水稻大穗形成及其调控的研究进展

刘立军 周沈琪 刘 昆 张伟杨 杨建昌

水稻大穗形成及其调控的研究进展

刘立军*周沈琪 刘 昆 张伟杨 杨建昌

扬州大学江苏省作物遗传生理重点实验室 / 江苏省粮食作物现代产业技术协同创新中心 / 江苏省作物基因组学和分子育种重点实验室 / 农业农村部耕地质量监测与评价重点实验室, 江苏扬州 225009

水稻每穗粒数是构成产量的关键因素, 现代高产水稻品种多表现为穗大粒多。增加每穗粒数、促进大穗形成是提高水稻产量的重要途径。本文概述了水稻每穗粒数形成与幼穗发育的关系, 并结合作者相关研究, 从水稻穗型的遗传调控、营养状况与氮肥管理、水分、温光条件和内源激素等方面分析了其对每穗粒数形成的影响。从根系形态生理与幼穗发育、水肥管理和温光条件以及植物激素间的相互作用调节颖花分化与退化的生理和分子机制等方面提出了未来加强水稻大穗形成研究的重点, 以期为大穗高产水稻品种选育和栽培调控提供依据。

水稻; 大穗; 颖花分化与退化; 水肥管理; 激素

水稻是世界上最重要的粮食作物之一, 全球一半人口以稻米为主食[1-3]。目前水稻单产年增长率仅为1.70%, 显著低于2050年将总产翻一番所需2.4%的年增长率[4-5]。水稻增产主要通过扩大种植面积和提高单位面积产量2条路径, 但由于耕地面积逐渐减少、水资源匮乏的现状, 靠扩大种植面积实现水稻增产的潜力非常有限[6]。因此, 提高单位面积产量是增加水稻总产量的有效途径。水稻产量由单位面积穗数、每穗粒数、结实率和粒重构成。单位面积穗数受空间条件制约, 存在一定程度上的饱和; 粒重主要受遗传因素控制, 表现相对稳定; 结实率不仅取决于水分和养分的供应, 还取决于成熟期的光温条件; 而每穗粒数的可变性与可调性较大, 高产水稻品种通常具有穗型大、每穗粒数多、单穗重量高(大穗)的特点[7-8]。因此, 增加每穗粒数、促进大穗形成是提高水稻产量的重要途径[9-11]。本文综述了水稻每穗粒数与幼穗发育的关系以及影响每穗粒数形成的主要因素, 提出了未来加强水稻每穗粒数研究的重点。旨在进一步揭示水稻大穗形成的机制, 为大穗高产水稻品种选育和栽培调控提供依据。

1 幼穗发育与每穗粒数形成

稻穗为圆锥花序, 由穗轴、一次枝梗、二次枝梗、小穗梗和小穗(小花)组成。每穗粒数在穗分化期形成并确定, 是小花分化、发育和退化等一系列复杂生理过程的最终体现[12-13]。国内外学者从形态和解剖等方面对稻穗的发育过程进行了详细研究[14]。水稻幼穗发育大致可分为2个阶段, 第一阶段自穗轴分化期(苞分化期)至颖花原基分化期结束, 此阶段主要分化一、二次枝梗和颖花; 第二阶段自雌雄蕊分化期至抽穗, 主要完成枝梗伸长、颖花发育、花器官形成、减数分裂和花粉粒充实完成等过程[15]。幼穗分化第一阶段主要影响颖花分化数的形成, 以二次枝梗及颖花原基分化期对颖花分化的促进作用最为显著, 且颖花分化数与二次枝梗分化数呈极显著正相关[16]。第二阶段主要影响颖花退化数, 以花粉母细胞减数分裂期对颖花退化的影响最为明显。花粉母细胞减数分裂期是水稻每穗颖花数从分化增加到退化减少的消长转折时期, 这一时期的幼穗正在快速伸长且接近全长, 绝大多数颖花已经分化完成, 劣势颖花正在急剧退化[17]。

