Quorum Sensing Is Required for the Colony Establishment of a Plant Phyllosphere Bacterium Rhodopseudomonas palustris Strain GJ-22.

The phyllosphere presents a hostile environment for many biocontrol agents; however, it is as significant as is the rhizosphere for plant health. Deploying biocontrol bacteria into the phyllosphere can efficiently suppress diseases; however, the lack of knowledge on the phyllosphere adaptive traits of biocontrol bacteria poses challenges. In this study, we demonstrated that Rhodopseudomonas palustris GJ-22 colonizes the phyllosphere by forming cell aggregates. The formation of cell aggregates required the production of exopolysaccharides (EPS), which depended on the function of the rpaI-rpaR quorum sensing (QS) mechanism, mediated by the signaling molecule p-coumaroyl-HSL (pC-HSL). The mutation of the EPS biosynthesis gene Exop1 or the signaling molecule biosynthesis gene rpaI compromised the ability of GJ-22 to tolerate reactive oxygen intermediates (ROIs), such as H2O2, in vitro and to form cell aggregates in vivo. Collectively, the results revealed that QS mediates EPS production and consequently leads to bacterial cell aggregation. IMPORTANCE Quorum sensing is used by various bacteria for coordinating the multiplication of bacterial cells in a group and for modulating the behaviors of surrounding microbial species. Host plants can benefit from this interspecies modulation, as it can disrupt the QS circuits of pathogenic bacteria. Some N-acyl homoserine lactone- (AHL-) producing bacteria that were introduced into the phyllosphere as biocontrol agents may establish AHL-based crosstalk with indigenous microbes to steer the nutritional and microecological conditions toward their own and the host plant's benefit. Here, we showed that biocontrol bacteria introduced into the phyllosphere require a functioning QS circuit to establish colonies and suppress pathogens. Furthermore, our findings provoked a broader investigation into the role of the QS circuit in beneficial microorganism-plant interactions.

A Sensitive Method for Detecting Beauveria bassiana, an Insecticidal Biocontrol Agent, Population Dynamics, and Stability in Different Substrates.

Beauveria bassiana is a well-known insecticidal biocontrol agent. Despite its broad field applications, its survival, colonization, and stability under field conditions remained unclear, mainly due to the lack of a quick and reliable detection method. In this study, we developed a quantitative real-time PCR technology to monitor the stability and population dynamics of B. bassiana in different substrates (water, soil, and on the cotton leaves surface), different spores of B. bassiana applied on Chinese cabbage leaves surface, and the lethality of Pieris rapae spraying with different spores of B. bassiana. Our results showed a decreased concentration of B. bassiana DNA in all three substrates from the 1st day till 9th day of post inoculation (dpi) period, possibly due to the death of B. bassiana. After this decrease, a quick and significant rebound of B. bassiana DNA concentration was observed, starting from the 11th dpi in all three substrates. The B. bassiana DNA concentration reached the plateau at about 13th dpi in water and 17th dpi in the soil. On cotton leaves surface, the B. bassiana DNA concentration reached the highest level at the 17th dpi followed by a small decline and then stabilized. This increase of DNA concentration suggested recovery of B. bassiana growth in all three substrates. We found that the most suitable killing effectiveness of P. rapae was the 1.0 × 107 spores/mL of B. bassiana. In summary, we have established a detection technology that allows a fast and reliable monitoring for the concentration and stability of B. bassiana under different conditions. This technology can benefit and help us in the development of proper management strategies for the application of this biocontrol agent in the field.

Microbiome homeostasis on rice leaves is regulated by a precursor molecule of lignin biosynthesis.

In terrestrial ecosystems, plant leaves provide the largest biological habitat for highly diverse microbial communities, known as the phyllosphere microbiota. However, the underlying mechanisms of host-driven assembly of these ubiquitous communities remain largely elusive. Here, we conduct a large-scale and in-depth assessment of the rice phyllosphere microbiome aimed at identifying specific host-microbe links. A genome-wide association study reveals a strong association between the plant genotype and members of four bacterial orders, Pseudomonadales, Burkholderiales, Enterobacterales and Xanthomonadales. Some of the associations are specific to a distinct host genomic locus, pathway or even gene. The compound 4-hydroxycinnamic acid (4-HCA) is identified as the main driver for enrichment of bacteria belonging to Pseudomonadales. 4-HCA can be synthesized by the host plant's OsPAL02 from the phenylpropanoid biosynthesis pathway. A knockout mutant of OsPAL02 results in reduced Pseudomonadales abundance, dysbiosis of the phyllosphere microbiota and consequently higher susceptibility of rice plants to disease. Our study provides a direct link between a specific plant metabolite and rice phyllosphere homeostasis opening possibilities for new breeding strategies.

Extraction Methods Determine the Quality of Soil Microbiota Acquisition.

The soil microbiome plays a key role in plant health. Native soil microbiome inoculation, metagenomic profiling, and high-throughput cultivation require efficient microbe extraction. Sonication and oscillation are the most common methods used to extract soil microbiomes. However, the extraction efficiency of these methods has not been investigated in full. In this study, we compared the culturable microbe numbers, community structures, and alpha diversities among the different methods, including sonication, oscillation, and centrifugation, and their processing times. The study results showed that sonication significantly increases the culturable colony number compared with oscillation and centrifugation. Furthermore, the sonication strategy was found to be the main factor influencing extraction efficiency, but increased sonication time can aid in recovery from this impact. Finally, the extraction processing times were found to have a significant negative relationship with α-diversity among the extracted microbiota. In conclusion, sonication is the main factor for enriching in situ microbiota, and increased extraction time significantly decreases the α-diversity of the extracted microbiota. The results of this study provide insights into the isolation and utilization of different microorganism sources.