MicroRNAs (miRNAs) and other endogenous small RNAs act as sequence-specific regulators of the genome, transcriptome, and proteome in eukaryotes. The interrogation of small RNA functions requires an effective, widely applicable method to specifically block small RNA function. Here, we report the development of a highly effective technology that targets specific endogenous miRNAs or small interfering RNAs for destruction in Arabidopsis thaliana. We show that the expression of a short tandem target mimic (STTM), which is composed of two short sequences mimicking small RNA target sites, separated by a linker of an empirically determined optimal size, leads to the degradation of targeted small RNAs by small RNA degrading nucleases. The efficacy of the technology was demonstrated by the strong and specific developmental defects triggered by STTMs targeting three miRNAs and an endogenous siRNA. In summary, we developed an effective approach for the destruction of endogenous small RNAs, thereby providing a powerful tool for functional genomics of small RNA molecules in plants and potentially animals.

Because of its excellent optical performance and electrical properties, TiO₂ has a wide range of applications in many fields. It is often considered to be physiologically inert to humans. However, some recent studies have reported that nano‐sized TiO₂ may generate potential harm to the environment and humans. In this paper the in vivo acute toxicity of nano‐sized TiO₂ particles to adult mice was investigated. Mice were injected with different dosages of nano‐sized TiO₂ (0, 324, 648, 972, 1296, 1944 or 2592 mg kg^–1). The effects of particles on serum biochemical levels were evaluated at various time points (24 h, 48 h, 7 days and 14 days). Tissues (spleen, heart, lung, kidney and liver) were collected for titanium content analysis and histopathological examination. Treated mice showed signs of acute toxicity such as passive behavior, loss of appetite, tremor and lethargy. Slightly elevated levels of the enzymes alanine aminotransferase and aspartate aminotransferase were found from the biochemical tests of serum whereas blood urea nitrogen was not significantly affected (P <0.05). The accumulation of TiO₂ was highest in spleen (P <0.05). TiO₂ was also deposited in liver, kidney and lung. Histopathological examinations showed that some TiO₂ particles had entered the spleen and caused the lesion of spleen. Thrombosis was found in the pulmonary vascular system, which could be induced by the blocking of blood vessels with TiO₂ particles. Moreover, hepatocellular necrosis and apoptosis, hepatic fibrosis, renal glomerulus swelling and interstitial pneumonia associated with alveolar septal thickening were also observed in high‐dose groups. Copyright © 2009 John Wiley & Sons, Ltd.