In plants, the level of accumulation of virus is dependent on the physiological-developmental state of the leaf and on the presence of competing viruses. Complementation of viral movement Virus infection begins with infection of a single cell of the plant. The infection, if not checked by plant defense responses, spreads to other parts of the plant. This spread occurs by movement from cell to cell and through the vascular system of the plant. Cell to cell spread, for most viruses and most plants, requires virus-encoded proteins called movement proteins.
Movement proteins do many things in plant cells.
They increase the diameter of connections between cells to allow larger molecules to pass through. They interact with elements of the cell skeleton. They cause the formation of tubules extending from cell membranes and become part of the tubules. Which of these activities are relevant to the movement of infection from cell to cell is unclear. The profile of activities of movement proteins suggests that the movement protein of one virus could facilitate the movement of another virus.