Study On The Mechanism Of Floral Transition Under Different Photoperiods In Maize

An efficient way for increasing the maize breeding level in China is to broaden Chinese maize germplasm with tropical and subtropical germplasms which contain abundant hereditary variation.To unveil the mechanisms of floral transition in maize under different photoperiods and overcome the photoperiod sensitivity which severely restricts the use of tropical and subtropical germplasms,this study observes the effects of photoperiod on maize and determines the photoperiod-sensitive phases in maize.We clone some key genes related to maize photoperiod sensitivity and flower development,analyze their function,and studied the effects of photoperiod on maize by gene regulatory network.The main results are as follows:1.Combining the phenotypes of stem apical meristem,leaf number and flowering time,we determine the photoperiod sensitive phase in maize tassel,and divide the photoperiod sensitive phase into photoperiod-sensitive inductive phase and photoperiod-sensitive flower development phase.Under Short-Day(SD) condition, the photoperiod-sensitive inductive phase and photoperiod-sensitive flower development phase of Huangzao 4 are during the expansion of the 4th～6th full leaf and 6th～8th full leaf;the photoperiod-sensitive inductive phase and photoperiod-sensitive flower development phase of CML288 are during the expansion of the 4th～7th full leaf and 7th～9th full leaf.Under Long-Day(LD) condition,the photoperiod-sensitive inductive phase and photoperiod-sensitive flower development phase of Huangzao 4 and CML288 are respectively prolonged one time and five times. The results indicate that CML288 is more sensitive to photoperiod than Huangzao 4.2.Leaf number,days to silking and ASI are used to determine photoperiods ensitivity phase in maize ear development.The results indicate that the ear photoperiod-sensitive phase of Huangzao 4 and CML288 are respectively during the second day of the 5th fully expanded leaf to the third day of the 9th fully expanded leaf and during the second of the 6th fully expanded leaf to the third day of the 10th fully expanded leaf under SD condition.Under LD condition,the time is during 8th～ 12th and 14th～18th 1 fully expanded eaf separately.3.Photoperiod affects the development of the tassel and ear during the photoperiod sensitive phase.The results show that Huangzao 4 and CML288 appear to be more vigorous,the length of tassel and the number of tassel tiller increase under LD.When the plants are transferred reciprocal between long-day(LD)(15hd^-1) and short-day conditions(SD)(9hd^-1) during the photoperiod-sensitive flower development phase,the length of tassel and the number of tassel tiller are affected. The maize ear development of CML288 is severely affected by transfer between long-day(LD)(15hd^-1) and short-day conditions(SD).Transferred from LD to SD during the period during the 14th fully expanded leaf to the 18th fully expanded leaf, the fecundity of ear is decreased,and the ear does not develop after the 19th fully expanded leaf.Transferred from SD to LD,the ear does not develop before the second day of 8th fully expanded leaf,the fecundity of ear is increased during the period from the 8th fully expanded leaf to the 11th fully expanded,leaf and ear developed normal after the 11th leaf fully expanded.4.ZmGI,ZmHd1(CO),ZmFT1(ZCNS),ZmFT2(ZCN18),ZmTFL1(ZCN1), ZmTFL3(ZCN3),ZmFD,ZmLFY and ZAP1 were isolated,or cloned,and analyzed. The result suggest that the component of the genetic network which controls flowering time and floral transition of plant in response to day length are conserved in monocots and dicots or in SD plants and LD plants.But the conservations of different gene in monocots or dicots are diversities.The results of homology analysis with its ortholog of rice and Arabidopsis are that ZmGI is 83%and 63%,ZmHd1 is 75%and 46%,ZmTFL3 is 94%and 73%,ZmFD is 42%and 25%,ZmLFY is 81%and 52%. The homology of amino acid of ZmGI between maize with rice and Arabidopsis is 91%and 69%.The homology of amino acid of ZAP1 between maize with Arabidopsis is 87%.5.The gene sequences are diversities in photoperiod-sensitive maize and photoperiod-insensitive maize.Seven amino acids are absented between the 66^th and the 74^th amino acids of ZmFT2 putative protein in Huangzao 4 compared with ZmFT2 in CML288.Two amino acids are defaulted and three amino acids are changed in putative protein of ZmLFY2 in Huangzao 4 compared with ZmLFY2 in CML288. Two amino acids are defaulted and five amino acids are changed in putative protein of ZmAP1 in CML288 compared with ZmAP1 in Huangzao 4.Moreover,30bp are lost between the 14^th and the 45^th bp outside the stop codon of ZmAP1 in CML288. 6.The results of RT-PCR about maize leaves and stem apical suggest that the expression patterns of ZmGI,ZmHd1 and ZmFT1 are regulated by photoperiod. Under SD conditions,the expression of ZmGI,ZmHd1 and ZmFT1 in maize leaf have diurnal fluctuation,peaking in light,in dark and before dawn respectively.Under LD, the expression of ZmGI and ZmHd1 are upregulated slightly,ZmFT1 is downregulated,while the expression of ZmFT1 in CML288 can not be detected.For the expression patterns of those genes in leaves during different development stages, the high expression periods of ZmGI and ZmHd1 in Huangzao 4 and CML288 are consistent with their photoperiod-sensitive inductive phase,and ZmFT1 is consistent with its photoperiod-sensitive phase.7.ZmFT1,ZmLFY,ZmAP1 and ZmTFL1 regulate the floral transition.FT mRNA levels in leaves during vegetative growth in SD reach its maximum around the floral transition.FT expression is reduced in LD which is consistent with the delay of flowering time.ZmLFY expression is detected in stem apical and reach maximum followed with the maximum expression of ZmFT1.ZmAP1 expression sharply increases after the maximum expression of ZmFT1 and ZmLFY.ZmTFL1 expression is mainly in shoots apical and is upregulated around the floral transition.These results suggest that ZmLFY and ZmAP1 are upregulated by ZmFT1,and then ZmLFY and ZmAP1 together initiate the transition from the vegetative meristem to inflorescence meristem.However,ZmTFL1 maintains the shoot apical meristem in indeterminate stage to develop leaves and branches.ZmFT1 is relative to photoperiod-sensitivity in maize.