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There has been increasing attention toward the influence of biofertilizers on the composition of microbial communities associated with crop plants. We investigated the impact of Azospirillum sp. B510, a bacterial strain with nitrogen-fixing ability, on the structure of bacterial and fungal communities within rice plant rhizospheres by amplicon sequencing at two sampling stages (the vegetative and harvest stages of rice). Principal coordinate analysis (PCoA) demonstrated a significant community shift in the bacterial microbiome when the plants were inoculated with B510 at the vegetative stage, which was very similar to the effect of chemical N-fertilizer application. This result suggested that the inoculation with B510 strongly influenced nitrogen uptake by the host plants under low nitrogen conditions. Least discriminant analysis (LDA) showed that the B510 inoculation significantly increased the N2-fixing Clostridium, Aeromonas and Bacillus populations. In contrast, there was no apparent influence of B510 on the fungal community structure. The putative functional properties of bacteria were identified through PICRUSt2, and this hinted that amino acid, sugar and vitamin production might be related to B510 inoculation. Our results indicate that B510 inoculation influenced the bacterial community structure by recruiting other N2-fixing bacteria in the absence of nitrogen fertilizer. [ABSTRACT FROM AUTHOR]

There has been increasing attention toward the influence of biofertilizers on the composition of microbial communities associated with crop plants. We investigated the impact of Azospirillum sp. B510, a bacterial strain with nitrogen-fixing ability, on the structure of bacterial and fungal communities within rice plant rhizospheres by amplicon sequencing at two sampling stages (the vegetative and harvest stages of rice). Principal coordinate analysis (PCoA) demonstrated a significant community shift in the bacterial microbiome when the plants were inoculated with B510 at the vegetative stage, which was very similar to the effect of chemical N-fertilizer application. This result suggested that the inoculation with B510 strongly influenced nitrogen uptake by the host plants under low nitrogen conditions. Least discriminant analysis (LDA) showed that the B510 inoculation significantly increased the N2-fixing Clostridium, Aeromonas and Bacillus populations. In contrast, there was no apparent influence of B510 on the fungal community structure. The putative functional properties of bacteria were identified through PICRUSt2, and this hinted that amino acid, sugar and vitamin production might be related to B510 inoculation. Our results indicate that B510 inoculation influenced the bacterial community structure by recruiting other N2-fixing bacteria in the absence of nitrogen fertilizer.

Azospirillum sp. B510, a free-living nitrogen-fixing bacterium isolated from the stems of rice (Oryza sativa cv. Nipponbare), was investigated to establish effective conditions for the colonization of rice plants. We analyzed the effects of the nitrogen sources KNO 3 , NH 4 Cl, urea (CO[NH 2 ] 2 ), and NH 4 NO 3 at different concentrations (0.01-10 mM) on this colonization. Nitrogen promoted plant growth in a concentration-dependent manner, with minor differences being observed among the different nitrogen sources. Bacterial colonization was markedly suppressed on media containing NH 4 + concentrations higher than 1 mM. Since concentrations of up to and including 10 mM NH 4 + did not exhibit any antibacterial activity, we analyzed several factors affecting the NH 4 + -dependent inhibition of endophytic colonization, including the accumulation of the reactive oxygen species H 2 O 2 and the secretion of the chemotactic substrate malic acid. The accumulation of H 2 O 2 was increased in rice roots grown on 1 mM NH 4 Cl. The amounts of malic acid secreted from NH 4 -grown rice plants were lower than those secreted from plants grown without nitrogen or with KNO 3 . Although the bacterium exhibited chemotactic activity, moving towards root exudates from plants grown without nitrogen and KNO 3 -grown plants, this activity was not observed with root exudates from NH 4 -dependent phenomena were markedly suppressed by the stabilization of medium pH using a buffer. These results demonstrate that the type and concentration of nitrogen fertilizer affects the colonization of rice plants by Azospirillum sp. B510.+ -grown plants. NH 4 + , but not NO 3 - , caused the acidification of growth media, which inhibited plant bacterial colonization. These NH 4 + -dependent phenomena were markedly suppressed by the stabilization of medium pH using a buffer. These results demonstrate that the type and concentration of nitrogen fertilizer affects the colonization of rice plants by Azospirillum sp. B510.