颖花退化是粮食减产的一个重要原因。阐明颖花分化与退化的机制是当今水稻栽培学家和育种学家关注的热点问题之一。前人对颖花分化与退化的认识归纳起来主要有以下4种理论。第一种是“资源限制”假设。在非胁迫条件下, 水稻节间伸长与幼穗原基分化发育之间存在“器官同伸”关系, 此时幼穗与茎鞘会发生养分竞争, 从而影响幼穗分化发育。在胁迫条件下, 处于下部的颖花养分供应变得不足[18-19], 缺少非结构性碳水化合物供应会导致其在强烈的养分竞争下营养不良, 最终劣势颖花退化[20]。但亦有研究表明, 即使在良好的营养供应条件下, 颖花仍会严重退化。因此, “资源限制”可能只是颖花退化的原因之一, 而非主要原因。第二种是“自组织过程”假设。该假设认为幼穗发育之初同一稻穗内部不同颖花相互竞争, 营养物质分配不均匀, 这种初始的资源不平衡在颖花分化前期表现为优势颖花分化能力提升, 在后期扩大并形成优势, 通过自催化过程获得更多的资源, 不能获得足够资源的劣势颖花则发生退化[21]。然而大多数谷物中的颖花退化并不是随机的, 如水稻小穗的颖花退化通常发生在穗基部和顶部。因此, 上述2种假说尚未找到更加有力的证据来支持。第三种是“植物激素平衡”理论。植物激素是影响颖花分化和退化的关键调节因子。一般认为, 生长素(indole-3-acetic, IAA)、细胞分裂素(cytokinins, CTK)和赤霉素(gibberellins, GAs)可促进颖花分化, 而脱落酸(abscisic acid, ABA)和乙烯(ethylene, ET)则会引起颖花退化[16]。第四种是“碳氮代谢平衡”理论。颖花分化和退化与植株器官的碳氮代谢平衡存在密切关系。有研究表明, 在颖花原基分化期, 水稻地上部的含氮量与颖花分化数呈显著正相关, 植株含氮量越高, 颖花分化数越多, 但颖花退化数也会增加[22]。在花粉母细胞减数分裂期, 每穗颖花数与穗分化期整株碳积累量呈直线相关, 植株体内积累的非结构性碳水化合物越多, 颖花退化率越低[22]。幼穗发育过程中的碳氮比例达到一个相互促进的平衡状态才能有效促进颖花分化, 减少退化, 增加颖花现存数[23]。

2 水稻穗型的遗传调控

转录组研究显示, 在22,000个基因中, 有357个基因在穗部发育中发挥了不同的作用, 表明仍有许多基因尚未被识别。在目前已被识别的基因中,()基因是第一个被克隆的控制水稻穗粒数的主效数量性状基因座, 它是与细胞分裂素氧化酶/脱氢酶高度同源的()基因。当表达水平降低时, CTK会在花序分生组织上大量积累, 增加枝梗数和籽粒数, 而活性CTK合成缺陷突变体的穗部枝梗以及籽粒数目显著下降[24]。()基因可直接通过调控的表达来影响水稻穗整体枝梗与籽粒的数目[25]。另外, 过表达以及干扰CTK应答的GATA转录因子都会降低水稻枝梗数以及穗粒数[26]。()基因编码一个赤霉素20-氧化酶, 参与GAs信号调控。该基因在水稻穗分生组织中通过调控()基因的转录反馈环, 增强CTK活性, 进而促进编码赤霉素20-氧化酶的的转录, 抑制活性GA1和GA3的积累, 从而增加穗粒数[27]。这些结果也从另一方面说明了CTK相关基因的表达对每穗粒数的形成起到了至关重要的调控作用。()、()和()是与水稻抽穗期感光性相关的3个主效数量性状基因座, 它能同时控制水稻株高、每穗粒数和抽穗期3个性状。单(敲除和)促进穗粒数形成, 而单(敲除和)抑制穗粒数形成。日照长短对上述3个基因的表达水平也有重要影响作用, 长日照下,促进表达, 并被和招募形成不同的抑制复合体, 产生不同程度的延迟抽穗或不抽穗。而短日照下,表达量很低,与抑制复合体间表现出竞争关系,表达量的增加可以不同程度地促进抽穗和穗粒数形成[28-29]。主要在幼嫩组织中表达, 它的高表达量能够显著增加一次枝梗数和二次枝梗数, 尤其是二次枝梗的数量。也有研究观察到,()能促进细胞分裂, 增加枝梗数和每穗籽粒数, 进而促进水稻增产[30]。除此之外, 每穗粒数还受到()、()、()和()等基因调控[28,31]。

