Driving Factors Of Rhizosphere Microbial Community Assembly And Its Relationship With Seed Germination In Casuarina Equisetifolia L.

Casuarina equisetifolia L.,an important windbreak forest with the characteristics of salinity and drought resistance on the southeast coast of China,plays an enormous role in protecting the coastal ecological environment.However,its natural regeneration is difficult,and the exact process of bacterial community assembly in the rhizosphere of C.equisetifolia is still largely unknown.Meanwhile,the ecological processes by which C.equisetifolia governs microbiome assembly,function,and dispersal remain largely unknown.Therefore,we set up the following experiments in an attempt to fill the key knowledge gaps.(1)We used 2,4-di-tert-butylphenol,methyl stearate,and arginine at different concentrations as chemoattractants to select chemotactic bacteria from 72 previously identified culturable C.equisetifolia rhizosphere bacteria by swarming assay.(2)To determine the effects of chemoattractant and chemotactic bacteria on the assembly of bacterial community in the rhizosphere of C.equisetifolia,field experiments were conducted with the combinations of "chemoattractant + sterile soil" and "chemoattractant + chemotactic bacteria + sterile soil",and the results were evaluated by 16 S r DNA sequencing and bioinformatics analysis.(3)Using 16 S r DNA and ITS sequencing,and bioinformatics analysis,we investigated the potential sources of C.equisetifolia rhizosphere chemotactic bacteria,as well as the bacterial and fungal communities in multiple compartment niches(bulk soil,rhizosphere soil,root endosphere,phylloplane,and leaf endosphere)of C.equisetifolia at three developmental stages in Hainan Province,China.(4)The effects of chemoattractant,chemotactic bacteria,and microbial communities shaped by them on seed germination of C.equisetifolia were investigated in pot experiments.The main results were as follows:(1)2,4-di-tert-butylphenol,methyl stearate,and arginine could all act as chemoattractants.The highest number of bacterial strains was observed to show chemotaxis at chemoattractant concentrations of 60 μM.(2)Both root exudates(2,4-di-tert-butylphenol,methyl stearate,and arginine)and chemotactic bacteria of C.equisetifolia attract bacteria to gather around them.Arginine-plus-Ochrobactrum sp.and-Pantoea sp.treatment yielded the bacterial community most similar to that of a C.equisetifolia.(3)The bacterial community of C.equisetifolia mostly originated from bulk soil.Microbiome assemblages along the soil-plant continuum were shaped by the compartment niches.Bacterial diversity and richness decreased from the soils to roots to leaves,with the highest network complexity found in the roots and the lowest found in the phylloplane.However,the fungal community was mainly derived from rhizosphere soil and air.Fungal diversity gradually increased from the soils to roots to phyllosphere,whereas fungal richness decreased from the soils to roots but increased from the roots to phyllosphere;the greatest network complexity was found in bulk soils and the lowest was found in the roots.(4)Chemoattractant,chemotactic bacteria,and microbial communities formed by them(except for the microbial community shaped by arginine)all delayed and inhibited seed germination of C.equisetifolia.Our results provide empirical evidence and new insights into the formation of bacterial communities in the rhizosphere and the control of microbial assembly and dispersal which has guiding significance for solving the difficulty in natural regeneration of C.equisetifolia.