When a plant is infested, viral RNA invades plant cell and proliferates. Inside an infested cell new RNA and viral protein molecules are synthesized, then proteins attach themselves to RNA. These complex molecules enter other cells and infest them. Viral RNA is able to enter neighbouring cell through a protoplasmatic connection – plasmodesms – tubes connecting cells. But RNA is able to travel along such tube only when it is bound with a special transport protein.
 

Group of scientists from Moscow State University Biology Faculty under the supervision of Atabekov I.G. have made an interesting observation: viral RNA bound to transport protein never proliferates. The researchers have performed the following experiment: individual plant cells, which don’t have a cell wall (protoplasts), were infested with RNA bound to transport protein. The virus didn’t proliferate. But the same thing didn’t work for the whole plant – the infection has spread further. So the scientists concluded that for normal proliferation it’s necessary for plant virus to travel through a plasmodesm. RNA-protein complex is likely to interact with some host-plant factors, which makes RNA proliferation possible.
 

The scientists suspect that the virus infests only those plants, whose cell membranes have a receptor protein able to interact with viral transport protein. To prove their hypotheses the scientists have created two artificial RNA-protein complexes. First complex consists of tobacco mosaic viral RNA and its transport protein, and the second - tobacco mosaic viral RNA and barley striate mosaic viral transport protein. Both complexes were introduced to three species of plants: tobacco, pigweed and spherical gomphrena. All these plants are sensitive to tobacco mosaic virus, but only pigweed suffers from barley striate mosaic virus.
 

Behaviour of first complex sprung no surprises on the scientists: RNA traveled through plasmodesms from an infested cell to its neighbour and restored its proliferating ability – all plants have been infested (their leaves were covered with small dark dots). And the second complex (RNA and alien transport protein) has come up to scientists’ expectations – only pigweed was infested, because barley and gomphrena cells do not have any special receptors able to interact with barley striate mosaic virus transport protein. The result was that RNA didn’t recover, and the plants were not infested.
 

The virus appears to activate only when its transport protein traveling from primary infected cells to healthy ones matches host receptor.
 

Why does the virus need that? It evidently needs this mechanism to be sure that it would flourish in its host plant. And why do we need to know that? We think that we should know that to find new ways of breeding virus-resistant plants.