Bergmann, G. E. | Search bioRxiv

bioRxiv2025

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

Flower-sourced assembly of seed microbiota remains an understudied ecological process. Here, we investigated the floral transmission pathway for bacterial acquisition by developing seeds of watermelon (Citrullus lanatus). Comparison of stigma- and seed-associated bacterial communities from field-grown C. lanatus revealed significant overlap: up to 40% of the bacterial diversity that was detected in seed was also found on stigmas. In a field pollinator exclusion experiment, honeybee visitation to flower stigmas had no significant effect on bacterial community composition in seeds. Among a collection of bacterial isolates from stigmas and seeds in the field, more than half (57%) were able to transmit to seeds after inoculation onto stigmas under laboratory conditions. Interestingly, for most bacterial strains, fruit set rates increased after floral inoculation, and in some cases even in the absence of transmission to the seed. We also found that bacterial isolates from watermelon stigmas and seeds had variable (i.e. positive or negative) effects on seed germination and seedling emergence. Our findings highlight the contribution of floral transmission to seed microbiota assembly and its consequences for plant fitness.

ecology↗

bioRxiv2020

Geographic variation in the seed mycobiome of Pseudotsuga menziesii var. menziesii across the Pacific Northwest, USA

Fungal symbionts occur in all plant tissues, and many aid their host plants with critical functions, including nutrient acquisition, defense against pathogens, and tolerance of abiotic stress. "Core" taxa in the plant mycobiome, defined as fungi present across individuals, populations, or time, may be particularly crucial to plant survival during the challenging seedling stage. However, studies on core seed fungi are limited to individual sampling sites, raising the question of whether core taxa exist across large geographic scales. We addressed this question using both culture-based and culture-free techniques to identify the fungi found in individual seeds collected from nine provenances across the range of Coastal Douglas-fir (Pseudotsuga menziesii var. menziesii), a foundation tree species in the Pacific Northwest and a globally important timber crop that is propagated commercially by seed. Two key findings emerged: 1) Seed mycobiome composition differed among seed provenances. 2) Despite spatial variation in the seed mycobiome, we detected four core members, none of which is a known pathogen of Douglas-fir: Trichoderma spp., Hormonema macrosporum, Mucor plumbeus and Talaromyces rugulosus. Our results support the concept of a core seed microbiome, yet additional work is needed to determine the functional consequences of core taxa for seedling germination, growth, survival and competition.

ecology↗