Lysobacter enzymogenes prevents Phytophthora infection by inhibiting pathogen growth and eliciting plant immune responses.

The Phytophthora pathogen causes enormous damage to important agricultural plants. This group of filamentous pathogens is phylogenetically distant from fungi, making them difficult to control by most chemical fungicides. Lysobacter enzymogenes OH11 (OH11) is a biocontrol bacterium that secretes HSAF (Heat-Stable Antifungal Factor) as a broad-spectrum antifungal weapon. Here, we showed that OH11 could also control a variety of plant Phytophthora diseases caused by three major oomycetes (P. sojae, P. capsici and P. infestans). We provided abundant evidence to prove that OH11 protected host plants from Phytophthora pathogen infection by inhibiting mycelial growth, digesting cysts, suppressing cyst germination, and eliciting plant immune responses. Interestingly, the former two processes required the presence of HSAF, while the latter two did not. This suggested that L. enzymogenes could prevent Phytophthora infection via multiple previously unknown mechanisms. Therefore, this study showed that L. enzymogenes could serve as a promising alternative resource for promoting plant resistance to multiple Phytophthora pathogens.

Biocontrol of tobacco black shank disease (Phytophthora nicotianae) by Bacillus velezensis Ba168.

Tobacco black shank (TBS) caused by Phytophthora nicotianae is destructive to almost all tobacco cultivars and is widespread in many tobacco-growing countries. Through lab study and field test, we isolated plant growth-promoting rhizobacteria (PGPR) strain Ba168 which is a promising biocontrol strain of TBS. Ba168 was isolated from 168 soil samples and identified as Bacillus velezensis by its genetic and phenotypic characteristics. A susceptibility test indicated that the P. nicotianae antagonistic materials of Ba168 in extracellular metabolites were composed of effective and stable proteins/peptides. P. nicotianae's growth was suppressed by the ammonium sulfate precipitation of Ba168 culture filtrates (ASPBa) at a minimum inhibitory concentration of 5 µg/mL. Extracellular conductivity, pH, and the wet/dry weights of P. nicotianae's mycelia, along with scanning electron microscope analysis, suggested that Ba168-derived proteins/peptides could effectively inhibit P. nicotianae by causing irreversible damage to its cell walls and membranes. Protein identification of ASPBa supported these results and identified many key proteins responsible for various biocontrol-related pathways. Field assays of TBS control efficacy of many PGPRs and agrochemicals showed that all PGPR preparations reduced the disease index of tobacco, but Ba168 was the most effective. These results demonstrated the importance of Bacillus-derived proteins/peptides in the inhibition of P. nicotianae through irreversible damage to its cell wall and membrane; and the effectiveness of PGPR strain B. velezensis Ba168 for biocontrol of the soil-borne disease caused by P. nicotianae.