| Ground-cover chrysanthemum,a cultivar group of Chrysanthemum morifolium,is widely used in landscape gardening due to its strong branching ability,large numbers of flowers,strong resistance and wide adaptability.Cultivars of ground-cover chrysanthemum with vertical architecture are common,while the creeping or prostrate type is rare.If the prostrate architecture of C.yantaiense(obtained from Yantai,abbreviated as YT),an outstanding trait with application value in landscape gardening,is introduced into ground-cover chrysanthemums,the ground-covered ability will be increased and greening costs will be reduced.The regulatory mechanism controlling the prostrate growth of chrysanthemum is still not clear.Therefore,YT,FH(a vertical cultivar of ground-cover chrysanthemum’Fanhuasijin’)and their F1 progenies were selected as plant materials to explore mechanism of the prostrate growth of chrysanthemum by combining with morphology,anatomy,hormone physiology,bioinformatics and molecular biology methods.The main results of the study were as following:(1)In stems of YT,the asymmetric cell growth of the bending part(cells in the upper side are significantly larger than those in the lower side)resulted in the transition of YT from erect growth to prostrate growth.The GSA of stem was gradually reduced from 150～180° to 90～105°.Compared with the upright cultivar FH,the gravitropic response of YT was weakened in a certain extent:the bending reaction time of YT is later than FH,as well as the time to restore upright growth.The weaker gravitropism of YT,which is lack of obvious endodermis that is an important tissue for gravity sensing,is largely responsible for the prostrate phenotype.(2)IAA is the key phytohormone that affecting the prostrate or erect growth of chrysanthemum.The IAA content and lateral auxin transport between the upper side and lower side in the stem are closely related to the formation of prostrate growth and the gravity response of YT.The content of IAA in the upper half of YT stem was significantly higher than that in the lower half,which led to the significantly bigger cortex parenchyma cells in the upper half than that in the lower half,and ultimately resulted in the transition from upright to prostrate.IAA and NAA smeared on the lower side of the stem led to the transition of stem from prostrate to upright growth and enhanced the graviresponse.However,IAA and NAA applying to the upper side of the stem enhanced the prostrate growth and reduced the graviresponse.The application of auxin transport inhibitors,NPA and TIBA,on the upper side,to a certain extent,prevented the lateral transport of auxin from the upper half to the lower half and ultimately reduced the stem GSA.After interfering the IAA lateral transport,the graviresponse of the stem was also impaired.(3)The F1 population of YTxFH with 511 progenies was constructed.16 extremely prostrate and 16 extremely erect progenies were selected from F1 population,respectively.All of the 32 screened individuals were identified as true hybrids by chromosome caryogram.The prostrate and erect bulks were built using the BSA method.The difference between parental lines in traits that were unrelated to GSA,including crown width,diameter of capitula,number of ray florets,length and width of petal ligule in ray floret,and the ray floret color(L*,a*and b*),was normalized by the bulking process.(4)The analysis of GSA,hormone content and transcriptional level of genes of prostrate and erect bulks were carried out during three developmental stages.The GSA of prostrate bulk gradually reduced from 163.10(Stage Ⅰ)to 98.9°(Stage Ⅲ),while the GSA of erect bulk was basically unchanged during three stages.The variation tendencies of IAA content in prostrate and erect bulk were different.At stageⅢ,when the GSA showed the biggest discrepancy between the prostrate bulk and erect bulk,the IAA content also presented the biggest difference.By comparative transcriptome analysis of prostrate and erect bulks at three developmental stages,49 candidate genes(LA1,TAC1,PGM1,PIN1,PIN3,PIN5,SAUR36,YUC10,and etc)related to the prostrate growth of chrysanthemum were screened out,which were involved in the regulation of branch angle,auxin synthesis,auxin signal transduction,polar auxin transport,gravity sensing and signal transduction,and the regulation of microtubules and cytoskeleton.(5)All the genes were divided into thirty modules by WGCNA.Darkorange2 and darkgreen module were closely related to the GSA and IAA content,respectively.In regulatory networks of these two modules,PB1,SAMS and UGT76 were identified as the most important hub genes.These three hub genes might be the new genes regulating the prostrate growth of chrysanthemum.(6)The hypothesis model of key genes regulating graviresponse was obtained through analyzing the expression patterns of 52 candidate genes during graviresponse.In YT,LAI was significantly up-regulated at 1 h after horizontal treatment,which inhibited the expression difference of PIN1,PIN3 and PIN5 between the upper and lower half,weakened the asymmetric distribution of auxin,and finally resulted in weaker gravitropism.TAC1,YUC10,CYP79B and SAUR36 have the similar function with LAI during graviresponse.SAMS and PGM1 were significantly up-regulated at 1 h or 3 h after horizontal treatment,inducing the asymmetric distribution of auxin.At the early stage of graviresponse,SAUR68,PIN1,PIN3 and PIN5 showed significantly higher expression in the lower half than the upper half,which could promote the asymmetry distribution of IAA.The expression level of PB1 in the upper half is significantly higher than the lower half,which promotes the accumulation of auxin in the lower half EXPAl is a downstream regulater in graviresponse pathway.This study provides abundant gene resources for researches on molecular mechanism of prostrate growth in chrysanthemum and also lays a solid foundation for fast and efficient breeding of novel prostrate varieties of ground-cover chrysanthemum. |
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