Effects of Phenotypic Variation on Biological Properties of Endophytic Bacteria Bacillus mojavensis PS17

Simple Summary Microorganisms play an important role in agriculture by protecting and stimulating the growth of plants. The phenotypic activities of microbial biological agents (MBA) can change under different environmental conditions. However, to adapt to these harsh conditions, genetic mutations take place in bacteria that are seen phenotypically, which might not be beneficial or less beneficial to the plants. Some adaptative mechanisms used by microorganisms, especially bacteria, to face these environmental factors lead to the appearance of subpopulations with different morphotypes that may be more adapted to survive in stressful conditions. Moreover, in favorable conditions, these subpopulations may become dominant among the overall bacterial population. In this study, Bacillus mojavensis undergoes phase variation when grown in a minimal medium, in which two colonies, opaque (morphotype I) and translucent (morphotype II), were generated. The characteristics of the generated morphotypes were determined and compared with those of their original strain. Overall, the results obtained showed that the phenotypic characteristics of morphotype I statistically differed from morphotype II. This phenomenon may be one of the factors behind the dissimilarities in the results between the laboratory and field data on the application of MBA. The use of microorganism-based products in agricultural practices is gaining more interest as an alternative to chemical methods due to their non-toxic bactericidal and fungicidal properties. Various factors influence the efficacy of the microorganisms used as biological control agents in infield conditions as compared to laboratory conditions due to ecological and physiological aspects. Abiotic factors have been shown to trigger phase variations in bacterial microorganisms as a mechanism for adapting to hostile environments. In this study, we investigated the stability of the morphotype and the effects of phenotypic variation on the biological properties of Bacillus mojavensis strain PS17. B. mojavensis PS17 generated two variants (opaque and translucent) that were given the names morphotype I and II, respectively. The partial sequence of the 16S rRNA gene revealed that both morphotypes belonged to B. mojavensis. BOX and ERIC fingerprinting PCR also showed the same DNA profiles in both morphotypes. The characteristics of morphotype I did not differ from the original strain, while morphotype II showed a lower hydrolytic enzyme activity, phytohormone production, and antagonistic ability against phytopathogenic fungi. Both morphotypes demonstrated endophytic ability in tomato plants. A low growth rate of the strain PS17(II) in a minimal medium was observed in comparison to the PS17(I) strain. Furthermore, the capacity for biocontrol of B. mojavensis PS17(II) was not effective in the suppression of root rot disease in the tomato plants caused by Fusarium oxysporum f. sp. radices-lycopersici stain ZUM2407, compared to B. mojavensis PS17(I), whose inhibition was almost 47.9 +/- 1.03% effective.