A complex balance: PLETHORA - JANUS ratiometry explains the PLETHORA dosage regeneration paradox

Molecular gradients are classically thought to encode positional information through concentration thresholds, but how transcription factor dosage is translated into regenerative competence remains unclear. In Arabidopsis roots, PLETHORA (PLT) transcription factors form a gradient that defines a regeneration competence zone and, following injury, rapidly re-establishes their transcriptional domain through an auto-activation. Yet, increasing PLT levels has a paradoxical effect: moderate increases expand the competence zone, whereas further increases impair regeneration. Here, combining mathematical modelling with experiments, we uncover the molecular basis of this non-monotonic response. Our model reveals that PLT activity depends not simply on its abundance, but on the balance between PLT protein complexes and unbound PLT. Following injury, PLT auto-activation rapidly restores the transcriptional domain, whereas increasing PLT dosage progressively shifts the equilibrium towards unbound PLT, reducing the availability of active PLT complexes. We identify and experimentally validate a strategy to extend regeneration competence by maintaining an appropriate ratio of PLT complexes to unbound PLT. Our findings reveal a ratiometric mechanism for decoding transcription factor dosage, in which excess regulator can attenuate rather than enhance activity. Such molecular balancing provides a mechanism by which a single transcription factor gradient can generate distinct, context-dependent outcomes during development and regeneration.