Plant microRNA Mobility and Extracellular Vesicles: Current Understanding, Controversies and Therapeutic Potential of Plant-to-Animal Communication

MicroRNAs (miRNAs) are a predominant class of small non-coding RNA (sRNA) which regulates target gene expression to occupy fundamental roles in growth, development, metabolism, stress responses, pathogen defence, and disease onset and progression. Traditionally viewed as intracellular regulators, the discovery that specific miRNAs can move between cells, tissues, organs, and even organisms has reshaped our understanding of the extent of their biological significance. In plants, miRNA mobility is a well-established component of long-distance signalling networks that coordinate phase changes across development, nutrient uptake and homeostasis, and responses to the external environment. In animals, extracellular miRNAs are transported within extracellular vesicles, protein complexes, and lipoproteins, where they mediate intercellular communication. More recently, the identification of extracellular RNA transport systems in both kingdoms has led to considerable investigation into cross-kingdom RNA communication. A growing body of evidence indicates that plant extracellular vesicles (PEVs) play important roles in the transport and protection of their harboured miRNA cargo. The strongest evidence supporting cross-kingdom RNA silencing originates from plant–pathogen systems, where PEVs transport regulatory sRNAs into invading pathogens to silence the activity of genes associated with virulence and pathogenicity. Alongside these exciting discoveries, PEVs are attracting even more attention as potential mediators of plant-to-animal communication, thus acting as natural nanocarriers for delivery of therapeutic RNAs. Several studies have reported that plant-derived miRNAs can be detected in animal tissues, wherein they maintain functionality to regulate the expression of targeted genes associated with cancer, viral infection, inflammation, and metabolism. However, the physiological significance of dietary plant miRNA uptake remains highly debated. While some investigations provide evidence supporting cross-kingdom regulation, other reports detail limited uptake, low bioavailability, and an insufficient final concentration in host tissues to elicit any form of biologically relevant effect. Consequently, plant-to-animal miRNA transfer remains one of the most actively debated topics in RNA biology. In this review, we outline the production and mobility of miRNA silencing signals in both the plant and animal systems, discuss the biogenesis and biological functions of PEVs, evaluate evidence supporting cross-kingdom RNA transfer, critically assess the controversies surrounding plant miRNA-directed regulation of mammalian target gene expression, and provide an informed discussion on the future directions in plant-to-animal miRNA transfer.