PHYTOCHROME INTERACTING FACTORS act upstream to coordinate microtubule dynamics and differential cell growth during Arabidopsis apical hook opening

Differential cell growth, a key evolutionary strategy, involves unequal cell expansion to bend organs like stems or roots, enabling directional growth to optimize access to resources and avoid harmful conditions. Apical hook development in darkness is an excellent model to study asymmetric elongation between the convex and concave sides of the hypocotyl during its two distinctive phases: formation and opening. While PHYTOCHROME INTERACTING FACTORS (PIFs) are recognized as essential for proper apical hook development, the underlying cellular basis remains largely unknown. Here, we show that PIF activity is necessary to sustain the coordination of cortical microtubule (cMT) organization (involved in orchestrating cell growth anisotropy) with cell expansion during apical hook opening in darkness under our imaging conditions. In the pifq mutant this coordination is lost, as both apical hook and apical hypocotyl cells undergo excessive elongation despite predominantly longitudinal cMT alignment and significant alignment strength, resulting in defective differential growth. RNA-seq reanalysis revealed the reduced expression of genes associated with cell wall biosynthesis and remodeling, as well as the upregulation of cMT remodeling factors. This work expands the classical view of PIFs as transcriptional regulators of elongation-related genes, identifying them as central coordinators of cytoskeletal organization and directional cell growth that underpins tissue curvature in darkness. These findings establish an epidermis-centered framework for PIF-dependent coupling of cMT organization and anisotropic growth, positioning them as candidate central regulators providing a foundation for future cross-layer analyses to generalize these mechanical interactions across tissues.