N-1-naphthylphthalamic acid (NPA) is a key inhibitor of directional (polar) transport of the hormone auxin in plants. For decades, it has been a pivotal tool in elucidating the unique polar auxin transport-based processes underlying plant growth and development. Its exact mode of action has long been sought after and is still being debated, with prevailing mechanistic schemes describing only indirect connections between NPA and the main transporters responsible for directional transport, namely PIN auxin exporters. Here we present data supporting a model in which NPA associates with PINs in a more direct manner than hitherto postulated. We show that NPA inhibits PIN activity in a heterologous oocyte system and that expression of NPA-sensitive PINs in plant, yeast, and oocyte membranes leads to specific saturable NPA binding. We thus propose that PINs are a bona fide NPA target. This offers a straightforward molecular basis for NPA inhibition of PIN-dependent auxin transport and a logical parsimonious explanation for the known physiological effects of NPA on plant growth, as well as an alternative hypothesis to interpret past and future results. We also introduce PIN dimerization and describe an effect of NPA on this, suggesting that NPA binding could be exploited to gain insights into structural aspects of PINs related to their transport mechanism.
This work was supported by Austrian Science Fund Grant FWF P21533-B20 (to L.A.); German Research Foundation Grant DFG HA3468/6-1 (to U.Z.H.); and European Research Council Grant 742985 (to J.F.). We thank Herta Steinkellner and Alexandra Castilho for N. benthamiana plants, Fabian Nagelreiter for statistical advice, Lanassa Bassukas for help with [ɣ32P]- ATP assays, and Josef Penninger for providing access to mass spectrometry instruments at the Vienna BioCenter Core Facilities. We thank PNAS reviewers for the many comments and suggestions that helped to improve this manuscript.
Abas L, Kolb M, Stadlmann J, et al. Naphthylphthalamic acid associates with and inhibits PIN auxin transporters. PNAS. 2021;118(1). doi:10.1073/pnas.2020857118
Abas, L., Kolb, M., Stadlmann, J., Janacek, D. P., Lukic, K., Schwechheimer, C., … Hammes, U. Z. (2021). Naphthylphthalamic acid associates with and inhibits PIN auxin transporters. PNAS. National Academy of Sciences. https://doi.org/10.1073/pnas.2020857118
Abas, Lindy, Martina Kolb, Johannes Stadlmann, Dorina P. Janacek, Kristina Lukic, Claus Schwechheimer, Leonid A Sazanov, Lukas Mach, Jiří Friml, and Ulrich Z. Hammes. “Naphthylphthalamic Acid Associates with and Inhibits PIN Auxin Transporters.” PNAS. National Academy of Sciences, 2021. https://doi.org/10.1073/pnas.2020857118.
L. Abas et al., “Naphthylphthalamic acid associates with and inhibits PIN auxin transporters,” PNAS, vol. 118, no. 1. National Academy of Sciences, 2021.
Abas L, Kolb M, Stadlmann J, Janacek DP, Lukic K, Schwechheimer C, Sazanov LA, Mach L, Friml J, Hammes UZ. 2021. Naphthylphthalamic acid associates with and inhibits PIN auxin transporters. PNAS. 118(1), e2020857118.
Abas, Lindy, et al. “Naphthylphthalamic Acid Associates with and Inhibits PIN Auxin Transporters.” PNAS, vol. 118, no. 1, e2020857118, National Academy of Sciences, 2021, doi:10.1073/pnas.2020857118.