Auxin is the first plant hormone—or "phytohormone"—ever identified, with its discovery dating to the late 19th and early 20th centuries. With its discovery, scientists began to understand how small, self-produced organic molecules could influence physiological processes in plants such as growth, cell division, flowering, fruit ripening, and stress responses.
The auxin family of phytohormones promotes cell elongation and root development and is involved in directional growth, such as growing toward light. It works by moving directionally within plant tissues to establish concentration gradients, thereby regulating key developmental processes. This directional transport is orchestrated by three protein families: the PIN, ABCB, and AUX1/LAX families.
While scientists previously understood how PIN proteins export auxin out of cells, the import mechanism by AUX1/LAX proteins remained unclear.
Now, however, research teams led by Prof. SUN Linfeng, Prof. LIU Xin, and Prof. TAN Shutang from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences have unveiled the structures of Arabidopsis AUXIN RESISTANT 1 protein (AUX1) and clarified the molecular mechanism by which this protein transports auxin into the cell via the proton concentration gradient. The study was published in Cell on May 15.
To elucidate the transport mechanism controlled by Arabidopsis AUX1, the researchers employed biochemical assays, structural biology, and computational simulations. Through microscale thermophoresis and isothermal titration calorimetry, they demonstrated that AUX1-mediated auxin binding and transport were proton-dependent and could be inhibited by the known small molecules 1-naphthoxyacetic acid (1-NOA) and 3-chloro-4-hydroxyphenylacetic acid (CHPAA).
Furthermore, using cryo-electron microscopy, the researchers determined high-resolution structures of AUX1 in three distinct states: apo (substrate-free), auxin-bound (IAA), and inhibitor-bound (CHPAA). These structures revealed, for the first time, the overall architecture of the AUX1/LAX family of proteins—which features an inward-facing conformation, with 11 transmembrane helices adopting a classical LeuT fold.
In the auxin-bound structure, several key residues involved in IAA recognition were identified—most notably His249 (H249), which underwent a marked side-chain reorientation upon substrate binding. The functional relevance of these residues was confirmed through site-directed mutagenesis, biochemical assays, and plant physiological experiments.
To further investigate the role of H249 in substrate recognition and proton coupling, the researchers, collaborating with Prof. ZHU Lizhe's team from the Chinese University of Hong Kong (Shenzhen), performed molecular dynamics simulations. Comparative analysis of the IAA- and CHPAA-bound structures showed that, while both ligands occupied a similar binding pocket, they exhibited distinct interaction patterns. These differences provided mechanistic insights into how CHPAA inhibits AUX1-mediated auxin transport.
This study, which reveals for the first time the molecular basis of auxin import mediated by the AUX1/LAX protein family in plants, represents an important breakthrough in understanding plant growth and development.
