Root exudates and chemotactic strains mediate bacterial community assembly in the rhizosphere soil of Casuarina equisetifolia L

Abstract

Rhizosphere bacterial diversity and community structure are important factors involving in plant growth. However, the exact process of how plant rhizosphere bacterial community structures is assembled remains unclear. To investigate the role of bacterial chemotaxis to rhizosphere secretions in the establishment of rhizosphere microbial community in Casuarina equisetifolia, we screened bacteria strains derived from the rhizosphere of Casuarina equisetifolia L. using top three chemicals of the plant root exudates (2,4-di-tert-butylphenol, methyl stearate, and arginine) as chemoattractant. Among 72 bacterial strains, five showed strong chemotaxis to 2,4-di-tert-butylphenol, six to methyl stearate, and eleven to arginine, with the highest bacterial chemotaxis occurring at a concentration of 60 μM. This indicates that arginine is a more important chemoattractant than 2,4-di-tert-butylphenol, methyl stearate in the establishment of rhizosphere microbial community in Casuarina equisetifolia. Bacterial community assembly analysis using different chemoattractants and chemoattractants-plus-bacteria combinations were then performed by burying laboratory prepared bags of sterlized soil into C. equisetifolia forest. Bacteria diversity and enrichment analyses using 16S rDNA sequencing at 7 and 14 days after burying showed that arginine-plus-Ochrobactrum sp. and Pantoea sp. treatment exhibited the greatest similarity to the natural forest bacterial community. Our date provides new insights into how chemoattractants and chemotactic bacteria strains shape the rhizosphere microbial community of C. equisetifolia, which constitutes foundational information for future management of these communities.

Keywords: bacterial diversity; casuarina equisetifolia L.; chemotaxis; hainan; root exudates.

Figure 1

Experimental design to determine the effects of different simulated chemoattractants and chemotactic strains on bacterial community assembly in the rhizosphere soils of a C. equisetifolia forest.

Figure 2

Effect of chemoattractants on the swarming motility of different bacteria. Swarming diameters were measured for chemoattractants at concentrations of 0, 30, 60, and 90 μM. (A): 2, 4-di-tert-butylphenol; (B): methyl stearate; (C): arginine. Different letters above the columns indicate significant difference (P < 0.05).

Figure 3

Effect of chemoattractants on the swarming motility of Bacillus cereus strain CP1 and Pseudomonas sp. (A–C) Bacillus cereus strain CP1; (D–F): Pseudomonas sp. (A, D) Chemoattractant 2, 4-di-tert-butylphenol; (B, E) chemoattractant methyl stearate; (C, F) chemoattractant arginine. Chemoattractant concentration from left to right: 0, 30, 60, and 90 μM.

Figure 4

Box plots of the effects of chemoattractants and chemoattractants-plus-chemotactic strains on the Chao1 richness and Shannon diversity of bacterial genera on days 7 (A, C) and 14 (B, D). Different lowercase letters indicate significant differences among treatments (P < 0.05).

Figure 5

Bacterial co-occurrence networks as affected by chemoattractant and chemoattractant-plus-chemotactic strain treatments at 7 and 14 days at the phylum level. Avg, Average degree of taxonomy.

Figure 6

Venn diagrams showing the numbers of shared and unique bacterial genera in chemoattractant and chemoattractant-plus-chemotactic strain treatments at 7 (A–D) and 14 (E–H) days and in rhizosphere soils of a C. equisetifolia forest. sg, specific genera. cg, common genera. ag, all genera.

Figure 7

Relative abundance of bacterial genera following chemoattractant and chemoattractant-plus-chemotactic strain treatments at 7 and 14 days.

Figure 8

Proportions of bacterial genera with high relative abundances in chemoattractant and chemoattractant-plus-chemotactic strain treatment groups at 7 and 14 days. Asterisks in red indicate a significant difference among treatments (P < 0.05).

Figure 9

NMDS ordinations based on weighted UniFrac distance matrices of bacterial communities in chemoattractant and chemoattractant-plus-chemotactic strain treatments at 7 (A) and 14 (B) days and in rhizosphere soils of a C. equisetifolia forest.