Predation pressure regulates plant growth promoting (PGP) attributes of bacterial species.

Aim: The present study aimed to investigate the effect of bacterivorous soil protists on plant growth promoting (PGP) attributes of bacterial species and their co-inoculative impact on rice seedling growth.

Methods And Results: The effect of protists on the PGP attributes of bacteria was tested using standard protocols. The results revealed that the plant-beneficial properties of plant growth promoting bacteria (PGPB) were altered in the presence of various protist species. A significant increase in the production of siderophore units (86.66%), ammonia (34.80 μmol mL-1), and phosphate solubilization index (PSI) (5.6) was observed when Bacillus cereus (Bc) and Pseudomonas fluorescens (Ps) were co-inoculated with unidentified species belonging to the family Kreyellidae (C5). In the case of Enterobacter cloacae co-inoculated with C5 (Kreyellidae), a higher amount of siderophore (51.33%), ammonia (25.18 μmol mL-1), and indole-3-acetic acid (IAA)-like substance (28.59 μg mL-1) production were observed. The biofilm-forming ability of B. cereus is enhanced in the presence of Tetrahymena sp. (C2Bc), unidentified Kreyellidae (C5Bc), and Colpoda elliotti (C12Bc), whereas E. cloacae showed higher biofilm formation in the presence of Tetrahymena sp. alone Although IAA production decreased under predation pressure, a significant increase in shoot length (64.24%) and primary root length (98.18%) in co-inoculative treatments (C12Bc and C5Bc) compared to bacteria alone (25% and 61.50% for shoots and roots, respectively) was observed. The results of enhanced PGP attributes and rice seedlings growth under predation pressure correlated with the enhanced bacterial activity under predation pressure and protist involvement in plant growth development.

Conclusions: Protists may act as regulators of the bacterial activities involved in plant growth promotion and thus enhance plant growth.