| Variegated leaf plants are a special group of colored-leaf plants with stable patterns of differently colored leaf areas,and are ideal materials for the study of plant chromatology,the mechanism of plants adapting to low light environment,and the mechanism of chloroplast development.Leaf variegations play extremely important roles in adapting to low light environment,resisting low temperature,avoiding being forage by herbivore and assisting self-pollinating.Apart from that,these plants are also points of interest as valuable ornamental plants.There has been a long history of classification on leaf variegation,but the classification systems are incompatible with each other and incomprehensive.The molecular mechanism of leaf variegation formation has not yet been analyzed.These problems limited our understanding of variegated leaf plants,and hindered the development and utilization of variegated leaf plant resources.In order to explore the application potential of variegated leaf plants and accumulate basic data for the upcoming researches on the molecular regulation mechanism and the evolutionary history of leaf variegation,firstly,this study obtained a relatively complete list of variegated leaf plants and a revised classification of leaf variegations through extensive sampling and morpho-anatomical observating:1.The obtained list includes a total of 1750 species of variegated angiosperm in80 families and 369 genera.According to the macroscopic characters of position,shape and color of leaf patches,variegations were classified into seven distinguishable patterns: fishbone-shaped,blotched,V-shaped,spotted,striped,reticulate,and pinnate.2.According to the anatomical observation of the leaves of 164 variegated species,leaf variegations are classified into five types,i.e.: ?,chlorophyll type;?,air space type;?,epidermis type;?,pigment type;and ?,appendages type.Type ? is the most common type,which covered approximately 56% of variegated species.The ‘variable palisade' type,‘chloroplast' type and ‘upper mesophyll' type are included in Type ?.The newly defined appendages type(Type ?)is different from Type ? in that its unevenly distributed adaxial appendages exceeds the epidermal cells.3.The causes of species diversity of variegated leaf plants are analyzed.It is believed that this diversity is closely related to the evolutionary status,life form,and growth environment.Herbaceous plants are prone to have leaf variegations due to shorter life cycle and exposure to unstable lighting environment.Secondly,taken Trifolium pratense of Fabaceae,a species with V-shaped Type ? variegation,as material,we examined its differentially expressed genes(DEGs)between variegated areas and green areas under comparative transcriptomics method,and analyzed the gene expression and influencing factors in the formation of the air space type variegation.Transcriptome sequencing analysis was carried out for the young leaflets and adult leaflets of variegated plants and green plants(green plants/young leaflets GY,variegated plants/young leaflets VY,green area of variegated plants/adult leaflets VA0,white area of variegated plants/adult leaflets VA1,the position corresponding to VA1 on the green plants/adult leaflets GA1).The following results are obtained:1.A total of 34771 expressed genes were detected and 43 candidate genes related to the formation of air space type variegation were obtained.3654 differentially expressed genes(DEGs)were obtained in the GY-vs-VY comparison group,of which2703 were up-regulated and 951 were down-regulated;2469 DEGs were obtained in the GA1-vs-VA1 comparison group,of which 1554 were up-regulated and 915 were down-regulated;and 1216 DEGs were obtained in the GA1-vs-VA1 comparison group,of which 473 were up-regulated and 743 were down-regulated.The identified candidate genes related to the formation of air space type variegation mainly involve the genes related to cell wall structure modification(CESA,CSL,EXP,FLA,PG,PGIP,PLL,PME,RGP,SKS and XTH family genes),photosynthesis(LHCB subfamily,RBCS,GOX and AGT family genes),redox(2OG and GSH family genes)and N-metabolism(Nod GS family genes),of which most genes related to cell wall structure modification.In addition,genes related to photooxidation,protein interaction and protease degradation systems were also involved.2.The down-regulated expression of light responsive LHCB subfamily genes and the up-regulated expression of genes related to cell wall structure modification are important conditions for the formation of air space type variegation in T.pratense.The up-regulated expression of ubiquitin-protein ligase enzyme(E3)related genes in the protease degradation systems are conducive to the formation of air space type variegation.It was found that the down-regulated expression of LHCB subfamily genes in photoreaction plays an important role in chloroplast development of leaf variegation,while the up-regulated expression of genes related to cell wall structure modification was the direct cause of the intercellular spaces between epidermal cells and palisade cells,and among palisade cells.Ubiquitin-protein ligase enzyme(E3)related genes were mainly up-regulated expression in both young and adult leaflets of variegated plants,which could remove damaged proteins and chloroplasts in the pathced.Therefore,it is suggested that the formation of air space type variegation in T.pratense is closely related to the expression of LHCB subfamily genes,and genes related to cell wall structure modification and protease degradation systems.Based on the fact that these family genes are necessary for plant growth and development,the formation pattern of leaf variegation in T.pratense may be a widely existing regulation mechanism in air space type variegations.In conclusion,this study provides a new viewpoint for the classification of leaf variegations in angiosperms,and the remodeling of variegation systems and data accumulation laid the foundation for in-depth study of the biological function,molecular mechanism and systematic evolution of leaf variegations.The foundings that the down-regulated expression of LHCB subfamily genes and the up-regulated expression of genes related to cell wall structure modification are of great significance for the formation of intercellular spaces in the V-shaped variegated areas of T.pratense,which would promote the understanding of other variegations and their origin and evolution. |
PDF Full Text Request