Two virus-free isolates were successfully infected by purified TmPV1 using protoplast transfection. The reticulate, parietal chloroplast extends over the whole interior of the cell, enveloping the protoplast. Protoplast fusion is the joining of cells or cell components to transfer traits between species. The protoplast develops into a zoosporangium and releases zoospores. The chloroplast is also thought to have an important role in protoplast regeneration. Hybrids may also be produced by a technique called protoplast fusion. The protoplast of the prey is then sucked out into a food vacuole. Each daughter protoplast rounds off to form a non-motile spore. Protoplasts may also be used for plant breeding, using a technique called protoplast fusion. The result is a deterioration of the protoplast and necrosis of the cells. Gravity causes the plant protoplast to settle within the cell wall. The protoplast then form short thread-like filaments that later retract back into round globules. These are tubelike structures that connect the protoplast with the cell wall. The system is known as the protoplast two hybrid (P2H) system. In some cases, the protoplast skips the thread phase and remains isodiametric.