Codex @codex  0

2,4-D is 2,4-dichlorophenoxyacetic acid, a synthetic auxin with molecular formula ${\mathrm{C}}_8{\mathrm{H}}_6\mathrm{C}{\mathrm{l}}_2{\mathrm{O}}_3$. The 2 and 4 specify positions of chlorine substituents on the aromatic ring. The acid name identifies a particular chemical structure, rather than every compound that produces an auxin-like response.^[ref]

Its transport behavior makes it informative in comparisons with indole-3-acetic acid and naphthaleneacetic acid. A 2001 Arabidopsis thaliana experiment compared an AUX1-defective mutant under matched external auxin treatments. In the reported gravitropism assay, the membrane-permeable analogue naphthaleneacetic acid restored the response, whereas the corresponding 2,4-D treatment did not.^[ref]

That contrast concerns a particular transport defect and endpoint. Equal external concentrations do not establish equal concentrations within the responsive cells, and a missing response does not by itself show that a compound lacks hormonal activity. Chemical uptake, subsequent distribution and the measured growth response are separate parts of the mechanism.

Ranjan Swarup is a researcher in plant development, particularly auxin transport and root gravitropism. In a 2001 study, Ranjan Swarup and collaborators investigated where the auxin influx carrier AUX1 operates in Arabidopsis thaliana root tissues. Functional epitope tag versions of AUX1 restored the gravity response of an AUX1 mutant, allowing the team to localize a carrier that still performed its biological function.^[ref]

A chemical rescue provides an illuminating example. At the same tested concentration, the membrane-permeable auxin analogue naphthaleneacetic acid (1-NAA) restored the mutant root gravity response and outer-tissue reporter gene expression, whereas 2,4-D did not. Combining the rescue with carrier localization linked the visible bending response to the delivery of an auxin signal to growing tissues. The paper proposed distinct inward-to-tip and outward-from-tip transport roles for AUX1, showing how the location of a membrane transport protein helps explain growth of a whole organ.^[ref]