Genetic and physiological characterization of the OsCem mutant in rice: formation of connected embryos with multiple plumules or multiple radicles

Abstract

Establishment of the apical-basal axis is a critical event in plant embryogenesis. Two functionally distinct primordia (the plumule and radicle) are meristems that originate from the lower and upper regions, respectively, of the embryo and are arranged along an apical-basal axis. One rice mutant (OsCem), which alters the embryo axis pattern to produce multiple plumules and/or radicles, was characterized. The OsCem mutant plants showed three different phenotypes: a multiple-shoot type, a heart-shaped Siamese embryo having multiple plumules and a common radicle; a multiple-root type, an upside-down Y-shaped Siamese embryo possessing multiple radicles and a common plumule; and a connected twin type, a connected Siamese twin having two sets of plumules and radicles. These diverse phenotypes demonstrated that the upper and lower regions of the embryo axis have a similar potential to complete the full spectrum of the developmental program of multiple plumules and radicles, respectively. Genetic analysis and molecular mapping based on simple sequence repeat markers has revealed that a recessive gene was involved in the control of the connected Siamese embryo formation. By using an F2 mapping population derived from a cross between the OsCem mutant and the variety 95-15, the OsCem locus was mapped primarily to the short arm of chromosome 3 of rice (Oryza sativa) at the interval between markers RM148 (6.4 cM) and RM468 (7.5 cM). Subsequently, the OsCem locus was fine mapped to the interval between markers M5 (1.6 cM) and M6 (1.0 cM).

Differentiation of Multiple Shoot Apical Meristems in Mutant Rice with One Embryo Causing Multiple Plumuples

“…细胞学机理, 有助于阐明正常生殖模式的发生 [1][2] 。一 粒种子内含有多个胚的现象在植物界相当普遍, 如 水稻、小麦、大麦和黑麦等大部分农作物都曾发现 [3] 。 1980-1990 期间, 我国科研工作者发现的多胚材料 有 100 多份 [4][5][6][7] , 并开展了大量的研究, 主要集中于 两个方面, 一是多(胚)芽萌发形态, 如多芽数目、多 芽频率、一籽多芽中多芽间的关系、多芽和多根数 目; 二是水稻多胚芽的来源, 可能存在双卵或多卵 受精、合子胚裂生 [11] 、助细胞受精发育成胚 [12][13] 和 某些珠心细胞或珠被细胞发育成胚 [14] 4 条途径。目 前一般认为水稻中最主要的是 1 个胚囊内含有多个 卵细胞, 多个卵一起受精发育成胚 [8][9][10] 。 水稻也存在单胚无芽突变体, 如 shootless1(shl1)、 shl2、shl3、shl4 等, 水稻单胚中存在根芽数目不一致 的现象 [15][16][17] 。Satoh 等 [17] 的研究发现, 水稻有根无芽 的突变体均是单胚, SHL 基因的突变导致茎尖生长 点的缺失, 但盾片和胚根的形成不受影响。 Kinae 等 [18] 分析了水稻中的 2 个突变体 apd1 和 shl1-2 apd1, apd1 在胚的顶部有一个不完全发育的芽, 胚的基部增大, 有一个或者两个胚根原基; shl1-2 apd1 表现为无芽, 但根的数目多于 2 条。此类突变体的细胞学与分子 机理探究促进对茎端分生组织分化的理解。然而, 水稻单胚多芽突变体仅见于 OsCem 的遗传与生理研 究 [19]…”

“…C1001B 单胚结构正常, 主苗是来自合子胚发育形成 的胚芽, 侧苗是由胚芽鞘腋部形成的 [12] 。双苗水稻 W3338 和 W255 的双苗可能来源于单胚中的多胚芽 生长点 [23] 。在水稻突变体 OsCem 中, 因胚胎初期生 长素的非正常分布致使胚胎产生多个轴, 导致胚胎 畸形--S 型、Y 型或并生型胚, 植物形态表现为多 苗或多根 [19] 。但它们都缺乏系统的胚胎发育观察。 1-6: Process of of embryonic development in the normal rice IR36; 1: spherical embryo at 1 day after pollination (DAP); 2: Coleoptile primordium protruded in a semi-circular shape (arrow) at 3 DAP; 3: annular coleoptile primordium (arrow) with a spindle-shaped hole (arrowhead) at 4 DAP; 4: scale primordium (arrow) emerged on both sides of coleoptile primordium at 5 DAP; 5: epiblast primordium extended (arrow) at 6 DAP; 6: "X"-shaped hole of coleoptile (arrow) formed at 10 DAP. 7-16: early embryonic development in the mutant line 4001; 7 and 8: pear-shaped and stick-shaped embryos at 3 DAP; 9-16: embryo shape of 4001 at 4 DAP; 9: semi-circular-shaped coleoptile primordium (arrow); 10: further development of semi-circular-shaped coleoptile primordium (arrow); 11: irregular semi-circular-shaped coleoptile primordium formedby enclosed and interlaced development (arrow); 12: split coleoptile primordium (arrow); 13-14: no coleoptile primordium formed; 15: two coleoptile primordia (arrow); 16: three coleoptile primordia (arrow).…”

“…和一个胚根。改变植物的轴向最终会导致植物体表 型的改变 [27][28][29] 。水稻中的两个单胚突变体 apd1 和 shl1-2 apd1 就是由于基因的突变, 改变了茎-根轴的 定位, 导致单胚多根产生 [18] 。生长素在植物的生长 和发育分化过程中也扮演着极为重要的作用 [30] 。茎 端分生组织和根端分生组织的形成 [31][32] 、叶序的分 化 [33] 都和生长素在一定位置上的分布梯度有关。在 胚胎发育过程中, 生长素是通过浓度梯度来定位胚 胎背-腹和茎-根轴的建立 [34][35] , 进而影响着茎端分 生组织或者根尖分生组织的定位。在对大麦胚胎体 外培养中, 破坏生长素在大麦胚胎中正常的极向运 输和梯度分布可以导致多根尖分生组织或者多茎端 分生组织的产生 [36] 。水稻突变体 OsCem 胚胎初期的 生长素分布水平改变, 导致胚胎产生多个轴, 植物 形态表现多苗或多根 [19]…”