Study on the functional role and molecular mechanism of PHYTOCHROME B

Abstract

Plants use the environmental signals of light and temperature to co-ordinate diverse physiological and developmental processes, and thereby increase their fitness. These signals change predictably in daily and seasonal cycles under natural conditions. Plant has developed a circadian clock which responds to the predictable, periodic rhythms of the environment and thus enables anticipation of daily and seasonal change. Conceptually, the circadian system is composed of the environmental input pathways, the circadian clock and physiological outputs. One of the important inputs to the circadian clock is the daily pattern of light and dark. The intimate connection between light and clock responses in plants enables timely and prepared regulation of various physiological outputs to cope with the periodic changes in the environment. Light indirectly regulates the circadian clock components through signaling cascades mediated by various signal transducers. In the current study it was found that Phytochrome B (PHYB), a red light photoreceptor, interacted directly with various circadian clock proteins in Arabidopsis and these interactions were changed by red and far-red light. Direct interactions between a photoreceptor and circadian clock proteins in a light quality-dependent manner provide an immediate way to convey light quality information to the clock, circumventing the signaling cascades. Temperature is also a crucial input to the circadian clock, and under natural conditions, information on light and temperature signals is integrated simultaneously by the plant circadian clock. Although it is well known there is crosstalk between two environmental signals in the regulation of common physiological processes, including germination, plant architecture and flowering time, the link between light and temperature in controlling the plant clock has not been well studied. In this study, expression of several clock genes was found to be dependent on temperature in a red-light specific manner. The phyB mutation showed the red-light specific temperature-insensitive response of clock genes. PHYB was found to play an important role in regulating temperature-dependent expression of clock genes. Thus, I suggest that red light is integrated to the clock system with temperature information, and PHYB is likely to function as a mediator between red light and temperature signaling in controlling the circadian clock. These findings extend previous understandings of the molecular mechanism for transmitting red light input to the clock and provide a new paradigm for PHYB's role in Arabidopsis circadian clock.