An endosymbiotic Paenibacillus sp. modulates disease severity caused by the common watermelon pathogen, Fusarium oxysporum f sp. niveum

Fungal plant pathogens can be affected by the bacteria they interact with in their environment, yet the characterization of these interactions beyond direct antagonism is lacking, especially in the case of endohyphal bacteria (EHB). Though limited in characterized examples, EHB can alter disease severity of their fungal host, providing either a potential tool or target for control. We screened isolates of Fusarium oxysporum f. sp. niveum (FON), an important soil-borne watermelon pathogen, using 16S PCR and fluorescence in situ hybridization microscopy to identify novel EHB. A symbiont of FON AS124 was identified to be a Paenibacillus sp. through genome sequencing and average nucleotide identity. To begin characterizing this relationship, we conducted watermelon infection assays using FON cured of its symbiont, the native association, and a coinoculation of fungi and bacteria. Disease severity was reduced in watermelon seedlings inoculated with the native association, though not in the coinoculation, and Paenibacillus sp. CB74 did not alone promote plant growth or inhibit fungal growth. This study shows an important functional outcome, reduced disease, for a novel symbiosis between FON and Paenibacillus sp. CB74, setting up further investigation into the mechanisms behind this outcome and the application of this interaction. ImportanceFungi pose a challenge in both the field and hospital as antifungal resistance rises and chemical control is increasingly scrutinized. In plant pathogenic fungi, endohyphal bacteria may present alternative targets or mechanisms of fungal control. These relationships are observed across diverse groups of fungi and bacteria, though few have been studied to the point of understanding impact. To contribute to the small but growing catalog of known endofungal bacterial relationships, we identified a novel symbiosis and began characterizing its functional outcomes with plant infection assays. The identified bacterial symbiont does alter disease severity of the fungal host offering a new system for both application and study of fungal pathogenesis.