Effects Of Root Heterogeneity On The Root-associated Bacterial Diversity And Structure Of Robinia Pseudoacacia L.

The primary functions of root system are nutrients absorption and transportation for plant.Fine roots(< 1 mm or 2 mm)and coarse roots(> 2 mm)mainly mediate the absorption and the transportation of nutrients,respectively.Root-associated microbiomes can contribute to plant development and nutrients absorption,in turn,they can be affected by root and soil together.Meanwhile,soil nutrient with spatial heterogeneity affects root traits(e.g.root diameter and activity).However,it is still unclear whether root diameter and spatial distribution(root heterogeneity)affect the diversity and structure of the root-associated bacterial communities(including rhizosphere and endosphere),as well as how root heterogeneity affect root-associated bacterial assembly.In this study,we selected R.pseudoacacia as a model plant to investigate the diversity and composition of root-associated bacterial community from different root diameters across various soil depths.Rhizospheric and root endophytic bacterial communities were analyzed via Illumina sequencing of the 16 S r RNA genes.In addition,we investigated the influence of rhizospheric microorganism on plant growth and root traits through plant experiment performed using R.pseudoacacia inoculated with rhizospheric soil slurry.The main results are as follows:1.The diversity and composition of root-associated bacterial community from three root diameters(RD1: < 1 mm;RD2: 1-2 mm;RD3: > 2 mm)were different significantly.Specifically,the α-diversity of rhizospheric and endophytic bacterial communities decreased with increasing root diameter.The enriched genera by different root diameters were different,Variovorax,Lysobacter,Rhizobium,etc.were mostly enriched by RD1 rhizosphere.Phyllobacterium,Inquilinus and Bacillus were mainly enriched in the RD2 rhizosphere.For the root endosphere,Bradyrhizobium,Variibacter,Variovorax,etc.were the enriched genera in RD1,while Psedonocardia,Inquilinus and Reyranella were mainly enriched by the RD2.Bosea was the main enriched genera of RD3-rhizosphere(the abundance: 31.9%)and endosphere(58.2%).2.Root depths(SD1: 0-20 cm;SD2: 20-40 cm;SD3: 40-60 cm;SD4: 60-100 cm)influenced the diversity and composition of rhizospheric and endophytic bacterial communities.The Shannon index of rhizospheric communities differed significantly between SD1 and SD2,and between SD1 and SD4,while OTU richness significantly differed among all root depths excluding between SD1 and SD2.However,no significant differences were observed in the alpha-diversity indices of endophytic communities among root depths.The composition of rhizospheric and endophytic bacterial communities were different significantly among root depths;and the composition of the bacterial community in the rhizosphere(R = 0.296,P < 0.001)was more affected by root depth compared with the endosphere(R = 0.017,P < 0.001),suggesting that the bacterial community of rhizosphere was more affected by soil environment than that of the endosphere.3.The network analysis indicated that the co-occurrence patterns were different among root diameters and depths.The rhizospheric bacterial network complexity decreased with root diameter in the order of RD1> RD2 > RD3,and varied with root depth in the order of SD1 > SD2 > SD3 > SD4.The endophytic bacterial network complexity varied with root diameter in the order of RD3 > RD1> RD2,and varied with root depth in the order of SD4 > SD3 > SD1 > SD2.Further,network keystone taxa were different among various root diameters.The proportions of keystone taxa in the RD1,RD2 and RD3 rhizosphere were 0.72%,2.67% and 3.00%,respectively.The proportions of keystone taxa in the RD1,RD2 and RD3 endosphere were 8.51%,11.67% and 0.58%.Taken together,the results indicated that the network with higher complexity had fewer keystone taxa.4.Plant experiment showed that the plant heights in the treatment inoculated with fine root-rhizosphere soil slurry(RD1-S)were slightly,but not significantly,higher than those in the treatment inoculated with coarse root-rhizosphere soil slurry(RD3-S).Moreover,the RD3-S resulted in smaller root diameter and specific root length,but larger root length,root dry weight,shoot dry weight,root density,root surface area,and root volume when compared with the RD1-S.These results indicated that the RD3-S plants obtained the more developed root system,revealing rhizospheric microbial community can affect the plant growth and their roots traits.In this study,we focused on the diversity and composition of root-associated bacteria from different root diameters and depths.The results demonstrated the relationship between root diameter as well as depth and bacterial community of rhizosphere and endosphere,and revealed that root heterogeneity influenced the root-associated bacterial community.The results provide theoretical support for maximizing microbial function and root adaptability through regulating root-associated microbiome.