## 1 Introduction
After dsRNA and hpRNA are taken up into a cell, they are fragmented by endoribonuclease Dicer to form small-interfering RNAs (siRNAs). The siRNA binds to the Argonaute (AGO) protein to form the RNA-induced silencing complex (RISC). In the process of RISC formation, the double-stranded siRNA is untied into single strands, which recognize and bind to the target mRNA with sequence complementary to that of the single strand. As a result, degradation and translational inhibition of target mRNA are induced; (**b**) Application of RNAi for insect and viral control. Administration of dsRNA that corresponds to a gene essential for pest growth or virus multiplication induces knockdown of expression of the gene and a decrease in expression of the gene-derived protein, resulting in suppression of pest growth or viral multiplication. the RNAi-based pesticide should not affect unintended (non-target) insects; this approach is favorable for ecosystem biodiversity. A second advantage is that the RNA, the effective factor of the insecticide, degrades reasonably quickly in the environment because of its inherent chemical instability and the ubiquitous presence of nucleases in the bioecological community [14,15]. Again, this feature of RNAi-based pesticides is fundamentally consistent with protection of ecosystems. Nevertheless, moderate control of degradability is an important aspect of the practical application of dsRNAs in pesticides applied topically to crops [16,17]. A third valuable characteristic of RNAi-based pesticides is that their application allows for flexible and fast responses to emerging pest diseases, and pests that have become resistant to pesticides already in use. Even if the target pest becomes resistant to the RNAi-based pesticide used, a new RNAi-based pesticide can, in principle, be developed quickly by simply targeting another region of the same gene or by changing the target gene in the pest [18]. In this regard, however, the emergence of a formidable resistant pest has recently been reported due to genetic mutations involved in the underlying mechanism of RNAi action [19]; this is discussed specifically below. It remains essential that the effects of the practical application of these innovative RNAi-based pesticides and antiviral agents on human health and ecosystems are evaluated (just as with conventional pesticides). However, the immediate practical issues for development and application of this technology are: (i) the requirement for substantial reduction of the production costs; (ii) improvement of administration methods to target organisms such as crop pests (this is also related to the product cost issue), and (iii) selection of RNA sequences that do not cause off-target effects, i.e., that do not exert effects on genes other than the specific gene of the target pest. The most urgent challenge is reduction of dsRNA production costs to a level where RNAi-based products can be put into practical commercial use. The amount of RNA required for use in crop fields and aquaculture ponds is enormous, and thus, ways must be found to produce extremely large amounts of dsRNA (with designed nucleotide sequences of a few hundred base pairs) in an economically viable way. This review presents an overview of recent production methods of dsRNA (including hairpin-type RNA species, hpRNA) using the representative microorganisms _Escherichia coli, Pseudomonas syringae, Corynebacterium_ (C.) _glutamicum_, and _Chlamydomonas (Chl.) reinhardtii_ as host strains, as well as research results using some other microorganisms. In _E. coli_, there are many reports of the production of hpRNAs with partial double-stranded structures and dsRNAs using the coliphage T7 transcription system. A unique system using the combination of _P. syringae_ and its bacteriophage phi6 has been developed to increase the stability of dsRNA products. A high-level dsRNA production system has recently been established using the industrial microorganism _C. glutamicum_, a \"Generally Recognized as Safe\" (GRAS) strain.