Key message

Pathways for assimilates.

Abstract

During their life cycle, plants alternate between a haploid stage, the gametophyte, and a diploid stage, the sporophyte. In higher plants, meiosis generates the gametophyte deeply embedded in the maternal tissue of the flower. The megaspore mother cell undergoes meiosis, and then, the surviving megaspore of the four megaspores produced undergoes mitotic divisions and finally gives rise to the female gametophyte, consisting of the egg cell, two synergids, the central cell, which due to the fusion of two nuclei is diploid (double haploid) in Arabidopsis and most angiosperms and the antipods, whose number is not fixed and varies significantly between species (Yadegari and Drews in Plant Cell 16(Suppl):S133–S141, 2004). The maternal tissues that harbor the female gametophyte and the female gametophyte are referred to as the ovule (Fig. 1). Double fertilization of the egg cell and the central cell by the two generative nuclei of the pollen leads to the diploid embryo and the endosperm, respectively (Hamamura et al. in Curr Opin Plant Biol 15:70–77, 2012). Upon fertilization, the ovule is referred to as the seed. Seeds combine two purposes: to harbor storage compounds for use by the embryo upon germination and to protect the embryo until the correct conditions for germination are encountered. As a consequence, seeds are the plant tissue that is of highest nutritional value and the human diet, by a considerable amount, consists of seeds or seed-derived products. Amino acids are of special interest, because plants serve as the main source for the so-called essential amino acids, that animals cannot synthesize de novo and are therefore often a limiting factor for human growth and development (WHO in Protein and amino acid requirements in human nutrition. WHO technical report series, WHO, Geneva, 2007). The plant embryo needs amino acids for general protein synthesis, and additionally they are used to synthesize storage proteins in the seeds of certain plants, e.g., legumes as a resource to support the growth of the seedling after germination. The support of the embryo depends on transport processes that occur between the mother plant and the seed tissues including the embryo. In this review, we will focus on the processes of unloading amino acids from the phloem and their post-phloem transport. We will further highlight similarities between amino acid transport and the transport of the main assimilate and osmolyte, sucrose. Finally, we will discuss similarities and differences between different plant species in terms of structural aspects but for the molecular aspects we are almost exclusively focusing on Arabidopsis.

Vascularization of the Arabidopsis ovule and seed. Plants expressing ER-localized mCherry under control of the companion cell-specific SUC2 promoter and ER-localized GFP under control of the sieve element marker PD1 as described (Müller et al. 2015) are shown to visualize the phloem in the funiculus and the chalazal regions. a Overview over an ovule. FG: female gametophyte. b A magnification of the region marked by a square in panel a. c Overview over a seed. ES: endosperm; E: embryo. d A magnification of the region marked by a square in panel c. The arrows in b and d point to the terminal companion cell and arrowheads to terminal sieve elements

中文翻译：

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漫长而曲折的道路：拟南芥种子中氨基酸的运输途径。

关键信息

同化途径。

抽象的

在生命周期中，植物在单倍体阶段（配子体）和二倍体阶段（子孢子体）之间交替。在高等植物中，减数分裂产生深嵌入花的母体组织中的配子体。大孢子母细胞经历减数分裂，然后，所产生的四个大孢子中幸存的大孢子经历有丝分裂，最终产生雌配子体，由卵细胞，两个协同体，中央细胞组成，由于两个细胞的融合核在拟南芥中是二倍体（双单倍体）多数被子植物和对足动物的数量是固定的，并且在物种之间差异很大（Yadegari和Drews in Plant Cell 16（Suppl）：S133-S141，2004）。带有雌配子体和雌配子体的母体组织称为胚珠（图1）。花粉的两个生成核对卵细胞和中央细胞的双重受精分别导致二倍体胚胎和胚乳（Hamamura等人，Curr Opin Plant Biol 15：70-77，2012）。受精后，胚珠被称为种子。种子具有两个目的：容纳萌发时胚胎使用的存储化合物，以及保护胚直到遇到正确的发芽条件。因此，种子是具有最高营养价值和人类饮食的植物组织，大部分由种子或种子衍生产品组成。氨基酸特别受关注，因为植物是所谓必需氨基酸的主要来源，动物无法从头合成氨基酸，因此通常是人类生长发育的限制因素（蛋白质中的WHO和《人类营养》，世卫组织技术报告系列，世卫组织，日内瓦，2007年。植物胚需要氨基酸来进行一般的蛋白质合成，此外，它们还用于合成某些植物种子（例如豆类）种子中的储存蛋白，以作为发芽后支持幼苗生长的资源。胚胎的支持取决于母体植物和包括胚胎在内的种子组织之间发生的转运过程。在这篇评论中 我们将重点研究从韧皮部中卸载氨基酸及其韧皮部后转运的过程。我们将进一步强调氨基酸转运与主要同化物和渗透压剂蔗糖的转运之间的相似性。最后，我们将在结构方面讨论不同植物物种之间的异同，但在分子方面，我们几乎只关注于拟南芥。

拟南芥胚珠和种子的血管化。如所描述的（Mülleret al.2015），在伴生细胞特异性SUC2启动子控制下表达ER定位的mCherry和在筛分元素标记PD1的控制下表达ER定位的GFP的植物显示出在韧皮部和沙棘中的韧皮部可视化地区。胚珠的概述。FG：雌配子体。b面板a中以正方形标记的区域的放大倍数。c种子概述。ES：胚乳；E：胚胎。d放大图c中以正方形标记的区域。b和d中的箭头 指向终端陪伴室，箭头指向终端筛网元素