Stark, F. G. | Search bioRxiv

bioRxiv2022

Pseudomonas isolates from ponds populated with duckweed prevent disease caused by pathogenic Pseudomonas species

Duckweeds are notoriously invasive plants. They are successful in inhabiting diverse environments, despite their lack of conventional immune pathways that are essential for disease resistance in other plant species. It is unclear how duckweeds thrive in the absence of these immune pathways. In this study, we investigated the effect of bacteria from duckweeds natural habitat on disease progression utilizing the duckweed-Pseudomonas pathosystem. Through nanopore sequencing of 16S and ITS rDNA amplicons we identified duckweed-associated bacterial and fungal genera present at three environmental sites. The pond filtrate from one of the three environmental locations primed duckweeds pathogen defenses leading to a reduction in disease symptoms. Furthermore, we were able to identify bacterial isolates from the filtrate that protect duckweed from disease symptoms upon Pseudomonas pathogen inoculation. The isolated protective bacteria belong to the Pseudomonas genus, and we demonstrated antagonistic interactions between the pathogen and beneficial strains in vitro and in vivo. The ability of our environmental isolates to protect against Pseudomonas pathogens appears to be plant/species specific as environmental strains showed no protective effect against Pseudomonas pathogens in Arabidopsis assays. Genome sequencing of the beneficial Pseudomonas strains showed the presence of several genes involved in bacterial competition. We have thus demonstrated that Pseudomonas species from duckweeds natural habitat can successfully antagonize other plant pathogens.

plant biology↗

bioRxiv2021

Pyrolyzed substrates induce aromatic compound metabolism in the post-fire fungus, Pyronema domesticum

Wildfires represent a fundamental and profound disturbance in many ecosystems, and their frequency and severity are increasing in many regions of the world. Fire affects soil by removing carbon in the form of CO2 and transforming remaining surface carbon into pyrolyzed organic material (PyOM). Fires also generate substantial necromass at depths where the heat kills soil organisms but does not catalyze the formation of PyOM. Pyronema species strongly dominate soil fungal communities within weeks to months after fire. However, the carbon pool (i.e. necromass or PyOM) that fuels their rise in abundance is unknown. We used a Pyronema domesticum isolate from the catastrophic 2013 Rim Fire (CA, USA) to ask if P. domesticum is capable of metabolizing PyOM. P. domesticum grew readily on agar media where the sole carbon source was PyOM (specifically, pine wood PyOM produced at 750 {degrees}C). Using RNAseq, we investigated the response of P. domesticum to PyOM and observed a comprehensive induction of genes involved in the metabolism and mineralization of aromatic compounds, typical of those found in PyOM. Lastly, we used 13C-labeled 750 {degrees}C PyOM to demonstrate that P. domesticum is capable of mineralizing PyOM to CO2. Collectively, our results indicate a robust potential for P. domesticum to liberate carbon from PyOM in post-fire ecosystems and return it to the bioavailable carbon pool. IMPORTANCEFires are increasing in frequency and severity in many regions across the world. Thus, its critically important to understand how our ecosystems respond to inform restoration and recovery efforts. Fire transforms the soil, removing many nutrients while leaving behind both nutritious necromass and complex pyrolyzed organic matter, which is often recalcitrant. Filamentous fungi of the genus Pyronema strongly dominate soil fungal communities soon after fire. While Pyronema are key pioneer species in post-fire environments, the nutrient source that fuels their rise in abundance is unknown. In this manuscript, we used a P. domesticum isolate from the catastrophic 2013 Rim Fire (CA, USA) to demonstrate that P. domesticum metabolizes pyrolyzed organic material, effectively liberating this complex pyrolyzed carbon and returning it to the bioavailable carbon pool. The success of Pyronema in post-fire ecosystems has the potential to kick-start growth of other organisms and influence the entire trajectory of post-fire recovery.

microbiology↗