Growth-promoting effects of self-selected microbial community on wheat seedlings in saline-alkali soil environments

Abstract

Saline-alkali land is a type of soil environment that causes poor crop growth and low yields. Its management and utilization are, therefore of great significance for increasing arable land resources, ensuring food security, and enhancing agricultural production capacity. The application of plant growth-promoting rhizobacteria (PGPR) is an effective way to promote the establishment of symbiotic relationships between plants and the rhizosphere microenvironment, plant growth and development, and plant resistance to saline-alkali stress. In this study, multiple saline-alkali-resistant bacteria were screened from a saline-alkali land environment and some of them were found to have significantly promotive effects on the growth of wheat seedlings under saline-alkali stress. Using these PGPR, a compound microbial community was selectively obtained from the root-zone soil environment of wheat seedlings, and the metagenomic sequencing analysis of wheat root-zone soil microbiomes was performed. As a result, a compound microbial agent with a Kocuria dechangensis 5-33:Rossellomorea aquimaris S-3:Bacillus subtilis BJYX:Bacillus velezensis G51-1 ratio of 275:63:5:1 was obtained through the self-selection of wheat seedlings. The synthetic compound microbial agent significantly improved the growth of wheat seedlings in saline-alkali soil, as the physiological plant height, aboveground and underground fresh weights, and aboveground and underground dry weights of 21-day-old wheat seedlings were increased by 27.39% (p < 0.01), 147.33% (p < 0.01), 282.98% (p < 0.01), 194.86% (p < 0.01), and 218.60% (p < 0.01), respectively. The promoting effect of this compound microbial agent was also greater than that of each strain on the growth of wheat seedlings. This microbial agent could also regulate some enzyme activities of wheat seedlings and the saline-alkali soil, thereby, promoting the growth of these seedlings. In this study, we analyze an efficient microbial agent and the theoretical basis for promoting the growth of wheat seedlings under saline-alkali stress, thereby, suggesting an important solution for the management and utilization of saline-alkali land.

Keywords: metagenome; plant growth-promoting rhizobacteria; saline-alkali soil environment; the rhizosphere microenvironment; wheat.

FIGURE 1

Colony, cell morphology, and phylogenetic tree analysis of selected strains. Strains were activated using LB media. The phylogenetic tree analysis of 16S rDNA sequences of strains was performed using MEGA 11.0. Colony (A), cell morphology (B), and phylogenetic tree (C) analysis of strain S-3; colony (D), cell morphology (E), and phylogenetic tree (F) analysis of strain BJYX; colony (G), cell morphology (H), and phylogenetic tree (I) analysis of strain G51-1; colony (J), cell morphology(K), and phylogenetic tree (L) analysis of strain YL-10; colony (M), cell morphology (N), and phylogenetic tree (O) analysis of strain G63-1.

FIGURE 2

Effects of strains on physiological plant heights of wheat seedlings using simulated saline-alkali soil. “0+” represents the normal field soil, “1+” represents the simulated saline-alkali soil. At the early stage of wheat germination, various microbial agents were applied, and 30 mL of water was applied every 2 days. The plant heights were measured at 7 days (A) and 14 days (B) after wheat growth. Each group was subject to 3–5 biological treatments. The averages of the data are presented. Error bars represent the standard deviations of the averages. *: p < 0.05, significant; **: p < 0.01, highly significant.

FIGURE 3

Analysis of taxonomic compositions at the phylum (A), genus (B), and species (C) levels, and the bacterial numbers of the added species obtained via metagenomic sequencing (D). The relative abundances (top 10) of different microbial community structures at the phylum, genus, and species levels with the three treatments (SSP_1, SSP_2, and SSP_3) are presented.

FIGURE 4

Agronomic traits of wheat seedlings under different treatment conditions. When the wheat sprouted, the strain inoculants were added, and then, the plants were watered every 2 days with 30 mL of water. The heights of the wheat seedlings after 7 days (A) and 21 days (B) were recorded. The fresh weights of the aboveground parts (C) and the underground parts (D), the dry weights of the aboveground parts (E) and the underground parts (F) after drying for 48 h, the complete wheat seedlings (G), and the roots of the wheat seedlings (H) after 21 days of growth are displayed. “MB” represents the self-selected mixed bacteria, and “MBE” represents the mixed bacteria of the selected strains based on an equal proportion. Each group was subjected to 3–5 biological treatments. The averages of the data are presented. Error bars represent the standard deviations of the averages. *: p < 0.05, significant; **: p < 0.01, highly significant.

FIGURE 5

Determination of enzyme activities in wheat leaves and roots treated with MBE, MB, 5-33, BJYX, G51-1, and S-3. The wheat leaves that were subjected to different treatments were selected and ground to determine SOD enzyme activity (A), POD enzyme activity (B), and the concentration of MDA (C). After cleaning, wheat roots were ground and filtered to determine the corresponding root SOD enzyme activity (D), POD enzyme activity (E), MDA concentration (F), and CAT enzyme activity (G). “MB” represents the self-selected mixed bacteria, and “MBE” represents the mixed bacteria based on an equal proportion. Each group was subjected to 3–5 biological treatments. The averages of the data are presented. Error bars represent the standard deviations of the averages. *: p < 0.05, significant; **: p < 0.01, highly significant.

FIGURE 6

Determination of enzyme activities in the saline-alkaline soil during wheat seedling growth. The activities of acid phosphatase (A), alkaline phosphatase (B), neutral phosphatase (C), peroxidase (D), sucrase (E), and urease (F) in the rhizosphere soil of wheat seedlings after 21 days of growth under different treatment conditions and those in the non-rhizosphere soil not treated with the strains were determined. “MB” represents the self-selected mixed bacteria, and “MBE” represents the mixed bacteria based on an equal proportion. “NR” represents the non-rhizosphere soil for “CK”. Each group was subjected to 3–5 biological treatments. The averages of the data are presented. Error bars represent the standard deviations of the averages. *: p < 0.05 significant; **: p < 0.01 highly significant.

FIGURE 7

Schematic diagram of the construction of a new type of microbial agent combination (MB) for wheat seedlings. The added and finally selected strains are noted.