The term auxin precursor refers to the raw material used in the synthesis of auxin.

Explanation:
Auxins are a class of plant hormones that play a crucial role in various aspects of plant growth and development, including cell elongation, root formation, and tropic responses. The most common and well-studied auxin is indole-3-acetic acid (IAA). Auxins are synthesized in the meristematic regions of plants, such as the shoot tips and root apices.

Auxin precursors are the substances that serve as the building blocks or raw materials for the synthesis of auxins. These precursors are converted into active auxins through various biochemical processes within the plant cells. The main precursor for auxin synthesis is the amino acid tryptophan, which is obtained from the plant's diet or synthesized de novo.

The synthesis of auxin from its precursor involves several enzymatic steps. Tryptophan is first converted into indole-3-pyruvic acid (IPA) by the enzyme tryptophan aminotransferase. This reaction takes place in the cytoplasm of plant cells. Then, IPA is further converted into IAA, the active form of auxin, through a series of enzymatic reactions in the peroxisomes and endoplasmic reticulum.

The conversion of tryptophan to auxin is regulated by various factors, including light, temperature, and hormonal signals. The levels of auxin precursors in plant cells can also influence the synthesis of auxins. Therefore, the availability of tryptophan and other precursors is crucial for the regulation of auxin biosynthesis and its physiological effects in plants.

In conclusion, auxin precursors are the raw materials used in the synthesis of auxins. The most important precursor is tryptophan, which is converted into the active form of auxin, indole-3-acetic acid (IAA), through a series of enzymatic reactions. The availability of these precursors plays a significant role in regulating auxin biosynthesis and its functions in plant growth and development.