In the context of genetic transformation of plants, a helper plasmid is a plasmid present in Agrobacterium that provides functions required by the bacteria for transferring foreign DNA to a plant cell. They have been extremely important in plant genetic engineering. Generally, helper plasmids are derivatives of the Ti plasmid that contain an active virulence region, but from which the T-DNA has been removed.
Central to the concept of developing the binary vector system for Agrobacterium-mediated plant cell transformation was the finding that the genes contained on the T-DNA are not required for its transfer to the plant cell, that this function is provided by gene products of the virulence region of the Ti plasmid, and that they are trans-acting (1). In practice, this means that foreign DNA to be transferred to the plant cell can be cloned between the T-DNA border sequences in any plasmid that is stable in Agrobacterium. Then, such a plasmid, known as the binary vector, can be transferred to Agrobacterium that contains a helper plasmid by conjugation, electroporation, or transformation (see Transfection). Helper plasmids are based on deletion derivatives of Ti plasmid that lack the T-DNA. The helper plasmid provides the functions required for transferring the foreign DNA located between the T-DNA borders of the binary vector to the plant cell. The most widely used binary vector/helper plasmid system is the pBin binary vector (2) used in combination with the LBA4404 helper plasmid (1).