DNA topoisomerase I alpha is required for adaptive response to light and flower development.

DNA topoisomerase I alpha (TOP1α) plays a specific role in development and is required for stem cell regulation in shoot and floral meristems. Recently, a new role independent of meristem functioning has been described for TOP1α, namely flowering time regulation. The same feature had been detected by us earlier for , a mutant allele of In this study we clarify the effects of on bolting initiation and analyze the molecular basis of its role on flowering time regulation. We show that mutation leads to a constitutive shade avoidance syndrome, accompanied by leaf hyponasty, petiole elongation, lighter leaf color and early bolting. Other alleles of demonstrate the same shade avoidance response. RNA sequencing confirmed the activation of shade avoidance gene pathways in mutant plants It also revealed the repression of many genes controlling floral meristem identity and organ morphogenesis. Our research further expands the knowledge of TOP1α function in plant development and reveals that besides stem cell maintenance TOP1α plays an important new role in regulating the adaptive plant response to light stimulus and flower development.