MED14-HY5 module orchestrates trade-off between cell-cycle activation and DNA damage response in Arabidopsis root

Root system architecture is critical for plant growth and resilience. In this study, we uncovered a novel mechanism in Arabidopsis thaliana by which the Mediator complex subunit MED14 and the transcription factor ELONGATED HYPOCOTYL 5 (HY5) cooperatively regulate root development by maintaining redox and genomic stability. We found that the reduction in the level of AtMED14 causes mis-regulation of a large number of genes required for root development and hence affects the primary root development in Arabidopsis seedlings. Based on transcriptome data, flavonoid levels were examined and found to be deficient in the roots of the med14 mutant. Flavonoids are essential metabolites, and our findings suggest that they play key role in regulating ROS homeostasis in roots. The med14 roots have enhanced accumulation of ROS, leading to DNA damage and eventually activating the DNA damage response in the meristematic zone. This study unravels the coordination of MED14 with HY5 in regulating the cell-cycle genes. Under normal condition, MED14 facilitates HY5-mediated activation of cell-cycle genes, especially CYCD1;1 , however, during genotoxic stress, there is repression of these genes, thereby halting the cell-cycle. In contrast, under non-stress conditions, MED14 and HY5 suppress the expression of DNA damage response genes NAC044 and NAC085 , but this repression is relieved under oxidative stress. Conversely, the loss of function in NAC genes leads to increased expression of CYCD1;1 , but cannot effectively downregulate the expression of CYCD1;1 under genotoxic conditions. Such control over activation and repression of these genes is crucial for preventing unwarranted cell cycle arrest and ensuring proper root meristem function. Thus, our findings reveal a previously uncharacterized role of MED14-HY5 module in balancing ROS homeostasis and DNA damage response to safeguard root growth and development, providing a critical insight into how plants cope with environmental and endogenous genotoxic threats.