And by this I don't mean anything bizarre like Marvel Comics's Venom.
This already happens in our world, in some way. Cattle such as sheep and cows eat grass. At a glance, most people would think that this is beneficial to the animal, while being solely detrimental to the plant. But the grass blades that have been munched on get a benefit as well.
Cattle saliva contain some substances that stimulate plant growth. In the absence of cattle, some kinds of grass might not grow as much, nor as healthy as they would otherwise.
Don't forget that that's not the only way in which cattle and grass interact. The waste of cows and sheep is broken down by fungi and bacteria, and the byproducts of that are important for grass.
Your species might do the same thing - by secreting different substances through different body parts, in response to different stimuli both from the plants themselves and the environment. Sentience in the animal species helps it plan and organize the application of its secretions on plants, thus acting more efficiently than if they did it by instinct alone. It also allows for actual farming.
Note that these secretions are hormones, so your fictional people are communing with plants in some way. Also notice that this does not preclude the use of technology. In fact, it synergizes very well with it. Being able to tell a plant to grow faster, form flowers, or to withold fruit formation at precise times of the year is great. Using that together with ploughs, dams and irrigation is even better.
A sample secretion as food for thought: ethylene. For humans, it is raw material for most of the plastic we produce:
Ethylene is widely used in the chemical industry, and its worldwide production (over 150 million tonnes in 2016) exceeds that of any other organic compound. Much of this production goes toward polyethylene, a widely used plastic containing polymer chains of ethylene units in various chain lengths.
But for plants, it is a hormone:
Ethylene is also an important natural plant hormone, and is used in agriculture to force the ripening of fruits.