Auxin signaling in algal lineages: fact or myth?

Auxin is of major importance throughout the life cycle of a plant, affecting several physiological and developmental processes, such as cell expansion and division. However, the evolutionary time point at which auxin became involved in such diverse processes is currently unclear. Despite some controversy, numerous reports demonstrate the presence of auxin in algal lineages and its effects on algal development, suggesting an early evolutionary origin of auxin-dependent mechanisms. Here, we review these reports and discuss in silico analyses of auxin signaling components. It seems that, at least in microalgae, the assumed major components of auxin signaling in land plants are absent. However, these microalgae might have alternative auxin signaling pathways that could account for their responses to auxin.