Virus Synergy in Transgenic Plants

Transgenic plants expressing viral genes offer novel means of engendering resistance to those viruses. However, some viruses interact synergistically with other viruses and it is now known that transgenic plants expressing particular genes of one virus may also mediate synergy with a second virus. Thus, our specific objectives were to (1) determine if transgenic plants resistant to one virus showed synergy with another virus; (2) determine what viral sequences were essential for synergy; and (3) determine whether one of more mechanisms were involved i synergy. This project would also enable an evaluation of the risks of synergism associated with the use of such transgenic plants. The conclusion deriving from this project are as follows: - There is more than one mechanism of synergy. - The CMV 2b gene is required for synergistic interactions. - Synergy between a potyvirus and CMV can break natural resistance limiting CMV movement. - Synergy operates at two levels - increase in virus accumulation and increase in pathology - independently of each other. - Various sequences of CMV can interact with the host to alter pathogenicity and affect virus accumulation. - The effect of synergy on CMV satellite RNA accumulatio varies in different systems. - The HC-Pro gene may only function in host plant species to induce synergy. - The HC-Pro is a host range determinant of potyviruses. - Transgenic plants expressing some viral sequences showed synergy with one or more viruses. Transgenic plants expressing CMV RNA 1, PVY NIb and the TMV 30K gene all showed synergy with at least one unrelated virus. - Transgenic plants expressing some viral sequences showed interference with the infection of unrelated viruses. Transgenic plants expressing the TMV 30K, 54K and 126K genes, the PVY NIb gene, or the CMV 3a gene all showed some level of interference with the accumulation (and in some cases the pathology) of unrelated viruses. From our observations, there are agricultural implications to the above conclusions. It is apparent that before they are released commercially, transgenic plants expressing viral sequences for resistance to one virus need to be evaluated fro two properties: - Synergism to unrelated viruses that infect the same plant. Most of these evaluations can be made in the greenhouse, and many can be predicted from the known literature of viruses known to interact with each other. In other cases, where transgenic plants are being generated from new plant species, the main corresponding viruses from the same known interacting genera (e.g., potexviruses and cucumoviruses, potyviruses and cucumoviruses, tobamoviruses and potexviruses, etc.) should be evaluated. - Inhibition or enhancement of other resistance genes. Although it is unlikely that plants to be released would be transformed with HC-Pro or 2b genes, there may be other viral genes that can affect the expression of plant genes encoding resistance to other pathogens. Therefore, transgenic plants expressing viral genes to engender pathogen-derived resistance should be evaluated against a spectrum of other pathogens, to determine whether those resistance activities are still present, have been lost, or have been enhanced!