What are examples of symbiotic relationships in plants?

Symbiosis is a biological relationship between two organisms. Symbiosis can be commensal, which means that one of the organisms is helped by the relationship and the other is neither helped or harmed. It can be parasitic, where one organism benefits but the other is harmed (albeit not enough to kill it immediately). Symbiosis can also be mutual, which is what we normally associate with this term. In such cases, both organisms benefit from the relationship.
In the plant world, examples of mutualism, the last type of symbiosis mentioned above, include lichens, mycorrhizal fungi, and companion planting. Lichens consist of two parts: a fungus that provides the structure and an alga that provides the photosynthesis to feed the plant. Mycorrhizal relationships involve fungi and tree roots. The fungus helps the tree extract nutrients from the soil while it feeds on the sugars the tree produces by photosynthesis. The trees grow better with their fungal partners.
Companion planting is a way that gardeners take advantage of symbiotic relationships between plants. For example, planting tomatoes in the same container as lettuce will protect the lettuce from too much direct sunlight and will provide a natural type of mulch for the tomato plant, which keeps the soil cooler and protects the tomato plant's leaves from diseases caused by splashing water and dirt.
https://treesforlife.org.uk/into-the-forest/habitats-and-ecology/ecology/symbiosis/