Novel constructs for efficient cloning of sRNA-encoding DNA and uniform silencing of plant genes employing artificial trans-acting small interfering RNA.

KEY MESSAGE: TAS atasiRNA-producing region swapping used one-step, high efficiency, and high fidelity directional TC-cloning. Uniform silencing was achieved without lethality using miRNA trigger- TAS overexpression fusion cassettes to generate 21-nt atasiRNA. Plant transgenic technologies are very important for basic plant research and biotechnology. Artificial trans-acting small interfering RNA (atasiRNA) represents an attractive platform with certain advantages over other silencing approaches, such as hairpin RNA, artificial microRNA (amiRNA), and virus-induced gene silencing (VIGS). In this study, we developed two types of constructs for atasiRNA-mediated gene silencing in plants. To functionally validate our constructs, we chose TAS1a as a test model. Type 1 constructs had miR173-precursor sequence fused with TAS1a locus driven by single promoter-terminator cassette, which simplified the expression cassette and resulted in uniform gene silencing. Type 2 constructs contained two separate cassettes for miR173 and TAS1a co-expression. The constructs in each type were further improved by deploying the XcmI-based TC-cloning system for highly efficient directional cloning of short DNA fragments encoding atasiRNAs into TAS1a locus. The effectiveness of the constructs was demonstrated by cloning an atasiRNA DNA into the TC site of engineered TAS1a and silencing of CHLORINA 42 (CH42) gene in Arabidopsis. Our results show that the directional TC-cloning of the atasiRNA DNA into the engineered TAS1a is highly efficient and the miR173-TAS1a fusion system provides an attractive alternative to achieve moderate but more uniform gene silencing without lethality, as compared to conventional two separate cassettes for miR173 and TAS locus co-expression system. The design principles described here should be applicable to other TAS loci such as TAS1b, TAS1c, TAS2, or TAS3, and cloning of amiRNA into amiRNA stem-loop.