下三方面需要加強。

(圖2)

YD-6: 揚稻6號; YY-2640: 甬優(yōu)2640; SPD: 穎花原基分化期; PMC: 花粉母細胞減數(shù)分裂期。該圖根據(jù)參考文獻[82]改制。**< 0.01。

YD-6: Yangdao 6; YY-2640: Yongyou 2640; SPD: spikelet primordium differentiation period; PMC: pollen mother cell meiosis period. The figure is adapted from the reference of [82].**< 0.01.

圖3 水稻幼穗發(fā)育過程中油菜素甾醇(BRs)作用的描述模型

Fig 3 A descriptive model for the role of brassinosteriods (BRs) in panicle growth and development of rice

黑色箭頭“→”表示增強, 紅色箭頭“→”表示抑制。IM activity: 分生組織活性; H2O2: 過氧化氫; O2: 氧氣; H2O: 水; AsA: 抗壞血酸; GSH: 谷胱甘肽; GSSH: 氧化型谷胱甘肽; D-mannose-1-P: 甘露糖-1-磷酸; GDP-D-mannose: 二磷酸鳥苷-二核苷酸-甘露糖; AO: 抗壞血酸氧化酶; APX: 抗壞血酸過氧化物酶; DHAR: 脫氫抗壞血酸還原酶; MDHAR: 單脫氫抗壞血酸還原酶; NADP: 氧化型煙酰胺腺嘌呤二核苷酸磷酸; NADPH: 還原型煙酰胺腺嘌呤二核苷酸磷酸; MDHA: 單脫氫抗壞血酸; DHA: 脫氫抗壞血酸; DKG: 2,3-二酮-L-古洛糖酸。該圖根據(jù)參考文獻[81]改制。

The black arrow “→” indicates enhancement, and the red arrow “→” indicates inhibition. IM activity: inflorescence meristem; H2O2: hydrogen peroxide; O2: oxygen; H2O: water; AsA: ascorbic acid; GSH: glutathione; GSSH: oxidized glutathione; D-mannose-1-P: D-mannose-1-phosphate; GDP-D-mannose: guanosine diphosphate-dinucleotide-mannose; AO: ascorbate oxidase; APX: ascorbate peroxidase; DHAR: dehydroascorbate reductase; MDHAR: monodehydroascorbate reductase; NADP: oxidized nicotinamide adenine dinucleotide phosphate; NADPH: reductive nicotinamide adenine dinucleotide phosphate; MDHA: monodehydroascorbate; DHA: dehydroascorbate; DKG: 2,3-diketogulonic acid. The figure is adapted from the reference of [81].

4.1 水稻根系形態(tài)生理與幼穗發(fā)育和大穗形成的關系及其機理

水稻根系形態(tài)生理指標主要包括根重、根數(shù)、根長、根系氧化力、激素種類與濃度、根系傷流液、根系分泌物組分與濃度。有研究表明, 每穗粒數(shù)多、產(chǎn)量高的品種在穗分化期有較高的根重[85]。但也有研究者認為穗分化期根系數(shù)量在一定范圍內(nèi)與每穗粒數(shù)呈正相關, 超過適宜范圍會造成無效消耗, 減少穎花分化、增加穎花退化, 造成每穗粒數(shù)減少, 即根系存在“冗余生長”, 這不利于促進水稻幼穗分化和產(chǎn)量提高[86-88]。更高的根系活力可以減少小穗退化, 促進每穗粒數(shù)形成[89]。我們以往的研究觀察到, 在花粉母細胞減數(shù)分裂期至花粉內(nèi)容物充實期, 水稻根系玉米素+玉米素核苷含量和根系活性水平(根系氧化力、根系傷流量、根系總吸收表面積和根系活躍吸收表面積)與每穗粒數(shù)呈顯著或極顯著正相關。上述結果暗示了水稻根系形態(tài)生理和幼穗分化發(fā)育以及激素含量間的關系復雜, 其潛在的調(diào)控機制還不清楚。今后需要繼續(xù)加強對根系形態(tài)生理變化特點的分析, 深入研究其與激素的協(xié)同作用對幼穗發(fā)育和大穗形成的影響。

