Study On The Function Of Exogenous Melatonin On The Circadian Rhythm Phenotype Of Arabidopsis Thaliana Under Low Temperature Stress

The rotation of the earth produces diurnal changes of environmental factors such as light and temperature that can be predicted by living organisms.In order to have competitive advantages in the using of periodically changing environmental factors,higher plants themselves also present a circadian rhythm with a cycle of about 24h.Circadian clock,as an endogenous time-keeping mechanism,can dynamically adjust its physiological and metabolic responses to adapt to the periodic changes of the surrounding environment.The corresponding fluctuation in the redox state of cells triggered by diurnal changes of environmental factors,has played an important role in the generation and maintenance of the circadian rhythm of plants.Under normal condition,a dynamic balanced interaction has been found to exist between the rhythmic changes of intracellular redox state and the regulation of circadian clock mechanisms.However,sudden changes in environmental factors,such as low temperature stresses,often lead to the happening of cellular peroxidative stresses and the over-accumulation of reactive oxygen species,ROS.At this circumstances,the circadian clock controlled the synthesizing of downstream antioxidant enzymes and their activities will be up-regulated to scavenge the excessive ROS in the cytoplasm of plants to avoid oxidative damages of macromolecules in plant cells.In the responding process of plants to oxidative stresses,the circadian rhythm has involved in the coordination of activities of the clock-controlled antioxidant system,and the regulating of the subsequent downstream anti-adversity responses.On the other hand,the circadian rhythm has also been responsible for the controlling of the balance of energy distribution between the basic metabolism and resistance activities.As a tryptophan derived chemical with definite antioxidant functions,melatonin could directly scavenge excessive ROS in the process of plants under abiotic stresses,which also could maintain proper vegetative growth activities,as well as the strength of photosynthetic activities,or the biological rhythmic phenotype of plants to enhance their tolerance to various abiotic stresses.The experiments in present work were firstly carried out to explore the effects of exogenous melatonin on the rhythmic oscillations ofcircadian clock genes and antioxidant enzyme genes in Arabidopsis thaliana seedlings under the condition of different temperatures.The results showed that melatonin could maintain or restore the rhythmic expression profiles of the circadian clock genes CCA1 and TOC1,as well as that of the antioxidant enzyme gene CAT2by affecting the redox homeostasis in the cell and cooperating with Prx-A gene;the rhythmic oscillations of other major antioxidant enzyme genes were closely related to the improvement of redox homeostasis,and then coordinated the energy distribution between growth and stress resistance to improve the low temperature tolerance of Arabidopsis thaliana.Besides,the Prx-A and Prx-B had involved in the modulation of melatonin's function in gene expression controlling via individual methods.Secondly,the effect of exogenous melatonin on the rhythmic oscillation of the key antioxidant enzyme activity of Arabidopsis thaliana seedlings under the condition of different temperatures was then explored.The results confirmed that melatonin had participated in the physiological responding of plant resistant to chilling stresses mainly by affecting the activities of key antioxidant enzymes or rebuilding the rhythmic oscillations of antioxidant enzymes.Thereby the Prx-A and Prx-B genes regulated different antioxidant enzymes in the process of changing the key antioxidant enzyme activities of Arabidopsis thaliana seedlings under low temperature stress induced by exogenous melatonin.Finally,the functions of CCA1 gene defection,that have involved in the regulation of TTFL mechanism,in the rhythmicity of the Prx-SO_2/3 rhythmic marker under different temperatures was also explored.The results revealed that there was no rhythmic oscillation of Prx-SO_2/3rhythmic marker that had been found in cca1 mutant of Arabidopsis thaliana seedlings under either normal or low temperature stressed conditions,which confirmed that the controlling mechanism of Prx-SO_2/3 rhythmic marker could mot be completely independent to CCA1clock based on the controlling of TTFL feedback loops.