Synthetic auxin herbicides like 2,4-dichlorophenoxyacetic acid (2,4-D) are widely used in agriculture to control dicot weeds, yet the identity of their transporters in plant cells has remained unresolved. While natural auxins – such as indole-3-acetic acid (IAA), key phytohormones that regulate plant growth and development – are known to be transported by PIN-FORMED (PIN) auxin exporters, it has been unclear whether structurally similar herbicides also rely on these transporters or diffuse passively.

A recent study led by Ulrich Hammes and Bjørn Pedersen addresses this long-standing question by demonstrating that several phenoxyacetic acid herbicides are actively transported by PIN proteins, particularly PIN8. Using solid-supported membrane electrophysiology (SSME, SURFE²R), the researchers recorded electrogenic transport of 2,4-D and 4-chlorophenoxyacetic acid (4-CPA). Transport was inhibited by the auxin efflux blocker NPA, confirming transporter-mediated export with measurable affinities comparable to that of IAA.

The study also employed cryo-electron microscopy (cryo-EM) to capture five conformational states of PIN8 bound to 2,4-D or 4-CPA, revealing a stepwise transport cycle with distinct ligand poses representing prebinding, binding, and release. Mutagenesis of residues involved in coordinating herbicide carboxyl or halogen groups – such as Asn117, Tyr150, and Ser55 – validated these structural snapshots and pinpointed the molecular determinants of substrate specificity.

Overall, this work reveals that phenoxyacetic acid herbicides utilize the same export machinery as endogenous auxins and resolves a major controversy in plant hormone transport biology. It also showcases SSME as a powerful tool for probing electrogenic transport in plant membrane proteins. By mapping herbicide transport at atomic resolution, the study lays the groundwork for structure-guided design of new auxin analogs and the engineering of herbicide-resistant crops.

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Find the full article here: Transport of phenoxyacetic acid herbicides by PIN-FORMED auxin transporters

Studying plant transporters? Learn more about Solid Supported Membrane-Based Electrophysiology (SSME) here: https://www.nanion.de/products/surfe2r-n1/