3 影响水稻每穗粒数形成的主要因素

3.1 营养状况与氮肥管理

水稻颖花现存数与氮素积累量和穗发育阶段的营养状况、生物积累量密切相关。水稻大穗形成的前提是充足的营养生长量和碳氮代谢水平的协调稳定, 这也是源强的直接表现[13]。当氮素或生物积累量减少时, 颖花分化数会随之减少。水稻幼穗发育过程中吸收的氮、磷、钾等矿物质营养元素主要用于构成碳水化合物、蛋白质和核酸。碳水化合物主要用来构建穗、枝梗和小花, 蛋白质和核酸则主要用于减少枝梗和颖花的退化、充实生殖器官[32]。在水稻由营养生长向生殖生长转化的阶段, 茎秆和幼穗处于共同旺盛生长时期, 互相竞争同化物[33-34]。叶片含糖量的增加是保证幼穗分化和水稻植株迅速生长的基础, 幼穗分化始期营养不足, 颖花分化数将明显减少, 影响结实粒数[8,35]。颖花干物质量越低, 退化就越多; 反之, 颖花干物质量越高, 枝梗和颖花退化越少, 每穗粒数则越多[11,36]。幼穗与茎秆竞争同化物的能力直接影响到穗粒数的形成。相对于茎秆来说, 幼穗是一个较弱的库。当水稻进入花粉母细胞减数分裂期后, 幼穗对营养物质的竞争能力大于茎秆[37]。此期过多无效分蘖和穗数将再次引起空间和养分的竞争, 最终导致秆细穗弱, 颖花发育不良。

每穗颖花数由颖花分化数和退化数决定。研究表明, 适宜的氮素水平可以显著提高水稻的颖花分化数, 从而增加每穗粒数[38-39]。氮肥的合理施用, 尤其是穗肥的合理用量与每穗粒数形成密切相关。作者研究观察到, 在0～216 kg hm–2氮素穗肥范围内, 增加施氮量可不同程度提高不同穗型超级稻品种一次枝梗和二次枝梗颖花分化数。颖花退化数和一次枝梗颖花退化率随穗肥施氮量的增加而增加, 而二次枝梗颖花退化率随穗肥施氮量增加呈先降后升的变化趋势。不同穗型水稻品种对穗肥的响应存在明显差异, 小穗型水稻品种每穗颖花数对穗肥的响应更大。如与不施穗肥相比, 施用穗肥后的小穗型品种南粳9108、中穗型品种扬两优6号和大穗型品种甬优1540每穗二次枝梗颖花现存数分别增加了25.0%～57.6%、10.5%～40.8%和7.1%～25.3% (图1)。

碳氮代谢平衡是水稻幼穗正常发育的基础, 一般认为, 氮代谢旺盛有利于营养生长, 而碳代谢旺盛有利于生殖生长[41-42]。在幼穗分化前期, 施用一定量的氮素穗肥对叶片的当时效应是“得氮耗糖”, 但由于叶片等营养器官的相对生长量变化, 在花粉母细胞减数分裂期后会出现“低氮高糖”现象, 这种现象有利于增加每穗颖花数。氮素营养可通过调节叶片光合作用和碳氮物质分配比例, 使一部分碳氮物质继续供应植株生长, 另一部分贮藏于幼穗、叶片等器官中, 并调节其相对生长[43-44]。李刚华等[45]观察到从穗分化开始到抽穗前20 d, 植株含氮量越高, 颖花分化数越多; 从抽穗前16 d开始, 植株体内积累的非结构性碳水化合物越多, 颖花退化率越低。在花粉母细胞减数分裂期前后, 幼穗急剧生长, 颖花急剧增大, 水稻从营养生长过渡到生殖生长, 此过程以碳积累为主, 需要大量碳水化合物, 此期施用适当比例氮肥可以防止穗下部的枝梗和颖花退化。水稻叶片需要保持一定的氮含量, 才有利于光合作用的进行。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应, 可使一些弱势颖花得以生存而减少退化[46]。氮肥一方面通过影响穗分化期植株的碳氮代谢平衡影响颖花量的多少, 另一方面还可通过调节幼穗中IAA和CTK的时空分布来调控颖花发育[47], 这也表明植物激素在养分管理调控水稻颖花发育的过程中发挥着重要作用。

