Physiological significance of the plant circadian clock in natural field conditions

For many decades, researchers have focused on the self-sustainable oscillations of plant circadian clocks, which can only be observed under artificial constant environmental conditions. However, plants have evolved under natural diurnal conditions where several major environmental cues such as light, temperature and humidity are dramatically changing and interacting with each other. Therefore, little is known about the roles of the plant circadian clock in natural field conditions. Molecular genetic analyses in Arabidopsis thaliana have revealed that some core circadian clock genes are required for the establishment of robust circadian rhythms under artificial diurnal conditions, and that others function only as self-oscillators. However, it is largely unknown yet how those robust rhythms can be obtained under natural diurnal conditions. Recently, an extensive time-course transcriptome analysis of rice (Oryza sativa) leaves in natural field conditions revealed that OsGIGANTEA, the sole rice ortholog of the Arabidopsis GIGANTEA gene, governs the robust diurnal rhythm of rice leaf transcriptomes even under natural diurnal conditions; rice Osgi mutants exhibited severely defective transcriptome rhythms under strong diurnal changes in environmental cues. This review focuses on the physiological significance of the plant circadian clock in natural field conditions.