Department of Biology, Indiana University, Bloomington, IN 47405, USA.
J Exp Bot. 2023 Apr 9;74(7):2389-2404. doi: 10.1093/jxb/erac512.
Extracellular RNA (exRNA) has long been considered as cellular waste that plants can degrade and utilize to recycle nutrients. However, recent findings highlight the need to reconsider the biological significance of RNAs found outside of plant cells. A handful of studies suggest that the exRNA repertoire, which turns out to be an extremely heterogenous group of non-coding RNAs, comprises species as small as a dozen nucleotides to hundreds of nucleotides long. They are found mostly in free form or associated with RNA-binding proteins, while very few are found inside extracellular vesicles (EVs). Despite their low abundance, small RNAs associated with EVs have been a focus of exRNA research due to their putative role in mediating trans-kingdom RNAi. Therefore, non-vesicular exRNAs have remained completely under the radar until very recently. Here we summarize our current knowledge of the RNA species that constitute the extracellular RNAome and discuss mechanisms that could explain the diversity of exRNAs, focusing not only on the potential mechanisms involved in RNA secretion but also on post-release processing of exRNAs. We will also share our thoughts on the putative roles of vesicular and extravesicular exRNAs in plant-pathogen interactions, intercellular communication, and other physiological processes in plants.
细胞外 RNA（exRNA）长期以来一直被认为是细胞废物，植物可以降解并利用它来回收营养物质。然而，最近的发现强调了需要重新考虑存在于植物细胞外的 RNA 的生物学意义。少数研究表明，exRNA 组是一个极其异质的非编码 RNA 群体，其大小从十几个核苷酸到几百个核苷酸不等。它们主要以游离形式或与 RNA 结合蛋白结合存在，而很少在细胞外囊泡 (EV) 内发现。尽管它们的丰度很低，但与 EV 相关的小 RNA 一直是 exRNA 研究的重点，因为它们可能在介导跨物种 RNAi 中发挥作用。因此，非囊泡 exRNA 直到最近才完全被忽视。在这里，我们总结了我们目前对构成细胞外 RNA 组的 RNA 种类的了解，并讨论了可以解释 exRNA 多样性的机制，不仅关注涉及 RNA 分泌的潜在机制，还关注 exRNA 释放后的处理。我们还将分享我们对囊泡和非囊泡 exRNA 在植物-病原体相互作用、细胞间通讯以及植物其他生理过程中的可能作用的看法。
