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Indole-3-pyruvate is the indole-substituted pyruvate anion named systematically 3-(1H-indol-3-yl)-2-oxopropanoate. It is the conjugate base of indole-3-pyruvic acid. The three-carbon side chain distinguishes it from indole-3-acetate, whose corresponding side chain has two carbons.^[ref]

A biological example is the plant route from tryptophan to the auxin indole-3-acetate. A transamination supplies indole-3-pyruvate; a YUCCA flavin monooxygenase then participates in its conversion to indole-3-acetate. The accepted net reaction uses NADPH and molecular oxygen and releases carbon dioxide, so the step is not simply a change in protonation.^[ref]

An original carbon inventory makes the one-carbon loss clear. The indole framework contributes eight carbon atoms, giving eleven in indole-3-pyruvate and ten in indole-3-acetate. Converting 20 nmol of substrate according to the stated reaction distributes $20(11)=220$ nmol of carbon atoms between $20(10)=200$ nmol in the organic product and 20 nmol in carbon dioxide. The balance checks the net stoichiometry, without establishing the provenance of individual oxygen atoms or a reaction rate.

Phtheirospermum japonicum YUC3 is a gene encoding a YUCCA flavin monooxygenase involved in auxin biosynthesis in the parasitic plant Phtheirospermum japonicum. Its role connects local gene expression with development of a haustorium, the attachment and invasion organ used in interactions with a host plant.^[ref]

The2016 study localized Phtheirospermum japonicum YUC3 expression to epidermal cells near the host-contact site during early haustorium formation. The pattern coincided with the DR5 auxin response reporter. Reducing Phtheirospermum japonicum YUC3 expression by RNA interference decreased haustorium formation, while expression at other epidermal positions could induce haustorium-like structures.^[ref]

The measurements asked complementary questions. Gene expression localized the candidate mechanism; perturbation tested its developmental contribution; a biochemical assay found approximately threefold greater free indole-3-acetic acid in roots overexpressing Phtheirospermum japonicum YUC3 than in empty-vector controls. A reporter gene pattern alone is not a direct chemical measurement of auxin, and a haustorium-like structure alone does not establish a successful host connection.^[ref]

A YUCCA flavin monooxygenase is a member of a plant enzyme family involved in auxin biosynthesis. Its indole-3-pyruvate monooxygenase activity converts indole-3-pyruvate to indole-3-acetate, the conjugate base of indole-3-acetic acid, with loss of carbon dioxide. The official reaction is EC 1.14.13.168.^[ref]

Let IPyA and IAA denote the two anionic organic participants in that reaction. The net balance isThe reaction uses NADPH and oxygen. Together with an upstream transamination step, it connects tryptophan to indole-3-acetic acid. The net transformation removes one carbon from the substrate's side chain; it does not merely change the substrate's concentration without changing its chemical identity.^[ref]

In a constructed ideal inventory, 40 nanomoles of indole-3-pyruvate, 20 of NADPH and 30 of oxygen permit at most 20 nanomoles each of indole-3-acetate and carbon dioxide through the stated reaction. That ceiling assumes no replenishment or side reactions; donor oxidation alone would not identify the organic product.

A developmental experiment supplies a different kind of evidence. In the parasitic plant Phtheirospermum japonicum, local YUC3 expression and altered gene expression were associated with changes in haustorium development. Such observations connect local auxin production to organ development, while a biochemical assay asks which reaction the enzyme performs.^[ref]