Department of Virology, Biological Faculty, Moscow State University, Moscow, 119234, Russia.
Faculty of Bioengineering and Bioinformatics, Moscow State University, Moscow, 119991, Russia.
Plant Sci. 2020 Oct;299:110602. doi: 10.1016/j.plantsci.2020.110602. Epub 2020 Jul 17.
The conducting sieve tubes of the phloem consist of sieve elements (SEs), which are enucleate cells incapable of transcription and translation. Nevertheless, SEs contain a large variety of RNAs, and long-distance RNA trafficking via the phloem has been documented. The phloem transport of certain RNAs, as well as the further unloading of these RNAs at target tissues, is essential for plant individual development and responses to environmental cues. The translocation of such RNAs via the phloem is believed to be directed by RNA structural elements serving as phloem transport signals (PTSs), which are recognized by proteins that direct the PTS-containing RNAs into the phloem translocation pathway. The ability of phloem transport has been reported for several classes of structured RNAs including viroids, genuine tRNAs, mRNAs with tRNA sequences embedded into mRNA untranslated regions, tRNA-like structures in the genomic RNAs of plant viruses, and micro-RNA (miRNA) precursors (pri-miRNA). Here, three distinct types of such RNAs are discussed, along with the proteins that may specifically interact with these structures in the phloem. Three-dimensional (3D) motifs, which are characteristic of imperfect RNA duplexes, are discussed as elements of phloem-mobile structured RNAs specifically recognized by proteins involved in phloem transport, thus serving as PTSs.
韧皮部的导筛管由筛分子（SE）组成，SE 是无核细胞，不能进行转录和翻译。然而，SE 中含有大量的 RNA，并且已经记录了通过韧皮部的长距离 RNA 运输。某些 RNA 在韧皮部的运输，以及这些 RNA 在靶组织中的进一步卸载，对植物个体发育和对环境线索的反应至关重要。这些 RNA 通过韧皮部的易位被认为是由充当韧皮部运输信号（PTS）的 RNA 结构元件指导的，这些 PTS 被识别并引导含 PTS 的 RNA 进入韧皮部易位途径。已经报道了几种结构 RNA 的韧皮部运输能力，包括类病毒、真正的 tRNA、在 mRNA 非翻译区嵌入 tRNA 序列的 mRNA、植物病毒基因组 RNA 中的 tRNA 样结构和 micro-RNA (miRNA) 前体（pri-miRNA）。在这里，讨论了三种不同类型的此类 RNA，以及可能在韧皮部中与这些结构特异性相互作用的蛋白质。不完美 RNA 双链的特征三维（3D）基序被讨论为参与韧皮部运输的蛋白质特异性识别的韧皮部移动结构 RNA 的元件，因此充当 PTS。
