Induction of pluripotent cells termed callus by auxin represents a typical cell fate change required for plant in vitro regeneration; however, the molecular control of auxin-induced callus formation is largely elusive. We previously identified four Arabidopsis auxin-inducible Lateral Organ Boundaries Domain (LBD) transcription factors that govern callus formation. Here, we report that Arabidopsis basic region/leucine zipper motif 59 (AtbZIP59) transcription factor forms complexes with LBDs to direct auxin-induced callus formation. We show that auxin stabilizes AtbZIP59 and enhances its interaction with LBD, and that disruption of AtbZIP59 dampens auxin-induced callus formation whereas overexpression of AtbZIP59 triggers autonomous callus formation. AtbZIP59-LBD16 directly targets a FAD-binding Berberine (FAD-BD) gene and promotes its transcription, which contributes to callus formation. These findings define the AtbZIP59-LBD complex as a critical regulator of auxin-induced cell fate change during callus formation, which provides a new insight into the molecular regulation of plant regeneration and possible developmental programs.

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Induction of a pluripotent cell mass, called callus, from detached organs is an initial step in in vitro plant regeneration, during which phytohormone auxin-induced ectopic activation of a root developmental program has been shown to be required for subsequent de novo regeneration of shoots and roots. However, whether other signals are involved in governing callus formation, and thus plant regeneration capability, remains largely unclear. Here, we report that the Arabidopsis calcium (Ca 2+ ) signaling module CALMODULIN IQ-MOTIF CONTAINING PROTEIN (CaM–IQM) interacts with auxin signaling to regulate callus and lateral root formation. We show that disruption of IQMs or CaMs retards auxin-induced callus and lateral root formation by dampening auxin responsiveness, and that CaM–IQM complexes physically interact with the auxin signaling repressors INDOLE-3-ACETIC ACID INDUCIBLE (IAA) proteins in a Ca 2+ -dependent manner. We further provide evidence that the physical interaction of CaM6 with IAA19 destabilizes the repressive interaction of IAA19 with AUXIN RESPONSE FACTOR 7 (ARF7), and thus regulates auxin-induced callus formation. These findings not only define a critical role of CaM–IQM-mediated Ca 2+ signaling in callus and lateral root formation, but also provide insight into the interplay of Ca 2+ signaling and auxin actions during plant regeneration and development.