Nutrients must cross the cell membranes of cells in order to be used in the cell or be incorporated into larger molecules. Polar molecules, including mineral ions, cross the membrane via specialized transport systems. In Chapter 34 you saw how water and ions move into plant roots by way of the apoplast (through cell walls and intercellular spaces) or symplast (directly through cells). The Casparian strip prevents water and ions from entering the xylem tissues of the roots (see Key Concept 34.1); therefore these nutrients must enter the symplast before they can be transported to other tissues. In most cases, ions are actively transported across the cell membrane of epidermal cells into the symplast because their concentrations in the soil solution are generally lower than their concentrations inside cells.

Plants have specialized transport systems for the uptake of specific ions (see Figure 35.2). For example, Arabidopsis thaliana has more than 50 genes that encode nitrate (NO₃^–) transporters, 6 genes encoding ammonium (NH₄⁺) transporters, and at least 4 genes for phosphate (PO₄^3–) transporters. Nutrient uptake is highly regulated because the levels of ions inside cells must be maintained at constant levels. The genes for ion transporters are regulated at the transcriptional level by the amounts of each nutrient inside cells: low nutrient levels stimulate transcription, whereas high levels repress transcription. In addition, the transporter proteins themselves are regulated (for example, by phosphorylation) to control their ion transport activity.

The incorporation of nutrients into more complex molecules is also regulated according to the plant’s needs. The enzymes involved in incorporating nitrate and ammonium into amino acids are regulated at the transcriptional and posttranscriptional levels to increase assimilation when available nitrogen is abundant. The uptake and assimilation of nitrogen are also stimulated by photosynthesis, and this ensures that the nitrogen status in the plant is coordinated with its carbon status.