Water accounts for over 80% of the mass of most plants. It is essential for photosynthesis, metabolism, and the transport of nutrients and other molecules. Inorganic nutrients are likewise crucial for plant growth and reproduction. For instance, nitrogen is a building block for key biomolecules, such as amino acids, while potassium is critical for the opening and closing of stomata. How do plants reliably absorb nutrients and large amounts of water?

Most plants acquire water and minerals from the soil in which they are rooted. Plant roots release hydrogen ions and carbon dioxide. Carbon dioxide reacts with water, forming a bicarbonate anion, and additional hydrogen ions. The hydrogen ions bind to negatively charged soil particles, which releases positively charged ions, like potassium, into the soil solution. This process is called cation exchange and makes nutrients available to the plant.

The root architecture and branching patterns are essential for efficient water and nutrient uptake. For instance, the outer layer of cells that is in contact with the soil forms tiny root hairs. These specialized cells have a large surface area that absorbs water through both active and passive processes.

For instance, water and dissolved nutrients may passively diffuse into the apoplast, which comprises all cell walls and spaces between cells. The space within the cell membrane is called the symplast. Water enters the symplast via osmosis across the cell membrane, while nutrients are taken up via active transport.

Once water and minerals enter the symplast, they move freely towards the center of the root through cell-to-cell connections. Water and nutrients in the apoplast, however, are blocked from entering the stele by Casparian strips. Casparian strips are layers of water-impermeable suberin in the cells of the endodermis, which surrounds the stele. These structures block the passage of potentially undesirable or toxic elements.

In order to enter the center of the root, all solutes need to cross a plasma membrane. Once they enter the symplast water and nutrients enter the stele, which distributes them throughout the plant to fulfill their essential roles.