Introduction Many nitrogen-fixing bacteria (NFB) can synthesize siderophores. However, the contribution of NFB siderophore synthesis to plant growth has not been evaluated, and its impact on NFB’s antifungal activity remains unclear. Methods The type of siderophore synthesized by the nitrogen-fixing bacterium (NFB) Kosakonia radicincitans GXGL-4A was determined. The effects of NFB siderophore production on plant growth were examined by inoculating cucumber with GXGL-4A and the mutant Δ gua A (which does not produce siderophores). The antifungal activities of NFB cell-free fermentation broth on the mycelial growth of Fusarium oxysporum , Fusarium proliferatum , Rhizoctonia solani , and Botrytis cinerea were evaluated. Recovery of the siderophore synthesis ability of Δ gua A upon the addition of exogenous guanylate was measured. Results The strain GXGL-4A produces carboxylate-type siderophores. The antifungal ability of Δ gua A was significantly reduced compared to that of GXGL-4A ( P 0.05). Compared with the group without NFB inoculation, GXGL-4A treatment significantly increased cucumber seedling height and plant dry weight ( P 0.05), but had no significant effect on root length and root dry weight ( P 0.05). There was no significant difference in seedling height between the experimental groups ( P 0.05). The siderophore production capability of Δ gua A could be partially restored via the addition of 2% (w/v) guanosine monophosphate to LB medium. NFB inoculation maintained a relatively stable, weakly alkaline rhizosphere (7.35–7.71), whereas the pH of the control group fluctuated more significantly (7.49–6.45). On the 3rd day after inoculation, the rhizosphere electrical conductivity of the Δ gua A treatment group (1,461 μs/cm) was significantly lower than that of the GXGL-4A treatment group (1,707 μs/cm) and the control group (2,973 μs/cm, P 0.001). Compared with the GXGL-4A treatment group, the leaf chlorophyll content of the Δ gua A treatment group was significantly decreased on the 9th day after inoculation ( P 0.05), whereas there were no significant differences in root activity, soil dehydrogenase activity, and alkaline protease activity ( P 0.05). Discussion The loss of siderophore synthesis ability in NFB leads to a significant decrease in their ability to promote plant growth and their own antifungal capacity. By utilizing NFB with high siderophore production capability, it is expected to play a greater role in sustainable agricultural production.
Wang et al. (Wed,) studied this question.
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