The phenotype and pathogenicity of Ralstonia solanacearum transformed under prolonged stress of excessive exogenous nitrogen

Abstract

Bacterial wilt, caused by soil‐borne pathogen Ralstonia solanacearum, is a serious disease in many plants such as Solanaceae. To investigate the effects of accumulated nitrogen in soil on the phenotype and pathogenicity of the R. solanacearum, a serial passage experiment (SPE) was designed. Specifically speaking, minimal medium supplied with a slight excess of ammonium sulphate (AS) or ammonium nitrate (AN) was used to simulate the nutrition of soil containing excess nitrogen. During the period of 30 SPE, the phenotype, pathogenicity and relative expression of nitrogen metabolism genes in R. solanacearum were monitored. Phenotypic analysis results illustrated that the colony morphology of R. solanacearum changed after long‐term culture, from high virulence colonies with strong fluidity to small, round non‐mucoid colonies; The strain after prolonged stress of excessive exogenous nitrogen was a no‐virulence phenotype conversion type (PC‐type). The time for a change in colony morphology to occur after exposure to exogenous AS or AN was significantly less than the untreated samples, which treated without exogenous nitrogen. The results of pathogenicity also demonstrated that the cultures treated with exogenous AN or AS reduced virulence more quickly than the control. The disease index of 10 SPE with AN treatment or AS treatment was 89% or 68% lower than that of the control, respectively. In addition, as the incubation time increased, the swimming motility and the number of biofilms formation of the cultures were significantly changed under both treatments in comparison to the untreated samples. Furthermore, the relative expression of the nitric oxide reductase norB gene in the cultures treated with AN was 1.51‐fold higher compared with the control after 30 SPE. These results indicated that excessive nitrogen supply in the environment could accelerate the transformation of R. solanacearum from high virulence wild‐type into a PC‐type, probably for the purpose of adapting to the adverse environment.

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“…The prolonged use of chemical fertilizers alters nutrient content in the soil 7 , diminishes the entry of rainwater into the soil by altering porosity 8 , 9 , induces pest attack 10 , reduces soil water holding capacity 9 , soil aggregation and friability 11 , and upsets soil pH 8 , resulting in eradication of beneficial microorganisms from soil that then affects plant immunity and soil nutrient availability 12 , 13 . Higher fertilizer inputs influence plant–microbe interactions, and the fast-release chemical fertilizers with high nitrogen content induce numerous bacterial and fungal diseases in plants 10 , 14 , 15 . These induced effects ultimately lead to cycles of increased pesticide use those impacts both living organisms and their environment.…”