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PHYTOCHROME INTERACTING FACTORS are required to coordinate microtubule dynamics and differential cell growth during Arabidopsis apical hook opening Background: Differential cell growth is a key strategy that enables plants to adjust their morphology in response to internal and external cues. Apical hook opening in dark-grown Arabidopsis seedlings serves as an excellent model to study how coordinated cell expansion generates organ bending. Several works highlight the influence of hormone signaling, cell wall mechanics, and environmental cues on cortical microtubule (cMT) arrangement in bending tissues. However, the precise genetic hierarchy governing these processes remains largely unknown. Results: Here, we reveal a two-step growth rate during apical hook opening, in which an initial phase of differential cell expansion is followed by a phase of more balanced growth. We show that cMT re-arrangement precedes and predicts cell expansion patterns, suggesting a central role in coordinating differential growth. Our findings indicate that PIF transcription factors regulate this process, as pifq mutants fail to align cMT reorganization with growth dynamics, resulting in premature hook opening. RNA-seq analysis further supports a role for PIFs in coordinating cell wall remodeling and microtubule-associated gene expression, including kinesins and cell wall-modifying enzymes. Conclusions: Our study uncovers a PIF-dependent regulatory mechanism that orchestrates cytoskeletal dynamics and cell expansion to control apical hook opening. ### Competing Interest Statement The authors have declared no competing interest. https://www.biorxiv.org/content/10.1101/2025.02.18.638861v1