Plant bodies are built from immobile cells, making the regulation of cell expansion essential for growth, development, and adaptation. In roots, cell elongation executes the movement of the root tips through the soil. This process is tightly controlled by numerous signaling pathways. Among these, gibberellin and auxin signaling stand out for their contrasting effects on root growth, interacting through complex cross talk at multiple regulatory levels. Here, we reveal the molecular basis of the auxin-gibberellin cross talk in the model plant Arabidopsis thaliana. We show that the auxin signaling pathway steers the expression of GIBBERELLIN 2-OXIDASES (GA2OXs), key gibberellin-deactivating enzymes in the root elongation zone (EZ). GA2OXs are negative regulators of root cell elongation; GA2OX8 overexpression decreases gibberellin levels and inhibits root cell elongation; in contrast, the ga2ox heptuple mutant roots show elevated gibberellin levels in the EZ and grow longer roots. Intriguingly, shoot-derived auxin can regulate GA2OX6 and GA2OX8 expression in roots, linking systemic auxin signaling to local gibberellin level modulation. Together, our findings identify GA2OX6 and GA2OX8 enzymes as key mediators of auxin-gibberellin cross talk, providing insights into their roles in root elongation. These results expand our understanding of how auxin integrates with gibberellin signaling to coordinate root development and growth dynamics.

Department of Experimental Plant Biology Charles University Prague 12844 Czech Republic

Institute of Experimental Botany of the Czech Academy of Sciences Prague 16502 Czech Republic

Laboratory of Growth Regulators Institute of Experimental Botany Czech Academy of Sciences and Faculty of Science Palacky University Olomouc Olomouc CZ 77900 Czech Republic

Sainsbury Laboratory Cambridge University Cambridge CB21LR United Kingdom

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Gibberellin-deactivating GA2OX enzymes act as a hub for auxin-gibberellin cross talk in Arabidopsis thaliana root growth regulation