Interaction between ciliate and plant growth promoting bacteria influences the root structure of rice plants, soil PLFAs and respiration properties

Plant growth and productivity depend on the complex and dynamic interaction between the plant roots and soil microorganisms. At present, the research on rhizosphere associated microbes is largely focused on bacteria and fungi; whereas the interaction of soil protists with plants and other microbes remains unexplored. The present study aimed to investigate the interactive impact of a ciliate (Kreyellidae, C5) and two plant growth-promoting bacteria (PGPB) i.e., Pseudomonas sp. (Ps) and Enterobacter sp. (Ec), on the growth of rice plants, soil microbial community composition and soil chemical properties. It was observed that the protist-PGPB interaction significantly modified the root structure leading to an enhanced outgrowth of lateral roots (272.08%-380.41%) and seminal roots (190.40%-250.45%), in addition to an increase in the primary root length (p < 0.05). The phospholipid fatty acid (PLFA) analysis indicated a striking shift in the overall soil microbial communities, especially in the Gram negative bacterial community and fungi, which were found to be decreased in treatments due to the presence of a predator. The combined ciliate-bacterial treatments further increased the microbial carbon biomass to 223.59% and 310.57% as compared to control and PGPB treatments respectively. A similar enhancement of dehydrogenase enzyme activity was observed in soil samples of rice plants on combined treatments. In contrast, the alkaline phosphatase and fluorescein diacetate enzyme activities were recorded to be more in soil samples treated with PGPB. The combined treatment of rice plants also enhanced the uptake of N and P moderately, as compared to PGPB treated plants. The colony-forming unit and most probable number were found to be significantly higher in PGPB treated soil and combined treatments (Tukey's HSD, p < 0.05). In conclusion, this is the first study that demonstrates significant modification of root structure and increased nutrient uptake by rice plants through interaction between ciliate and PGPB.