Symbiosis means a relationship between two species that offers mutual benefit upon its members. The biological and evolutionary success of each species is linked to and enhanced by that of the other.
Humans or all warm blooded animals are directly effected by the plant world.
In order for a plant to survive, it must rely on photosynthesis.
This process requires carbon dioxide and water and in the presence of sunlight plants make oxygen and glucose (sugar).
In order for a warm blooded animal such as a human to survive, it must rely on respiration.
This process requires exactly what the plant gives off. Humans use oxygen and glucose to produce carbon dioxide and water.
So they breathe, we breathe, they breathe, we breathe. And all is well.