Final answer:
The phloem is the plant structure that transports water, nutrients, proteins, and mRNA. It is part of the vascular system, which also includes the xylem that transfers water and dissolved minerals.
Step-by-step explanation:
The structure in plants that is responsible for transporting water, nutrients, proteins, and mRNA is called the phloem. The phloem is one of the two types of vascular tissue found in plants and is crucial for the distribution of sugars, proteins, and other solutes throughout the plant. In contrast, the xylem is the vascular tissue that primarily transports water and dissolved minerals from the roots to the stems and leaves. Both xylem and phloem tissues are integral components of the plant's vascular system, which functions in the transportation of essential substances. Furthermore, the phloem consists of living sieve elements or conducting cells, companion cells, phloem parenchyma, and phloem fibers, and it operates along with xylem in a close relationship to enable efficient transport within plants.