4.2 水肥管理和溫光條件對水稻穎花分化與退化的調(diào)控機制

水稻穗的大小主要由遺傳因素決定, 但其生長周期內(nèi)的環(huán)境因素, 如高溫[90]、二氧化碳濃度[91]等對穗大小也有很大影響。目前, 栽培措施對穎花分化及退化的影響已有大量研究, 但相關研究仍存在許多薄弱之處, 如氮素穗肥影響水稻穎花分化和退化的機制、干濕交替等節(jié)水灌溉方式對水稻穎花分化和退化的具體影響方式、穗分化期高溫對水稻穎花分化及退化過程的影響及其與器官碳氮代謝及活性氧產(chǎn)生之間的關系還有待進一步研究。另外, 已往研究多集中于穎花分化數(shù)和退化數(shù)的表觀觀察, 極少涉及幼穗發(fā)育中穎花形成與退化過程的研究。因此, 仍需深入研究明確水分和氮素管理對水稻穎花分化與退化的調(diào)控機理, 以期為增加每穗粒數(shù)提供理論依據(jù)。

4.3 植物激素間的相互作用調(diào)節(jié)穎花分化與退化的生理和分子機制

植物激素可調(diào)節(jié)植物的生長發(fā)育和養(yǎng)分分配, 是植物適應不同環(huán)境的首要作用因子。作者觀察到, 外源噴施BRs可顯著提高幼穗內(nèi)源激素的含量、能量水平和抗氧化能力, 并顯著降低幼穗中MDA和H2O2水平, 降低穎花退化率。表明BRs可通過調(diào)控能量和抗氧化能力水平, 進而調(diào)控水稻穎花的發(fā)育[82]。但在逆境下外源BRs調(diào)控水稻植株內(nèi)源激素變化并影響穗發(fā)育過程的途徑和機制、BRs是否參與穎花分化退化對不同氮水平的響應及其調(diào)控過程仍需進一步探明。此外, 水稻激素含量變化具有一定規(guī)律性, 且易受諸多環(huán)境因素的影響, 各激素及其互作效應對水稻穎花分化與退化、籽粒灌漿、產(chǎn)量和品質(zhì)等具有重要調(diào)節(jié)作用。目前對植物激素間的相互作用對水稻耐受性的響應已有了初步的研究, 比如ABA和BRs在調(diào)控植物響應干旱的機制上存在拮抗作用; 擬南芥的CTK受體在ABA信號轉(zhuǎn)導和滲透脅迫反應中起負調(diào)控作用[92]。但是對于在不同水肥管理條件下, 植物激素間的互作效應調(diào)節(jié)穎花分化與退化的機制是怎樣的？是協(xié)同作用還是拮抗作用？仍然缺乏研究。

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Research progress on the formation of large panicles in rice and its regulation

LIU Li-Jun*, ZHOU Shen-Qi, LIU Kun, ZHANG Wei-Yang, and YANG Jian-Chang

Jiangsu Key Laboratory of Crop Genetics and Physiology / Jiangsu Co-Innovation Centre for Modern Production Technology of Grain Crops / Jiangsu Key Laboratory of Crop Genomics and Molecular Breeding / Key Laboratory of Arable Land Quality Monitoring and Evaluation, Ministry of Agriculture and Rural Affairs, Yangzhou University, Yangzhou 225009, Jiangsu, China

The spikelet number per panicle is a key factor that constitutes the grain yield in rice. Modern high-yielding rice varieties mostly show high spikelet number per panicle. Increasing the spikelet number per panicle and promoting the formation of large panicles are important ways to improve rice yield. This paper reviewed the relationship between the formation of spikelet number per panicle and young panicle development in rice. Combined with the author’s related research, the mechanisms underlying genetic regulation in rice panicle size, the effects of nutritional status and nitrogen fertilizer management, water, temperature, light, and endogenous hormones on the formation of spikelet number per panicle in rice were reviewed. We put forward the future research focus on strengthening the formation of large panicles in rice from the aspects of root morphophysiology and young panicle development, water and nitrogen management, temperature and light conditions, and the physiological and molecular mechanisms of interaction between plant hormones regulating spikelet degeneration. The purpose of this study was to provide a basis for the selection and cultivation of high-yielding rice varieties with large panicles.

rice; large panicle; spikelet differentiation and degeneration; water and nutrient management; hormone

10.3724/SP.J.1006.2023.22035

本研究由國家自然科學基金項目(32071947, 31871557)資助。

This study was supported by the National Natural Science Foundation of China (32071947, 31871557).

通信作者(Corresponding author):劉立軍, E-mail: ljliu@yzu.edu.cn, Tel: 0514-87972133

2022-06-07;

2022-09-05;

2022-09-23.

URL: https://kns.cnki.net/kcms/detail/11.1809.S.20220922.1443.002.html

This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).