Adaptive Root Of Mangrove Plant Avicennia Marina:Structure And Transcriptome Analyses

Mangrove is a type of woody plant community that resides in tropical,subtropical estuaries and coastal intertidal zones.Mangrove is also a special forest type that adapts to the transitional area between land and ocean.Avicennia marina is a kind of common mangrove species,and the pioneer species in community succession.A.marina lives in relatively low oxygen environment in coastal wetlands and their root system is highly specialized to the tidal wetland environment.According to the different growth locations and directions,the root system of A.marina is divided into four parts.Cable roots run horizontally and radially more than 10 m from the tree trunk.Pneumatophores grow vertically upward from the cable roots and expose their tips into the air.Anchor roots grow vertically down to a depth of about 1 m from the cable roots.Feeding roots originate from the pneumatophores that are just under the ground surface and grow horizontally.Most of A.marina pneumatophore studies are mainly in the anatomical and physiological levels,and fewer researchers study the pneumatophore development process and the negative geotropism.In my project,the first step is to observe the pneumatophore and anchor root spatial distribution in the cable root from the Zhangjiang River Mangrove Reserve,Fujian Province,China.Then we used paraffin section to determine different developmental stages of pneumatophore and anchor root.Finally,with the transcriptome sequencing(RNA-seq)and bioinformatics methods to analyze the molecular mechanism of the development of the different developmental stages of pneumatophore and anchor roots.In addition,using the paraffin section method,we obverse anatomical differences of the tips of the shoot,pneumatophore and anchor root.At last,by RNA-seq and bioinformatics methods to analyze the molecular mechanism of pneumatophore negative geotropism.At the same time,it also can provide a theoretical basis for screening the molecular markers of pneumatophore negative geotropism.Following is the results and conclusions:1.Using the Illumina Hi-Seq 2500 sequencing platform,we assembled the raw data of different tissues transcriptomes of A.marina,and obtained 83131 unigenes,and then annotated the A.marina transcriptome according to the NCBI-NR database.The results of the annotation were 25795 unigenes.Also,the principal component analysis of A.marina transcriptome revealed that all the samples had good repeatability,indicative of high reliability of the dataset.2.We determined the specific developmental stages of pneumatophore by paraffin sections.It was revealed that the pneumatophore formation was a complex dynamic process that included the pneumatophore initial and pneumatophore emergence stages.First,the pneumatophore root primordium originated from the cable root;then the cells undergone several anticlinal and periclinal divisions and finally exposed from the cable root.In this sense,the pneumatophore was another kind of lateral roots developed from the cable root.By analyzing the transcriptome of different developmental stages of pneumatophore and anchor roots,the specifically expressed genes of different developmental stages of anchor roots revealed that the combination of brassinosteroid and auxin.These hormones promoted the formation of the anchor root origin from the cable root.Under the same token,anchor root was another kind of lateral root which was developed from the cable root and it is an adaptation to the hypoxic environment in coastal wetlands.3.We used the paraffin section to obverse anatomical differences of the negative geotropism shoot and pneumatophore from the geotropic anchor root.All of these tissues had epidermis,cortex and vascular cylinder,both pneumatophore and anchor root have arenchyma,and pneumatophore arenchyma area was bigger than anchor root arenchyma,but for the shoot,there was no arenchyma.The xylem development patterns were different in the three tissues,anchor root xylem development was endarch,and shoot and pneumatophore xylem development were exdarch.Through the longitudinal section of three tissues,we found that pneumatophore and anchor root have root caps,but no cap in shoot tip.Thus,the pneumatophore was a kind of special root structure between the anchor root and stem,an adaptation to the hypoxic environment in coastal wetlands.RNA-seq analysis also found that the potential molecular mechanism of pneumatophore negative geotropism.By comparing different growth of negative geotropism shoot,negative geotropism pneumatophore and geotropism anchor root and horizontally cable root.Gene Ontology(GO)enrichment results showed that pneumatophore negative geotropism was mainly regulated by the calcium signal.KEGG result of specific expression genes and different expression genes are the same,they show that negative geotropism was regulated by the interactions of statolish,calcium pathway and auxin pathway.During the pneumatophore and anchor root initiation,where the root growth direction was determined,starch plays an important role.In conclusion,my research was the first to complete RNA-seq of A.marina's different growth directions root tissues.It had revealed that the mechanism of Avicennia marina pneumatophore development and negative geotropism.This research also provides the genetic resources and molecular basis for the further study of root development and geotropism.