**1. Introduction**

In recent years, non-protein-coding RNA transcripts have been associated to regulatory functions in plants and animals, particularly micro RNAs (miRNAs), a class of endogenous single-stranded molecules of ~22 nucleotides, transcribed by polymerase II from MIR genes, involved in negative regulatory activities at the posttranscriptional level in plants and mammals [1–3]. The sophisticated mechanism used by miRNAs for the regulation of their targets is based on perfect or near-perfect complementarity binding of miRNAs at the ORF, 5'UTR and 3'UTR sites of mRNA, resulting in repression of translation or degradation of the messenger [4]. Since the identification of the first miRNA in *Caenorhabditis elegans* [5], until now according to the miRBase version 22.1 database (http://www.mirbase.org, accessed April 2021), a total of 38,589 hairpin precursors (pre-miRNAs) from 217 organisms have been recorded. In plants some crucial functions of miRNAs have been established such as response to developmental signals, auxin responsive factors (ARFs), pathogen infection, cell division, metabolism, etc. [6, 7]. In human, these small transcripts have been found to endogenously participate in some diseases such as diabetes, cancer, heart disease, tumors, atherosclerosis, or biomarkers in early stages of particular diseases [8, 9]. Recently, several

studies have shown that plant miRNAs can enter the gastrointestinal tract through food in humans, be identified in various tissues and circulatory system, perceiving genes in humans as potential regulatory targets [10]. Because exogenous miRNAs and endogenous miRNAs have no distinguishing characteristics from each other, therefore, it will be recognized as an endogenous miRNA [11]. The cross-kingdom regulation by miRNAs has developed a series of investigations because it is important to determine the effects of miRNAs from plants on gene expression in humans when they enter the human body through food [11–13].

This chapter will provide a brief overview of the evidence about the impact of exogenous miRNAs and their direct influence on various biological processes in human, a cross-kingdom approach.

## **2. Biogenesis and mechanisms of miRNAs**

miRNAs are single-stranded strands of approximately (18–24) base pairs evolutionarily conserved in diverse species [14], they are transcribed by RNA polymerase II (pol II) giving rise to a miRNA-primary, which possesses a stem-loop structure [3]. The first step occurs in the nucleus and is carried out by cleavage by the RNase III enzyme Drosha and the double-stranded RNA-binding protein (dsRNA) DGCR8, generating a product of approximately 60 nt called pre-miRNA that contains an overhang of 2–3 nt [15]. It is subsequently exported to the cytoplasm by the premiRNA/Exportin5/Ran-GTP complex, in the cytoplasm GTP is hydrolyzed to GDP inducing the release of the pre-miRNA [16], then processed by the Dicer RNaseIII protein producing an RNA duplex of approximately 22 nt [17]. The double strand is incorporated into the RNA-induced silencing complex (RISC), a protein nuclease complex, an Argonaute protein (Ago2), and a double-stranded RNA-binding protein, upon incorporation one strand of the duplex is degraded and the other remains as a mature miRNA, with the faculty to regulate the expression of a target mRNA [17, 18]. While, in plants, the pri-miRNA synthesized by pol II in the nucleus is processed by an enzyme of the RNase III family DICER-LIKE1 (DCL1), resulting in an miRNA/ miRNA duplex chain. To stabilize and protect from degradation these duplexes are 2′-O-methylated at the 3′-ends by a Hua Enhancer 1 (HEN1) methyltransferase [19]. Finally, one strand of the duplex is incorporated into AGO1 in the cytoplasm to form the RISC complex [20]. One of the determining mechanisms in the interaction of miRNA and its target mRNA is the seed region (nt 2–8, [21]). The seed region appears to be the most important site for miRNA recognition of its target [22]. In addition to taking advantage of its utility to predict regulatory targets, in relation to the characteristics in the mRNA sites necessary for the effective recognition of the miRNA [23].
