Effects of Enterobacter cloacae HG-1 on the Nitrogen-Fixing Community Structure of Wheat Rhizosphere Soil and on Salt Tolerance

Abstract

The present study investigated the physiological and biochemical characteristics of Enterobacter cloacae HG-1 isolated from saline-alkali soil. We further studied the effect of this strain on the salt tolerance of wheat and on the community structure of nitrogen-fixing bacteria in rhizosphere soil. We determined that the investigated strain had high nitrogen fixation activity and produced iron carriers, 1-aminocyclopropane-1-carboxylic acid deaminase, and plant hormones. The metabolites of this strain contained 2,3-butanediol, [R-(R*, R*)], 2-heptanone, and other growth-promoting and antibacterial substances. The strain was also highly salt-tolerant (10% NaCl). After the inoculation of wheat with the HG-1 strain, we recorded increases in root length, plant height, fresh weight, and dry weight of 19.15%, 18.83%, 16.67%, and 17.96%, respectively, compared with uninoculated plants ( P < 0.05). Compared with the leaves of uninoculated plants, the proline concentration in the leaves of inoculated plants increased by 12.43% ( P < 0.05), the malondialdehyde level decreased by 27.26% ( P < 0.05), K + increased by 20.69%, Ca 2+ increased by 57.53% and Na + decreased by 31.43% (all P <0.05). Furthermore, we detected that inoculation with the HG-1 strain did not affect the species composition of nitrogen-fixing bacteria in wheat rhizosphere soil at the phylum level. However, the average relative abundance of Proteobacteria was significantly increased, whereas the abundance of Verrucomiorobia was significantly decreased compared with uninoculated plants. At the genus level, we detected 32 genera in control samples and 27 genera in inoculated samples, and the species diversity and relative abundance of samples inoculated with the HG-1 strain decreased compared with uninoculated plants. Inoculated samples had lower abundances of Azospirillum , Rhodomicrobium , and Anabaena . Our study demonstrated that the inoculation of wheat with E. cloacae HG-1 could promote the growth of wheat under salt stress and increase salt stress tolerance. The results of this study investigating the interaction among soil, plants, and microorganisms supplement agricultural microbial databases and could provide a reference for the development of microbial-based saline soil improvement programs.

2,3-Butanediol synthesis from glucose supplies NADH for elimination of toxic acetate produced during overflow metabolism

“…Acetate accumulates and then vanishes during the growth of E. cloacae SDM E. cloacae, a Voges-Proskauer-positive strain with the ability to produce 2,3-BD (Fig. 1a), is widely distributed in gut and rhizosphere soil 13 . 2,3-BD released by E. cloacae was reported to promote the growth of Arabidopsis thaliana 14,15 .…”

“…To further identify the catabolic fate of acetate, 13 C-labeled sodium acetate (~10 mM) was added into the culture system of E. cloacae SDM at 2 h and the variation of 13 C-labeled acetate ( 13 C 1 -CH 2 COOH) was detected. Our results showed that exogenous 13 Clabeled acetate also vanished, and most of the 13 C-labeled acetate was reduced to 13 C-labeled ethanol by E. cloacae SDM (Fig. 1e-g; Supplementary Table S1).…”

“…Elimination of exogenously added 13 C acetate by these two mutant strains was also assayed. E. cloacae SDM (ΔbudABC) and E. cloacae SDM (ΔadhE) could not metabolize most of the 13 C-labeled acetate to 13 C-labeled ethanol as E. cloacae SDM (Fig. 1e-g), further supporting that 2,3-BD biosynthesis and ethanol production participate in the elimination of acetate.…”

“…Formate-lyase (PFL) becomes the main source of acetyl-CoA under anaerobic condition 23 . Formate dehydrogenase (FDH) catalyzed 13 C acetate by E. cloacae SDM and its derivatives. f Production of 13 C ethanol by E. cloacae SDM and its derivatives.…”

“…f Production of 13 C ethanol by E. cloacae SDM and its derivatives. g Yield of 13 C ethanol from 13 C acetate by E. cloacae SDM and its derivatives. 13 C sodium acetate at a concentration about 10 mM was added into the culture system of E. cloacae SDM, E. cloacae SDM (ΔadhE), and E. cloacae SDM (ΔbudABC) at 2 h. After 4 h of culture, the concentrations of 13 C ethanol and 13 C acetate were assayed.…”