The developmental sensitivity of plant shoots to their environment is mirrored underground. When a plant experiences drought, it produces more roots, and these roots penetrate farther into the soil, in part because they produce fewer lateral branches. By producing deeper roots, a plant increases its chances of reaching moister soil.
The root cap appears to play an important role in allowing roots to sense and respond to the amount of water in the soil. Because the root cap is not well connected to the plant’s vascular system, the water status of its cells provides a good indication of the moisture content of the soil. As soils dry, root cap cells produce a hormone called abscisic acid, abbreviated as ABA (Table 31.1). Abscisic acid stimulates root elongation, leading to roots that penetrate deeper into the soil. It also triggers stomata to close, reducing the demand for water.
Abscisic acid stimulates root elongation by suppressing ethylene synthesis. Ethylene slows root elongation by influencing the orientation of cellulose in the cell wall. Cells treated with ethylene expand in diameter rather than in length. Thus, by inhibiting the production of ethylene, the production of ABA in drought-