Data-Driven Modeling of Intracellular Auxin Fluxes Indicates a Dominant Role of the ER in Controlling Nuclear Auxin Uptake

TLDR

Auxin is a master regulator of plant development, with nuclear signaling triggered by Aux/IAA degradation. The study aims to determine how auxin movement between the nucleus and surrounding compartments is regulated. Using mathematical modeling combined with time‑course assays and single‑cell phenotyping, the authors identify ER‑to‑nucleus auxin flux as a major pathway controlling nuclear auxin levels. Auxin diffusion into the nucleus is restricted, and the ER’s homeostatic auxin pool and its flux to the nucleus govern nuclear auxin concentrations and downstream responses.

Abstract

In plants, the phytohormone auxin acts as a master regulator of developmental processes and environmental responses. The best characterized process in the auxin regulatory network occurs at the subcellular scale, wherein auxin mediates signal transduction into transcriptional programs by triggering the degradation of Aux/IAA transcriptional repressor proteins in the nucleus. However, whether and how auxin movement between the nucleus and the surrounding compartments is regulated remain elusive. Using a fluorescent auxin analog, we show that its diffusion into the nucleus is restricted. By combining mathematical modeling with time course assays on auxin-mediated nuclear signaling and quantitative phenotyping in single plant cell systems, we show that ER-to-nucleus auxin flux represents a major subcellular pathway to directly control nuclear auxin levels. Our findings propose that the homeostatically regulated auxin pool in the ER and ER-to-nucleus auxin fluxes underpin auxin-mediated downstream responses in plant cells.