图1 穗肥施氮量对不同穗型超级稻品种一、二次枝梗颖花分化和退化的影响

Nanjing 9108、Yangliangyou 6和Yongyou 1540分别代表小穗型(每穗粒数130粒左右)、中穗型(每穗粒数220粒左右)和大穗型(每穗粒数300粒左右)品种南粳9108、扬两优6号和甬优1540。0N、54N、108N、162N和216N分别代表穗肥施氮量为0、54、108、162和216 kg hm–2。DiN、SN、DeN和DR分别代表分化数、退化数、现存数和退化率。该图根据参考文献[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时期。花粉母细胞减数分裂期对水分最为敏感, 此期遭受水分胁迫可能造成颖花退化数增加或花粉不育而影响总颖花量和结实粒数, 不利于形成大穗[40,48]。有研究表明, 水分胁迫可显著提高水稻叶鞘中氮的比例, 水分的缺失使得叶片光合作用的同化产物优先运往新生叶片和正在伸长的茎秆, 而后运往穗部, 此时营养物质供应不足就会导致颖花分化数减少、退化数增加[49]。因此, 自减数分裂开始至抽穗开花结束, 生产上一般采用水层灌溉的方法。但也有研究表明, 长期淹水条件下根层土壤环境恶化会影响水稻根系和地上部的生长发育, 不利于颖花发育和最终穗粒数形成[13,50]。

我们最近的研究表明, 与传统淹水灌溉相比, 轻干湿交替灌溉(土壤落干期土水势不低于-15 kPa或中午叶片水势不低于-1.2 MPa)有利于促进颖花分化、减少颖花退化, 最终有利于促进水稻大穗形成[40]。其主要原因是土壤落干后复水使根系产生一种积极的自我调节能力(补偿效应), 即水稻受到轻度水分胁迫时, 某些生理功能略有降低或生长受阻, 但在复水后却能超过正常供水状态[48,51-53]。具体表现为增加了根系纵向伸长和横向扩展, 提高了根系生物量和根毛数量以及根系吸收表面积, 扩大了根系对水分和养分的吸收范围, 增加了穗分化前期单茎干物质和非结构性碳水化合物的积累量, 进而有利于增加水稻二次枝梗颖花分化数、降低二次枝梗颖花退化数与退化率, 提高水稻颖花现存数, 最终获得高产(表1)。

表1 穗分化期水分处理对不同穗型水稻品种颖花分化和退化的影响

CF: 常规灌溉; AWMD: 干湿交替灌溉; DiN: 分化数; DeN: 退化数; SN: 现存数; DR: 退化率。标以不同字母表示在= 0.05水平上差异显著, 同栏内同一品种间比较。该表根据参考文献[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 温光条件

温度和光照是影响水稻每穗粒数的重要环境因子。幼穗分化期最适温度为26～30℃, 温度过高或过低均不利于每穗粒数形成。幼穗分化期遇高温胁迫会减少水稻颖花分化数、增加退化数[33,55]。花粉母细胞减数分裂期对环境条件最为敏感, 此期受到高温胁迫会导致IAA、CTK含量下降, ET释放速率增加, 花粉代谢紊乱[56-57]。水稻在幼穗分化期遇高温将积累大量氧化物, 氧离子、过氧化氢(H2O2)、丙二醛(MDA)等氧化物含量上升。随着高温胁迫时间的增加, 活性氧的生成速度已经远远大于清除速度, 过氧化物酶(POD)、过氧化氢酶(CAT)和超氧化物歧化酶(SOD)等抗氧化酶活性受到抑制, 细胞膜受到伤害而影响水稻器官正常建成, 导致颖花退化数增加[58]。幼穗发育过程中, 花粉母细胞减数分裂期后2～3 d是水稻对低温最为敏感的时期, 这一时期如遇17℃以下的低温环境, 会降低颖花分化数和现存数、增加颖花退化数[59]。穗分化期低温还会影响花粉粒的正常发育, 导致籽粒充实不良[48]。总之, 极端高温或极端低温均不利于颖花分化, 且低温天气对颖花退化的影响明显大于高温天气[60]。日照时数和光照强度对一、二次枝梗数和颖花数的形成有较大影响作用, 水稻幼穗分化期如没有足够的光强, 将影响营养物质的积累, 导致枝梗分化速度减缓、颖花退化数增加[3,6]。

