Why are plants not affected by animal viruses such as retroviruses? What prevents them from being infected?
Virus are extremely host-dependent because they need to enter the host, move inside of it, use its replication mechanisms and codon usage. There are virus that cross the family boundry (ToMV for example) but crossing the kingdom boundry is less probable. For example, animal cells don't have a cell wall, which is a major change and requires a change in strategy for entrance to plant cells.
For retrovirus, they simply are virus that can integrate into the host genome. Several articles [1, 2, 3] describe retroelements that have been classified as retrovirus, retrovirus-like particles, and retroelements encoding coat proteins. This can lead to the conclusion that there are retrovirus in plants.
It is worth noticing that virus are not the major pathogens of plants (fungi are) and resistance to virus can be acquired even without understanding the mechanism. This leads to a lower level of interest, funds, and researchers in the area, which leads to a lower rate of discovery of new virus.
In conclusion, we have not found a virus that crosses the kingdom barrier but have families of virus that contain both plant and animal virus.