Phloem and RNA: Identification and Characterization of Distinct Phloem-Allocated Non-coding RNA Molecules

该项目的总体目标是研究通过显花植物的脉管系统分配的tRNA半体的生物起源和功能。在植物中，韧皮部作为营养素和激素的系统范围的递送途径是公认的。然而，在过去的十年里，这一观点受到了挑战，因为发现韧皮部也分配特定的RNA分子。这种韧皮部递送的RNA包括信使RNA（mRNA）、沉默诱导的RNA（siRNA）和微RNA（miRNA）。此外，这些韧皮部分配的RNA分子被证明是影响生长的重要长距离信号。在我们的研究过程中，我们确定了相当数量的全长和片段的小非编码RNA（ncRNA），如核糖体RNA（rRNA），转移RNA（tRNA），小核RNA（snoRNA），和其他未知的小ncRNA在韧皮部。虽然我们对PS ncRNA的研究在分析和鉴定转录本的数量方面并不全面，但我们发现了韧皮部渗出物中发现的tRNA库中的一个有趣的偏倚。一些独特的和必需的tRNA缺失，这表明选择性地递送到韧皮部流中。此外，我们仅在韧皮部中检测到大量特定的tRNA片段（tRNA一半）。强调我们的发现的意义，我们表明，与天然叶RNA池，天然韧皮部RNA池抑制在一个非特异性的时尚翻译。与后来在人类细胞上进行的tRNA半研究一致，我们可以证明tRNA半有助于这种抑制。在其他生物体如酵母和人类细胞中也观察到这种tRNA的一半。许多研究表明，tRNA的一半出现在发育开关和压力条件下。除此之外，tRNA半体在所有生物体中的作用仍有待证明。在酵母中，一种专门的RNase T2和在酵母中，一种与RNase A相关的酶，称为血管生成素，在其反密码子环处切割特定的tRNA并产生tRNA的一半。在植物中不存在血管生成素相关的RNA酶，然而，编码RNA酶T2相关酶的基因是丰富的（例如，在A. thaliana）。到目前为止，这些酶还没有测试它们对tRNA的特异性。因此，产生tRNA半体的植物tRNA内切核酸酶仍有待鉴定和表征。对tRNA半体产生酶的鉴定和分析将提供增加或减少活植物中tRNA半体产生的手段，并揭示它们在植物生长中的潜在作用。i）研究经由韧皮部分配的tRNA半体的功能，ii）鉴定并功能性表征产生酶的tRNA半体，和iii）建立不同于siRNA和miRNA的韧皮部小ncRNA的综合数据库。

The general aim of the project is to study the biogenesis and function of tRNA halves allocated via the vasculature of flowering plants. In plants it is well established that the phloem serves as system-wide delivery pathway for nutrients and hormones. However, in the last decade this view was challenged by the discovery that the phloem also allocates specific RNA molecules. Such phloem delivered RNAs include messenger RNA (mRNA), silencing-induced RNA (siRNA) and micro RNA (miRNA). Furthermore, these phloem allocated RNA molecules were shown to act as important long distance signals effecting growth. In the course of our studies we identified a considerable number of full length and fragments of small non-coding RNAs (ncRNA) such as ribosomal RNA (rRNA), transfer RNA (tRNA), small nuclear RNA (snoRNA), and other unknown small ncRNA in the phloem. Although our study on the PS ncRNAs was not comprehensive with respect to the number of analyzed and identified transcripts, we uncovered an interesting bias in the tRNA pool found in the phloem exudate. A number of distinct and essential tRNAs are missing, which suggests a selective delivery into the phloem stream. In addition, we detected high amounts of specific tRNA fragments (tRNA halves) exclusively in the phloem. Emphasizing the significance of our finding we demonstrated that, in contrast to the native leaf RNA pool, the native phloem RNA pool inhibits in a non-specific fashion translation. In line with a later tRNA halves study performed on human cells, we could show that tRNA halves contribute to this inhibition. Such tRNA halves are also observed in other organisms such as yeast and human cells. A number of studies suggest that tRNA halves appear during developmental switches and under stress conditions. Apart from this a role for the tRNA halves remains to be shown in all organisms. In yeast a specialized RNase T2 and in mammalians an RNase A related enzyme, named angiogenin, cut specific tRNAs at their anticodon loop and produce tRNA halves. In plants angiogenin related RNase enzymes do not exist, however, genes encoding RNases T2 related enzymes are abundant (e.g. five in A. thaliana). Up to now these enzymes are not tested regarding their specificity towards tRNAs. Hence, the plant tRNA endonuclease(s) producing tRNA halves remains to be identified and characterized. The identification and analysis of the tRNA halves producing enzyme will provide the means to increase or decrease the production of tRNA halves in living plants and to unravel their potential role in plant growth.The proposed aims are: i) to study the function of tRNA halves allocated via the phloem, ii) to identify and functionally characterize the tRNA halves producing enzyme, and iii) to build a comprehensive database of the phloem small ncRNAs distinct from siRNA and miRNA.