Floral nectar microbiota of Persea americana inhibits pathogens and improves plant fitness

The floral nectar-living microorganisms contribute to flower protection and influence mutualistic relationships between plants and pollinators. Avocado floral nectar is not attractive to the introduced pollinator Apis mellifera. More than 99% of the flowers produced by avocado trees are not able to set fruits, falling to the ground; the interaction between floral microbiota with plant roots is an emerging ecological topic to study. Here, we examined the richness and abundance of bacteria and fungi from avocado nectar and screened the culturable fraction for their antagonistic activity against the avocado pathogens Phytophthora cinnamomi and Colletotrichum gloeosporioides, and against the most devastating honeybee pathogens Ascosphaera apis and Paenibacillus larvae. Experimentally, we also analyzed the effects of the microbial isolates on plant growth and the activation of the jasmonic acid (JA) defense-responses in the model plant Arabidopsis thaliana. Pseudomonas, Acinetobacter, Protomyces and Aureobasidium were the most abundant genera. From 43 isolates evaluated, 20 bacteria, and three yeasts showed differential inhibitory activity against the plant and honeybee pathogens, promoted the growth of A. thaliana seedlings and induced defense responses. Collectively, our findings highlight the selectivity of avocado nectar over its microbiota, which could directly impact the plant fitness and contribute to its pollinators health.