Low temperature inhibits RNA silencing-mediated defence by the control of siRNA generation.

Temperature dramatically affects plant-virus interactions. Outbreaks of virus diseases are frequently associated with low temperature, while at high temperature viral symptoms are often attenuated (heat masking) and plants rapidly recover from virus diseases. However, the underlying mechanisms of these well-known observations are not yet understood. RNA silencing is a conserved defence system of eukaryotic cells, which operates against molecular parasites including viruses and transgenes. Here we show that at low temperature both virus and transgene triggered RNA silencing are inhibited. Therefore, in cold, plants become more susceptible to viruses, and RNA silencing-based phenotypes of transgenic plants are lost. Consistently, the levels of virus- and transgene-derived small (21-26 nucleotide) interfering (si) RNAs-the central molecules of RNA silencing-mediated defence pathways-are dramatically reduced at low temperature. In contrast, RNA silencing was activated and the amount of siRNAs gradually increased with rising temperature. However, temperature does not influence the accumulation of micro (mi) RNAs, which play a role in developmental regulation, suggesting that the two classes of small (si and mi) RNAs are generated by different nuclease complexes.