Plants with roots, stems, and leaves are vascular plants, which means they have xylem and phloem.
Xylem is a specialized tissue that takes the water and minerals from the roots to the rest of the plant.
Phloem, on the other hand, conducts the products of photosynthesis (carbohydrates) from the leaves to where they are needed.
Carbohydrates such as glucose store chemical energy, and are broken down to obtain ATP, which provides the energy needed for metabolic reactions.
This means that the energy needed by the roots of a plant is obtained from the carbohydrates conducted by the phloem.