A government, in this case the Brazilian government, cannot effectively control what people, particularly people who are not its citizens, do in other countries. If people are able to obtain and ship outside of Brazil supplies of the plant, then the Brazilian government cannot stop them doing research on it.
However, the Brazilian government can largely control what happens in Brazil. It can ban or restrict cultivation or harvesting of the plant. (Let's call it Athelas.) It can pass a law requiring an Athelas harvester's license, and only approve licenses for citizens of Brazil who agree not to sell or transfer any Athelas to anyone outside Brazil. Indeed it can require that any Athelas harvester have a contract with a firm doing research in Brazil.
Whether such restrictions would be effective cannot be said in advance. They might be evaded. They might not be allowed under the Brazilian Constitution. But such restrictiosn might serve the purpose that the Brazilian government has in mind.