Contribution of floral transmission to the assembly of bacterial communities in watermelon seeds

Seed-associated bacteria can impact germination and seedling development. Some of these bacteria are acquired via the flower stigma. Evidence of floral transmission during seed microbiota assembly is mostly limited to seed-borne pathogens. Here, we investigated floral transmission during seed microbiota assembly of field-grown watermelon (Citrullus lanatus). We performed multiple experiments to 1) compare stigma- and seed-associated bacterial community composition of C. lanatus, 2) determine if insect pollination affects seed microbiota composition, 3) show that non-pathogenic bacteria can transmit florally, and 4) screen the impacts of such bacteria on seed vigor. A microbiota survey of C. lanatus fields showed that 2-40% of seed-associated bacterial taxa were also detected on stigmas, making up on average 40.4% of the seed bacterial community's relative abundance. In a field pollinator exclusion experiment, we found that honeybee visitation of flowers did not significantly alter bacterial community composition in seeds. By inoculating bacterial strains onto stigmas, we validated that some but not all strains (57% of those tested) could transmit to seeds. Finally, we found that inoculating florally transmitted bacteria onto watermelon seeds had variable (i.e. positive or negative) effects on germination and seedling emergence. Our findings demonstrate the contribution of floral transmission to seed microbiota assembly in watermelon. Moreover, florally transmitted bacteria can impact fruit and seedling development, demonstrating a role of floral transmission to seeds that affects plant fitness.