Analyses Of The Origin And Evolution Of Plant Tm-1-like Genes

The origin of land plants is one of the most important events during the evolution of life on earth,and played critical roles in the formation of modern terrestrial ecosystem.Land plants originated from green algae,which lives in water.From peaceably aquatic surroundings to harshly terrestrial environments,the ancestor of land plants had to face significant stresses different from those in water,including drought,glexible temperature,and enhanced ultraviolet radiation.In addition,land plants should also stay in microbial environment,which is also different from those in water.Therefore,land plants need to evolve new genes and phenotypic novelties for adapting to terrestrial environments during their colonization of land.The tomato(Solarium lycopersicum L.)gene Tm-1,conferring resistance to Tomato mosaic virus(ToMV),was bred from the wild tomato S.habrochaite.Although the Tm-1 protein does not share any functional domain with previously known resistance(R)proteins,it physically binds ToMV replication proteins and functionally inhibits the RNA-dependent RNA replication of ToMV.Sequence similarity searches revealed that homologs of the tomato Tm-1 gene were present in nearly all of the sequenced land plant genomes and showed highly conserved.In addition,plant homologous proteins of Tm-1 in plants possess similar domain strutures,contaning an uncharacterized UPF0261 domain in N-terminal and a TIM-barrel-like signal transduction(TBST)domain in C-terminal,respectively.Although the functions in diease resistance of the tomato Tm-1 gene have been revealed,the origin and evolutionary mechanisms of plant Tm-1-like(Tm-1L)genes reminds largely unknown.In addition,the function of this gene in other plant species need father detectation.In this thesis,the methods of bioinformatics,including sequence similarity and phylogeny,have been used to illustrate the origin mechanism of the plant Tm-1L gene by performing an extensive search of its homologs in current sequence databases and by analyzing their phylogeny.We further investigated the tissue expression patterns and expression responding to some abiotic stresses of maize(Zea mays L.)ZmTm-1L gene using RT-PCR.The results of this thesis will not only provide a theoretical foundation for molecular mechanisms of the origin and evolution of the land plant Tm-IL gene,but also provide reference for functional detection of this gene in other plant species.The main researching contents and findings were as follows:(1)Sequence similarity analysis revealed that homologs of the Tm-1L gene were present in all of the sequenced streptophyte groups,including charophyte algae and land plants.However,no homologs were detected in any other plant genomes,including chlorophyte green algae and red algae,both of which were believed to be the ancestor of streptophytes.(2)The plant Tm-IL protein is predicted to contain an uncharacterized N-terminal UPF0261 domain and a C-terminal TIM-barrel-like(TBST)domain.Further homologous searches revealed that proteins containing both of these two domains are only present in streptophytes while homologous genes in other species only encode one domain.(3)Phylogenetic analyses based on homologs of these two domains revealed that the ancestor of land plants acquired UPF0261-and TBST-domain-encoding genes via two independent horizontal gene transfer(HGT)events before the origin of land plants from charophytes.Subsequently,gene fusion occurred between these two horizontally acquired genes and resulted in the origin of the Tm-1L gene in streptophytes.(4)Co-evolution analyses of UPF0261 and TBST proteins revealed that these two genes co-evolved in bacteria,archaea and fungi,suggesting that there should be functional complementarity and interaction between the proteins of UPF0261 and TBST.(5)In conclusion of bioinformatics analyses,we reported a novel mechanism of gene origination by analysing the origin and evolution of the plant Tm-1L gene,through which the recipient organism may acquire genes with functional interaction through two different HGT events and further fused them into one functional gene.(6)In order to evaluate the potential function of the Tm-IL gene in maize,tissue expression analyses were performed respectively at the seelding and tasseling stages,and the results showed that the highest expression level occured in the young stem and ear at the seelding and tasseling stages,respectively.(7)Expression pattern analyses were also performed for maize Tm-1L gene under different abiotic stresses,and the results illustrated that the expression were significantly enhanced under drought,cold,heat and salt stresses.In addition,the expression of this gene was also induced by ABA.All these findings suggested that maize Tm-1L gene might be involved in the responding to abiotic stresses in plants.