Mechanism On Photoperiodic Control Of Flowering In Luculia Gratissima ‘Xiangfei'

Luculia gratissima,which belongs to Rubiaceae,is an evergreen perennial shrub or small tree with flowering from August to December.Due to its pink color,aroma and long flowering period,it has a high ornamental value.However,the flowering mechanism so far is still unclear.In this study,the morphological differentiation of L.gratissima ‘Xiangfei' flower buds was observed under the inductive and non-inductive photoperiods.The contents of endogenous substances such as soluble sugar,soluble protein and hormone(IAA,ABA,ZT,GA3)were determined in five development stages of buds,including vegetative phase,undifferentiation phase,bract primordium differentiation phase,inflorescence primordium differentiation phase,small flower primordium differentiation phase.The sequencing libraries were constructed,and the geneexpression data were obtained at four developmental stages of inductive and non-inductive buds using RNA-seq high-throughput sequencing.Reference genes of flower development were selected and validated by q RT-PCR.Two homologous genes involved in circadian rhythm were obtained by RACE technique.The Specific tissue rhythmic expression analysis of the two genes was performed by q RT-PCR under the inductive photoperiod.The results could not only provide a theoretical basis for the artificial regulation of the flowering period of the genus,but also support reference for flowering mechanism study of the perennial short-day woody plants.The main conclusions were as follows:(1)‘Xiangfei' is a qualitative or obligatory short-day plant,the critical day-length is about14 h,the suitable day-length for flowering is 10-12 h,and the limited inductive photoperiod is30 d short-day.Under the induced photoperiod,all of plants completely achieved the transition to flowering after treatment for 16 d.After treatment for 30 days,the floral determination of plants reached a stable maintenance state and the floral reversion would not occur when transferred to the non-inductive photoperiod.Under the uninduced photoperiod,plants were always in a vegetative growth.(2)During floral transition process of ‘Xiangfei',lower levels of sugars were beneficial to floral transition.Sugars took part in ABA regulation pathway and integrated into the flowering networks.T-6-P,as a signal molecule of sugars,mediated Lg FTL to regulate floral transition.Higher levels of ABA was beneficial to flowering.ABA could regulate Lg COL to mediate floral transition.GA could regulate Lg LFY and Lg FTL to mediate floral transition.Through mediating expression of Lg FKF1 and Lg PRR7,photoperiod regulated Lg FTL to control flowering transition.Finally,Lg SOC1,Lg FTL and Lg LFY integrated signals from sugars,hormones and photoperiod to mediate expression of Lg AP1 and Lg FUL,and promoted floral transition.(3)Lg ACT+Lg EF1 was determined as the best stable reference gene combinations for q RT-PCR of gene expression studies on flower development of ‘Xiangfei'.(4)Full-length c DNA sequence of Lg PRR7 and Lg FKF1 were obtained from ‘Xiangfei'.The characteristics of encoded proteins of Lg PRR7 and Lg FKF1 coincided with those of other reported plants,which could meet requirements of transcription regulation of the genes.Compared with rhythmic expression of PRR7 and FKF1 homologous genes in other plants,Lg PRR7 was more conservative,while Lg FKF1 had its own specificity.