Circadian clock-coordinated response to chilling stress in rice

Chilling is a major abiotic stress affecting rice growth and development. Circadian clock is an endogenous timer enabling plants to cope with ever changing surroundings. However, little is known about coordination of circadian clock in rice chilling tolerance. In this study, we found that seedlings only subjected to nighttime-chilling stress exhibited chlorosis phenotype, with yellower leaves compared to those undergoing cold only at daytime. Chlorophyll content and photosynthesis efficiency in seedlings under nighttime-chilling stress were significantly lower than that under daytime-chilling stress. Transcriptome analysis distinguished the COLD-REGULATED (COR) genes between daytime and nighttime. COR genes responding differentially to daytime- and nighttime-chilling were non-randomly distributed among rice chromosomes, clustering mainly in 8 chromosome arms, Chr. 1L, 1S, 2L, 2S, 3L, 3S, 4L and 7L. Among them, key genes for chlorophyll metabolism and photosynthesis were repressed by chilling stress during Zeitgeber time (ZT) at night but promoted by stress during ZT time in day, which was confirmed by qRT-PCR. Our study revealed impacts of circadian clock on rice chilling response, implying that manipulation of circadian clock has great potential to determine the magnitude of chilling response, and would be helpful for breeding new rice varieties with chilling tolerance.