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ARGONAUTE (AGO) proteins are integral parts of regulatory pathways under the control of small RNA (sRNA) that are fundamental for the proper functioning of eukaryotic cells. AGOs, as highly specialized platforms binding specific sRNA, coordinate gene silencing through interaction with other protein factors (forming the RNA-induced silencing complex, RISC), contributing to endonucleolytic cleavage of the target mRNA and/or influencing the translation process. The increasing number of evidence confirms the participation of AGO proteins in several other cellular processes, such as i.e.: transcription regulation, sequestration, RNA-dependent methylation of DNA, repair of DNA damages, synthesis of siRNA independent of DCL (DICER-like) proteins, or co-transcriptional regulation of MIRNA genes expression and intron splicing. Particular plant species are characterized by the presence of a different number of AGO proteins, in many cases of yet unknown regulatory and/or biological function. This review article covers the current knowledge about the functions of AGOs in cell biology and plant development.

The genetic material is constantly subjected to DNA damage which is caused by physiological processes occuring in the cell and is exposed to exogenous DNA damaging agents. Eucariotic cells have developed a system called the DNA damage response (DDR), which is responsible for maintaining genomic inegrity. DNA damage can lead to senescence, DNA repair as well as to cell death. The key protein in the DDR pathway is p53. This protein undergoes numerous posttranslational modifications and can be involved in the activation of many genes and proteins leading to survival or cell death. In cell senescence the p53 protein leads to the induction of p21, which causes cell cycle arrest. In apoptosis p53 participates in the activation of caspases, which are responsible for the degradation of many proteins.

Telomeres are guanine-rich repeated sequences located at the ends of chromosomes. The loss of telomeric repeats after each cell division may function as a biological clock limiting the cell proliferation ability. Telomerase is a RNA-dependent DNA polymerase that synthesisezes telomeric DNA and thus enables cancer cells an unlimited proliferative potential. Human telomerase is a ribonucleoprotein complex composed of reverse transcriptase-hTERT, RNA component-hTR (functioning as a template for the telomeric DNA addition) and associated proteins. Telomerase activity is present in most malignant cells but undetectable in most normal cells. The enzyme and its altered activity distinguishing cancer cells, is an attractive molecular target for anti-cancer therapy. One of the most promising methods for modulation of the telomerase activity is RNA interference. Many investigators showed that targeting different subunits of telomerase (mainly hTERT) with siRNA had inhibitory effects on expression and activity of the enzyme and cells proliferation. siRNA targeting telomerase has the possibility to became effective anti-cancer agent especially in an adjuvant therapy.

The discovery of the RNAi phenomenon drastically changed our thinking about the mechanisms affecting the expression of genetic information. It demonstrated that not only proteins but also small regulatory RNAs (srRNAs) are the key players in this process. In addition, it was shown that srRNAs can influence viral infections. They are able to mediate viral RNA degradation or to inhibit viral RNA replication and translation. The collected data suggest that srRNAs will soon become an important tool in our struggle with viruses.