Spinach Seed Microbiota Associated with <i>Globisporangium ultimum</i> Disease Suppression Are Culturable and Prevalent

Four microbial genera associated with spinach (Spinacia oleracea) seeds were recently described as indicators of suppressiveness against Globisporangium ultimum (previously known as Pythium ultimum) damping-off using amplicon sequencing. These were the bacterial genus Massilia as well as the basidiomycetous dimorphic yeasts Vishniacozyma, Filobasidium and Papiliotrema. Here, we examined the culturability of spinach seed microbiota using two spinach seed lots. Seeds were split into true seed and pericarp compartments and these were used to prepare suspensions that were plated on R2A and PDA plates to culture bacteria and fungi, respectively. We found that 49% of bacterial and 52% of fungal genera were culturable, including all four genera correlating with suppressiveness and most high-prevalence genera, while this was not the case for most low-prevalence genera. Most high-prevalence genera were found in both seed compartments (true seed and pericarp) with little variation between seed compartment communities, while more than 65% of microbial composition variation was explained by the factor whole seed sample, indicating high intraspecific seed microbiota variability. Given the culturability and ubiquity of indicator genera, our results are a stepping stone towards the use of seed microbiota for disease suppression.