WUSCHEL RELATED HOMEOBOX5 and 7 maintain callus development by promoting cell division in Arabidopsis

In tissue culture, a high concentration of auxin in the callus induction medium (CIM) stimulates cell division and subsequent callus formation, which acquires root primordium-like characteristics necessary for cell pluripotency. In Arabidopsis, WUSCHEL-RELATED HOMEOBOX5 (WOX5) and its closest homolog WOX7, which are abundant in the middle cell layer of mature callus, play a crucial role in maintaining pluripotency by promoting auxin accumulation and enhancing cytokinin sensitivity. However, the mechanism by which WOX5/7 regulate callus formation remains unclear. In this study, we found that mutations in WOX5/7 resulted in a significant down-regulation of genes involved in the G2M and S phases during callus induction. Loss-of-function mutants of WOX5/7 exhibited reduced callus formation, which was correlated with decreased expression of CYCB1;1 compared to the wild-type. Furthermore, we provided evidence that WOX5 physically interacts with PHYTOCHROME A SIGNAL TRANSDUCTION1 (PAT1), which spatio-temporally co-expresses with WOX5 in early-induced callus, and up-regulates a subset of cycle-regulating genes targeted by PAT1. Collectively, our findings suggest a critical role for the WOX5-PAT1 protein complex in regulating cell cycle progression, thereby promoting the continuous growth capacity of pluripotent callus.