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MicroRNAs (miRNAs) constitute a class of small non-coding endogenous RNAs that regulate gene expression by targeting mRNAs for degradation or translational repression. MiRNAs are intensively investigated and they have been found to be a pivotal component of developmental regulation processes. Recent studies showed the non-cell autonomous function of several miRNAs. We analyzed the accumulation pattern of selected miRNAs in Arabidopsis thaliana embryonic tissues. The majority of the investigated miRNAs showed uniform accumulation across the embryo suggesting their possible role at this developmental stage. In the case of miR167 however, we detected a gradient-like expression profile which in earlier studies has been considered to be the hallmark of the non-cell autonomous activity of miRNAs. Using reporter assay we analyzed the expression patterns of the four MIR167 precursor genes. We found that two of the precursor genes, MIR167A and MIR167B, also showed an overlapping gradient-like expression patterns in the embryo. These data indicate that in addition to non-cell autonomous activity of some miRNAs, the gradient-like expression patterns can be generated also by the specific expression characteristic of miRNA precursor genes.

Virus infections induce the expression of ARGONAUTE1 (AGO1) mRNA and in parallel enhance the accumulation of miR168 (regulator of AGO1 mRNA). Here, we show that in virus-infected plants the enhanced expression of AGO1 mRNA is not accompanied by increased AGO1 protein accumulation. We also show that the induction of AGO1 mRNA level is a part of the host defence reaction, whereas the induction of miR168, which overlaps spatially with virus-occupied sectors, is mediated mainly by the Tombusvirus p19 RNA-silencing suppressor. The absence of p19 results in the elimination of miR168 induction and accompanied with the enhanced accumulation of AGO1 protein. In transient expression study, p19 mediates the induction of miR168 and the down-regulation of endogenous AGO1 level. P19 is not able to efficiently bind miR168 in virus-infected plants, indicating that this activity is uncoupled from the small RNA-binding capacity of p19. Our results imply that plant viruses can inhibit the translational capacity of AGO1 mRNA by modulating the endogenous miR168 level to alleviate the anti-viral function of AGO1 protein.

The seasonal dynamics of picophytoplankton communities in shallow turbid alkaline pans in Hungary was studied between July 2006 and May 2007. Similarly to other aquatic environments in the temperate zone, dominance of picocyanobacteria was observed in summer and that of picoeukaryotes in winter. The mild winter in 2006–2007, with midday water temperatures of 5–10°C, resulted in large winter phytoplankton blooms (maximum chlorophyll a concentration 800 μg l−1) in the shallow pans. The phytoplankton was composed of single-celled picoeukaryotes and had a maximum of 108 × 106 cells ml−1 in Budos-szek pan, 50 × 106 cells ml−1 in Kelemen-szek pan in April 2007, and 47 × 106 cells ml−1 in Zab-szek pan in March 2007. In order to explain the winter dominance of picoeukaryotes, we isolated picoeukaryotic and picocyanobacterial strains and determined the temperature and light dependence of their photosynthesis. Under temperatures