The structure that has a symbiotic relationship with fungi in mycorrhizae is the root of a plant.

Here's why:

* Mycorrhizae are a type of symbiotic relationship between fungi and plant roots.

* The fungi provide the plant with increased access to water and nutrients (especially phosphorus), which are absorbed from the soil.

* In return, the plant provides the fungi with carbohydrates produced through photosynthesis.

This mutually beneficial partnership helps both organisms thrive.