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.

Secondary metabolites from methylotrophic bacteria: their role in improving plant growth under a stressed environment.

Climate change is considered a natural disaster that causes the ecosystem to fluctuate and increase temperature, as well as the amount of UV radiation (UV-A and UV-B) on the Earth's surface. Consequently, greenhouse gases such as chlorofluorocarbons, methane, nitrogen oxide, and carbon dioxide have become obstacles to the development of sustainable agriculture. To overcome environmental stress such as phytopathogens, drought, salinity, heavy metals, and high-low temperatures, the utilization of microorganisms is a viable option. The synthesis of secondary metabolites by methylotrophic bacteria improves plant metabolism, enhances tolerance, and facilitates growth. The genus Methylobacterium is a pink-pigmented facultative methylotrophs which abundantly colonizes plants, especially young leaves, owing to the availability of methanol. Secondary metabolites such as amino acids, carotenoids, hormones, antimicrobial compounds, and other compounds produced by methylotrophic bacteria enhance plant metabolism under stress conditions. Therefore, in this review, we discuss the role of secondary metabolites produced by methylotrophic bacteria and their role in promoting plant growth under stress.

Predatory activity of Acanthamoeba sp genotype T4 on different plant growth-promoting bacteria and their combined effect on rice seedling growth.

Heterotrophic protists play a crucial role in plant growth promotion via nutrient cycling and shift in microbial community composition in the soil ecosystem. Selective predation pressure by protists contributes to the evaluation of plant beneficial traits in rhizospheric bacteria. However, not always all plant growth-promoting bacterial (PGPB) strains are benefitted by predation. This study aimed to examine the predatory effect of Acanthamoeba sp genotype T4 on a range of PGPB strains and their combined impact on early rice seedling growth. Acanthamoeba sp isolated from rice rhizosphere soils were used to assess predation against several PGPB such as Pseudomonas, Bacillus, Enterobacter, Morganella, Stenotrophomonas, Providencia, and Lysinibacillus on Nutrient Yeast Extract agar (NYE) plate. The controlled experiment on the germinated rice seeds (Oryza sativa L.) grown in Petri dishes containing each PGPB strain and Acanthamoeba sp was performed to evaluate the combined impact on plant performance. The PGPB-Acanthamoeba combined treatments in Petri dishes showed significant rice seedling growth compared to PGPB alone, non-PGPB and control. Our results indicated the positive but different impact of Acanthamoeba sp with different PGPB species on early rice plant growth. Further in-depth research should be carried out with diverse protists and PGPB species to assess which protist species can be linked to enhancement of indigenous soil PGPB for improved plant growth.