A normal Arabidopsis plant (on the left) and a plant in which ta-siRNA formation is compromised (right)
Throughout their life, plants form leaves and side roots. The development of these two kind of organs has in common to be fine-tuned by small RNA molecules called ta-siRNAs. A study conducted by the scientists Dr. Alexis Maizel, CellNetworks Junior Group Leader, and Virginie Jouannet at Heidelberg University's Centre for Organismal Studies now highlights how and where within the plant cell these ta-siRNAs are produced. The results of this study have been published in the "EMBO Journal."
Microscopic image of plant cells. AGO7 (in green) concentrates in foci in the cytoplasm of whereas DNA (in blue) is in the nucleus
The formation of plant organs depends on the presence of proteins that allow cells to divide and take on new shapes and characteristics. The most straightforward route to protein production begins when genes are activated and transcribed into messenger RNAs that are then translated into proteins. However, cells often fine-tune their population of proteins by producing short interfering RNAs (siRNAs): small molecules that dock onto messenger RNAs and cause them to be broken down before they can be used for protein production. ta-siRNAs are created from longer RNA molecules which are whittled down by a complex of other molecules. One essential component of this cutting machine is a protein called AGO7. Scientists already knew that ta-siRNA fine tune the formation of leaves and the growth of side roots by blocking production from other genes; it was however unknown where in the plant cell the ta-siRNA were produced. The Heidelberg scientists have uncovered that AGO7 accumulates in foci, called siRNA bodies, located in the cytoplasm of the cells. "Interestingly these siRNA bodies contain all other enzymes required for the formation of ta-siRNAs, they form therefore hotspots for the formation of siRNAs" says Virginie Jouannet, PhD student in the Maizel group. In addition, the scientists could show that AGO7 was no longer fulfilling its functions when displaced away from the siRNA bodies, resulting in problems in the development of the plant.
Microscopic image of a plant cell. AGO7 (in green) concentrates in foci. The network of membranes used by the cell to secrete proteins is in red
Two other observations caught the attention of the researchers. "First, these siRNA bodies are closely linked to the network of membranes that the cell uses to secrete proteins. Second, interestingly these foci also host viruses and plants defend themselves against virus using siRNAs" says Dr. Alexis Maizel. "These results reveal a hitherto unknown role for membranes in processing of RNA and suggest that the generation of siRNA can only occur in specific locations of the cell." adds Dr. Maizel.
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Dr. Alexis Maizel
Center for Organismal Studies (COS)
+49 6221 54 64 56
alexis.maizel [ aT ] cos.uni-heidelberg.de