Apart from lichens, can you give some examples of symbiosis in plants?
Mistletoe growing on trees is a nice example of a parasitic symbiosis. As you know, symbiosis implies not only mutually beneficial co-existence; for example, it can take a form when one of the species just does not get hurt by their symbiont and does not benefit from it either (commensalism). Or, there is parasitic symbiosis, when a symbiont lives on the account of its host, harming it. A mistletoe is exactly such kind of species.