The interplay between autophagy and the carbon/nitrogen ratio as key modulator of the auxin-dependent chloronema-caulonema developmental transition in Physcomitrium patens.

Plant growth and development are tightly coordinated with metabolic status and environmental nutrient availability, yet how intracellular recycling pathways integrate these cues with hormonal signaling remains poorly understood outside angiosperms. Here, we used the model bryophyte Physcomitrium patens to address the role of autophagy in modulating auxin-induced caulonemata development in response to conditions altering the carbon/nitrogen (C/N) ratio. Loss of autophagic function enhanced caulonemata differentiation and colony expansion under ammonium-free and nitrogen-deficient conditions, phenocopying wild-type responses to nutrient starvation even under mild C/N imbalance. These developmental changes were associated with altered carbon homeostasis, increased endogenous indole-3-acetic acid (IAA) levels, reduced gene expression of the auxin exporter PpPINA, and up-regulation of auxin-responsive RSL transcription factors. In contrast, autophagy-deficient mutants show reduced sensitivity to exogenous sugars and exhibit hypersensitivity to prolonged auxin treatment, resulting in growth inhibition and cell death. Autophagic flux analyses revealed that caulonemata-inducing conditions, including auxin supply and nitrogen limitation, generally suppress autophagic activity, particularly during the dark period. Together, our results identify autophagy as a central integrator of metabolic and hormonal signals during early moss development, acting to restrain auxin-driven differentiation in favor of nutrient remobilization and growth homeostasis.