Diurnal Oscillation of Circadian Clock Gene Transcripts in Rice Leaves under Osmotic Stress Conditions

Abstract

The circadian clock is an endogenous oscillator evolved for perceiving and responding to environmental stimuli by generating transcriptional, metabolic and physiological changes synchronized with the day and night cycle. In higher plants, circadian clock is closely related with increased fitness, growth and survival. This research investigated how drought and salt stress conditions affected the expression patterns of circadian clock-associated genes during reproductive stage of KDML 105 rice cultivar. The KDML 105 rice plants were subjected to drought or salt stressed conditions by adding 20% polyethylene glycol 6000 or 200 mM NaCl to the nutrient solution, respectively. Flag leaves were collected at 4 hour intervals for 48 hours. Sampling was started at 08.00 am after plants were immediately subjected to drought and salt stress (0-48 h) and after they were under stress conditions for 5 days (120-168 h). Changes in relative transcript levels of OsCCA1/LHY, OsPRR1/TOC1, OsFKF1 and OsGI were determined using reverse transcription quantitative PCR. The results showed that drought and salt stress decreased the levels of water potential in leaves. Moreover, the number of filled grains/panicle and their weight were significantly reduced in stressed plants. Expression of most clock associated-genes in flag leaves were altered when rice plants were exposed to drought and high salinity for 0-48 hours. A significant increase in peak expression of clock genes was observed during this period. A shift of peak time of OsCCA1/LHY and OsGI was observed during 120-168 h of salt stress period. These results indicate that circadian clock may involve in the control of short term responses of rice to osmotic stress by adjusting their oscillation of transcript levels.