Topological difference of core regulatory networks induces different entrainment characteristics of plant and animal circadian clocks.

The plant circadian rhythm is quickly entrained to the change of a light stimulus but the mammalian circadian rhythm shows a relatively slow entrainment. Where does such a different entrainment feature of plants and mammals originate? To answer this question, we have investigated circadian regulatory networks of various species and identified the respective core structures of plants and animals. The core circadian regulatory network of plants is composed of two coupled negative feedback loops while the core network of animals consists of coupled negative and positive feedback loops. In addition, the way of regulation (gene transcription or protein degradation) induced by a light stimulus differs depending on species. Mathematical simulations revealed that the topological difference of the core regulatory networks as well as the different way of regulation induced by a light stimulus leads to the different entrainment characteristics of plant and animal circadian clocks.