3.4 内源激素

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

CTK主要是一种由根系合成、通过输导组织运输到地上部分对植物生长发育起调控作用的植物激素[40,47,62]。CTK除具诱导细胞分裂的作用外, 还参与延缓叶片衰老[63-64]、顶端优势[65]、根系增殖[24]和生殖生长[66]等各种生理代谢过程。有研究表明, 水稻幼穗发育与CTK含量有明显的同步性, 颖花分化期高浓度的CTK及其与IAA的比值有利于延长分化时间, 促进颖花分化, 抑制颖花退化[67]。CTK促进颖花发育的同时, 还能抑制IAA从分化较早的颖花中向外输出, 削减幼穗生长的顶端优势, 使小穗生长具有一致性, 从而稳定颖花分化数、减少退化数[63,65]。除了CTK和IAA, GAs也是一种促进植物生长发育的激素。有学者观察到, 当水稻遭受盐胁迫时, 喷施GAs可以有效抑制颖花退化[68]。但亦有研究发现, 水稻在花粉母细胞减数分裂期遭受干旱时, 颖花退化数显著增加, 但此时幼穗中的GAs含量并没有下降[69]。因此, 水稻逆境胁迫下GAs水平对颖花分化与退化调控还有待进一步研究。

ET和ABA常被认为是抑制型植物激素[70-71]。前人观察到, 高浓度的ET会诱发玉米和水稻等禾谷类作物的颖花(小花)退化或种子败育。内源ABA和ET还可能相互拮抗调控水稻颖花发育, 较高的ABA与ET比值能减少颖花退化[50,72]。此外, PAs、SA、JA对水稻颖花发育也有重要调控作用[63,66,73-74]。PAs广泛存在于细胞各处并参与植物细胞代谢的整个过程, 如细胞分裂、DNA复制、转录和翻译、细胞的生长衰老以及对环境胁迫的响应等。PAs通过与ET之间的拮抗作用响应适度的土壤干旱, 从而调控水稻颖花退化[35]。在高温胁迫下, 喷施SA能显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, 减少小穗退化[75-76]。JA是植物体内一种重要的脂类激素。研究表明, JA可通过激活()、()和()等已克隆的茉莉素合成及转导基因的表达来调控颖花的分化与退化[77]。

BRs主要由油菜素内酯(brassinolid, BL)和油菜素甾酮(castasterone, CS)及其衍生物组成, 对花粉育性、颖花分化与退化有明显调控作用[78]。其中, 24-表油菜甾酮(24-epocastasterone, 24-epiCS)和28-高油菜素内酯(28-homobrassinolide, 28-homoBL)被认为是水稻体内重要的BRs[79-80]。作者观察到, 颖花原基分化期幼穗中较高含量的BRs (24-epiCS和28-homoBL)能促进颖花分化, 提高花粉母细胞减数分裂期幼穗中BRs含量有利于减少颖花退化(图2)。BRs主要通过以下三方面调控颖花分化和退化。(1)增加BRs含量可促进()和()基因表达, 从而提高花序分生组织的活性, 延迟小穗分生组织分化, 延长枝梗发育时间, 有利于促进颖花分化, 最终增加每穗颖花数。(2) BRs生物合成量的增加提高了水稻具有MYB结构域蛋白的()基因表达, 并直接触发幼穗中糖利用相关基因的表达, 可使幼穗从营养组织中获得更多的碳同化物, 从而增加颖花分化数并降低颖花退化率。(3) BRs含量的提高发育幼穗中抗氧化系统的活性, 减少活性氧对幼穗细胞的伤害, 最终促进颖花分化, 并减少颖花退化。BRs调控幼穗发育的机制分析见图3[81]。

4 研究与展望

增加每穗粒数、促进水稻大穗形成、增加库容一直是水稻高产栽培和育种的重要目标。目前, 此方面已有大量研究, 并取得明显进展。近年来, 以甬优系列为代表的大穗型水稻品种在生产上应用广泛,多地多年均表现高产稳产, 代表性品种甬优1540、甬优2640和甬优12的每穗粒数超过300粒[83-84], 远高于大面积生产中常用高产水稻品种的每穗粒数, 但这些品种大穗形成的机制仍然不是十分明确。基于已有研究, 笔者认为将来对水稻大穗形成机制的研究有以下三方面需要加强。

(图2)

YD-6: 扬稻6号; YY-2640: 甬优2640; SPD: 颖花原基分化期; PMC: 花粉母细胞减数分裂期。该图根据参考文献[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.

