The Mechanism Study On Water Lily Circadian Clock

Water lilies(Nymphaeaceae)are beautiful aquatic flowering plants that are distributed worldwide.These plants are found in the aquatic sections of nearly every botanic gardens because of their highly valued ornamental features.Water lilies are not only widely used for interior decoration and bouquet.Benefiting from its medical effects,the extrations of Water lilies have been utilized as an ingredient in many products,including beneficial and cosmetic substances,soap,perfume,hand cream,flower tea bags,and traditional medicine.Besides,Water lilies have been reported as a biomonitor and materials of phytoremediation,which can uptake metal ions form coastal acid sulfate soils and reduce the land population.On the other hand,Water lilies occupy a significant evolutionary status,and identified as one of the three branches of Basal angiosperms,together with Amborellales and Austrobaileyales.The origin and rapid radiation of flowering plants increased the angiosperm biodiversity.Flowers open at sunrise while close at sunset,establishing a circadian floral rhythmic movement pattern to facilitate pollination,being traits of a reproductive syndrome.A proper floral circadian movement requires the coordination of endogenous factors and environmental stimulus.The recent advances have characterized the crucial regulatory factors,such as circadian clock,photoperiod,light and temperature for establishing floral circadian movement pattern,whereas,the underlying mechanisms are not clear yet.It is crucial for understanding the origin and early evolutionary trajectory of flowering plants to get insight into the mechanism of this phenomenon for the basal angiosperm lineages.To explore the mechanism of flower circadian clock in Water lilies,we adopted the flowers of Nymphaea colorata Peter and Nymphaea caerulea,which are belong to one of the most ancient angiosperm lineages-Nymphaeales as the study model system.The Nymphaea colorata Peter flowers opened after the sunrise at 6:00am,and reached the maximal opening angle at 9:00am;after 15:00pm,the flowers graduadly closed and reached to the minimum angle at 18:00pm.Accompanied with 7:00am light-on,Nymphaea caerulea cut flowers gradually opened;at 10:00am,the opening angle rapidly increased;until 14:00pm,opening angle reached the maximum;after 14:00pm,flowers were gradually closed;at 18:00pm,the rapidly closure slowed;at 23:00pm,flowers were nearly completely closed.Thus,with the manual manipulation of growth condition,the cut flowers displayed a stable opening-closure rhythmic movement pattern,as shown by the key time points are:7:00am(initiation),10:00am(rapid opening),14:00pm(maximal opening)and 18:00pm(rapidly closure slowed).Due to the shortage of Nymphaea colorata Peter materials,we purchased the fresh cut flowers of Water lily cultivar Nymphaea caerulea(called Water lily below)as experimental materials.To explore the most sensitive part of flowers which is responsive to rhythmic movement regulation,we removed different parts of Water lily flowers to observe whether it will influence the opening and closing rules.We demonstrated that the sepals and stamens take small influence in circadian clock in Water lilies,while the stamens exhibit irreversible opening process.Interestingly,after the upper part of petals were removed,the basal tissue of petals(0-1 cm)behave in action to circadian regulation accompanied with cell expansion in Water lilies,which indicates that the basal part of Water lily flowers is the key tissue for circadian rhythm response.Transcriptome profiles generated from basal petals at different day-time points showed a global molecular mechanism of floral rhythm movement,in coordination of photosynthesis,vesicle trafficking,phytohormoneulation,cell wall modification,light signaling and so on.Photosynthesis and ion transportation related genes were upragulated when the light turned on,indicating that the rapid response of photosynthesis and ion transportation are the initial reactions for further flower opening.Associating with rapid flower opening,COPI vesicle coat,Auxin and cell wall bionenesis related genes were highly upregulated,suggesting that the rapidly flower opening accompanied by materials trafficking,and the important role of auxin in regulation of anisotropic cell expansion during floral rhythm movement.Cell wall bionegesis genes even mantian high transcriptional leval antil the closure prosess finished,implying that dramatic degradation and re-establishment of cell wall happened during flower opening and closure.The transcription and translation were decreased when flowersopenwhereas increased when flowers close,further suggesting that transcription and translation processes might negatively regulate floral rhythm movement.A large number of kinases were induced during Water lily flower closure.Combining with previous reports showed the correlation between receptor kinases(RLKs)and cell wall remodeling,we speculated that the possibility of the coordinating role between receptor kinase and cell wall remodeling during Water lily flower closure.This study provides a general understanding of molecular mechanism for flower rhythm action in Water lily,which makes profound significance for the understanding of Basal Angiosperm's flowring characters.