Bacterial endophyte traits in vitro do not predict protection from a fungal pathogen in planta
Matthew Bakker: USDA ARS
<div><span style="font-family:'Palatino Linotype';">All plants are colonized by an array of microbiota, with impacts on host function that can span a continuum from beneficial to pathogenic. Increasingly, endophytes (those microbiota that reside inside plant tissues) are seen as potentially useful symbionts for conferring disease suppression or abiotic stress tolerance. Commonly used approaches to identify putatively beneficial symbionts include plate competition assays and co-inoculation on plant substrates in the lab. However, few studies have directly contrasted functional roles inferred from <i>in vitro </i>tests with functional outcomes <i>in planta</i>.</span><span style="font-family:'Palatino Linotype';"> <span>This can lead to a failure of candidate biocontrol agents when transferred to more realistic field conditions. Our objective was to test whether traits displayed by bacterial endophytes <i>in vitro </i>would be predictive of disease outcomes <i>in planta</i>. Using two common <i>in vitro </i>assays, we contested bacterial endophytes isolated from wheat plants against <i>Fusarium graminearum</i>, a fungal pathogen that causes Fusarium head blight. A subset of isolates, ranging from weakly to strongly antagonistic in the <i>in vitro</i> assays, were selected for an <i>in planta </i>assay. All assays were performed under variable abiotic conditions to test the climatic resilience of the plant-fungal-bacterial interactions. We found that the degree of pathogen inhibition detected in the two <i>in vitro </i>assays was not predictive of pathogen loads in the <i>in planta </i>assay. This was true across temperature and carbon dioxide conditions chosen to reflect potential future climate change scenarios. Additionally, climatic resilience within the plant-fungal-bacterial interaction varied among bacterial isolates. We suggest that field or <i>in planta </i>trials of the impacts of endophytes on plant performance should include strains that both do and do not show <i>in vitro </i>activity.</span></span></div>
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