图3 水稻幼穗发育过程中油菜素甾醇(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

黑色箭头“→”表示增强, 红色箭头“→”表示抑制。IM activity: 分生组织活性; H2O2: 过氧化氢; O2: 氧气; H2O: 水; AsA: 抗坏血酸; 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: 二磷酸鸟苷-二核苷酸-甘露糖; AO: 抗坏血酸氧化酶; APX: 抗坏血酸过氧化物酶; DHAR: 脱氢抗坏血酸还原酶; MDHAR: 单脱氢抗坏血酸还原酶; NADP: 氧化型烟酰胺腺嘌呤二核苷酸磷酸; NADPH: 还原型烟酰胺腺嘌呤二核苷酸磷酸; MDHA: 单脱氢抗坏血酸; DHA: 脱氢抗坏血酸; DKG: 2,3-二酮-L-古洛糖酸。该图根据参考文献[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 水稻根系形态生理与幼穗发育和大穗形成的关系及其机理

水稻根系形态生理指标主要包括根重、根数、根长、根系氧化力、激素种类与浓度、根系伤流液、根系分泌物组分与浓度。有研究表明, 每穗粒数多、产量高的品种在穗分化期有较高的根重[85]。但也有研究者认为穗分化期根系数量在一定范围内与每穗粒数呈正相关, 超过适宜范围会造成无效消耗, 减少颖花分化、增加颖花退化, 造成每穗粒数减少, 即根系存在“冗余生长”, 这不利于促进水稻幼穗分化和产量提高[86-88]。更高的根系活力可以减少小穗退化, 促进每穗粒数形成[89]。我们以往的研究观察到, 在花粉母细胞减数分裂期至花粉内容物充实期, 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、根系伤流量、根系总吸收表面积和根系活跃吸收表面积)与每穗粒数呈显著或极显著正相关。上述结果暗示了水稻根系形态生理和幼穗分化发育以及激素含量间的关系复杂, 其潜在的调控机制还不清楚。今后需要继续加强对根系形态生理变化特点的分析, 深入研究其与激素的协同作用对幼穗发育和大穗形成的影响。

4.2 水肥管理和温光条件对水稻颖花分化与退化的调控机制

水稻穗的大小主要由遗传因素决定, 但其生长周期内的环境因素, 如高温[90]、二氧化碳浓度[91]等对穗大小也有很大影响。目前, 栽培措施对颖花分化及退化的影响已有大量研究, 但相关研究仍存在许多薄弱之处, 如氮素穗肥影响水稻颖花分化和退化的机制、干湿交替等节水灌溉方式对水稻颖花分化和退化的具体影响方式、穗分化期高温对水稻颖花分化及退化过程的影响及其与器官碳氮代谢及活性氧产生之间的关系还有待进一步研究。另外, 已往研究多集中于颖花分化数和退化数的表观观察, 极少涉及幼穗发育中颖花形成与退化过程的研究。因此, 仍需深入研究明确水分和氮素管理对水稻颖花分化与退化的调控机理, 以期为增加每穗粒数提供理论依据。

4.3 植物激素间的相互作用调节颖花分化与退化的生理和分子机制

植物激素可调节植物的生长发育和养分分配, 是植物适应不同环境的首要作用因子。作者观察到, 外源喷施BRs可显著提高幼穗内源激素的含量、能量水平和抗氧化能力, 并显著降低幼穗中MDA和H2O2水平, 降低颖花退化率。表明BRs可通过调控能量和抗氧化能力水平, 进而调控水稻颖花的发育[82]。但在逆境下外源BRs调控水稻植株内源激素变化并影响穗发育过程的途径和机制、BRs是否参与颖花分化退化对不同氮水平的响应及其调控过程仍需进一步探明。此外, 水稻激素含量变化具有一定规律性, 且易受诸多环境因素的影响, 各激素及其互作效应对水稻颖花分化与退化、籽粒灌浆、产量和品质等具有重要调节作用。目前对植物激素间的相互作用对水稻耐受性的响应已有了初步的研究, 比如ABA和BRs在调控植物响应干旱的机制上存在拮抗作用; 拟南芥的CTK受体在ABA信号转导和渗透胁迫反应中起负调控作用[92]。但是对于在不同水肥管理条件下, 植物激素间的互作效应调节颖花分化与退化的机制是怎样的？是协同作用还是拮抗作用？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

本研究由国家自然科学基金项目(32071947, 31871557)资助。

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

通信作者(Corresponding author):刘立军, 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/).